Orcas Moon Cottages Preliminary Technical …Orcas+Moon+TIR.pdfOrcas Moon Cottages Preliminary Technical Information Report Job # 12-248 Page 1-2 on-site runoff, provide flow control,

Orcas Moon Cottages Preliminary Technical Information Report

Job # 12-248 i

Table of Contents

SECTION 1 PROJECT OVERVIEW......................................................................................... 1-1

Existing Conditions Exhibit ................................................................................................................................. 1-3 Developed Conditions Exhibit ............................................................................................................................ 1-4

SECTION 2 CONDITIONS AND REQUIREMENTS SUMMARY .............................................. 2-1

SECTION 3 OFFSITE ANALYSIS ........................................................................................... 3-1

3.1 Level I Downstream Analysis ..................................................................................................... 3-1 Task 1: Define and Map the Study Area ........................................................................................................... 3-1 Task 2: Resource Review ................................................................................................................................... 3-1 Task 3 and Task 4: Field Inspection and Drainage System Description ......................................................... 3-6 Task 5: Mitigation of Existing or Potential Problems ........................................................................................ 3-7

SECTION 4 FLOW CONTROL AND WATER QUALITY ANALYSIS AND DESIGN ..................... 4-1

4.1 Hydraulic Analysis ....................................................................................................................... 4-1 Existing Conditions ............................................................................................................................................. 4-1 Developed Conditions ........................................................................................................................................ 4-2

4.2 Individual Lot BMP Requirements ............................................................................................. 4-3 COK Policy L-2 Acknowledgment ....................................................................................................................... 4-4

4.3 Vault Performance ...................................................................................................................... 4-6 4.4 Water Quality Analysis and Design ............................................................................................ 4-7

SECTION 5 CONVEYANCE DESIGN ..................................................................................... 5-1

SECTION 6 SPECIAL REPORTS AND STUDIES .................................................................... 6-1

SECTION 7 OTHER PERMITS .............................................................................................. 7-1

SECTION 8 TESC ANALYSIS AND DESIGN .......................................................................... 8-1

SECTION 9 BOND QUANTITIES AND FACILITIES SUMMARY .............................................. 9-1

SECTION 10 OPERATIONS AND MAINTENANCE ................................................................. 10-1

APPENDIX

City Correspondence with Wes Ayers for Drainage Complaints


Job # 12-248 Page 1-1

Section 1 Project Overview

The project is located at 4XX 20th Ave, Kirkland, WA. More generally, the site is located in NE

¼ of Section 32, Township 26 N, Range 5 E, W.M. Please see the vicinity map below.

Refer to the Existing & Developed Conditions Exhibits included on the following pages. The site

consists of a single parcel (#389010-0050) totaling approximately 6.64 acres pending lot line

adjustment (LLA #TBD). The site is generally forested with several streams conveying water

through the site. Two streams divide the parcel creating two discharge locations from the site.

A third stream, along the eastern property boundary, meanders on and off of the project and

combines with flows from one of the on-site streams. In the southwestern portion of the site is

a Type 3 wetland and associated buffers. Downstream drainage converges along the south

side of Forbes Creek Drive, within a 1/4–mile of the site, creating one threshold discharge

area. In general, drainage from the western basin sheet flows toward one of the on-site stream

and discharges to the roadside ditch along the south side of Forbes Creek Drive. Drainage from

the eastern basin sheet flows towards the on-site streams and flows north towards a road side

ditch along the south side of Forbes Creek Drive as well. The developed condition will collect


Job # 12-248 Page 1-2

on-site runoff, provide flow control, water quality treatment and discharge to the public

stormwater infrastructure along the south side of Forbes Creek Drive.

According to the USDA Natural Resource Conservation Service Web Soil Survey, soils are a

combination of Indianola loamy sand with 5-10% slopes and Kitsap silt loam with slopes

ranging from 2-30%. The steep slopes make these soils unfavorable for infiltration. A Web Soil

Survey is included in Section 3.

The site is bound by Forbes Creek Drive to the north, generally forested land with a few single

residences to the east and west and 20th Avenue to the south. The project proposes to remove

trees within the development area and subdivide the existing parcel (pending LLA) into 15

cottage lots with associated access and utilities. A Developed Conditions Exhibit is included in

Section 3 for reference.

Stormwater elements will be designed in accordance with the 2009 King County Surface Water

Design Manual (KCSWDM) as adopted and amended by the City of Kirkland. The proposed

detention facilities are designed to meet the Level II Flow Control Standard. Water quality

treatment facilities are designed according to the Basic Treatment requirement and additional

requirements established by the City of Kirkland. Stormwater Low Impact Development (LID)

features will be designed in accordance with Kirkland Policy L-2. Additional information related

to stormwater management elements is provided in later sections of this report.


Job # 12-248 Page 2-1

Section 2 Conditions and Requirements Summary

Core and special requirements listed in Section 1.2 of the 2009 KCSWDM are listed below.

Core Requirement #1: Discharge at the Natural Location

See Section 3. In the existing conditions, a portion of the stormwater runoff sheet flows

towards the western on-site stream and discharges to the roadside ditch along the south side

of Forbes Creek Drive. The remaining portion of the site sheet flows towards the two eastern

on-site streams and discharges to the roadside ditch along the south side of Forbes Creek

Drive. The western stream flows combine with the flows from the eastern streams along the

south side of Forbes Creek Drive within a ¼-mile. In the developed conditions, runoff will be

collected onsite, conveyed to a detention vault along the northern portion of the site and

discharge to the public infrastructure along the south side of Forbes Creek Drive.

Core Requirement #2: Off-site Analysis

See Section 3. A Level 1 Downstream Analysis prepared by The Blueline Group is included in

the Offsite Analysis.

Core Requirement #3: Flow Control

See Section 4. The site will provide a detention vault meeting Level 2 Flow Control

requirements per Chapter 5 of the 2009 King County Stormwater Design Manual (KCSWDM).

Core Requirement #4: Conveyance System

See Section 5. The conveyance system will be designed according to the 2009 KCSWDM to

convey the 100-year, 24-hour storm event without overtopping. A conveyance analysis will be

provided at final engineering.

Core Requirement #5: Erosion and Sediment Control

See Section 8. The temporary erosion and sedimentation control (TESC) plan will consist of

temporary measures (rock entrance, inlet protection, silt fence, sediment trap, etc.) as well as

permanent measures (hydroseeding and landscaping). A TESC plan will be submitted with the

final engineering plans.

Core Requirement #6: Operations and Maintenance

See Section 10. Operations and Maintenance information will be provided for the proposed

stormwater facilities with the final TIR.


Job # 12-248 Page 2-2

Core Requirement #7: Financial Guarantees and Liability

A City of Kirkland Improvement Evaluation Worksheet will be included with the final TIR.

Core Requirement #8: Water Quality

Water quality will be by a combination detention/wetvault and is designed to comply with the

2009 KCSWDM’s Basic Water Quality Menu. See Water Quality of Section 4 for further

information.

Special Requirement #1: Other Adopted Area-Specific Requirements: There are no known

additional requirements for the subject project.

Special Requirement #2: Floodplain/Floodway: The subject site does not contain or is

adjacent to a flood hazard area. This Special Requirement is therefore, not applicable.

Special Requirement #3: Flood Protection Facilities: The subject site does not rely on an

existing flood protection facility and will not modify or construct a new flood protection

facility. This Special Requirement is therefore, not applicable.

Special Requirement #4: Source Control: The subject project is a residential development

and not a commercial development. This Special Requirement is not applicable.

Special Requirement #5: Oil Control: The subject project is a single family development and

not a high-use development. This Special Requirement, therefore, is not applicable.


Job # 12-248 Page 3-1

Section 3 Offsite Analysis

A Level I Downstream Analysis is included in this section.

3.1 LEVEL I DOWNSTREAM ANALYSIS

TASK 1: DEFINE AND MAP THE STUDY AREA

A Downstream Drainage Exhibit and Downstream Drainage Photographs are included at the

end of this section.

TASK 2: RESOURCE REVIEW

The best available resource information, including King County iMAP and City of Kirkland

resource maps, were reviewed for existing or potential problems. The following is a summary

of the findings from the information used in preparing this report.

According to the USDA soil survey, the onsite soils are a combination of Indianola

loamy sand with 5-10% slopes and Kitsap silt loam with slopes ranging from 2-30%.

The site contains a single drainage basin that is a part of the Forbes Creek Drainage

Basin (City of Kirkland).

The site contains a wetland (City of Kirkland). Refer to critical areas report prepared by

Talasaea Consultants, Inc. included in Section 6 of this report.

The site contains several streams (City of Kirkland). Refer to critical areas report

prepared by Talasaea Consultants, Inc. included in Section 6 of this report.

The site is not located within a floodplain (City of Kirkland).

The site is located in an Erosion Hazard Area (King County iMap).

A portion of the site is located in a High Landslide Hazard Area (City of Kirkland).

The site is not located in a Seismic Hazard Area (City of Kirkland and King County iMap).

The site and its downstream path has one relevant drainage complaints on record or

in adjacent parcels – refer to Task 5 on the following pages and e-mail correspondence

with Wes Ayers included in the Appendix section of this report.


Job # 12-248 Page 3-2

City of Kirkland Sensitive Areas Map

SITE


Job # 12-248 Page 3-3

City of Kirkland Landslide and Hazards Area Map

SITE


Job # 12-248 Page 3-4

King County iMap Erosion Hazards Area Map

PROJECT SITE

EROSION HAZARD AREA


Job # 12-248 Page 3-5


Job # 12-248 Page 3-6

TASK 3 AND TASK 4: FIELD INSPECTION AND DRAINAGE SYSTEM DESCRIPTION

A field inspection was conducted on Friday, July 29, 2016, a sunny day with temperatures

around 80°F. Please reference the Downstream Drainage Exhibit, and Downstream Drainage

Photographs included at the end of this section.

Onsite Basin The site is generally forested with no existing onsite drainage facilities. As such, stormwater

generated onsite typically sheets flows following existing grades. Onsite there are several

streams conveying runoff from the upstream area. Two streams divide the parcel creating two

discharge locations from the site. A third stream, along the eastern property boundary,

meanders on and off of the project and combines with flows from one of the on-site streams.

The USDA NRCS Web Soil Survey indicates the onsite soils are a combination of Indianola

loamy sand with 5-10% slopes and Kitsap silt loam with slopes ranging from 2-30%. The

drainage path is described below; see Existing Conditions Exhibit in Section 1 of this report.

Upstream Basin The existing parcel contains several streams which are fed from the areas upstream of the

project. In locations where the proposed development will interact with stream paths, the

stream paths have been tightlined. In regions where the streams exist as surface flow, the

project will employ setbacks based on the type of stream, additionally, 20th Avenue slopes

south, away from the project. As such, upstream inputs are considered negligible.

Existing Downstream Drainage Path Runoff sheet flows and shallow concentrated flows northeast and/or flows to one of the two

onsite streams. Runoff ultimately discharges to a roadside ditch along the south side of Forbes

Creek Drive (Photo 2 and 3). Runoff continues to a Type II birdcage structure located at the

low point of Forbes Creek drive (Photo 4). The birdcage structure then tightlines the flows north

under Forbes Creek Drive where it then discharges to an open channel (Photo 5 and 6). Runoff

continues north until it converges with Forbes Creek (Photo 7). Forbes Creek continues west

under Market Street where the downstream analysis was concluded, ¼ mile from the project

(Photo 8). The flow path appeared to be functioning well and there were no signs of overtopping

or cause for concern.


Job # 12-248 Page 3-7

TASK 5: MITIGATION OF EXISTING OR POTENTIAL PROBLEMS

At the time of the site investigation, no problems were found with the existing system. Existing

catch basins and pipes require no immediate corrective maintenance. An inquiry was made

with the City of Kirkland for records of any relevant drainage complaints within the downstream

path, and one relevant drainage complaint occurred within the preceding ten-year period.

Drainage complaint SW-06-113 indicates a property adjacent to the downstream drainage

path, 10454 Forbes Creek Drive, reported the since Forbes Creek 11 was built they are

receiving additional runoff into the existing wetland making it wetter and the drainage coming

off of the adjacent hillside has increased. The project’s flow control and water quality facilities

will be designed to minimize the impacts of development to the downstream system. Runoff

from the developed area will be conveyed via the public tight-line system prior to discharging

to the roadside swale along the south side of Forbes Creek Drive, and therefore is not

anticipated to have further impact on the previously reported drainage issue adjacent to the

downstream path.

An erosion and sedimentation control plan will be designed to reduce the discharge of

sediment-laden runoff from the site. The plan is comprised of temporary measures (rock

entrance, filter fence, straw mulch, etc.) as well as permanent measures (hydroseeding and

landscaping). All ESC facilities will be periodically inspected and maintained as necessary

during construction to minimize impacts to the downstream system.

BLUELINE

Off-site Analysis Drainage System TableSurface Water Design Manual, Core Requirement #2

Basin Name: Lake Washington / Cedar River Subbasin Name: Forbes Creek

SymbolDrainage Component Type, Name and Size

Drainage Component Description

SlopeDistance from site discharge

Existing Problems Potential ProblemsObservations of field inspector, resource

reviewer, or resident

see mapType: sheet flow, swale,

stream, channel, pipe pond; Size: diameter, surface area

drainage basin, vegetation, cover, depth, type of

sensitive area, volume% 1/4 mi = 1320 ft.

tributary area, likelihood of problem, overflow pathways,

pontential impacts

Photo 1Onsite sheet flow and shallow concentrated

flowforest ~15% 0 ft none none No problems observed

Photo 2&3 Channelized flow Road side ditch

collects flows from forest and pavement

1.00% 0 ft none none No problems observed

Photo 4&5Type II Birdcage CB and

55-foot 12-inch pipeConveys flows under Forbes Creek Drive

1.00% 55 ft none none No problems observed

Photo 6&7 Unnamed ditch/streamDischarge from 12-

inch pipe flows ~450 ft north

~1% 500 ft none none No problems observed

Photo 8 Forbes Creek

Connection from unnamed stream

travels West in Forbes Creek

1,320 ft none none No problems observed

constrictions, under capacity, ponding, overtopping, flooding, habitat or organism

destruction, scouring, bank sloughing, sedimentation, incision, other erosion


Job # 12-248 Page 3-10

Downstream Drainage Photographs

Note: See the Downstream Drainage Exhibit for numbered locations of pictures.

Photo 1: Runoff from the western stream running through the site discharges from concrete

culvert and enters roadside ditch along the south side of Forbes Creek Drive – looking north.

Photo 2: Runoff is conveyed east via roadside ditch to a low point along the south side of

Forbes Creek drive – looking east.


Job # 12-248 Page 3-11

Photo 3: Runoff from the eastern on-site streams discharge to the roadside ditch along the

south side of Forbes Creek Drive and is conveyed west to the low point in the ditch – looking

west.

Photo 4: A Type II catch basin with birdcage structure located at the low point of the roadside

ditch along the south side of Forbes Creek Drive collects flows from streams discharging to

the ditch in addition to the outlet from the Forbes Creek 11 project – looking west.


Job # 12-248 Page 3-12

Photo 5: Flows from the birdcage structure are conveyed north underneath Forbes Creek

Drive via a tight-line system and discharge along the north side of the road to a drainage

channel – looking south.

Photo 6: Runoff discharging from the tight-line system is conveyed north toward Forbes

Creek – looking north.


