WESTPARK STORMWATER BYPASS RECONSTRUCTION

WESTPARK STORMWATER BYPASS RECONSTRUCTIONDraft Drainage Study ReportAugust 2019

PM&E Project No. 18-16

Prepared For:The Municipality of AnchorageProject Management and Engineering

Prepared By: AWR Engineering, LLC

4011 Arctic Blvd, Suite 106 Anchorage, AK 99503

Draft Drainage Study Report

Westpark Stormwater Bypass Reconstruction PM&E Project Number 18‐16

August 2019

Prepared for: The Municipality of Anchorage

Project Management and Engineering 4700 Elmore Road

Anchorage, Alaska 99507

Prepared by:

AWR Engineering, LLC 4011 Arctic Boulevard, Suite 106

Anchorage, Alaska 99503 (907) 441‐2973 COA: AECL 1470

In cooperation with: Huddle AK, LLC – Public Involvement

Stephl Engineering, LLC – Storm Drain Inspections CRW Engineering Group, LLC – Basemapping

Shannon and Wilson, Inc. – Geotechnical Considerations Bettisworth North Architects and Planners, Inc. – Bioswale Section Figure

MOA Project Number 18‐16 Westpark Stormwater Bypass Reconstruction

Draft Drainage Study Report Page i of iv August 2019

Table of Contents

Executive Summary ............................................................................................................................................ iii

1. Introduction ............................................................................................................................................... 1

2. Data Used and Collected for this Study ....................................................................................................... 2

3. Project Area Description ............................................................................................................................. 3

3.1. Stormwater Runoff and Drainage Patterns .................................................................................................. 5

3.2. Emergency Overflow Details ......................................................................................................................... 5

3.3. Stormwater Treatment ................................................................................................................................. 7

4. Analysis Approach and Hydraulic Modeling ................................................................................................ 8

4.1. Approach Overview ...................................................................................................................................... 8

4.2. Design Rainfall Events ................................................................................................................................... 8

4.3. Storm Drain Design Criteria .......................................................................................................................... 9

4.4. Modeling Details ........................................................................................................................................... 9

4.5. Baseline Levels of Service ........................................................................................................................... 14

4.6. Model Components – Proposed Conditions ............................................................................................... 15

4.7. Analyses and Modeling Limitations ............................................................................................................ 15

5. Alternatives Evaluation .............................................................................................................................. 16

5.1. Alternative Evaluation Criteria .................................................................................................................... 16

5.2. Alternative 1 – Extended Bypass ................................................................................................................ 17

5.3. Alternative 2 – Shortened Bypass ............................................................................................................... 20

5.4. Alternative 3 – Bioswale ............................................................................................................................. 23

5.5. Hydraulic Performance Summary ............................................................................................................... 28

5.6. Other Alternatives Considered ................................................................................................................... 31

6. Public Involvement Summary .................................................................................................................... 32

7. Recommendations ..................................................................................................................................... 33

8. Summary ................................................................................................................................................... 33


Draft Drainage Study Report Page ii of iv August 2019

List of Figures Figure 1: Project Vicinity ............................................................................................................................................... 1

Figure 2: Project Area ................................................................................................................................................... 4

Figure 3: Cook Inlet Storm Drain Outlet Structure ....................................................................................................... 5

Figure 4: Emergency Overflow Schematic .................................................................................................................... 6

Figure 5: Emergency Overflow and Channel ................................................................................................................ 7

Figure 6: Modeled Subbasins ...................................................................................................................................... 10

Figure 7: Baseline Model Schematic ........................................................................................................................... 12

Figure 8: Levels of Service ........................................................................................................................................... 15

Figure 9: Extended Bypass Schematic......................................................................................................................... 19

Figure 10: Shortened Bypass Schematic ..................................................................................................................... 22

Figure 11: Bioswale Schematic ................................................................................................................................... 26

Figure 12: Bioswale Typical Section ............................................................................................................................ 27

List of Tables Table 1: Design Rainfall Events ..................................................................................................................................... 8

Table 2: Alternative 1 Extended Bypass ‐ Estimated Project Cost .............................................................................. 18

Table 3: Alternative 2 Shortened Bypass ‐ Estimated Project Cost ............................................................................ 21

Table 4: Alternative 3 Bioswale ‐ Estimated Project Cost ........................................................................................... 25

Table 5: Modeling Results Summary 10‐year Event ................................................................................................... 29

Table 6: Modeling Results Summary 100‐year Event ................................................................................................. 30

List of Appendices

Appendix A: Outfall Pipe and Discharge Structure Condition Assessment

Appendix B: Modeling Details

Appendix C: Construction Cost Estimates

Appendix D: Bioswale Conceptual Design Drawings

Appendix E: Public Involvement Support Documents


Draft Drainage Study Report Page iii of iv August 2019

Executive Summary

AWR Engineering, LLC (AWR) is assisting the Municipality of Anchorage (MOA) Project Management and

Engineering (PM&E) department with evaluation of alternatives to disconnect an existing stormwater emergency

overflow pipe that currently discharges to a local pond near the Westpark Subdivision. Some residents are

concerned that this discharge could adversely impact the water quality of the pond. The purpose of this drainage

study report (DSR) is to identify alternatives and provide recommendations to disconnect the existing emergency

overflow without adversely impacting drainage conditions for the surrounding area.