Job # 12-248 Page 3-13

Photo 7: Runoff from the drainage channel discharges to Forbes Creek and is conveyed west

toward Lake Washington – looking west.

Photo 8: Forbes Creek is conveyed underneath 98th Avenue, past the quarter-mile

downstream point, and continues west prior to discharging to Lake Washington – looking

north.


Job # 12-248 Page 4-1

Section 4 Flow Control and Water Quality Analysis and Design

Runoff from the developed site will be collected on site via a network of catch basins and pipes

and will be routed to one of two individual detention vaults along the northern portion of the

developed area. Please see the Developed Conditions Exhibit in Section 1 of this report.

4.1 HYDRAULIC ANALYSIS

The drainage analysis was modeled using the King County Runoff Time Series (KCRTS). Per

the USDA Natural Resource Conservation Service Web Soil Survey, included in Section 3 of

this report, soils in the vicinity are Indianola loamy sand. The site soils are till soils which are

typically not suitable for infiltration.

The project was modeled with the following parameters:

Rainfall Region: Seatac

Scale Factor: 1.0

EXISTING CONDITIONS

The existing basin boundary is established by the project’s limits of disturbance. This area is

approximately 2.24-acres and includes 0.18-acres of frontage improvements, and 0.09 acres

of access improvements. Please see the Existing Conditions Exhibit in Section 1 of this report.

The site is generally forested and contains an access road for the property to the east. In

general, a portion of the site runoff sheet flows towards the northern property boundary and

enters the public tight-line conveyance system and the remaining portion sheet flows towards

the on-site streams.

The site lies within a Level 2 Flow Control Area which dictates that the existing condition be

modeled in the historic (forested) condition, matches durations for 50% of the 2-year through

the 50-year peaks, and matches 2 and 10-year peaks. The existing areas are summarized

below:

EXISTING CONDITIONS Forest

Parcel 1.97 ac

Frontage 0.18 ac

Access 0.09 ac

Total Forest 2.24 ac

TOTAL EXISTING CONDITIONS 2.24 ac


Job # 12-248 Page 4-2

DEVELOPED CONDITIONS

Proposed development will include the creation of 15 cottage lots with associated utilities and

infrastructure, drainage tract with flow control/water quality treatment facilities, sensitive area

tract, and frontage improvements. The existing discharge location from the site will be

preserved. The developed basin boundary areas are the same as the existing basin boundary

areas, 2.102-acres. The developed site conditions include impervious surfaces such as roofs,

patios, driveways, sidewalk, and pervious landscaped/lawn area.

As the site is within a Level 2 Flow Control Area the development is required to match

developed discharge durations to pre-developed durations for the range of pre-developed

discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow. Also match

developed peak discharge rates to pre-developed peak discharge rates for the 2- and 10-year

return periods. The detention vault pass all of these requirements. The areas used to compute

the drainage calculations associated with the developed conditions are summarized as

follows:

DEVELOPED CONDITIONS Impervious To Vault

Lot Cover 0.82 ac

Frontage 0.12 ac

Impervious to Vault 0.94 ac Pervious to Vault

Lot Lawn 1.05 ac

ROW Lawn 0.06 ac

Pervious to Vault 1.11 ac

Total to Vault 2.05 ac

Impervious Bypass

Lot Cover 0.05 ac Access Cover 0.06 ac

Impervious Bypass 0.11 ac Pervious Bypass

Site Lawn 0.05 ac Access Lawn 0.03 ac

Pervious Bypass 0.08 ac

Total Bypass 0.19 ac

TOTAL DEVELOPED CONDITIONS 2.24 ac


Job # 12-248 Page 4-3

4.2 INDIVIDUAL LOT BMP REQUIREMENTS

To meet Low Impact Development (LID) requirements for the Full Drainage Review as outlined

in the City of Kirkland Policy L-2, the project is required to apply LID BMPs to an area equal to

at least 10% of the lot since each lot area is less than 11,000sf. Native growth retention credit

as shown on the proposed plans and tabulated on the following page will be used to the meet

the requirement.

Large portions of the site have slopes that are over 40% and as such are considered steep

slopes. These steep slopes make full and limited dispersion not possible on this project.

Additionally, the proximity of the steep slopes to the developed area also make full and basic

infiltration not possible per the geotechnical engineer’s recommendation. The proposed

project graded has slopes ranging from 5-22% to accommodate the existing slopes. These

proposed slopes are too steep to use rain gardens and permeable pavements.

The cottage project requirements restrict the house sizes and implement open space

requirements. These requirements reduce the impervious area and increase the pervious

area. Additionally, the remainder of the site area will be preserved as native open space and

meets the Native Growth Retention Credit (NGRC). The NGRC requires a preservation ratio of

3.5 square feet of native growth area to 1 square foot of mitigated area. A summary of the

required and proposed LID areas is provided below.

10% of total lot area = 27,820 SF x 0.10 = 2,782 SF

Native Growth Retention required = 2,782 SF x 3.5 = 9,737 SF

Native Growth Space preserved = 217,286 SF (approximately 20,000 SF < 15% slope)

1 Full Drainage Review refers to projects creating 5,000ft2 or more new impervious surface area (not replaced). See policy D-3 for full definition.

CITY OF KIRKLAND 123 FIFTH AVENUE � KIRKLAND, WASHINGTON 98033-6189 � (425) 587-3800

DEPARTMENT OF PUBLIC WORKS PRE-APPROVED PLANS POLICY

Policy L-2: FEASIBILITY OF STORMWATER LOW IMPACT DEVELOPMENT (LID) FOR FULL DRAINAGE REVIEW1 PROJECTS

Applicants for projects meeting the threshold for full drainage review must evaluate the Feasibility and Applicability of dispersion, infiltration, and other stormwater LID options. All stormwater LID BMPs must be designed and installed according to the 2009 King County Surface Water Design Manual (KCSWDM), COK Addendum, and the PW Pre-Approved Plans. Listed below are the Stormwater LID BMP options from the 2009 KCSWDM (listed in order of preference):

• Dispersion (Full or Basic) • Infiltration (Full or Basic)

• Limited Infiltration • Rain Garden • Permeable Pavement • Rainwater Harvesting • Vegetated Roof • Reduced Impervious Surface Credit

The reduction in impervious surface area below maximum lot coverage must be assured through

recorded covenant and/or alternative design of impervious surface area. Reduction techniques include: restricted footprint, wheel strip driveways, minimum disturbance foundation, and open grid

decking over pervious surface.

• Native Growth Retention Credit Credit for preserving native growth at the rate of 1 sq ft impervious requires 3.5 sq ft of native vegetated surface – in other words, for every 3.5 sq ft of native vegetation area preserved, 1 sq ft

of target impervious surface may be credited as mitigated. EVALUATION Evaluate the feasibility of Dispersion BMPs:

• Full Dispersion is feasible if the proposed project has 50-100 feet of native vegetated flowpath (depending on the dispersion method) to disperse the runoff from the new and/or replaced impervious surface area. A SW Adjustment Form (Policy D-11) may be necessary if less than 100ft flowpath, with approval on a case-by-case basis.

• Basic Dispersion is feasible if the project has 25-50 feet of vegetated flowpath (does not have to be native vegetation) to disperse the runoff. Other constraints affecting feasibility are steep slopes (greater than 15%), sensitive areas, and the potential to cause or aggravate flooding or erosion problems to neighboring problems. If dispersion is feasible, use the design criteria for dispersion BMPs in the flow control section of Chapter 5 in the 2009 KCSWDM.

If Dispersion BMPs are not feasible, evaluate the feasibility of Infiltration BMPs. Potential constraints to consider are soil type, ground water level, and steep slopes (15% or greater). If possible, infiltration facilities should be designed with an overflow connection to the public storm drainage system. Use the design criteria for infiltration BMPs in the flow control section of Chapter 5 in the 2009 KCSWDM.

Last Revised: 03/2016

Pre-Approved Plans Policy L-2

March, 2016

2

If the BMPs involving dispersion or infiltration are not feasible, then evaluate the feasibility of the other LID BMPs (rainwater harvesting, vegetated roof, reduced impervious surface credit, and native growth retention credit). SOIL REPORT A Soil Report is required for infiltration facilities, bioretention facilities, and pervious pavement for projects triggering a full drainage review. A Soil Report includes (2009 KCSWDM Sec 5.4, page 5-57):

1. At least two soil logs for each proposed infiltration location

• Borings shall extend at least 5 feet below the bottom of the LID facility • A description of the soil series and the textural class of each horizon through the depth

of the log, and • Notes of any evidence of a high groundwater table, such as mottling.

2. Level of maximum wet-season water table 3. Measured infiltration rates and a recommended design infiltration rate. Three infiltration tests,

equally spaced within the limits of proposed facility, per proposed infiltration facility, except for drywells that only require one test.

4. Soil reports must be prepared by or under the direction of a licensed onsite sewage system designer, civil engineer, engineering geologist, or geotechnical engineer.

BMP AREA REQUIREMENTS The minimum amount of impervious area routed to the storm LID BMPs varies based on the amount of impervious coverage in the developed condition:

1. For a lot up to 11,000sf, route runoff from an impervious surface area equal to at least 10% of the lot to one or more storm LID BMPs.

2. For a lot between 11,000 and 22,000sf, route runoff from an impervious surface area equal to at least 20% of the lot to one or more storm LID BMPs. If total impervious area is less than 20% of the lot, route all impervious area to one or more LID BMPs.

3. For a lot larger than 22,000sf, the amount depends on the % impervious surface coverage of the site/lot:

• For projects with 45% to 65% impervious coverage in the developed condition, route runoff from at least 20% of the lot area or 40% of the target impervious surface area (whichever is less) to one or more storm LID BMPs.

• For projects with more than 65% impervious coverage in the developed condition, route runoff from at least 10% of the lot area or 20% of the target impervious surface area (whichever is less) to one or more storm LID BMPs.

ADDITIONAL INFORMATION Regardless of stormwater LID feasibility, the applicant must meet all flow control and water quality treatment requirements applicable to the project. LID BMPs can be counted towards those requirements, but flow control BMPs (LID or other) must be applied for all new and replaced impervious areas. City policy is to require the installation of stormwater LID to the maximum extent feasible. The City acknowledges stormwater LID may not work on some sites, due to topography, soil, or other site specific conditions. If the evaluation indicates standard LID options are not feasible, please contact City surface water staff at (425) 587-3800 to discuss site specifics. Policy D-11 contains a Stormwater Adjustment Form that must be completed if standard storm LID BMPs are not feasible. If standard LID options are not feasible, at a minimum amended soil will be required in all landscaped areas and/or additional landscape/trees as appropriate.


Job # 12-248 Page 4-6

4.3 VAULT PERFORMANCE

Per the KCRTS printout, provided below, the live volume required at the maximum stage of

8.52’ is 26,460 cubic feet. The provided vault volume will exceed the minimum required. The

proposed vault will provide 2 – 17.5’ x 96’ cells with a depth of 8.7’ totaling 29,232

cubic feet. The proposed vault is therefore, adequately sized to accommodate for the required

flow control.

Live Storage Volume

Required = 26,460 cubic feet

Provided = 29,232 cubic feet


Job # 12-248 Page 4-7

Retention/Detention Facility Type of Facility: Detention Vault Facility Length: 90.00 ft Facility Width: 35.00 ft Facility Area: 3150. sq. ft Effective Storage Depth: 8.40 ft Stage 0 Elevation: 0.00 ft Storage Volume: 26460. cu. ft Riser Head: 8.40 ft Riser Diameter: 12.00 inches Number of orifices: 2 Full Head Pipe Orifice # Height Diameter Discharge Diameter (ft) (in) (CFS) (in) 1 0.00 0.63 0.031 2 5.50 1.25 0.072 4.0 Top Notch Weir: None Outflow Rating Curve: None Stage Elevation Storage Discharge Percolation (ft) (ft) (cu. ft) (ac-ft) (cfs) (cfs) 0.00 0.00 0. 0.000 0.000 0.00 0.01 0.01 32. 0.001 0.001 0.00 0.02 0.02 63. 0.001 0.001 0.00 0.03 0.03 95. 0.002 0.002 0.00 0.04 0.04 126. 0.003 0.002 0.00 0.05 0.05 158. 0.004 0.002 0.00 0.19 0.19 599. 0.014 0.005 0.00 0.34 0.34 1071. 0.025 0.006 0.00 0.48 0.48 1512. 0.035 0.007 0.00 0.62 0.62 1953. 0.045 0.008 0.00 0.76 0.76 2394. 0.055 0.009 0.00 0.91 0.91 2867. 0.066 0.010 0.00 1.05 1.05 3308. 0.076 0.011 0.00 1.19 1.19 3749. 0.086 0.012 0.00 1.33 1.33 4190. 0.096 0.012 0.00 1.48 1.48 4662. 0.107 0.013 0.00 1.62 1.62 5103. 0.117 0.013 0.00 1.76 1.76 5544. 0.127 0.014 0.00 1.90 1.90 5985. 0.137 0.015 0.00 2.05 2.05 6458. 0.148 0.015 0.00 2.19 2.19 6899. 0.158 0.016 0.00 2.33 2.33 7340. 0.168 0.016 0.00 2.47 2.47 7781. 0.179 0.017 0.00 2.61 2.61 8222. 0.189 0.017 0.00 2.76 2.76 8694. 0.200 0.018 0.00 2.90 2.90 9135. 0.210 0.018 0.00 3.04 3.04 9576. 0.220 0.018 0.00 3.18 3.18 10017. 0.230 0.019 0.00 3.33 3.33 10490. 0.241 0.019 0.00


Job # 12-248 Page 4-8

3.47 3.47 10931. 0.251 0.020 0.00 3.61 3.61 11372. 0.261 0.020 0.00 3.75 3.75 11813. 0.271 0.021 0.00 3.90 3.90 12285. 0.282 0.021 0.00 4.04 4.04 12726. 0.292 0.021 0.00 4.18 4.18 13167. 0.302 0.022 0.00 4.32 4.32 13608. 0.312 0.022 0.00 4.47 4.47 14080. 0.323 0.022 0.00 4.61 4.61 14522. 0.333 0.023 0.00 4.75 4.75 14963. 0.343 0.023 0.00 4.89 4.89 15404. 0.354 0.023 0.00 5.04 5.04 15876. 0.364 0.024 0.00 5.18 5.18 16317. 0.375 0.024 0.00 5.32 5.32 16758. 0.385 0.024 0.00 5.46 5.46 17199. 0.395 0.025 0.00 5.50 5.50 17325. 0.398 0.025 0.00 5.51 5.51 17357. 0.398 0.025 0.00 5.53 5.53 17420. 0.400 0.026 0.00 5.54 5.54 17451. 0.401 0.028 0.00 5.55 5.55 17483. 0.401 0.030 0.00 5.57 5.57 17546. 0.403 0.033 0.00 5.58 5.58 17577. 0.404 0.037 0.00 5.59 5.59 17609. 0.404 0.038 0.00 5.60 5.60 17640. 0.405 0.039 0.00 5.75 5.75 18113. 0.416 0.046 0.00 5.89 5.89 18554. 0.426 0.052 0.00 6.03 6.03 18995. 0.436 0.057 0.00 6.17 6.17 19436. 0.446 0.061 0.00 6.32 6.32 19908. 0.457 0.065 0.00 6.46 6.46 20349. 0.467 0.068 0.00 6.60 6.60 20790. 0.477 0.072 0.00 6.74 6.74 21231. 0.487 0.075 0.00 6.89 6.89 21704. 0.498 0.078 0.00 7.03 7.03 22145. 0.508 0.080 0.00 7.17 7.17 22586. 0.518 0.083 0.00 7.31 7.31 23027. 0.529 0.086 0.00 7.46 7.46 23499. 0.539 0.088 0.00 7.60 7.60 23940. 0.550 0.091 0.00 7.74 7.74 24381. 0.560 0.093 0.00 7.88 7.88 24822. 0.570 0.095 0.00 8.02 8.02 25263. 0.580 0.097 0.00 8.17 8.17 25736. 0.591 0.099 0.00 8.31 8.31 26177. 0.601 0.102 0.00 8.40 8.40 26460. 0.607 0.103 0.00 8.50 8.50 26775. 0.615 0.412 0.00 8.60 8.60 27090. 0.622 0.977 0.00 8.70 8.70 27405. 0.629 1.710 0.00 8.80 8.80 27720. 0.636 2.500 0.00 8.90 8.90 28035. 0.644 2.780 0.00 9.00 9.00 28350. 0.651 3.040 0.00 9.10 9.10 28665. 0.658 3.280 0.00 9.20 9.20 28980. 0.665 3.500 0.00 9.30 9.30 29295. 0.673 3.700 0.00 9.40 9.40 29610. 0.680 3.900 0.00