Surface water runoff in the project area is collected in storm drain pipes and generally directed to the south, toward

Dimond Boulevard. Under heavy flow events, pipes become surcharged, and excess water that cannot enter the

pipes is diverted to a local pond via an emergency overflow structure. The emergency overflow discharges surface

water just upstream of the pond, and it flows into the pond via an open channel. Since its construction in 2016, the

overflow is known to have sent stormwater to the adjacent pond twice.

This project analyzed multiple options for disconnecting the overflow and eliminating or mitigating adverse

drainage impacts to the adjacent areas. Hydrologic and hydraulic analyses and modeling were completed to

establish the baseline performance of the drainage system in the project area for the 10‐year and 100‐year rainfall

events. The baseline scenario includes both existing and anticipated future conditions in the project drainage basin.

Baseline performance was quantified by applying a level of service rating to various locations throughout the project

drainage basin. Alternatives were then analyzed by updating the baseline analyses as needed to reflect the

proposed changes. Three primary alternatives were identified.

1. Alternative 1 – Extended Bypass. This alternative involves removing the existing emergency overflow and

constructing a stormwater bypass route that would provide additional conveyance capacity. The bypass

route includes a combination of new pipes and upsizing existing pipes. The alternative also includes a

stormwater detention basin near Dimond Boulevard. This alternative was developed to accommodate the

goal of not increasing surface flow or ponding from the baseline conditions at any location within the basin,

which required additional conveyance capacity both upstream and downstream of the existing emergency

overflow location. This alternative includes approximately 4,900 feet of new storm drain pipe as well as a

detention basin. Acquisition of six undeveloped future parcels is expected to be needed for construction of

the detention basin. This alternative would maintain or improve the baseline level of service at all locations

throughout the project area drainage basin. The estimated total project cost of Alternative 1 is $10 million.

2. Alternative 2 – Shortened Bypass. This alternative was developed to reflect a minimal bypass option with a

reduced pipe length compared to Alternative 1 and no detention facilities. It includes 3,350 feet of new

storm drain pipe and is expected to require acquisition of one undeveloped future parcel. This alternative

would worsen the baseline level of service in two locations in the project drainage area, resulting in

additional stormwater being directed onto privately owned land, which is considered to be unacceptable.

The estimated total project cost of Alternative 2 is $4.7 million.

3. Alternative 3 – Bioswale. This alternative would leave the existing emergency overflow in place but re‐direct

discharge from the overflow into a constructed bioswale. The bioswale would provide treatment for

stormwater before it discharges to the pond. Treatment would be provided via a combination of filtration,


Draft Drainage Study Report Page iv of iv August 2019

sedimentation, infiltration, and evapotranspiration. The bioswale would be separated from the pond by a

vegetated buffer strip such that any swale overflow during flood events would be filtered through the

vegetation prior to entering the pond. Because the bioswale does not send additional water into

downstream receiving pipes, this alternative does not change the levels of service from baseline conditions.

The estimated total project cost of Alternative 3 is $2.1 million.

Available funding for this project is currently $2.1 million. The feasibility of obtaining additional funding through

bonds or state grants is uncertain due to limited available funding and a significant number of competing capital

projects.

The recommended alternative for disconnecting the existing overflow is Alternative 3 – Bioswale. This option

eliminates a direct connection between the overflow and the pond, and does not cause adverse drainage impacts

to the surrounding area. It is also the only alternative that is currently fundable and could be designed and

constructed in the near future.


Draft Drainage Study Report Page 1 of 34 August 2019

1. Introduction

AWR Engineering, LLC (AWR) is assisting the Municipality of Anchorage (MOA) Project Management and

Engineering (PM&E) department with evaluation of alternatives to disconnect an existing stormwater emergency

overflow pipe that currently discharges to a local pond near the Westpark Subdivision. Some residents are

concerned that this discharge could adversely impact the water quality of the pond.

The project area is located in west Anchorage, north of Dimond Boulevard and west of Sand Lake Road. The project

vicinity is shown in Figure 1. Surface water runoff in the project area is collected in storm drain pipes and generally

directed to the south, toward Dimond Boulevard. Under heavy flow events, pipes become surcharged, and excess

water is diverted to a local pond via an emergency overflow structure. The emergency overflow discharges surface

water just upstream of the pond, and it flows into the pond via an overland channel. A more detailed description

of area drainage patterns is provided in Section 3.

The purpose of this drainage study report (DSR) is to identify alternatives and provide recommendations to

disconnect the existing emergency overflow, without adversely impacting drainage conditions for the surrounding

area.

Figure 1: Project Vicinity



2. Data Used and Collected for this Study

A variety of data was collected and/or referenced in the development of this study. A description of the significant

data sources is provided below.

Topographic Information. Topographic information in the project area was based on the 2015 Anchorage

area LIDAR data obtained from the MOA’s online Geographic Data and Information Center. The LIDAR data

was used to delineate the project drainage basin, characterize ground surface slopes, and develop

parameters for the hydrologic and hydraulic analysis and modeling work.

Aerial Imagery. This study utilized 2015 aerial imagery of the project area that was also obtained from the

MOA’s online Geographic Data and Information Center. The imagery was used for developing modeling

parameters and for supporting graphics and figures.

Drainageway Mapping. Drainageway mapping was obtained from the MOA’s online Geographic Data and

Information Center and was used to represent the type and location of drainage infrastructure in the project

area including culverts, storm drains, and open channels.

Record Drawings. Record drawings of the area were provided by the MOA and were used to obtain specific

attributes of the drainage network including storm drain size, depth, and slope. Record drawings were also

used to obtain information regarding the depth and location of other utilities such as water and sewer lines.

In cases where record drawings included soil boring information, that information was used to characterize

the area soil conditions.

Visual Observations. The project team made site observations to confirm area drainage patterns and the

location of portions of the existing drainage network. Observations and photos from other stakeholders

taken both before and after the start of this study were also used to document area conditions.

Utility and Property Mapping. A utility and property basemap was developed by CRW Engineering Group,

LLC for this project. This basemap was developed using aerial photography (described above), Geographic

Information System (GIS) data, and utility facility maps. A field survey was not performed. GIS shape files,

including right‐of‐way, easement data, and utility facilities were obtained from the MOA Geographic Data

and Information Center. Utilities included in the basemap are Chugach Electric Association (CEA),

Anchorage Water and Wastewater Utility, ENSTAR Natural Gas, Alaska Communications, and General

Communication Inc. The approximate location of utilities shown in the base map were confirmed using

aerial photography, utility facility maps, and field observations. If the GIS information was significantly

different than what was observed, it was adjusted. For instance, CEA electric poles were adjusted to match

field observations and the CEA facility map.

Pipe Condition Information. The project team facilitated video inspection of the storm drain pipes in the

lower portion of the project drainage basin as well as an approximately 1,200 foot long pipe that directs

stormwater under Dimond Boulevard and into Cook Inlet. The pipe under Dimond Boulevard was

constructed via horizontal directional drilling (also known as boring) in 2006. Municipal Street Maintenance

completed the inspection of the pipes in the project drainage basin, and Stephl Engineering coordinated



the inspection of the pipe under Dimond. The inspections did not find any significant structural deformities

or other issues that would cause notable flow blockage or poor performance of the system. Additional

information is presented in Appendix A.

3. Project Area Description

The project area is shown in Figure 2 and generally includes the surface area that contributes stormwater runoff to

the Dimond Boulevard storm drain pipe as well as the area surrounding the existing South Pond. Much of the project

area was previously a gravel pit that has been developed into medium density residential housing. The project area

also includes an undeveloped site owned by the Anchorage School District (ASD), a privately owned undeveloped

site in the northwest corner, and an area previously used as a material disposal site called the Lucy Pit.

The area includes several residential subdivisions. The largest subdivision in the project area is the Westpark

Subdivision, which was developed in several phases beginning in 2005 and completing in 2013. The Sonoma Glen

Subdivision is located south of the Westpark Subdivision and is also being constructed in phases. One phase is

complete, and other phases are either under construction or planned for the future. The Heather Wood Estates

Subdivision is located west of Sonoma Glen and was completed in 2018. The Westgate subdivision is north of

Heather Wood Estates and was completed in 2015. There is ongoing residential development in Sonoma Glen,

Westgate, and Heather Wood Estates. Muirwood Park Subdivision is located at the north end of the project area,

and is fully developed.



Figure 2: Project Area



3.1. Stormwater Runoff and Drainage Patterns

Within the project drainage basin, the ground surface is generally sloped from north to south. Surface water runoff

is directed to inlets and catch basins and then into subsurface stormwater conveyance pipes (storm drains). The

primary storm drain trunk line for the area runs along Westpark Drive and through the Sonoma Glen Subdivision to

Dimond Boulevard. From there, water is directed across Dimond Boulevard via a 28‐inch (outside diameter) pipe

that was installed via horizontal directional drilling at depths up to 110 feet. The pipe discharges water into Cook

Inlet through a beehive‐style outlet structure. The outlet structure is shown in Figure 3.

Surface water runoff from Muirwood Park Subdivision, Westgate Subdivision, Westpark Subdivision, and Sonoma

Glen Subdivision is directed into the main storm drain trunk line via connecting storm drain pipes as shown in Figure

2. Most surface water runoff from the Heather Wood Estates subdivision is directed to a local infiltration facility,

and the Heather Wood Estates area is only expected to contribute surface water to the main trunk line during heavy

rain events.

Figure 3: Cook Inlet Storm Drain Outlet Structure

3.2. Emergency Overflow Details

The storm drain pipes in the project area are generally corrugated polyethylene pipe with diameters ranging from

12 to 24 inches. The storm drains have fairly shallow slopes, especially at the lower end of the basin, closest to



Dimond Boulevard. Under heavy flow events, pipes become surcharged and unable to accept all of the area’s

surface water runoff. To help mitigate this problem, an emergency overflow was constructed in 2016 that diverts

excess water from the main storm drain trunk line to an adjacent pond. A schematic of how the overflow functions

is provided in Figure 4.

Figure 4: Emergency Overflow Schematic

The emergency overflow outlet is a beehive‐style structure that discharges stormwater into an open channel. The

open channel then conveys water into the pond. The emergency overflow discharge structure and the open channel

are shown in Figure 5.



Figure 5: Emergency Overflow and Channel

Since its construction, the overflow is known to have sent stormwater to the adjacent pond twice. The first

occurrence was on August 8, 2016. On this day, rainfall records at nearby Ted Stevens Anchorage International

Airport (TSAIA) show a 24‐hour rainfall of 1.06 inches and a three‐hour total of 0.91 inches. The second occurrence

was on August 12‐13, 2017, and rainfall records at TSAIA show a 24‐hour rainfall total of 0.95 inches, with a one‐

hour record of 0.63 inches.