Job # 12-248 Page 4-9

9.50 9.50 29925. 0.687 4.080 0.00 9.60 9.60 30240. 0.694 4.260 0.00 9.70 9.70 30555. 0.701 4.430 0.00 9.80 9.80 30870. 0.709 4.600 0.00 9.90 9.90 31185. 0.716 4.750 0.00 10.00 10.00 31500. 0.723 4.910 0.00 10.10 10.10 31815. 0.730 5.060 0.00 10.20 10.20 32130. 0.738 5.200 0.00 10.30 10.30 32445. 0.745 5.340 0.00 Hyd Inflow Outflow Peak Storage Stage Elev (Cu-Ft) (Ac-Ft) 1 0.68 0.10 8.15 8.15 25676. 0.589 2 0.33 0.09 7.50 7.50 23626. 0.542 3 0.40 0.08 7.14 7.14 22506. 0.517 4 0.33 0.06 6.25 6.25 19702. 0.452 5 0.35 0.03 5.54 5.54 17439. 0.400 6 0.25 0.02 4.76 4.76 15009. 0.345 7 0.25 0.02 2.34 2.34 7365. 0.169 8 0.31 0.01 1.72 1.72 5430. 0.125 Hyd R/D Facility Tributary Reservoir POC Outflow Outflow Inflow Inflow Target Calc 1 0.10 0.07 ******** ******* 0.14 2 0.09 0.03 ******** 0.14 0.11 3 0.08 0.04 ******** ******* 0.10 4 0.06 0.03 ******** ******* 0.08 5 0.03 0.04 ******** ******* 0.05 6 0.02 0.03 ******** ******* 0.05 7 0.02 0.03 ******** ******* 0.04 8 0.01 0.03 ******** ******* 0.04 ---------------------------------- Route Time Series through Facility Inflow Time Series File:dev.tsf Outflow Time Series File:rdout POC Time Series File:dsout Inflow/Outflow Analysis Peak Inflow Discharge: 0.679 CFS at 6:00 on Jan 9 in Year 8 Peak Outflow Discharge: 0.099 CFS at 16:00 on Jan 9 in Year 8 Peak Reservoir Stage: 8.15 Ft Peak Reservoir Elev: 8.15 Ft Peak Reservoir Storage: 25676. Cu-Ft : 0.589 Ac-Ft Add Time Series:bypass.tsf Peak Summed Discharge: 0.142 CFS at 9:00 on Jan 9 in Year 8 Point of Compliance File:dsout.tsf Flow Frequency Analysis Time Series File:rdout.tsf Project Location:Sea-Tac


Job # 12-248 Page 4-10

---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) (ft) Period 0.094 2 2/09/01 22:00 0.099 8.15 1 100.00 0.990 0.023 7 1/07/02 6:00 0.094 7.83 2 25.00 0.960 0.083 3 3/06/03 22:00 0.083 7.15 3 10.00 0.900 0.016 8 8/26/04 9:00 0.063 6.26 4 5.00 0.800 0.023 6 1/08/05 6:00 0.029 5.54 5 3.00 0.667 0.029 5 1/20/06 2:00 0.023 4.96 6 2.00 0.500 0.063 4 11/24/06 11:00 0.023 4.77 7 1.30 0.231 0.099 1 1/09/08 16:00 0.016 2.34 8 1.10 0.091 Computed Peaks 0.097 8.04 50.00 0.980 Flow Frequency Analysis Time Series File:dsout.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) Period 0.113 2 2/09/01 18:00 0.142 1 100.00 0.990 0.046 6 1/05/02 16:00 0.113 2 25.00 0.960 0.102 3 3/06/03 18:00 0.102 3 10.00 0.900 0.043 7 8/26/04 2:00 0.075 4 5.00 0.800 0.042 8 10/28/04 16:00 0.055 5 3.00 0.667 0.055 5 1/18/06 16:00 0.046 6 2.00 0.500 0.075 4 11/24/06 6:00 0.043 7 1.30 0.231 0.142 1 1/09/08 9:00 0.042 8 1.10 0.091 Computed Peaks 0.132 50.00 0.980 Flow Duration from Time Series File:rdout.tsf Cutoff Count Frequency CDF Exceedence_Probability CFS % % % 0.001 28567 46.587 46.587 53.413 0.534E+00 0.004 6586 10.740 57.327 42.673 0.427E+00 0.007 6536 10.659 67.986 32.014 0.320E+00 0.009 5068 8.265 76.251 23.749 0.237E+00 0.012 3782 6.168 82.418 17.582 0.176E+00 0.015 3815 6.221 88.640 11.360 0.114E+00 0.017 2666 4.348 92.988 7.012 0.701E-01 0.020 1625 2.650 95.638 4.362 0.436E-01 0.022 1501 2.448 98.085 1.915 0.191E-01 0.025 804 1.311 99.397 0.603 0.603E-02 0.028 40 0.065 99.462 0.538 0.538E-02 0.030 12 0.020 99.481 0.519 0.519E-02 0.033 11 0.018 99.499 0.501 0.501E-02 0.036 3 0.005 99.504 0.496 0.496E-02 0.038 11 0.018 99.522 0.478 0.478E-02 0.041 27 0.044 99.566 0.434 0.434E-02 0.043 31 0.051 99.617 0.383 0.383E-02 0.046 23 0.038 99.654 0.346 0.346E-02 0.049 22 0.036 99.690 0.310 0.310E-02 0.051 22 0.036 99.726 0.274 0.274E-02


Job # 12-248 Page 4-11

0.054 12 0.020 99.746 0.254 0.254E-02 0.057 24 0.039 99.785 0.215 0.215E-02 0.059 8 0.013 99.798 0.202 0.202E-02 0.062 14 0.023 99.821 0.179 0.179E-02 0.064 15 0.024 99.845 0.155 0.155E-02 0.067 8 0.013 99.858 0.142 0.142E-02 0.070 9 0.015 99.873 0.127 0.127E-02 0.072 5 0.008 99.881 0.119 0.119E-02 0.075 7 0.011 99.892 0.108 0.108E-02 0.078 7 0.011 99.904 0.096 0.962E-03 0.080 12 0.020 99.923 0.077 0.766E-03 0.083 11 0.018 99.941 0.059 0.587E-03 0.085 3 0.005 99.946 0.054 0.538E-03 0.088 11 0.018 99.964 0.036 0.359E-03 0.091 10 0.016 99.980 0.020 0.196E-03 0.093 6 0.010 99.990 0.010 0.978E-04 Flow Duration from Time Series File:dsout.tsf Cutoff Count Frequency CDF Exceedence_Probability CFS % % % 0.002 28633 46.694 46.694 53.306 0.533E+00 0.005 6918 11.282 57.976 42.024 0.420E+00 0.008 7361 12.004 69.980 30.020 0.300E+00 0.011 5157 8.410 78.390 21.610 0.216E+00 0.014 4761 7.764 86.155 13.845 0.138E+00 0.017 3070 5.007 91.161 8.839 0.884E-01 0.021 2103 3.430 94.591 5.409 0.541E-01 0.024 1580 2.577 97.167 2.833 0.283E-01 0.027 896 1.461 98.629 1.371 0.137E-01 0.030 286 0.466 99.095 0.905 0.905E-02 0.033 111 0.181 99.276 0.724 0.724E-02 0.036 53 0.086 99.362 0.638 0.638E-02 0.040 30 0.049 99.411 0.589 0.589E-02 0.043 42 0.068 99.480 0.520 0.520E-02 0.046 52 0.085 99.565 0.435 0.435E-02 0.049 34 0.055 99.620 0.380 0.380E-02 0.052 29 0.047 99.667 0.333 0.333E-02 0.055 27 0.044 99.711 0.289 0.289E-02 0.059 19 0.031 99.742 0.258 0.258E-02 0.062 28 0.046 99.788 0.212 0.212E-02 0.065 12 0.020 99.808 0.192 0.192E-02 0.068 15 0.024 99.832 0.168 0.168E-02 0.071 9 0.015 99.847 0.153 0.153E-02 0.074 11 0.018 99.865 0.135 0.135E-02 0.078 9 0.015 99.879 0.121 0.121E-02 0.081 8 0.013 99.892 0.108 0.108E-02 0.084 10 0.016 99.909 0.091 0.913E-03 0.087 9 0.015 99.923 0.077 0.766E-03 0.090 8 0.013 99.936 0.064 0.636E-03 0.093 10 0.016 99.953 0.047 0.473E-03 0.097 8 0.013 99.966 0.034 0.342E-03 0.100 7 0.011 99.977 0.023 0.228E-03 0.103 5 0.008 99.985 0.015 0.147E-03 0.106 5 0.008 99.993 0.007 0.652E-04


Job # 12-248 Page 4-12

0.109 2 0.003 99.997 0.003 0.326E-04 0.112 1 0.002 99.998 0.002 0.163E-04 ---------------------------------- Route Time Series through Facility Inflow Time Series File:dev.tsf Outflow Time Series File:rdout POC Time Series File:dsout Inflow/Outflow Analysis Peak Inflow Discharge: 0.679 CFS at 6:00 on Jan 9 in Year 8 Peak Outflow Discharge: 0.099 CFS at 16:00 on Jan 9 in Year 8 Peak Reservoir Stage: 8.15 Ft Peak Reservoir Elev: 8.15 Ft Peak Reservoir Storage: 25676. Cu-Ft : 0.589 Ac-Ft Add Time Series:bypass.tsf Peak Summed Discharge: 0.142 CFS at 9:00 on Jan 9 in Year 8 Point of Compliance File:dsout.tsf Flow Frequency Analysis Time Series File:rdout.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) (ft) Period 0.094 2 2/09/01 22:00 0.099 8.15 1 100.00 0.990 0.023 7 1/07/02 6:00 0.094 7.83 2 25.00 0.960 0.083 3 3/06/03 22:00 0.083 7.15 3 10.00 0.900 0.016 8 8/26/04 9:00 0.063 6.26 4 5.00 0.800 0.023 6 1/08/05 6:00 0.029 5.54 5 3.00 0.667 0.029 5 1/20/06 2:00 0.023 4.96 6 2.00 0.500 0.063 4 11/24/06 11:00 0.023 4.77 7 1.30 0.231 0.099 1 1/09/08 16:00 0.016 2.34 8 1.10 0.091 Computed Peaks 0.097 8.04 50.00 0.980 Flow Frequency Analysis Time Series File:dsout.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) Period 0.113 2 2/09/01 18:00 0.142 1 100.00 0.990 0.046 6 1/05/02 16:00 0.113 2 25.00 0.960 0.102 3 3/06/03 18:00 0.102 3 10.00 0.900 0.043 7 8/26/04 2:00 0.075 4 5.00 0.800 0.042 8 10/28/04 16:00 0.055 5 3.00 0.667 0.055 5 1/18/06 16:00 0.046 6 2.00 0.500 0.075 4 11/24/06 6:00 0.043 7 1.30 0.231 0.142 1 1/09/08 9:00 0.042 8 1.10 0.091


Job # 12-248 Page 4-13

Computed Peaks 0.132 50.00 0.980 Flow Duration from Time Series File:rdout.tsf Cutoff Count Frequency CDF Exceedence_Probability CFS % % % 0.001 28567 46.587 46.587 53.413 0.534E+00 0.004 6586 10.740 57.327 42.673 0.427E+00 0.007 6536 10.659 67.986 32.014 0.320E+00 0.009 5068 8.265 76.251 23.749 0.237E+00 0.012 3782 6.168 82.418 17.582 0.176E+00 0.015 3815 6.221 88.640 11.360 0.114E+00 0.017 2666 4.348 92.988 7.012 0.701E-01 0.020 1625 2.650 95.638 4.362 0.436E-01 0.022 1501 2.448 98.085 1.915 0.191E-01 0.025 804 1.311 99.397 0.603 0.603E-02 0.028 40 0.065 99.462 0.538 0.538E-02 0.030 12 0.020 99.481 0.519 0.519E-02 0.033 11 0.018 99.499 0.501 0.501E-02 0.036 3 0.005 99.504 0.496 0.496E-02 0.038 11 0.018 99.522 0.478 0.478E-02 0.041 27 0.044 99.566 0.434 0.434E-02 0.043 31 0.051 99.617 0.383 0.383E-02 0.046 23 0.038 99.654 0.346 0.346E-02 0.049 22 0.036 99.690 0.310 0.310E-02 0.051 22 0.036 99.726 0.274 0.274E-02 0.054 12 0.020 99.746 0.254 0.254E-02 0.057 24 0.039 99.785 0.215 0.215E-02 0.059 8 0.013 99.798 0.202 0.202E-02 0.062 14 0.023 99.821 0.179 0.179E-02 0.064 15 0.024 99.845 0.155 0.155E-02 0.067 8 0.013 99.858 0.142 0.142E-02 0.070 9 0.015 99.873 0.127 0.127E-02 0.072 5 0.008 99.881 0.119 0.119E-02 0.075 7 0.011 99.892 0.108 0.108E-02 0.078 7 0.011 99.904 0.096 0.962E-03 0.080 12 0.020 99.923 0.077 0.766E-03 0.083 11 0.018 99.941 0.059 0.587E-03 0.085 3 0.005 99.946 0.054 0.538E-03 0.088 11 0.018 99.964 0.036 0.359E-03 0.091 10 0.016 99.980 0.020 0.196E-03 0.093 6 0.010 99.990 0.010 0.978E-04 Flow Duration from Time Series File:dsout.tsf Cutoff Count Frequency CDF Exceedence_Probability CFS % % % 0.002 28633 46.694 46.694 53.306 0.533E+00 0.005 6918 11.282 57.976 42.024 0.420E+00 0.008 7361 12.004 69.980 30.020 0.300E+00 0.011 5157 8.410 78.390 21.610 0.216E+00 0.014 4761 7.764 86.155 13.845 0.138E+00 0.017 3070 5.007 91.161 8.839 0.884E-01


Job # 12-248 Page 4-14

0.021 2103 3.430 94.591 5.409 0.541E-01 0.024 1580 2.577 97.167 2.833 0.283E-01 0.027 896 1.461 98.629 1.371 0.137E-01 0.030 286 0.466 99.095 0.905 0.905E-02 0.033 111 0.181 99.276 0.724 0.724E-02 0.036 53 0.086 99.362 0.638 0.638E-02 0.040 30 0.049 99.411 0.589 0.589E-02 0.043 42 0.068 99.480 0.520 0.520E-02 0.046 52 0.085 99.565 0.435 0.435E-02 0.049 34 0.055 99.620 0.380 0.380E-02 0.052 29 0.047 99.667 0.333 0.333E-02 0.055 27 0.044 99.711 0.289 0.289E-02 0.059 19 0.031 99.742 0.258 0.258E-02 0.062 28 0.046 99.788 0.212 0.212E-02 0.065 12 0.020 99.808 0.192 0.192E-02 0.068 15 0.024 99.832 0.168 0.168E-02 0.071 9 0.015 99.847 0.153 0.153E-02 0.074 11 0.018 99.865 0.135 0.135E-02 0.078 9 0.015 99.879 0.121 0.121E-02 0.081 8 0.013 99.892 0.108 0.108E-02 0.084 10 0.016 99.909 0.091 0.913E-03 0.087 9 0.015 99.923 0.077 0.766E-03 0.090 8 0.013 99.936 0.064 0.636E-03 0.093 10 0.016 99.953 0.047 0.473E-03 0.097 8 0.013 99.966 0.034 0.342E-03 0.100 7 0.011 99.977 0.023 0.228E-03 0.103 5 0.008 99.985 0.015 0.147E-03 0.106 5 0.008 99.993 0.007 0.652E-04 0.109 2 0.003 99.997 0.003 0.326E-04 0.112 1 0.002 99.998 0.002 0.163E-04


Job # 12-248 Page 4-15

4.4 WATER QUALITY ANALYSIS AND DESIGN

A combined detention/basic wetvault will provide water quality treatment for all area collected

to comply with the Basic Water Quality treatment requirement. The wetvault is designed

according to Section 6.4.2 of the KCSWDM. A summary of the water quality facility sizing

calculations is provided below.