Under heavy rain events, stormwater could also enter the adjacent pond via overland flow from a vertical sag point

on Westpark Drive. Surface water could collect in the sag, and when the ponding depth exceeds the height of the

adjacent pathway, water would spill over the pathway and flow toward the pond. The sag location is shown on

Figure 2.

3.3. Stormwater Treatment

Stormwater can contain pollutants from surface contaminants such as animal waste, lawn fertilizers, and gasoline

or oil leaks/spills. Generally, most surface pollutants are mobilized during a rainfall “first flush” which is either the

first portion of a large rain event or an entire small rain event. Local requirements for stormwater treatment are

based on this understanding and are driven by the joint MOA and Alaska Department of Transportation and Public

Facilities (DOT&PF) Alaska Pollutant Discharge Elimination System (APDES) Permit. This permit establishes that the

MOA and DOT&PF should provide stormwater treatment for the first 0.52 inches of rainfall in a 24‐hour period,

following 48 hours of no precipitation. This is considered Anchorage’s water quality treatment event.



Stormwater pollutants generally bind to sediment, and most stormwater treatment facilities work by removing

sediments and the associated pollutants from the water via sedimentation, infiltration, and/or filtration. These

treatment requirements are included in the current drainage design criteria provided by the MOA. Modeling results

for the project drainage basin confirmed that runoff from the water quality event does not enter the local pond.

Modeling details are discussed in Section 4.

There are five oil and grit separators (OGS) within the project drainage basin. One is providing pre‐treatment for

the Heather Wood Estates infiltration system. The remaining four OGS units are designed to capture and remove

pollutants in various locations throughout the basin. The existing OGS units are shown in Figure 2.

4. Analysis Approach and Hydraulic Modeling

The sections below discuss the overall approach to the hydrologic and hydraulic evaluations and provide details

regarding model development.

4.1. Approach Overview

As previously discussed, the purpose of this project is to develop alternatives and recommendations for how to

disconnect the existing emergency overflow without adversely impacting drainage conditions for the surrounding

area. To accomplish that goal, a hydrologic and hydraulic model was created that represents the baseline drainage

conditions for the project drainage area. A hydrologic and hydraulic model is a computer simulation of rainfall on a

defined area. The model represents rain falling on the earth’s surface and shows how rainfall is either infiltrated or

runs off the surface and is collected in conveyance facilities like pipes and channels. The model also shows how

stormwater moves through the conveyance network and where surface ponding is expected to occur.

Once a baseline model was established, it was updated to represent various proposed alternatives, and the

performance of each alternative was compared to the baseline to determine if there were adverse impacts. It

should be noted that this modeling approach focused on a general, system‐wide performance analysis and was not

intended to reflect a detailed design for each scenario.

4.2. Design Rainfall Events

The MOA Design Criteria Manual Chapter 2 Drainage (DCM) specifies that storm drain pipes should be designed to

convey runoff from the 10‐year, 24‐hour rainfall event and provide safe passage for runoff at the 100‐year, 24‐hour

rainfall event. The DCM also provides a design distribution (hyetograph) for these rainfall events. The baseline

hydrologic and hydraulic model as well as the proposed alternative models were evaluated for both the 10‐year

and the 100‐year events using the design distribution. These events are shown in Table 1.

Table 1: Design Rainfall Events

Return Period (years)

24‐Hour Rainfall Total (Inches)

10 2.28

100 3.59



The design rainfall events in the DCM are based on rainfall data collected at TSAIA. Because the rainfall amounts

tend to increase closer to the mountains, the DCM also provides orographic factor multipliers to be applied to base

rainfall amounts based on geographic location. The orographic factor for this project area is one, so no multiplier

was needed.

It should be noted that rainfall events used for this analysis are based on the current DCM dated December 2017

DCM. This document replaced the 2007 DCM, and the design rainfall events included in the current DCM are notably

larger than the ones in the 2007 DCM.

4.3. Storm Drain Design Criteria

Where proposed alternatives include new or reconstructed storm drain pipes, the modeling approach followed the

general requirements of the DCM regarding minimum pipe slope, required cover over the pipe, and the maximum

length between manholes.

4.4. Modeling Details

Hydrologic and hydraulic modeling was completed using Autodesk’s Storm and Sanitary Analysis program version

2016 (SSA). The discussion below outlines the modeling methodology and discusses how the baseline model was

established. Additional modeling details are presented in Appendix B.

Hydrology Method. There are several options for transforming rainfall into excess runoff within SSA. The non‐linear

reservoir method was selected for this analysis. This is the same method utilized in EPA’s SWMM software, and it

is an industry‐standard approach for estimating stormwater runoff in primarily urban areas.

Infiltration Method. When using the non‐linear reservoir method in SSA, there are three options for simulating

infiltration in pervious areas of the basin. For this analysis, Horton’s method was selected because it is generally a

reliable method to use when detailed surface soils information is not available. Horton’s method computes

infiltration based on the maximum infiltration rate, minimum infiltration rate, and a decay constant. Values for

these infiltration properties were estimated based on soil data obtained from record drawings and using

recommended values provided in the DCM.

Hydraulic Routing Method. Within SSA, several options are available for routing excess runoff though the receiving

conveyance network. Hydrodynamic routing was selected for this analysis because it is the only option that can

account for pressurized flow and backflow. Pressurized flow and/or backflow are common in cases where system

capacity is limited and pipe surcharging occurs, which is expected during high flow events in the project area.