Vb = fVr

where: Vb = wetpool volume, cf

f = volume correction factor (3.0 for “basic” wetvaults)

Vr = volume of runoff from mean annual storm, ft3

Vr = (0.9Ai + 0.25Atg + 0.10Atf + 0.01Ao) x R

where: Ai = area of impervious surface, ft2

Atg = area of till grass, ft2

Atf = area of till forest, ft2

R = rainfall from mean annual storm, ft

(0.039’ from Figure 6.4.1.A in the Appendix)

Total area = 0.59 acres

Ai = 1.088 acres (40,869 ft2)

Atg = 1.014 acres (48.349 ft2)

Vr = [ 0.9 (40,869) + 0.25 (48,349)] x 0.039 = 1,905 ft3

Vb = 3 (1,905) = 6,282 ft3

One cell of the vault will provide 4’ of water quality storage. The water quality volume

will be 17.5’ x 96’ x 4’ = 6,720 ft3

Required Water Quality Volume = 5,717 ft3

Provided Water Quality Volume = 6,720 ft3


Job # 12-248 Page 4-16

4.5 DISPERSION

The project will install a gabion dispersion system downstream of the vault to match existing

drainage patters. The dissipater will mitigate erosion of high velocity flows discharging form

the proposed vault. The vault output flows are below:

---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) Period 0.113 2 2/09/01 18:00 0.142 1 100.00 0.990 0.046 6 1/05/02 16:00 0.113 2 25.00 0.960 0.102 3 3/06/03 18:00 0.102 3 10.00 0.900 0.043 7 8/26/04 2:00 0.075 4 5.00 0.800 0.042 8 10/28/04 16:00 0.055 5 3.00 0.667 0.055 5 1/18/06 16:00 0.046 6 2.00 0.500 0.075 4 11/24/06 6:00 0.043 7 1.30 0.231 0.142 1 1/09/08 9:00 0.042 8 1.10 0.091 Computed Peaks 0.132 50.00 0.980

Per the 2009 KCSWDM section 4.2.2, flow dispersal shall be used to satisfy Core

Requirements 1 in certain cases where a flow dispersion system will promote slope stability

and aid in matching the existing discharge condition. Discharge velocities are expected to be

less than 5 fps for all stream protection duration flows and will only reach into the velocity

specified for gabion dispersion during vault overflow events. Due to steep slopes present at

the site, however, a gabion dispersion was selected due to the possibility of high velocities

during the vault overflow condition.


Job # 12-248 Page 5-1

Section 5 Conveyance Design

Conveyance analysis of the proposed storm drain system will be included with the final

engineering submittal.

The existing 12” CMP culvert which collects drainage from development above 20th Ave and a

portion of the project site was observed to overtop during a site visit by a consulting ecologist.

The Blueline Group performed a conveyance analysis of the existing culvert to determine how

the overtopping should be addressed. Upon analysis is was determined the existing pipe has

enough capacity to convey the input flows. A trash rack is recommended for the culvert inlet

in conjunction with regular maintenance of the pipe to maintain optimal flows.

EXISTING CULVERT CONVEYANCE

MANNING’S EQUATION; CAPACITY OF 12” CMP Pipe @ 15.3% = 7.54 cfs Q = 1.486/n * A * R2/3 * S1/2

n = roughness coefficient = 0.024 A = cross sectional area of pipe = π (D/2)2 = π (1.00 ft/2)2 = 0.785 R = wetted perimeter of pipe

R2/3 = (D/4)2/3 = (1/4)2/3 = 0.397 S = slope S1/2 = (0.153 ft/ft)1/2 = 0.391

Q = (1.486/0.024) * 0.785 * 0.397 * 0.391 = 7.54 cfs

KCRTS INPUT FLOWS

Upstream Areas were determined using GIS data. A map of the input area is included on the

following page. The 100-year input flows were calculated using KCRTS software and 15-minute

timesteps.

LAND COVER AREA (AC)

ROOFTOPS 1.24PAVING 0.97PERVIOUS 4.85

TOTAL 7.06

Capacity Required 5.38 cfs

Capacity Provided 7.54 cfs

STORM EVENT FLOW (CFS)

2 YEAR 1.28 5 YEAR 1.69 10 YEAR 2.57 25 YEAR 3.25 50 YEAR 4.67

100 YEAR 5.38


Job # 12-248 Page 6-1

Section 6 Special Reports and Studies

A Geotechnical Report prepared by Associated Earth Sciences, Inc dated July 28, 2016 and a

critical areas report dated July 8, 2016 by Talasaea Consultants, Inc. are included in this

section.

a s s o c i a t e de a r t h s c i e n c e si n c o r p o r a t e d

Associated Earth Sciences, Inc. 911 5th AvenueKirkland, WA 98033P (425) 827 7701 F (425) 827 5424

Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report

LONDO FORBES CREEK Kirkland, Washington

Prepared For: ORCAS MOON, LLC

Project No. KE160384A July 28, 2016

July 28, 2016

Project No. KE160384A

Orcas Moon, LLC

P.O. Box 2710

Redmond, Washington 98073

Attention: Mr. Robert Londo

Subject: Subsurface Exploration, Geologic Hazard, and

Geotechnical Engineering Report

Londo Forbes Creek

20th Avenue and 4th Place

Kirkland, Washington

Dear Mr. Londo:

We are pleased to present copies of the above-referenced report. This report summarizes the

results of our subsurface exploration, geologic hazard, and geotechnical engineering studies

and offers recommendations for the design and development of the proposed project. At the

time of this report, site grading, structural plans, and construction methods have not been

finalized and the recommendations presented herein are preliminary.

We have enjoyed working with you on this study and are confident that the recommendations

presented in this report will aid in the successful completion of your project. If you should

have any questions or if we can be of additional help to you, please do not hesitate to call.

Sincerely,

ASSOCIATED EARTH SCIENCES, INC.


Bruce L. Blyton, P.E.

Senior Principal Engineer

BLB/pc - KE160384A2- Projects\20160384\KE\WP

Kirkland Office I 911 Fifth Avenue I Kirkland, WA 98033 P I 425.827.7701 Fl 425.827.5424

Everett Office I 2911 Yz Hewitt Avenue, Suite 2 I Everett, WA 98201 P I 425.259.0522 F I 425. 827.5424

Tacoma Office I 1552 Commerce Street, Suite 102 I Tacoma, WA 98402 P I 253.722.2992 F I 253.722.2993

www .aesgeo.com

SUBSURFACE EXPLORATION, GEOLOGIC HAZARD, AND GEOTECHNICAL ENGINEERING REPORT

LONDO FORBES CREEK


Prepared for: Orcas Moon, LLC

P.O. Box 2710 Redmond, Washington 98073

Prepared by: Associated Earth Sciences, Inc.

911 5th Avenue Kirkland, Washington 98033

425-827-7701 Fax: 425-827-5424

July 28, 2016 Project No. KE160384A

Subsurface Exploration, Geologic Hazard, and Londo Forbes Creek Geotechnical Engineering Report Kirkland, Washington Project and Site Conditions

July 28, 2016 ASSOCIATED EARTH SCIENCES, INC. JPL/pc – KE160384A2 – Projects\20160384\KE\WP Page 1

I. PROJECT AND SITE CONDITIONS 1.0 INTRODUCTION This report presents the results of our subsurface exploration, geologic hazard, and geotechnical engineering study for the Londo Forbes Creek project, located near the intersection of 20th Avenue and 4th Place in Kirkland, Washington (Figure 1). The approximate locations of explorations referenced for this study are shown on the “Site and Exploration Plan,” Figure 2. Interpretive exploration logs are included in the Appendix. At the time of this report, site grading, structural plans, and construction methods have not been finalized and the recommendations presented herein are preliminary. As the nature, design, and locations of the site improvements and lots are planned, the conclusions and recommendations contained in this report should be reviewed and modified, or verified, as necessary. 1.1 Purpose and Scope The purpose of this study was to provide subsurface data to be utilized in design and construction of the site improvements and residences at the above-referenced site. Our study included a review of available geologic literature and exploration logs, and performing geologic studies to assess the type, thickness, distribution, and physical properties of the subsurface sediments and shallow ground water conditions. A geologic hazards assessment and a geotechnical engineering study were also completed to determine suitable geologic hazard mitigation techniques, the type of suitable foundations, allowable foundation soil bearing pressures, anticipated foundation settlements, erosion considerations, drainage considerations, and construction recommendations. This report summarizes our fieldwork and offers geologic hazard mitigation and development recommendations based on our present understanding of the project. 1.2 Authorization Written authorization to proceed with this study was granted by Mr. Robert Londo of Orcas Moon, LLC. Our study was accomplished in general accordance with our scope of work letter dated July 21, 2016. This report has been prepared for the exclusive use of Orcas Moon, LLC and its agents for specific application to this project. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering and engineering geology practices in effect in this area at the time our report was prepared. No other warranty, express or implied, is made. It must be understood that no recommendations or engineering design can yield a guarantee of stable slopes. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner.



2.0 PROJECT AND SITE DESCRIPTION This report was completed with an understanding of the project based on our discussions with you, and review of a “Cottage Plan” and associated cross sections, prepared by Blueline and dated July 12, 2016. We understand that you are currently planning 16 single-family cottage residences, with associated grading, access, and utilities, at the subject site. Rockeries, ranging up to approximately 10 feet in exposed height, are planned to face fills placed for roadways and building pads. The property was situated north of the intersection of 20th Avenue and 4th Place in Kirkland, Washington (King County Parcel Nos. 3890100050 and 3890100055). The approximately 7-acre property generally slopes down to the north and is situated on the south flank of the Forbes Creek valley. The total elevation change across the property was on the order of 120 feet. Incised depressions on the slope appeared to serve as collectors of surface runoff above and for the upper third of the subject property. Locally, the depressions contained slopes on the order of 40 to 50 percent. We were informed that three sections of corrugated metal pipe were laid in the incised depressions and extend down the slope with water exiting the pipes near Forbes Creek Drive. It is our understanding that the pipe was installed to carry runoff water from 20th Avenue to the south of the site. The property is accessed via a roughly graded road entering from Forbes Creek Drive along the northern property boundary. The site contains remnants of a demolished house and pump house. The site contains a moderate growth of native vegetation consisting of maple and evergreen trees, blackberry bushes, ferns, and short grass. While on-site, we did not observe bowed trees or similar conditions that would indicate creep or downslope movement of the existing slope. The only significant erosion features we observed were along the previously mentioned incised depressions running north-south on the face of the slope. 3.0 SUBSURFACE EXPLORATION Our previous field study, completed in 2005, included excavating 10 exploration pits to gain information about the site. The various types of sediments, as well as the depths where characteristics of the sediments changed, are indicated on the exploration logs presented in the Appendix. The depths indicated on the logs where conditions changed may represent gradational variations between sediment types. Our explorations were approximately located in the field by measuring from known site features. The conclusions and recommendations presented in this report are based on the subsurface explorations completed for this study. The number, locations, and depths of the explorations were completed within site and budgetary constraints. Because of the nature of exploratory



work below ground, extrapolation of subsurface conditions between field explorations is necessary. It should be noted that differing subsurface conditions may sometimes be present due to the random nature of deposition and the alteration of topography by past grading and/or filling. The nature and extent of any variations between the field explorations may not become fully evident until construction. If variations are observed at that time, it may be necessary to re-evaluate specific recommendations in this report and make appropriate changes. 3.1 Exploration Pits Exploration pits were excavated with a trackhoe. The pits permitted direct, visual observation of subsurface conditions. Materials encountered in the exploration pits were studied and classified in the field by a geotechnical engineer from our firm. All exploration pits were backfilled immediately after examination and logging. Selected samples were then transported to our laboratory for further visual classification and testing, as necessary. 4.0 SUBSURFACE CONDITIONS Subsurface conditions at the subject site were inferred from the field explorations referenced for this study, visual reconnaissance of the site, and review of applicable geologic literature. The approximate locations of the explorations are shown on Figure 2. As shown on the field logs, the exploration pits generally encountered medium dense sand with varying amounts of silt and gravel or stiff to hard silts. Minor amounts of fill may occur at some locations, particularly those in the northern portion of the site, adjacent to the roadway, or within utility trenches across the property and in the vicinity of previous structures. The following section presents more detailed subsurface information. 4.1 Stratigraphy Topsoil Topsoil consisting of loose, moist, dark brown, silty sand was encountered in most of the explorations. The topsoil ranged in thickness from about 0.5 to 1.5 feet. This material is unsuitable for structure or pavement support. Fill We observed fill soils covering buried, approximately 12-inch–diameter, corrugated pipes laid along the steep site slopes. The pipes appeared to be between 1.5 and 2 feet below existing site grades at the locations we observed. Fill may also be encountered around utilities and



foundation areas associated with the demolished structure. This material is unsuitable for structure or pavement support. Recessional Outwash Sediments encountered below the topsoil layer consisted of medium dense, fine to medium sand with varying quantities of silt. We interpret these sediments to be representative of recessional outwash. The recessional outwash consists of sediments that were deposited by meltwater streams that emanated from the retreating glacial ice during the latter portion of the Vashon Stade of the Fraser Glaciation ending approximately 12,500 years ago. Where glacial sediments are exposed at the ground surface throughout the Puget Sound region, the upper several feet of these sediments typically become weathered. The recessional outwash sediments generally extended about 4 to 5 feet below existing grades, but in exploration pit EP-1, extended to the bottom of the exploration at 16 feet. When properly prepared, the recessional outwash will be suitable for the support of foundations. Advance Outwash An advance outwash deposit consisting of medium dense to very dense sand containing variable amounts of disseminated silt, interbeds of clayey silt, and few amounts of scattered gravel was encountered below the topsoil and recessional deposits. The advance outwash deposit was generally encountered between 4 to 6 feet below existing grades in exploration pits EP-6 and EP-9, or approximately the middle third of the slope. The advance outwash was deposited ahead of the advancing Vashon-age glacial ice sheet in meltwater streams and subsequently overridden by several thousand feet of ice. Consequently, these materials are medium dense to very dense, possess high shear strength, and have low compressibility characteristics. The advance outwash deposit is suitable for direct foundation support. Transitional Beds A hard, clayey silt and silty clay deposit containing trace amounts of fine sand interpreted to be transitional beds was generally encountered in the upper portions (south end) of the property. The glaciolacustrine clayey silt and silty clay was deposited in freshwater lakes or slow-moving rivers far ahead of the advancing Vashon-age glacial ice sheet and was also overridden by several thousand feet of ice. These materials are hard, have low compressibility characteristics, and are relatively impermeable. The transitional beds are considered suitable for support of shallow foundations with proper preparation. The transitional beds are typically highly moisture-sensitive and susceptible to disturbance when wet. Care should be taken not to disturb planned load-bearing surfaces that are composed of the transitional beds during periods of wet site or weather conditions.



4.2 Hydrology Ground water seepage was only encountered in exploration pit EP-5 at the time of our field study in June 2005. It should be noted that fluctuations in the level of the ground water may occur due to the time of the year, variations in the amount of precipitation, and changes in site development. Seepage may also occur at random depths and locations in unsupervised or non-uniform fills.

Subsurface Exploration, Geologic Hazard, Londo Forbes Creek and Geotechnical Engineering Report Kirkland, Washington Geologic Hazards and Mitigations


II. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards is based on review of the City of Kirkland maps of environmentally critical areas. The subject site is classified as a high landslide hazard area with a potential for severe erosion hazards when devegetated. 5.0 LANDSLIDE HAZARDS AND MITIGATION Our explorations encountered medium dense to dense, glacially consolidated sediments on the slopes at relatively shallow depths. In our opinion, the aforementioned slope geometry, drainage/ground water, and geologic conditions present a relatively low to moderate landslide hazard risk for the site. No springs or seeps were observed at the time of our visit and the site did not exhibit obvious indications of past or present slope instability. Landslide hazards can be mitigated by implementing the following measures:

1. Control storm water crossing the site.

2. Proper grading, compaction, and benching of subgrade soils

3. Permanent slopes in cut and structural fill must be limited to 2H:1V (Horizontal:Vertical). In areas where it is not feasible to construct a 2H:1V slope, engineered walls should be constructed.

4. Limit site clearing only to areas to be developed.

The basemap used for Figure 2 includes shaded areas indicating slopes across the subject site greater than 40 percent in grade. This basemap also shows stream and wetland areas, delineated by others, along with associated buffers. Based on our explorations, it is our opinion that the wetland/stream buffers also provide a suitable setback from significant site slopes for the proposed improvements, provided the recommendations in this report are followed. 6.0 SEISMIC HAZARDS AND MITIGATIONS Earthquakes occur regularly in the Puget Lowland. Most of these events are small and are not felt by humans. However, large earthquakes do occur, as evidenced by the 2001, 6.8-magnitude event; the 1965, 6.5-magnitude event; and the 1949, 7.2-magnitude event. The 1949 earthquake appears to have been the largest in this region during recorded history and



was centered in the Olympia area. Evaluation of earthquake return rates indicates that an earthquake of the magnitude between 5.5 and 6.0 is likely within a given 20-year period. Generally, there are four types of potential geologic hazards associated with large seismic events: 1) surficial ground rupture, 2) seismically induced landslides, 3) liquefaction, and 4) ground motion. The potential for each of these hazards to adversely impact the proposed project is discussed below. 6.1 Surficial Ground Rupture The nearest known fault traces to the project site are the South Whidbey Island-Lake Alice Fault located approximately 4 miles to the north, and the Seattle Fault located approximately 5 miles to the south. Recent studies of both the Seattle Fault and the South Whidbey Island-Lake Alice Fault indicate that they are active faults capable of generating surface ruptures. The recognition of these faults is relatively new, and data pertaining to them are limited, with the studies still ongoing. According to the U.S. Geological Society (USGS) studies, the recurrence interval of movements along these faults is unknown, but is speculated to be on the order of 1,100 years. Due to the distance from the site to the known fault zones, and due to the long recurrence interval that is suspected for these fault systems, the risk for damage to the project during the expected life of the structures due to surface faulting is expected to be low, in our opinion. 6.2 Seismically Induced Landslides It is our opinion that the risk of damage to the proposed structures by seismically induced landsliding is low due to the presence of medium dense to very dense compacted soils observed at depth beneath the surface of the site. 6.3 Liquefaction Liquefaction is a temporary loss in soil shear strength that can occur when loose granular soils below the ground water table are exposed to cyclic accelerations, such as those that occur during earthquakes. The observed site soils were relatively dense and unsaturated and are not expected to be prone to liquefaction. A detailed liquefaction hazard analysis was not performed as part of this study, and none is warranted, in our opinion. 6.4 Seismic Site Class In our opinion the subsurface conditions at the site are consistent with seismic Site Class D in accordance with the 2012 International Building Code (IBC), and the publication ASCE 7 referenced therein, the most recent version of which is ASCE 7-10.



7.0 EROSION HAZARDS AND MITIGATION The on-site sediments contain a high percentage of silt and fine sand and are sensitive to erosion. In order to control erosion and reduce the amount of sediment transport off the site during construction, the following recommendations should be followed.

1. Construction activity should be scheduled or phased as much as possible to reduce the amount of earthwork activity that is performed during the winter months.

2. The winter performance of a site is dependent on a well-conceived plan for control of

site erosion and storm water runoff. The project temporary erosion and sediment control (TESC) plan should include ground-cover measures, access roads, and staging areas. The contractor must implement and maintain the required measures. A site maintenance plan should be in place in the event storm water turbidity measurements are greater than the Washington State Department of Ecology (Ecology) standards.

3. TESC measures for a given area to be graded or otherwise worked should be installed

soon after ground clearing. The recommended sequence of construction within a given area after clearing would be to install sediment traps and/or ponds and establish perimeter flow control prior to starting mass grading.

4. During the wetter months of the year, or when large storm events are predicted during

the summer months, each work area should be stabilized so that if showers occur, the work area can receive the rainfall without excessive erosion or sediment transport. The required measures for an area to be “buttoned-up” will depend on the time of year and the duration the area will be left un-worked. During the winter months, areas that are to be left un-worked for more than 2 days should be mulched or covered with plastic. During the summer months, stabilization will usually consist of seal-rolling the subgrade. Such measures will aid in the contractor’s ability to get back into a work area after a storm event. The stabilization process also includes establishing temporary storm water conveyance channels through work areas to route runoff to the approved treatment facilities.

5. All disturbed areas should be revegetated as soon as possible. If it is outside of the

growing season, the disturbed areas should be covered with mulch, as recommended in the erosion control plan. Straw mulch provides a cost-effective cover measure and can be made wind-resistant with the application of a tackifier after it is placed.

6. Surface runoff and discharge should be controlled during and following development.

Uncontrolled discharge may promote erosion and sediment transport.



7. Soils that are to be reused around the site should be stored in such a manner as to reduce erosion from the stockpile. Protective measures may include, but are not limited to, covering with plastic sheeting, the use of low stockpiles in flat areas, or the use of silt fences around pile perimeters.

8. On-site erosion control inspections and turbidity monitoring (when required) should be

performed in accordance with Ecology requirements. Weekly and monthly reporting to Ecology should be performed on a regularly scheduled basis. Temporary and permanent erosion control and drainage measures should be adjusted and maintained, as necessary, for the duration of project construction.

It is our opinion that with the proper implementation of the TESC plans and by field-adjusting appropriate mitigation elements (best management practices [BMPs]) throughout construction, as recommended by the erosion control inspector, the potential adverse impacts from erosion hazards on the project may be mitigated.

Subsurface Exploration, Geologic Hazard, and Londo Forbes Creek Geotechnical Engineering Report Kirkland, Washington Design Recommendations


III. DESIGN RECOMMENDATIONS 8.0 INTRODUCTION Our explorations indicate that, from a geotechnical standpoint, the subject site is suitable for the proposed development provided the recommendations contained herein are properly followed. The bearing stratum is relatively shallow and spread-footing foundations may be utilized. We understand that the distribution of foundations loads of the proposed residences will be typical; concentrated loads on the order of 2 kips per lineal foot of foundation can be expected. Consequently, the native dense outwash soils, hard transitional bed silts, or structural fills bearing on the native soils are capable of providing suitable building support. 9.0 SITE PREPARATION Site preparation of planned building, road, and structural fill areas should include removal of all trees, brush, debris and any other deleterious material. Additionally, the upper organic topsoil should be removed and the remaining roots grubbed. Areas where loose surficial soils exist due to grubbing operations should be considered as fill to the depth of disturbance and treated as subsequently recommended for structural fill placement. Loose topsoil should be stripped down to the underlying, medium dense to dense outwash soils and hard transitional bed silts. Since the density of the soil is variable, random soft pockets may exist and the depth and extent of stripping can best be determined in the field by the geotechnical engineer or his representative. We recommend that road areas be proof-rolled with a loaded dump truck to identify soft spots; soft areas should be overexcavated and backfilled with structural fill. 9.1 Temporary Slopes In our opinion, stable construction slopes should be the responsibility of the contractor and should be determined during construction. For estimating purposes, we anticipate that temporary, unsupported cut slopes in the unsaturated, medium dense recessional outwash soils and stiff silts can be made at a maximum slope of 1.5H:1V, and in the unsaturated native advance outwash sands and gravels and the very stiff to hard silts at 1H:1V. As is typical with earthwork operations, some sloughing and raveling may occur and cut slopes may have to be adjusted in the field. In addition, WISHA/OSHA regulations should be followed at all times.



9.2 Moisture-Sensitive Soils The on-site soils contain a high percentage of fine-grained material which makes them moisture-sensitive and subject to disturbance when wet. The contractor must use care during site preparation and excavation operations so that the underlying soils are not softened. If disturbance occurs, the softened soils should be removed and the area brought to grade with structural fill. Consideration should be given to protecting access and staging areas with an appropriate section of crushed rock or asphalt treated base (ATB). If crushed rock is considered for the access and staging areas, it should be underlain by engineering stabilization fabric to reduce the potential of fine-grained materials pumping up through the rock and turning the area to mud. The fabric will also aid in supporting construction equipment, thus reducing the amount of crushed rock required. We recommend that at least 10 inches of rock be placed over the fabric; however, due to the variable nature of the near-surface soils and differences in wheel loads, this thickness may have to be adjusted by the contractor in the field. 10.0 STRUCTURAL FILL Due to the slopes on the site, structural fill will be necessary to establish desired grades. All references to structural fill in this report refer to subgrade preparation, fill type, placement, and compaction of materials as discussed in this section. If a percentage of compaction is specified under another section of this report, the value given in that section should be used. 10.1 Subgrade Keying and Benching If fill is to be placed on slopes steeper than 5H:1V, the base of the fill should be tied to firm, stable subsoil by appropriate keying and benching, which would be established in the field to suit the particular soil conditions at the time of grading. The keyway will act as a shear key to embed the toe of the new fill into the hillside. Generally, the keyway for hillside fills should be at least 8 feet wide and cut into the lower, dense sand or stiff silt. Level benches would then be cut horizontally across the hill following the contours of the slope. No specific width is required for the benches, although they are usually a few feet wider than the dozer being used to cut them. All fills proposed over a slope should be reviewed by our office prior to construction. We recommend that AESI observe exposed subgrades prior to fill placement. Should wet subgrade conditions be present, we recommend that the wet subgrade areas for fills planned along the slopes be equipped with subfill drains. Subfill drains may consist of a 1- to 2-foot-thick section of free-draining aggregate placed below the fill and covered with a geotextile



fabric. The aggregate should be compacted to 95 percent of the modified Proctor maximum density using American Society for Testing and Materials (ASTM):D 1557 as the standard or to a firm and unyielding condition as determined by the geotechnical engineer or his representative. The subfill drains will allow hydrostatic forces, if present, to disperse. 10.2 Fill Subgrade Preparation After overexcavation/stripping has been performed to the satisfaction of the geotechnical engineer/engineering geologist, the upper 12 inches of exposed ground should be recompacted to 90 percent of the modified Proctor maximum density using ASTM:D 1557 as the standard or to a firm and unyielding condition as determined by the geotechnical engineer or his representative. If the subgrade contains too much moisture, adequate recompaction may be difficult or impossible to obtain and should probably not be attempted. In lieu of recompaction, the area to receive fill should be blanketed with washed rock or quarry spalls to act as a capillary break between the new fill and the wet subgrade. Where the exposed ground remains soft and further overexcavation is impractical, placement of an engineering stabilization fabric may be necessary to prevent contamination of the free-draining layer by silt migration from below. 10.3 Structural Fill Placement and Compaction After the recompacted ground is tested and approved or a free-draining rock course is laid, structural fill may be placed to attain desired grades. Structural fill is defined as non-organic soil, acceptable to the geotechnical engineer, placed in maximum 10-inch loose lifts with each lift being compacted to at least 95 percent of the modified Proctor maximum density using ASTM:D 1557 as the standard. In the case of roadway and utility trench filling, the backfill should be placed and compacted in accordance with current local or county codes and standards. The top of the compacted fill should extend horizontally outward a minimum distance of 3 feet beyond the location of the perimeter footings or roadway edge before sloping down at an angle of 2H:1V. 10.4 Moisture-Sensitive Fill Materials Soils in which the amount of fine-grained material (smaller than the No. 200 sieve) is greater than approximately 5 percent (measured on the minus No. 4 sieve size) should be considered moisture-sensitive. Use of moisture-sensitive soil in structural fills should be limited to favorable dry weather conditions. The on-site soils generally contained significant amounts of silt and are considered moisture-sensitive. In addition, construction equipment traversing the site when the soils are wet can cause considerable disturbance. Due to the sloping, potentially wet conditions at the subject site, and the proposed structures, road ways, utilities, and rockeries planned for these slope conditions, a select import material consisting of a clean,



free-draining gravel and/or sand should be used. Free-draining fill consists of non-organic soil with the amount of fine-grained material limited to 5 percent by weight when measured on the minus No. 4 sieve fraction. We recommend that imported structural fill conform to Washington State Department of Transportation (WSDOT) Specification 9-03.14(1) (gravel borrow) or similar as determined by the geotechnical engineer. 10.5 Structural Fill Testing The contractor should note that any proposed fill soils must be evaluated by Associated Earth Sciences, Inc. (AESI) prior to their use in fills. This would require that we have a sample of the material 48 hours in advance of filling activities to perform a Proctor test and determine its field compaction standard. A representative from our firm should inspect the stripped subgrade and be present during placement of structural fill to observe the work and perform a representative number of in-place density tests. In this way, the adequacy of the earthwork may be evaluated as filling progresses and problem areas may be corrected at that time. It is important to understand that taking random compaction tests on a part-time basis will not assure uniformity or acceptable performance of a fill. As such, we are available to aid the owner in developing a suitable monitoring and testing frequency. 11.0 FOUNDATIONS Spread footings may be used for building support when founded on medium dense recessional outwash soils, dense to very dense advance outwash soils, stiff to hard transitional beds, or structural fill placed as previously discussed. We recommend that an allowable bearing pressure of 2,000 pounds per square foot (psf) be utilized for design purposes, including both dead and live loads. An increase of one-third may be used for short-term wind or seismic loading. Perimeter footings should be buried at least 18 inches into the surrounding soil for frost protection. However, all footings must penetrate to the prescribed bearing stratum and no footing should be founded in or above loose, organic, or existing fill soils. It should be noted that the area bounded by lines extending downward at 1H:1V from any footing must not intersect another footing or intersect a filled area which has not been compacted to at least 95 percent of ASTM:D 1557. In addition, a 1.5H:1V line extending down from any footing must not daylight because sloughing or raveling may eventually undermine the footing. Thus, footings should not be placed near the edge of steps or cuts in the bearing soils. Anticipated settlement of footings founded on medium dense to very dense outwash soils, stiff to hard transitional bed silts, or approved structural fill should be on the order of 1 inch. However, disturbed soil not removed from footing excavations prior to footing placement