Subbasins. The project basin was divided into subbasins based on drainage patterns and land cover homogeneity,

which was determined through visual inspection of aerial imagery. Subbasins are shown in Figure 6. Each subbasin

has a defined outlet point, which was defined as either an adjacent subbasin or the lowest inflow point to the

conveyance network within the subbasin.



Figure 6: Modeled Subbasins



A description of each subbasin property is provided below, and the selected values for each entity are provided in

Appendix B.

Area. This is the area of the subbasin in acres. This information was obtained from computing polygon areas

of defined subbasins using GIS software.

Width. This is the width of the overland flow path. It was generally approximated by dividing the subbasin

area by the average overland flow length. These lengths were determined based on topography and land

cover. Lengths were limited to a maximum of 300 feet in the more developed areas and 500 feet in the

more vegetated areas.

Slope. This is the average subbasin slope in percent. This information was obtained from a spatial analysis

of the subbasin topography using GIS software.

Percent Impervious. This is the percent of the subbasin with impervious surfaces such as roads, driveways,

parking lots, and roof tops. This value was obtained through a combination of area calculations and visual

inspection of aerial imagery.

Pervious and Impervious Roughness. These are the Manning’s “n” values for overland flow on pervious and

impervious surfaces. The roughness coefficients vary with land cover type and were selected based on

recommended values in the DCM.

Pervious and Impervious Depression Storage. These are the depression storage depths for pervious and

impervious surfaces in inches. The depression storage depths vary with land cover type and were selected

based on recommended values in the DCM.

Internal Routing. This option allows the user to specify the relationship between pervious and impervious

surfaces within each subbasin. For each subbasin, runoff can be modeled as flowing from an impervious

surface to a pervious surface (e.g. driveway runoff flowing to lawns), from a pervious surface to an

impervious surface (e.g. lawn runoff flowing to driveways), or both surfaces can be routed directly to the

outlet. For this project, runoff from the two surfaces is generally routed directly to the subbasin outlet with

an exception for rooftop runoff. A portion of the rooftop runoff was directed to pervious areas to simulate

house downspouts that discharge into lawn areas.

Conveyance Network. The main features of the conveyance network include the drainage system location,

connectivity, manhole geometry, storm drain pipe geometry, and open channel geometry. These parameters were

obtained from record drawings, topographic data, and drainageway mapping. The drainage network is shown

schematically in Figure 7.



Figure 7: Baseline Model Schematic



The network parameters are described below, and selected values for each parameter are provided in Appendix B.

Manning’s Roughness Coefficient. This is the Manning’s “n” value for pipes and open channels. These values

were selected based on recommended values in the DCM and on other published values from various

hydraulic texts. The values vary based on pipe type and on the surface type of an open channel (e.g. rock

versus grass).

Entrance and Exit Losses. These are the coefficients used to account for inefficient flow patterns at pipe

inlets and outlets. Inefficient flow patterns result from flow contracting and rapidly changing directions,

such as in manholes and where an open channels transitions into a culvert. Coefficients were selected based

on appropriate representations of the pipe end treatment and the layout of pipes entering each manhole.

Orifice/Weir Properties. There is an orifice/weir structure located in the storm drain system near the

intersection of Lucy Street and 80th Avenue. This facility was installed to control the rate at which flow from

upstream of Lucy Street is allowed to enter the downstream system. The system geometry was determined

from record drawings, and appropriate model parameters were selected to reflect this geometry.

Storage Properties. Stormwater storage areas were represented in the model using storage curves, which

provide a relationship between surface area, depth, and volume of storage. The baseline model includes

two storage areas. One is a small detention area located near the north end of Park West Circle, and this

storage curve was based on topographic data. The other is a constructed subsurface chamber system in the

Heather Wood Estates subdivision, and this storage curve was based on the chamber geometry (obtained

via record drawings) and corresponding storage curves provided in SSA.

Overflow Channels. Overflow channels were used in places where excess stormwater is expected to flow

down the street surface from one manhole/inlet to the next when the subsurface pipes are surcharged.

Overflow channels were represented using the geometry of the road surface which was obtained from as‐

builts.

Ponding Areas. Ponding areas were used to represent low points (such as vertical sags) where surface water

is expected to pond if it cannot enter the conveyance system, rather than flow down the street. Ponding

areas were established based on model iterations that correlated the ponded water depth with the area

topography.

Westpark Drive Sag. Under baseline conditions, stormwater is expected to collect and pool in the vertical

sag along Westpark Drive. When ponded water reaches approximately the height of the adjacent pathway,

it spills over the pathway and flows downhill toward the local pond. This process was simulated in the model

using a broad‐crested weir with a crest elevation correlated to the spill‐over elevation. The spill‐over

elevation was determined based on area topography.

Initial and Boundary Conditions. For initial conditions, it was assumed that manhole sumps were full and detention

areas were empty at the start of the simulation. The boundary conditions were determined by testing the model

sensitivity to changes in the receiving water elevations and by examining area tide fluctuations. This information

was used to determine how the receiving waters are expected to interact with the modeled conveyance system. It



was determined that the receiving waters are not expected to significantly impact upstream water surface

elevations even under extreme conditions. The three outfall points in the model were assigned a free boundary

condition to reflect no flow restriction.