could result in increased settlements. All footing areas should be inspected by AESI prior to placing concrete to verify that the design bearing capacity of the soils has been attained and that construction conforms to the recommendations contained in this report. Such inspections may be required by the City of Kirkland. Perimeter footing drains should be provided as discussed under the section on “Drainage Considerations.” 12.0 LATERAL WALL PRESSURES All backfill behind walls or around foundations should be placed following our recommendations for structural fill and as described in this section of the report. Horizontally backfilled walls, which are free to yield laterally at least 0.1 percent of their height, may be designed using an equivalent fluid equal to 35 pounds per cubic foot (pcf). Fully restrained, horizontally backfilled, rigid walls that cannot yield should be designed for an equivalent fluid of 50 pcf. Walls that retain sloping backfill at a maximum angle of 2H:1V should be designed for 55 pcf for yielding conditions and 75 pcf for restrained conditions. If parking areas are adjacent to walls, a surcharge equivalent to 2 feet of soil should be added to the wall height in determining lateral design forces. Undrained walls/structures must be designed for combined soil and hydrostatic pressures (85 pcf for yielding walls, 100 pcf for unyielding walls with horizontal backfill) and for buoyant/uplift forces. In accordance with the 2012 IBC, retaining wall design should include seismic design parameters. Based on the site soils and assumed wall backfill materials, we recommend a seismic surcharge pressure in addition to the equivalent fluid pressures presented above. A rectangular pressure distribution of 4H and 8H psf (where H is the height of the wall in feet) should be included in design for “active” and “at-rest” loading conditions, respectively. The resultant of the rectangular seismic surcharge should be applied at the midpoint of the walls. 12.1 Wall Backfill The lateral pressures presented above are based on the conditions of a uniform backfill consisting of either the on-site glacial sediments, or imported sand and gravel compacted to 92 percent of ASTM:D 1557. A higher degree of compaction is not recommended, as this will increase the pressure acting on the walls. A lower compaction may result in unacceptable settlement behind the walls. Thus, the compaction level is critical and must be tested by our firm during placement. The recommended compaction of 92 percent of ASTM:D 1557 applies to any structural fill placed behind the wall within a distance equal to the wall height and up to the elevation of the top of the wall. Structural fill used to construct slopes behind retaining walls should be compacted to at least 95 percent of ASTM:D 1557 if the fill is placed above the elevation of the top of the wall. Surcharges from adjacent footings, heavy construction equipment, or sloping ground must be added to the above-recommended lateral pressures.



Footing drains should be provided for all retaining walls, as discussed under the “Drainage Considerations” section of this report. 12.2 Wall Drainage It is imperative that proper drainage be provided so that hydrostatic pressures do not develop against the walls. This would involve installation of a minimum, 1-foot-wide blanket drain for the full wall height (excluding the uppermost 1 foot of backfill) using imported washed gravel against the walls. The wall drain material must be hydraulically connected to the footing drain pipe. Wall foundation drains are discussed in Section 15.0 of this report. 12.3 Passive Resistance and Friction Factor Lateral loads can be resisted by friction between the foundation and the natural, medium dense to very dense sediments or supporting structural fill soils, or by passive earth pressure acting on the buried portions of the foundations. The foundations must be backfilled with compacted structural fill to achieve the passive resistance provided below. We recommend the following allowable design parameters.

• Passive equivalent fluid = 250 pcf • Coefficient of friction = 0.30

13.0 ROCKERIES Rockeries may be used to prevent erosion of slopes; however, they are not engineered structures and should not be used in place of retaining walls. Buildings and roads should be set back from rockeries so that a 1H:1V line extending up from the rear base of the rockery does not intersect the footing or pavement. Rockery construction quality depends largely on the skill of the builder. Although rockeries are commonly used, they should be considered a long-term maintenance item. Care must be exercised in selecting a rock source since some of the material presently being supplied is soft and disintegrates in a relatively short period of time. Samples of rock can be tested by AESI prior to their use in rockeries. It is our understanding that rockery walls will be used as a facing for geogrid-reinforced fill slopes. The following notes present rockery design and construction considerations. A typical rockery detail for geogrid-reinforced slopes (Figure 3) is included in this report. In addition, the contractor should confirm that the proposed configuration conforms to current City of Kirkland specifications.



A) The base of the rockery should be started by excavating a trench to a minimum depth of 12 inches below subgrade into firm, undisturbed ground. If loose, soft, or disturbed materials exist at the base rock location, they should be removed and replaced with free-draining sand and gravel or crushed rock. This backfill material should be compacted to a minimum of 90 percent of ASTM:D-1557. The gradation of the sand and gravel should be such that not more than 5 percent by weight should be finer than the No. 200 sieve, based on the minus No. 4 sieve.

B) The base rock should have a minimum width (perpendicular to the line of the

rockery) of 40 percent of the height of the rockery. All rocks should also meet the following weight requirements:

Height of Rockery Minimum Weight of Rock

Above 5 feet 200/6,000 pounds, graded, top/bottom rocks 5 feet or less 200/2,000 pounds, graded, top/bottom rocks

C) The rock material should all be as nearly rectangular as possible. No stone

should be used which does not extend through the wall. The rock material should be hard, sound, durable, and free from weathered portions, seams, cracks, or other defects. The rock density should be a minimum of 160 pcf.

D) Rock selection and placement should be such that there will be minimum voids

and, in the exposed face of the wall, no open voids over 8 inches across in any direction. The rocks should be placed in a manner such that the longitudinal axis of the rock will be at right angles or perpendicular to the rockery face. Each rock should be placed so as to lock into two rocks in the lower tier. After setting each rock course, all voids between the rocks should be chinked on the back with quarry rock to eliminate any void sufficient to pass a 2-inch-square probe. The rockery should be limited to 8 feet in height.

E) A drain consisting of rigid, perforated, polyvinyl chloride (PVC) pipe enclosed in a

12-inch-wide pea gravel trench should be placed behind the lower course of rock to remove water and prevent the buildup of hydrostatic pressure behind the wall. The remainder of the wall backfill should consist of quarry spalls with a maximum size of 4 inches and a minimum size of 2 inches. This material should be placed to a 12-inch-minimum thickness between the entire wall and the cut material. The backfill material should be placed in lifts to an elevation approximately 6 inches below the top of each course of rocks as they are placed until the uppermost course is placed. Any backfill material falling onto the



bearing surface of a rock course should be removed before the setting of the next course.

F) Any asphalt paving should be sloped to drain away from the rockery. In

addition, the areas above rockeries should be planted with grass as soon as possible after rockery construction to reduce erosion.

14.0 FLOOR SUPPORT Slab-on-grade floors may be constructed either directly on the medium dense to very dense natural sediments, or on structural fill placed over these materials. Areas of the slab subgrade that are disturbed (loosened) during construction should be recompacted to an unyielding condition prior to placing the pea gravel, as described below. If moisture intrusion through slab-on-grade floors is to be limited, the floors should be constructed atop a capillary break consisting of a minimum thickness of 4 inches of washed pea gravel. The pea gravel should be overlain by a 10-mil (minimum thickness) plastic vapor retarder. 15.0 DRAINAGE CONSIDERATIONS The underlying, glacially compacted soils are relatively impermeable and water will tend to perch atop this stratum. Additionally, traffic across these soils when they are damp or wet will result in disturbance of the otherwise firm stratum. Therefore, prior to site work and construction, the contractor should be prepared to provide temporary drainage and subgrade protection, as necessary. All retaining and perimeter footing walls should be provided with a drain at the footing elevation. The drains should consist of rigid, perforated, PVC pipe surrounded by washed pea gravel. The level of the perforations in the pipe should be set approximately 2 inches below the bottom of the footing, and the drains should be constructed with sufficient gradient to allow gravity discharge away from the buildings. All retaining walls should be lined with a minimum, 12-inch-thick, washed gravel blanket provided to within 1 foot of finish grade, and which ties into the footing drain. Roof and surface runoff should not discharge into the footing drain system, but should be handled by a separate, rigid, tightline drain.



Exterior grades adjacent to walls should be sloped downward away from the structures to achieve surface drainage. Final exterior grades should promote free and positive drainage away from the buildings at all times. Water must not be allowed to pond or to collect adjacent to foundations or within the immediate building areas. It is recommended that a gradient of at least 3 percent for a minimum distance of 10 feet from the building perimeters be provided, except in paved locations. In paved locations, a minimum gradient of 1 percent should be provided unless provisions are included for collection and disposal of surface water adjacent to the structures. Additionally, pavement subgrades should be crowned to provide drainage toward catch basins and pavement edges. Crawl space areas should be provided with drains at low points to prevent water from accumulating. 16.0 PROJECT DESIGN AND CONSTRUCTION MONITORING At the time of this report, site grading, structural plans, and construction methods have not been finalized and the recommendations presented herein are preliminary. We are available to provide additional geotechnical consultation as the project design develops and possibly changes from that upon which this report is based. We recommend that AESI perform a geotechnical review of the plans prior to final design completion. In this way, our earthwork and foundation recommendations may be properly interpreted and implemented in the design. This plan review is not included in the current scope of work and budget. We are also available to provide geotechnical engineering and monitoring services during construction. The integrity of the foundations depends on proper site preparation and construction procedures. In addition, engineering decisions may have to be made in the field in the event that variations in subsurface conditions become apparent. Construction monitoring services are not part of this current scope of work. If these services are desired, please let us know and we will prepare a cost proposal.

UNINCORPORATED

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APPENDIX

CRITICAL AREAS REPORT

ORCAS MOON PROPERTY KIRKLAND, WASHINGTON

Prepared For: ORCAS MOON, LLC

Prepared By: TALASAEA CONSULTANTS, INC.

21 July 2016

Critical Areas Report

Orcas Moon Property Kirkland, Washington

Prepared For: Orcas Moon, LLC

13809 209th Avenue NE Woodinville, Washington 98072

Prepared By: Talasaea Consultants, Inc.

15020 Bear Creek Road NE Woodinville, Washington 98077

(425) 861-7550

21 July 2016

Orcas Moon Project Critical Areas Report

21 July 2016 Copyright © 2016 Talasaea Consultants, Inc. 518B Critical Areas Report (2016-06-21).docx Page i

EXECUTIVE SUMMARY

PROJECT NAME: Orcas Moon Project

CLIENT: Orcas Moon, LLC

SITE LOCATION: Property is northwest of the intersection of 28th Avenue and 5th Street and south of Forbes Creek Drive in Kirkland, Washington. The Public Land Survey System location of the property is the southwest ¼ of Section 32, T26N, R5E, Willamette Meridian.

PROJECT STAFF: Bill Shiels, Principal; Ann Olsen, Senior Project Manager; David R. Teesdale, Senior Wetlands Ecologist

FIELD SURVEY: Site was evaluated and critical areas delineated on 8 and 19 April 2016.

DETERMINATION: The Orcas Moon property is located within a City of Kirkland Primary Basin (Forbes Creek). Two wetlands and five streams were identified on the Orcas Moon Property. One wetland was identified offsite to the west of the property. The onsite wetlands were all rated as City of Kirkand Type 3 wetlands. The offsite wetland was rated as a City of Kirkland Type 2 wetland. Type 2 wetlands within a Primary Basin have a 75-foot standard buffer. Type 3 wetlands within a Primary Basin have a 50-foot standard buffer. The streams were rated as City of Kirkland Class B waters. Class B waters within a Primary Basin have a 60-foot standard buffer.

HYDROLOGY: Hydrology for Wetlands A and C is provided by shallow groundwater seepage on a slope. Hydrology for Wetland B is supported entirely by stream flow from Stream 4, which is supported by Wetland C.

SOILS: Three soil types are mapped on the property. These are Kitsap silt loam (2 to 8 percent slope), Kitsap silt loam (15 to 30 percent slope), and Indianola loamy fine sand (4 to 15 percent). These soils are not listed as hydric by the National Technical Committee on Hydric Soils.

VEGETATION: Vegetation within Wetland A is a mixture of sparse herbaceous and scrub-shrub species with a significant portion of bare soil present. Species include skunk cabbage (Lysichiton americanum), piggyback plant (Tolmea menziesii), slough sedge (Carex obnupta), field and tall horsetail (Equisetum arvense and E. telmateia), lady fern (Athyrium filix-femina), salmonberry (Rubus spectabilis), and young red alder (Alnus rubra). Vegetation within Wetland B includes American brooklime (Veronica americana), lady fern, piggyback plant, and slough sedge. Vegetation within Wetland C is mostly scrub-shrub species, comprised predominantly of salmonberry, lady fern, skunk cabbage, slough sedge, and red alder.

PROPOSED DEVELOPMENT: The Client proposes to develop the Orcas Moon Project as a cottage unit development. Sixteen (16) units of cottages will be constructed in two separate groups on the property. Spreading the development out into two different groups allows the project to maximize the buildable area outside of steep slope zones. The two cottage unit groups will be arranged around rain gardens, which will handle all stormwater runoff from paved parking and foot trail systems as well as rooftop runoff.

The proposed development will not directly impact wetlands or streams on the subject property. However, it will be necessary to reduce the critical areas buffers in nine (9) locations. This is permitted under KMC §90.60(2)(a) and §90.100(1)(a) for buffer averaging. Sufficient area is available onsite to provide additional buffer area that is equal in functions and services to the buffer areas being reduced. Further enhancement of buffer vegetation will not be required.


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TABLE OF CONTENTS

Chapter 1. Introduction ................................................................................................ 1

1.1 Report Purpose .................................................................................... 1 1.2 Statement of Accuracy ......................................................................... 1

Chapter 2. General Property Description and Land Use ............................................. 1

2.1 Project Location .................................................................................... 1 2.2 General Property Description ............................................................... 1 2.3 Land Use and Zoning ........................................................................... 1

Chapter 3. Methodology .............................................................................................. 2

3.1 Background Information Reviewed ....................................................... 2

3.2 Field Investigation................................................................................. 3

Chapter 4. Results ....................................................................................................... 3

4.1 Analysis of Existing Information ............................................................ 3 4.1.1 USFWS Wetlands Online Mapper (National Wetlands Inventory) ........ 4 4.1.2 Natural Resources Conservation Service Web Soil Survey ................. 4 4.1.3 StreamNet and SalmonScape GIS Databases ..................................... 4 4.1.4 King County GIS Database .................................................................. 4 4.1.5 City of Kirkland Critical Areas Map ....................................................... 4 4.2 Analysis of Existing Site Conditions...................................................... 5 4.2.1 Wetlands .............................................................................................. 5 4.2.2 Streams ................................................................................................ 6

Chapter 5. Regulatory Review ..................................................................................... 7

5.1 City of Kirkland Critical Areas Regulations ........................................... 7 5.2 State and Federal Regulations ............................................................. 8

Chapter 6. Proposed Project ....................................................................................... 8

6.1 Project Description ............................................................................... 8 6.2 Project impacts ..................................................................................... 8 6.3 Proposed Mitigation .............................................................................. 9

Chapter 7. Summary ................................................................................................... 9

Chapter 8. References .............................................................................................. 10


21 July 2016 Copyright © 2016 Talasaea Consultants, Inc. 518B Critical Areas Report (2016-06-21).docx Page iii

LIST OF FIGURES

Figure 1 – Vicinity Map Figure 2 – Site Map Figure 3 – NWI Map – Kirkland Quadrangle Figure 4 – NRCS Soils Data (from City of Kirkland) Figure 5 – King County Critical Areas GIS Data Figure 6 – City of Kirkland Critical Areas Figure 7 – Wetland and Stream Map Figure 8 – Site Development Concept

APPENDICES

Appendix A. City of Kirkland Wetland Rating Forms (Plate 26)


21 July 2016 Copyright © 2016 Talasaea Consultants, Inc. 518B Critical Areas Report (2016-06-21).docx Page 1

Chapter 1. INTRODUCTION

1.1 Report Purpose This report is the result of a critical areas study of the Orcas Moon Project property (referred to hereinafter as “Project Site” or “Site). The Site is located within the Forbes Creek basin of Kirkland (Figure 1). The purpose of this report is to identify, categorize, and describe existing site conditions, such as wetlands, streams, or other critical habitats, and their respective buffers. The report has been prepared to comply with the requirements of Kirkland Municipal Code Chapter 90 – Drainage Basins.

This report will provide and describe the following information:

General property description;

Methodology for critical areas investigation;

Results of critical areas background review and field investigation; and

Regulatory review.

1.2 Statement of Accuracy Critical areas characterizations and ratings were conducted by trained professionals at Talasaea Consultants, Inc., and adhered to the protocols, guidelines, and generally accepted industry standards available at the time the work was performed. The conclusions in this report are based on the results of analyses performed by Talasaea Consultants and represent our best professional judgment. To that extent and within the limitation of project scope and budget, we believe the information provided herein is accurate and true to the best of our knowledge. Talasaea does not warrant any assumptions or conclusions not expressly made in this report, or based on information or analyses other than what is included herein.

Chapter 2. GENERAL PROPERTY DESCRIPTION AND LAND USE

2.1 Project Location The Project Site is located northwest of the intersection of 20th Avenue and 5th Street in the City of Kirkland, Washington (Figure 2). The Site extends northward from 20th Avenue to Forbes Creek Drive. The Site includes two tax parcels: Parcel A (3890100055), and Parcel B (3890100050). The Site encompasses approximately 7.1 acres. The Public Land Survey System location of the Site is southwest ¼ of Section 32, T26N, R5E, Willamette Meridian.

2.2 General Property Description The Site is currently undeveloped and forested with second-growth mixed coniferous and deciduous trees. The topography of the Site is moderately sloped with five ravines extending generally in a north-south orientation. The Site generally slopes downward from 20th Avenue to Forbes Creek Drive.

2.3 Land Use and Zoning The Site is zoned RS-12.5 or Single Family Residential. The Site is currently undeveloped. However, a single-family residence and an associated outbuilding did exist on Parcel A prior to 1936 (date of earliest aerial photo available). This residence



was still visible as of 1952. This residence was removed from Parcel A, although its driveway is still present. Properties to the northeast and south are developed as single-family residential. Properties to the west and southeast are currently undeveloped.

Chapter 3. METHODOLOGY

The critical areas analysis of the Site involved a two-part effort. The first part consisted of a preliminary assessment of the Site and the immediate surrounding area using existing published environmental information. This information includes:

1. Wetland and soils information from resource agencies; 2. Critical areas information from the City of Kirkland and King County; 3. Orthophotography and LIDAR imagery; and, 4. Relevant studies completed or ongoing in the vicinity of the Site.

The second part consisted of site investigations where direct observations and measurements of existing environmental conditions were made. Observations included plant communities, soils, hydrology, and stream conditions. This information was used to help characterize the site and define the limits of critical areas onsite and offsite for regulatory purposes (see Section 3.2 – Field Investigation below).

3.1 Background Information Reviewed Background information from the following sources was reviewed prior to field investigations:

US Fish and Wildlife Service (USFWS) Wetlands Online Mapper (National Wetlands Inventory)(U.S. Fish and Wildlife Service) (www.wetlandsfws.er.usgs.gov/wtlnds/launch.html);

Natural Resources Conservation Service, Web Soil Survey (Natural Resources Conservation Service)(www.websoilsurvey.nrcs.usda.gov/app);

Natural Resources Conservation Service National Hydric Soils List by State (Natural Resources Conservation Service)(www.soils.usda.gov/use/hydric/lists/state.html);

City of Kirkland GIS database (City of Kirkland, 2015);

King County GIS database (King County 2015);

King County iMap online mapping program (King County);

LIDAR data from King County GIS (2006);

Orthophotography from Earth Explorer (2016);

WDFW Priority Habitats and Species (PHS) Database on the Web (Washington State Department of Fish and Wildlife)(wdfw.wa.gov/mapping/phs); and

Washington Department of Natural Resources Natural Heritage GIS database, 2015.

Fish usage data from SalmonScape (http://apps.wdfw.wa.gov/salmonscape/map.html) and StreamNet

http://www.wetlandsfws.er.usgs.gov/wtlnds/launch.html

http://www.websoilsurvey.nrcs.usda.gov/app

http://www.soils.usda.gov/use/hydric/lists/state.html

http://wdfw.wa.gov/mapping/phs

http://apps.wdfw.wa.gov/salmonscape/map.html



(http://www.streamnet.org/data/interactive-maps-and-gis-data/)

3.2 Field Investigation The Site was evaluated and critical areas delineated on 8 and 19 April 2016. The boundaries of wetlands and the ordinary high water mark (OHWM) of streams were flagged in the field for later professional surveying.

The wetland delineation utilized the routine approach described in the Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Western Mountains, Valleys, and Coast Region (U.S. Army Corps of Engineers, 2010). The ordinary high water mark (OWHM) for any streams found on the Site was determined and delineated using the methodology described by Washington State Department of Ecology’s “Determining the Ordinary High Water Mark on Streams in Washington State”. (Olson and Stockdale 2008). Wetlands and streams were classified according to City of Kirkland Municipal Code, Chapter 90 – Drainage Basins.

Plant species were identified according to the taxonomy of Hitchcock and Cronquist (Hitchcock, et al. 1969). Taxonomic names were updated and plant wetland status was assigned according to North American Digital Flora: National Wetland Plant List, Version 2.4.0 (Lichvar, et al. 2012). Wetland classes were determined using the U.S. Fish and Wildlife Service’s system of wetland classification (Cowardin, et al. 1979). Vegetation was considered hydrophytic within a suspected wetland area if greater than 50% of the dominant plant species had a wetland indicator status of facultative or wetter (i.e., facultative, facultative wetland, or obligate wetland).

Wetland hydrology was determined based on the presence of hydrologic indicators listed in the Corps regional supplement. These indicators are separated into Primary Indicators and Secondary Indicators. To confirm the presence of wetland hydrology, one Primary Indicator or two Secondary Indicators must be demonstrated. Indicators of wetland hydrology may include, but are not necessarily limited to; drainage patterns, drift lines, sediment deposition, watermarks, stream gauge data and flood predictions, historic records, visual observation of saturated soils, and visual observation of inundation.

Soils on the Site were considered hydric if one or more of the hydric soil indicators listed in the Corps’ Regional Supplement were present. Indicators include: presence of organic soils; reduced, depleted or gleyed soils, or redoximorphic features in association with reduced soils.

Wetlands were rated using the City of Kirkland’s wetland rating system. The wetland datasheets are contained in Appendix A.

Chapter 4. RESULTS

4.1 Analysis of Existing Information The following sources provided information on site conditions based on data compiled from resource agencies and local government. For the purposes of this report, the term “vicinity” will mean an area within ¼ mile of the Project Site.



4.1.1 USFWS Wetlands Online Mapper (National Wetlands Inventory) The USFWS Wetlands Online Mapper maps six wetland units within the vicinity of the Site (Figure 3). No wetlands are indicated on or extending onto the site. Three of the wetlands are palustrine forested (one is indicated as palustrine forested/scrub-shrub), two are palustrine unconsolidated bottom, and one is a palustrine scrub-shrub wetland.

4.1.2 Natural Resources Conservation Service Web Soil Survey Three soil types are mapped on the property (Figure 4). These are Kitsap silt loam (KpB, 2 to 8 percent slope), Kitsap silt loam (KpC, 15 to 30 percent slope), and Indianola loamy fine sand (InC, 4 to 15 percent).

The Kitsap series is made up of moderately well drained soils that formed in glacial lake deposits, under a cover of conifers and shrubs. These soils are on terraces and strongly dissected terrace fronts. The surface layer and subsoil are very dark brown and dark yellowish brown silt loam.

The Indianola series is made up of somewhat excessively drained soils that formed under conifers in sandy, recessional, stratified glacial drift. These undulating, rolling, and hummocky soils are on terraces. These soils are generally brown, dark yellowish-brown, and light olive-brown loamy fine sand.

The Kitsap and Indianola soil series are not listed as hydric by the National Technical Committee on Hydric Soils.

4.1.3 StreamNet and SalmonScape GIS Databases StreamNet and SalmonScape maintain data concerning the usage or potential usage of streams in the Pacific Northwest. StreamNet maps Coho (Oncorhynchus kisutch) as utilizing Forbes Creek for rearing and migration. No other salmonid species are mapped within the vicinity of the Site. SalmonScape maps four species utilizing or having the potential to utilize Forbes Creek. These are fall chinook (O. tshawytscha), coho, winter steelhead (O. mykiss), and sockeye (O. nerka). Coho are indicated as documented rearing. Sockeye are indicated as documented presence. Both fall chinook and winter steelhead are indicated as modeled presence1.

4.1.4 King County GIS Database King County GIS does not map any features on the Site. However, it does map some features within the vicinity of the Site (Figure 5). These features include two water bodies, two streams, a floodway, and a flood plain. One of the streams is associated with the floodway and floodplain, and is identified as Forbes Creek. The second stream is unnamed on the King County GIS database.

4.1.5 City of Kirkland Critical Areas Map The City of Kirkland does not map any wetlands on the Site (Figure 6). However, it does map two wetlands in the vicinity of the Site. One wetland is located near the

1 “Modeled presence” indicates that physical parameters of a particular stream may support the presence of a salmonid species, but no actual documentation of their presence exists.



southwest property corner on an adjacent parcel. The other wetland is associated with Forbes Creek to the north of the Site.

The City of Kirkland also maps five streams on the Site, including Forbes Creek to the north of the property. At least four more streams are mapped on properties to the east and west of the Site.

Finally, the City of Kirkland maps a floodplain and floodway in the general vicinity of Forbes Creek.

4.2 Analysis of Existing Site Conditions Two wetlands and five streams were identified during our evaluation of the Site (Figure 7). An additional wetland was identified off site to the west, but was not delineated. It was, however, rated using the City of Kirkland’s wetland rating system (Plate 26).

4.2.1 Wetlands 4.2.1.1 Wetland A Wetland A is an approximately 5,900 sf wetland located near the southwestern corner of the Site (Parcel A). It appears to have been created by a slump in the recent past, based on the age of the alders growing within Wetland A. The wetland is a slope wetland that provides hydrology for one of the five onsite streams.

Vegetation within Wetland A consists primarily of skunk cabbage (Lysichiton americanum), piggyback plant (Tolmea menziesii), slough sedge (Carex obnupta), field and tall horsetail (Equisetum arvense and E. telmateia), lady fern (Athyrium filix-femina), salmonberry (Rubus spectabilis), and young red alder (Alnus rubra).

Wetland A was rated using the City of Kirkland’s wetland rating system. The wetland scored 21 points, which satisfies the criteria for characterization as a Type 3 wetland. Type 3 wetlands located within a Primary Basin (Forbes Creek) have a 50-foot standard buffer. Wetland buffers may be modified through buffer averaging, provided that the minimum buffer width at any one point is not less than 33 feet and that the total area of the averaged buffer is not less than the area of the standard buffer.

4.2.1.2 Wetland B Wetland B is a very small (approximately 170 sf) wetland that formed within an old concrete cistern. The cistern is constructed within the ravine for one of the onsite streams (Stream 4) and may have provided water for the residence that existed on Parcel A. Over time, this cistern has silted in and wetland vegetation has become established. Vegetation in Wetland B consists of American brooklime (Veronica americana), lady fern, piggyback plant, and slough sedge.

Wetland B scored 17 points using the City of Kirkland wetland rating system. This satisfies the criteria for characterization as a Type 3 wetland. Type 3 wetlands located within a Primary Basin have a 50-foot standard buffer. Wetland buffers may be modified through buffer averaging, provided that the minimum buffer width at any one point is not less than 33 feet and that the total area of the averaged buffer is not less than the area of the standard buffer.



4.2.1.3 Wetland C (Off Site) Wetland C is a slope wetland that is located to the west of the southwest property corner. This wetland was not delineated since it resides off property. However, we estimate its size to be approximately 6,200 sf. Vegetation consists predominantly of salmonberry, lady fern, skunk cabbage, slough sedge, and red alder. Wetland C is the headwaters of one of the onsite streams (Stream 4).

Wetland C scored 25 points using the City of Kirkland wetland rating system. This satisfies the criteria for characterization as a Type 2 wetland. Type 2 wetlands located within a Primary Basin have a 75-foot standard buffer. Due to the location of this wetland, buffer averaging will likely not be possible.

4.2.2 Streams 4.2.2.1 Stream 1 Stream 1 starts at the outfall of a stormwater pipe located on the north side of 20th Avenue. The stream flows onto the Site at the southeast property corner and flows in a northerly direction for approximately 70 feet. Then, the stream flows off property to the east. The stream channel is in a deeply incised ravine that extends from the stormwater outfall.

Stream 1 satisfies the criteria for categorization as a City of Kirkland Class B stream. Class B streams within a Primary Basin have a 60-foot standard buffer. This buffer may be reduced to 39.6 feet through buffer averaging, provided that the total area of the reduced buffer is not less than the area of the standard buffer.

4.2.2.2 Stream 2 Stream 2 starts at the outfall of two stormwater pipes located on the north side of 20th Avenue, approximately 170 feet west of the stormwater outfall for Stream 1. As with Stream 1, Stream 2 flows within a deeply incised ravine. The stream flows aboveground for approximately 390 feet where it flows into a buried pipe. The pipe extends to the northeast for approximately 160 feet. The outfall of this pipe is within the channel for Stream 5.

Stream 2 satisfies the criteria for categorization as a City of Kirkland Class B stream. Class B streams within a Primary Basin have a 60-foot standard buffer. This buffer may be reduced to 39.6 feet through buffer averaging, provided that the total area of the reduced buffer is not less than the area of the standard buffer. There is no buffer requirement for the piped portion of Stream 2.