Future Development. The Heather Wood Estates, Westgate, and Sonoma Glen Subdivisions were considered fully

built‐out for the baseline conditions model. Information obtained from record drawings was used to assign subbasin

and conveyance network properties for these areas. In locations where final street and home elevations have not

been set, elevations were approximated based on preliminary site grading plans provided by the area designers.

In addition to the subdivisions, the school site and the undeveloped parcel in the northwest corner of the basin

were assumed to be developed for the baseline conditions model. Based on development restrictions in the DCM,

it was assumed that when these areas are developed in the future, the resulting increases in peak discharge from

those sites will be limited to a maximum of 1.05 times the pre‐development peak discharge for both the 10‐year

and 100‐year events. Subbasin hydrographs were developed to reflect this increase.

Model Testing. The model validity was tested using the rainfall event from August 2017 which caused the overflow

to activate. The model was run using hourly rainfall from TSAIA, and the model results matched observations that

the overflow discharged stormwater during this event.

4.5. Baseline Levels of Service

The results of the baseline modeling showed that the drainage system performance varies by location and by the

design rainfall event that was evaluated. To quantify the performance, a level of service rating was developed that

could be applied to individual locations within the project drainage basin. The various levels of service are shown in

Figure 8 and described below.

Level A: This level of service indicates that stormwater is contained within the storm drain pipes. The pipes

may be surcharged, but water is not reaching the ground surface.

Level B: This level of service indicates that water is not staying inside the subsurface storm drain pipes, but

is contained within the Municipal right‐of‐way (ROW). In other words, water may be ponding or flowing

down the street.

Level C: This level of service indicates that water is flowing or ponding in areas outside of the ROW, on

adjacent properties, but that it is not impacting houses or other buildings.

Level D: This level of service indicates that water is ponding or flowing outside of the ROW and is expected

to be reaching the location of homes or other buildings.

This rating system was applied throughout the project drainage basin. Because the performance varied based on

the design rainfall event, separate ratings were given for the 10‐year event and the 100‐year event. The results are

shown in Tables 5 and 6 in Section 5.5.



Figure 8: Levels of Service

4.6. Model Components – Proposed Conditions

Modeling proposed conditions was generally completed by starting with the baseline hydraulic model and updating

select parameters to reflect a proposed alternative. Additional details are provided with discussion of proposed

alternatives in Section 5 and in Appendix B.

4.7. Analyses and Modeling Limitations

The analyses and associated modeling work completed for this project have the following limitations.

The methods and values selected for this project’s analyses follow industry best practices and are expected

to provide reasonable estimates of runoff characteristics and receiving system performance for the rainfall

events that were evaluated. However, the results of this study are an approximation of actual conditions.

The analyses and modeling completed for this project reflect an appropriate level of effort for an area‐wide

evaluation and associated alternative development. These analyses and models are not intended to reflect

a detailed design and should not be used to support a detailed design without further refinement.

Model calibration to measured flows has not been completed. The timing of this project along with the

timing of area rainfall events did not provide adequate opportunity for model calibration.

The reported levels of service under all modeling scenarios are approximate. The process of assigning levels

of service required correlating model‐predicted excess water volumes and flows to the surface topography

and roadway geometry. As discussed in Section 2, surface topography is based on LIDAR data and roadway

geometry is based on as‐built information. These data sources provide a good approximation of surface

conditions but are not expected to be exact.



This project’s analyses and modeling were based on limited available information regarding expected future

development in the project drainage basin. In some locations, road and home elevations have not yet been

finalized, and the information used to support this project is based on the best information currently

available from the area designers. For the undeveloped ASD site, future development is based on a

conceptual site layout provided by ASD.

5. Alternatives Evaluation

This section presents the criteria that were used to select and evaluate potential alternatives, and provides a

description and evaluation of each primary alternative considered. A brief discussion of alternatives that were

considered but determined to be infeasible is also provided.

5.1. Alternative Evaluation Criteria

This project evaluated many options for disconnecting the existing emergency overflow. The following criteria were

used to evaluate the performance and feasibility of each primary alternative and to make recommendations for

moving forward.

Disconnecting the Emergency Overflow. Primary alternatives focused on eliminating stormwater discharge

to the local pond and/or improving the water quality prior to water entering the pond.

Hydraulic Performance. Primary alternatives were evaluated to determine if the baseline levels of service

described in Section 4.5 would change as a result of the alternative implementation.

Land Availability. Primary alternatives were limited to those in which access to required land space was

expected to be attainable, either by staying within rights‐of‐way, purchasing property, or executing an

intergovernmental agreement for use of the property.

Long‐term Maintenance. Primary alternatives excluded facilities that were expected to have high long‐term

maintenance costs.

Stakeholder Input. Stakeholder input on the primary alternatives was important for evaluating potential

community support for the alternative and the associated feasibility for success. Preliminary stakeholder

input is discussed for each alternative, and a detailed discussion of the project’s public involvement process

is presented in Section 6.

Cost and Fundability. The total available funding for this project is $2.1 million. This is the funding available

for all project costs including design, permitting, administration, ROW acquisition, utility conflicts, and

construction. Preliminary cost estimates were developed for the primary alternatives to compare the cost

to the available funding.

Other Considerations. Other considerations specific to each alternative were evaluated as needed.

Based on these criteria, three primary alternatives were evaluated. Many additional alternatives were considered

but determined to be not feasible, and these are discussed in Section 5.6.



5.2. Alternative 1 – Extended Bypass

Description. This alternative involves removing the existing emergency overflow and constructing a stormwater

bypass route that would provide additional conveyance capacity. The bypass route includes a combination of new

pipes and upsizing existing pipes. The alternative also includes a stormwater detention basin near Dimond

Boulevard. This alternative was developed to accommodate the goal of not increasing surface flow or ponding from

the baseline conditions at any location within the basin. A summary of the alternative features is provided below,

and this alternative is conceptually illustrated in Figure 9.

2,300 LF of 30‐ to 42‐inch pipe

2,600 LF of 48‐inch pipe

Berm adjacent to Westpark Drive sag to keep water from overflowing the curb and flowing to the pond

Detention basin at Dimond that requires acquisition of 6 lots

Hydraulic Analysis. The hydraulic evaluation of this alternative was generally completed by modifying the baseline

model to reflect the proposed changes to the drainage network. When the emergency overflow was removed,

additional conveyance capacity was needed both upstream and downstream of the overflow to eliminate increases

in surface flow and ponding. When conveyance capacity was increased, this resulted in more water reaching the

south end of the basin more rapidly, and a detention area was added to the model to store water and meter it out

slowly as downstream capacity becomes available.

Model support files are provided in Appendix B.

It should be noted that a design of the proposed detention area at the south end of the drainage basin has not been

completed. The approximate size of the detention area is based by a rough correlation of required storage volumes

and an approximate detention area geometry. A detailed basin design would require consideration of features such

as safety benching, maintenance access, landscaping, pre‐treatment, a settling forebay, etc. Therefore, the

expected footprint of this facility is approximate.

Results and Evaluation Criteria. The evaluation criteria presented in Section 5.1 are discussed below for this

alternative.

Disconnecting the Emergency Overflow. This alternative removes the existing overflow to the local pond and sends

stormwater to the south, eventually into Cook Inlet.

Hydraulic Performance. This alternative maintains or improves the baseline level of service throughout the project

drainage basin. The results are shown in Tables 5 and 6 in Section 5.5.

Land Availability. This alternative requires acquisition of approximately six future parcels for construction of the

stormwater detention facility. This area is located in the lower portion of the basin on undeveloped land that has

not yet been subdivided. Acquisition of these future parcels has not been discussed with the property owner.

Long‐term Facility Maintenance. The long‐term maintenance of this alternative is expected to be fairly low, with

the exception of the proposed detention facility. The detention facility will require periodic inspection, sediment

and trash removal, maintenance of control structures, and maintenance of vegetation.



Stakeholder Input. Preliminary public input regarding this alternative indicates that stakeholders are not supportive

of this option due to the high cost and the associated long project timeline that would be needed to try to secure

funding. The project’s Stakeholder Working Group indicated that they did not support construction of new

stormwater infrastructure north of the existing emergency overflow. This section will be updated after public input

on the draft DSR is received.

Cost and Fundability. The estimated total project cost of Alternative 1 is $10 million, as shown in Table 2 below.

Supporting information is provided in Appendix C. This cost exceeds available funding by more than $8 million.

PM&E projects are typically funded through municipal bonds. Each year, PM&E’s capital improvement program

requests generally $30 to $40 million in bond funding to support road and drainage projects across the MOA. The

estimated cost of Alternative 1 would require roughly one third to one quarter of that funding to be spent on a

single project, which is not expected to be feasible. Bonding the project over several years was also considered, but

was not expected to be achievable given the large number of competing capital improvement projects. State

funding for municipal projects is not expected to be available due to the state’s current fiscal situation.

Table 2: Alternative 1 Extended Bypass ‐ Estimated Project Cost

Item Description Cost

A Construction $5,300,000

B Utilities $100,000

C ROW Acquisition $950,000

D Subtotal A+B+C $6,350,000

E Administration and Overhead $2,700,000

F Design $1,000,000

G Project Total D+E+F $10,050,000 Notes: 1. Construction estimate includes a 30% contingency. 2. Utility costs are unknown and are approximated. 3. Numbers shown in this table are rounded.

Additional Work to Advance this Alternative. Advancement of this alternative will require a design survey of the

project area, a geotechnical investigation and recommendations, a complete design of the proposed improvements,

coordination for necessary permits and approvals, and acquisition of necessary land, as discussed in this section.



Figure 9: Extended Bypass Schematic



5.3. Alternative 2 – Shortened Bypass

Description. Due to the high cost of Alternative 1, this alternative was created to represent a minimal bypass

concept with a shorter alignment and no detention facilities. Similar to Alternative 1, this alternative involves

removing the existing emergency overflow and routing stormwater toward Dimond Boulevard via a combination of

new bypass pipes and upsizing existing pipes in the drainage network. However, there are no system upgrades to

the north of the existing overflow, and this alternative does not include a detention area. A summary of the

alternative features is provided below, and the alternative is conceptually illustrated in Figure 10.

3,350 LF of 30‐ to 42‐inch pipe

Berm adjacent to Westpark Drive sag to keep water from overflowing the curb and flowing to the pond

Acquisition of one parcel

Hydraulic Analysis. The hydraulic evaluation of this alternative was generally completed by modifying the baseline

model to reflect the proposed changes to the drainage network. Model support files are provided in Appendix B.


alternative.

Disconnecting the Emergency Overflow. This alternative removes the existing overflow and sends water to the

south, eventually into Cook Inlet.