4.2.2.3 Stream 3 Stream 3 starts near the northwest corner of the Site in an area of a previous soil slump (the same slump that likely created Wetland A). There are at least three pipe outfalls mapped near the headwaters of Stream 3. As with Stream 1 and 2, the pipes carry stormwater from the development to the south of 20th Avenue. Stream 3 begins as three separate seeps that coalesce towards the northern tip of Wetland A. At this point, it flows in a deeply incised ravine for approximately 220 feet. The stream then enters a



buried pipe that extends to the northeast for approximately 280 feet. The pipe then discharges into a roadside ditch along Forbes Creek Road.

Stream 3 satisfies the criteria for categorization as a City of Kirkland Class B stream. Class B streams within a Primary Basin have a 60-foot standard buffer. This buffer may be reduced to 39.6 feet through buffer averaging, provided that the area of the reduced buffer is not less than the area of the standard buffer. There is no buffer requirement for the piped portion of Stream 3.

4.2.2.4 Stream 4 The headwaters for Stream 4 are within Wetland C off property to the west. Stream 4 flows onto the Site approximately 130 feet north of the northwest property corner and flows within a deeply incised ravine for approximately 100 feet (this aboveground portion of Stream 4 includes Wetland B). At this point, the stream enters a buried pipe. The pipe extends to the northeast for approximately 140 feet and discharges into a roadside ditch along Forbes Creek Road.

Stream 4 satisfies the criteria for categorization as a City of Kirkland Class B stream. Class B streams within a Primary Basin have a 60-foot standard buffer. This buffer may be reduced to 39.6 feet through buffer averaging, provided that the area of the reduced buffer is not less than the area of the standard buffer. There is no buffer requirement for the piped portion of Stream 4.

4.2.2.5 Stream 5 Stream 5 starts off property to the east. Prior to the subdivision along Forbes Creek Road adjacent to the east of the Site, Stream 5 did not flow onto the subject property. Stream 5 is collected offsite in a pipe and shunted along the south side of the aforementioned subdivision. This pipe discharges into a deeply incised ravine that flows in a westerly direction on to the Site, then in a northwesterly direction towards Forbes Creek Road. As previously mentioned, the piped portion of Stream 2 discharges into the ravine for Stream 5.

Stream 5 satisfies the criteria for categorization as a City of Kirkland Class B stream. Class B streams in a Primary Basin have a 60-foot standard buffer. This buffer may be reduced to 39.6 feet through buffer averaging, provided that the area of the reduced buffer is not less than the area of the standard buffer.

Chapter 5. REGULATORY REVIEW

5.1 City of Kirkland Critical Areas Regulations Wetlands and streams on the Site are subject to City of Kirkland critical areas regulations under Chapter 90 – Drainage Basins. City of Kirkland currently uses its own wetland rating and water typing systems. The wetland rating system seems to be based on the Washington Department of Ecology’s (WDOE) Washington State Wetland Rating System for Western Washington (1993), which is not comparable with the current WDOE Washington State Wetland Rating System for Western Washington (2014). Similarly, their method of water typing for streams is not comparable with the



current Washington Department of Natural Resources (WDNR) water typing system, which is promulgated in WAC 222-16-030.

Wetland buffers are determined based on the wetland’s rating and whether it is located within a Primary Basin. Primary Basins are defined as the basin that supports one of Kirkland’s major stream systems. Similarly, stream buffers are based on the stream’s class and whether it is located within a Primary Basin.

5.2 State and Federal Regulations Wetlands and streams on the Site are subject to applicable State and Federal regulations. Wetland impacts are regulated at the Federal level by Sections 404 and 401 of the Clean Water Act. The U.S. Army Corps of Engineers (Corps) is responsible for administering compliance with Section 404 via the issuance of Nationwide or Individual Permits for any fill or dredging activities within wetlands under Corps jurisdiction. Any project that is subject to Section 404 permitting is also required to comply with Section 401 Water Quality Certification, which is administered by the Washington State Department of Ecology (WDOE). No dredging or filling of onsite wetlands is proposed for the current site development plan. Therefore, the project will not need to apply for any Section 404 Nationwide or Individual Permits or Section 401 Water Quality Certification.

Any work within, over, or under the Ordinary High Water Mark of a stream requires a Hydraulic Project Approval (HPA) from the Washington Department of Fish and Wildlife (WDFW), pursuant to the State Hydraulic Code (Chapter 77.55 RCW).

Chapter 6. PROPOSED PROJECT

6.1 Project Description Orcas Moon, LLC is proposing to develop the Orcas Moon property with 16 units of cottage housing (Figure 8). The site development will be divided into two separate groups based on available land that is not constrained by steep slopes. For the purposes of this report, the groups will be called Group 1 and Group 2. Group 1 is located in the southwestern portion of the Site adjacent to 20th Avenue. Group 2 is located in the southeastern portion of the Site, also adjacent to 20th Avenue. Group 1 will include 10 cottage units, and Group 2 will provide 6 cottage units. Parking for Groups 1 and 2 will be provided off of the north side of 20th Avenue in two separate locations. Access to the Group 1 and 2 cottage units will be provided by loop walkways. The loop walkway for the Group 1 units will have an approximately 5,200 sf rain garden open space and the loop walkway for Group 2 will have an approximately 3,960 sf rain garden open space as defined by their respective loops. Both Group 2 and 3 walkways will incorporate switchbacks across steeper sloped areas to ensure ADA compliance.

6.2 Project impacts The project has been designed to avoid all direct impacts to wetlands and streams on the Site. However, it will be necessary to impact wetland and stream buffers in order to provide the required yard setbacks for the cottage units, construction of some of the parking areas, and walkways (Figure 9). Buffers will be reduced in these areas of



impact using buffer averaging. In all, there will be nine areas of buffer reduction for a total reduction area of 12,047 sf.

6.3 Proposed Mitigation The proposed mitigation for the buffer reduction will be through buffer averaging. Sufficient area is available on the Site to provide meaningful buffer averaging. We have identified 6 areas on the Site that will provide additional buffer area. The total area of buffer addition is approximately 12,490 sf for a net increase of approximately 246 sf.

The areas proposed for buffer addition are well vegetated and similar to the areas of buffer reduction. The functions and services provided by the lost buffer area will be compensated by the functions and services provided by the additional buffer areas. Enhancement will not be provided since the buffer increase areas are of equal value to the areas proposed for buffer reduction.

Chapter 7. SUMMARY

The Orcas Moon property is an approximately 7.1 acre assemblage of two tax lots, located in Kirkland, Washington. The property is currently undeveloped and forested. Two wetlands and five streams were identified and delineated on the property. One wetland was identified off property to the west. Orcas Moon, LLC proposes to development of 16 units of cottage housing on the property. The units will be constructed in two groups across the property to take advantage of limited relatively level areas. Approximately 2 acres of the 7-acre Site will be developed. The remaining portion (approximately 70 percent of the total Site size) will remain in its natural state.



Chapter 8. REFERENCES

City of Kirkland, 2016. Chapter 90 – Drainage Basins. Retrieved from http://www.codepublishing.com/wa/kirkland.html.

Cowardin, Lewis M., Virginia Carter, Francis C. Golet, and Edward T. LaRoe. 1979. Classification of Wetlands and Deepwater Habitats of the United States. Washington, DC: U.S. Department of the Interior, Fish and Wildlife Service.

Google Earth. 2016. Google Earth. https://www.google.com/earth/.

Hitchcock, C. Leo, Arthur Cronquist, Marion Owensby, and J. W. Thompson. 1969. Vascular Plants of the Pacific Northwest. Seattle: University of Washington Press.

Iowa State University. 1995. Hydric Soils of Washington State. U.S. Department of Agriculture, Natural Resources Conservation Service.

King County. 2010. GIS Database. Accessed mo da, year. <http://www.kingcounty.gov/operations/GIS/GISData.aspx>.

Lichvar, R.W. 2012. The National Wetland Plant List. ERDC/CRREL TR-12-11, Hanover, NH: U.S. Army Corps of Engineers, Cold Regions Research and Engineering Laboratory. http://acwc.sdp.sirsi.net/client/search/asset:asset?t:ac=$N/1012381.

Munsell Color. 1988. Munsell Soil Color Charts. Grand Rapids, Michigan: X-RiteCorp.

Natural Resources Conservation Service. 2012. National Technical Committee for Hydric Soils - Hydric Soils List. http://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/use/hydric/.

Olson, P, and E Stockdale. 2008. Determining the Ordinary High Water Mark on Streams in Washington State. Olympia: Washington Department of Ecology.

Reed, P. B. 1988. National List of Plant Species that Occur in Wetlands: Northwest (Region 9). Report 88, USF & WS Biol.Update.

Reed, P. B. 1996. Supplement to National List of Plant Spcies that Occur in Wetlands (Region 9). National Wetlands Inventory. US Fish and Wildlife Service.

Soil Survey Staff. 2013. Web Soil Survey. December. Accessed September 2014. http://websoilsurvey.nrcs.usda.gov/.

StreamNet. 2016. "StreamNet Mapper." StreamNet. Accessed July 2016. http://map.streamnet.org/website/bluesnetmapper/viewer.htm.

U.S. Army Corps of Engineers. 2010. Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Western Mountains, Valleys, and Coast Region

http://www.codepublishing.com/wa/kirkland.html



(Version 2.0). U. S. Army Corps of Engineers, Vicksburg, MS: U.S. Army Engineer Research and Development Center.

U.S. Department of Agriculture. 1991. Hydric Soils of The United States. Washington, DC: Soil Conservation Service.

U.S. Department of the Interior, Fish and Wildlife Service. year. National Wetlands Inventory Map. Washington D.C., month day.

United States Geological Service. 2014. EarthExplorer. http://earthexplorer.usgs.gov/.

Washington State Department of Fish and Wildlife. 2016. Salmonscape. Accessed July 2016. wdfw.wa.gov/mapping/salmonscape/index.html.


21 July 2016 Copyright © 2016 Talasaea Consultants, Inc. 518B Critical Areas Report (2016-06-21).docx Figures

Figures

Figure 1 – Vicinity Map Figure 2 – Site Map Figure 3 – NWI Map – Kirkland Quadrangle Figure 4 – NRCS Soils Data (from City of Kirkland) Figure 5 – King County Critical Areas GIS Data Figure 6 – City of Kirkland Critical Areas Figure 7 – Wetland and Stream Map Figure 8 – Site Development Concept


21 July 2016 Copyright © 2016 Talasaea Consultants, Inc. 518B Critical Areas Report (2016-06-21).docx Appendix A

Appendix A

City of Kirkland Wetland Rating Forms (Plate 26)


Job # 12-248 Page 7-1

Section 7 Other Permits

No other permits related to the site storm drainage are required.


Job # 12-248 Page 8-1

Section 8 TESC Analysis and Design

The temporary erosion and sedimentation control plan is designed to reduce the discharge of

sediment-laden runoff from the site. The plan will be comprised of temporary measures (rock

entrance, filter fence, straw mulch, etc.) as well as permanent measures (hydroseeding and

landscaping). A TESC plan will be submitted with the final engineering plans.

The following BMPs will be applied to prevent erosion and trap sediments within the project

site and are shown on sheet TP-01:

Mark Clearing Limits / Minimize Clearing – Prior to any site clearing or grading, the

clearing limits are to be marked in the field. The trees to remain will have tree

protection measures installed per City of Kirkland detail on sheet TD-01, to be provided

at final engineering.

Minimize Sediment Tracked Offsite – A stabilized construction entrance shall be

installed as the first step in clearing and grading. The construction entrance is to be

installed per City of Kirkland Standard Plan No. CK-E.01 at the location shown on sheet

TP-01, to be provided at final engineering.

Control Sediment – Perimeter protection to filter sediment from sheet wash shall be

located downslope of all disturbed areas and shall be installed prior to upslope

grading. The silt fence will be installed along the boundary of the site to retain all

sediment on site. Additionally, storm drain inlet protection measures will be applied to

all catch basins within the project vicinity. See sheet TP-01 for more details, to be


Stabilize Exposed Soils – Temporary and permanent cover measures shall be provided

to protect all disturbed areas. Cover measures include the use of surface roughening,

mulch, erosion control nets and blankets, plastic covering, seeding, and sodding. See

sheet TD-01 for more details, to be provided at final engineering.

Control Runoff– All drainage will remain in a sheet flow condition during construction.

Silt fence will be installed along the boundary of the site to control runoff on site. See

sheet TD-01 for more details, to be provided at final engineering.


Job # 12-248 Page 8-2

Control Dewatering– Any runoff generated by dewatering shall be treated by releasing

the water to a well vegetated, gently sloping area. See notes on sheet TD-01, to be


Control Other Pollutants – Pollutants shall be controlled per TESC notes shown on

sheet TD-01, to be provided at final engineering.

Final Stabilization – Prior to final construction approval, the project site shall be

stabilized to prevent sediment-laden water from leaving the site after project

completion. All disturbed areas shall be vegetated or otherwise permanently stabilized.

See sheet TD-01 for more detail, to be provided at final engineering.


Job # 12-248 Page 9-1

Section 9 Bond Quantities and Facilities Summary

A City of Kirkland Improvement Evaluation Packet (IEP) will be included in the final TIR.


Job # 12-248 Page 10-1

Section 10 Operations and Maintenance

An operation and maintenance manual will be provided with the final TIR.


Job # 12-248 Page A

Appendix

1

Tim Ho

From: Wes Ayers <[email protected]>Sent: Thursday, July 28, 2016 10:22 AMTo: Yannick MetsSubject: RE: 12-248 Forbes Creek Plat

Follow Up Flag: Follow upFlag Status: Completed

Hi Yannick, I have one complaint to note:

1. SW‐06‐113 – located at 10454 Forbes Creek Drive ‐ Property owner wants to develop site and has wetland/stream buffer issues that regulate the extent of encroachment. Homeowner indicates that since Forbes Creek 11 was built they are receiving more runoff into the existing wetland making it wetter. They also indicate that the drainage coming off of the adjacent hillside has increased.

If you have any other questions, please let me know. Thank you,

Wes Ayers City of Kirkland Public Works Surface Water Engineering Analyst (425) 587-3859 [email protected]

Public Works Department

Caring for your infrastructure to keep Kirkland healthy, safe, and vibrant

From: Yannick Mets [mailto:[email protected]] Sent: Thursday, July 28, 2016 8:36 AM To: Wes Ayers <[email protected]> Cc: Todd Oberg <[email protected]> Subject: 12‐248 Forbes Creek Plat Wes, I am working on another project in Kirkland and need to verify whether there are any downstream drainage complaints or particular conditions to note for the site and it’s downstream path. The project is currently undeveloped (PIN# 389010‐0050 and 0055) and is located in the Forbes Creek Drainage Basin. The site runoff currently is collected on‐site via various streams which concentrate to a single discharge point at the northern property boundary. Flows are conveyed via a tight‐line system north underneath Forbes Creek Drive and

2

daylight to a stream along the north side of the road. The stream continues north and combines with Forbes Creek in Juanita Bay Park. Forbes Creek continues approximately another ½ mile prior to discharging into Lake Washington. I have attached a downstream drainage exhibit for reference. If you need any additional information or have any questions please let me know. Thanks, Yannick Yannick Mets | Engineer BLUELINE | www.thebluelinegroup.com d 425.250.7262 | o 425.216.4051 | f 425.216.4052 LAND MATTERS : : Civil Engineering : : Land Use Planning

Documents

Orcas Moon Cottages Preliminary Technical …Orcas+Moon+TIR.pdfOrcas Moon Cottages Preliminary Technical Information Report Job # 12-248 Page 1-2 on-site runoff, provide flow control,