Hydraulic Performance. This alternative would worsen the baseline level of service in two locations in the project

drainage basin. This would result in additional stormwater being directed onto privately owned land, which is

considered unacceptable. The results are shown in Tables 5 and 6 in Section 5.5. Note that the final elevation of

future roads and homes in the impacted locations may impact the resulting levels of service.

Land Availability. This alternative requires acquisition of one privately‐owned, future parcel to accommodate the

bypass pipe alignment. The parcel is part of a tract that is not yet subdivided, and acquisition of this parcel has not

been discussed with the property owner.

Long‐term Facility Maintenance. The long‐term maintenance of this alternative is expected to be fairly low, since it

generally only includes subsurface storm pipes.

Public Input. Preliminary public input regarding this alternative indicates that area stakeholders are generally

supportive of the alternative concept, but have concerns regarding the cost and the associated timeline for getting

the project constructed. This section will be updated after public input on the draft DSR is received.

Cost and Fundability. The estimated total project cost of Alternative 2 is $4.7 million, as shown in Table 3 below.

Supporting information is provided in Appendix C. This cost exceeds available funding by $2.6 million. The feasibility

of obtaining additional bond funding for this project is unknown, but is expected to be low based on the large

number of capital projects competing for limited resources. The fact that this alternative worsens the baseline level

of service further reduces the likelihood for bond funding. State funding for the project is not expected to be

available given the state’s current fiscal situation.



Table 3: Alternative 2 Shortened Bypass ‐ Estimated Project Cost



B Utilities $100,000

C ROW Acquisition $165,000


E Administration and Overhead $1,200,000

F Design $650,000

G Project Total D+E+F $4,715,000 Notes: 1. Construction estimate includes a 30% contingency. 2. Utility costs are unknown and are approximated. 3. Numbers shown in this table are rounded.

Additional Work to Advance this Alternative. Advancement of this alternative will require design survey of the


coordination for necessary permits and approvals, and acquisition of necessary land, as discussed in this section.



Figure 10: Shortened Bypass Schematic



5.4. Alternative 3 – Bioswale

Description. This alternative involves leaving the existing stormwater emergency overflow in place, but re‐rerouting

discharge from that overflow into a new bioswale designed to treat stormwater before it reaches the pond. The

bioswale would be approximately 1,000 feet long and would generally wrap around the perimeter of the local pond.

A bioswale is a flat‐bottom, open channel that is constructed at a fairly shallow slope and is designed to provide

stormwater treatment through filtration, infiltration, sedimentation, and evapotranspiration. Because pollutants in

stormwater tend to bind to sediment, stormwater treatment facilities generally focus on removing sediment and

their associated pollutants. Bioswales provide sediment removal through multiple processes, making them effective

and widely used for stormwater treatment. The bottom of a bioswale is constructed of engineered soil, which is a

mixture of sand, topsoil, and organics, and select vegetation is planted along the floor and the sides of the swale.

As water flows through the bioswale, the shallow slope and vegetation promote slow velocities which allows for

filtration and sedimentation to occur. Slow velocity also allows for water percolation through the engineered soil,

providing filtration and infiltration. If needed, the bioswale design can include check dams to increase residence

time of the swale and promote slower movement through the facility. Plants in the bioswale also utilize stormwater

for natural evapotranspiration processes.

Bioswales are typically designed to capture and treat stormwater runoff from small, frequent rain events so that

they provide treatment for the first flush described in Section 3.3. However, in this case, the first flush will not be

directed toward the bioswale. It will continue to bypass the emergency overflow as it does under existing

conditions, and water will only enter the bioswale under heavy rain events when the existing emergency overflow

is utilized. As discussed in Section 3.2, the overflow has been utilized twice since its construction in 2016.

This alternative would not require modification of the existing drainage network, other than what is required to re‐

route discharge from the overflow to the bioswale. A summary of the alternative features is provided below, and

the alternative is conceptually illustrated in Figures 11 and 12.

Approximately 1,000 feet of bioswale

Redirecting water from the emergency overflow via a riprap flume or other similar feature

Drainage easement from the ASD

Conceptual plan and profile drawings of the bioswale are provided in Appendix D.

Hydraulic Analysis. This alternative does not change the hydraulic performance from the baseline conditions, so a

hydraulic model for evaluation of the drainage impacts to the surrounding area was not needed.

The expected performance of the bioswale will vary based on specific details that will be determined during detailed

design. These include but are not limited to bioswale length, bioswale longitudinal slope, the type of vegetation in

the swale, and the native soil characteristics which influence both infiltration rates and the stability of the bioswale

side slopes. Based on currently available information, approximately 1,000 feet of bioswale is expected to provide

treatment for events equivalent to the previous two events that activated the overflow. Under extreme events, the

bioswale is expected to overtop. To accommodate the potential for overtopping, the edge of the bioswale will be

separated from the pond by approximately 20 to 120 feet. When the bioswale overtops, excess water will flow

through this wide strip of existing vegetation, providing additional treatment before water enters the pond.




alternative.

Disconnecting the Emergency Overflow. This project leaves the existing emergency overflow in place but sends

overflow water to the bioswale instead of directing it immediately toward the pond. The bioswale provides

treatment for stormwater prior to discharge, as discussed above.

Maintaining the baseline Level of Service. This alternative does not change the levels of service from the baseline

conditions.

Land Availability. This alternative requires acquisition of a drainage easement from the ASD and potentially from

MOA Real Estate Services. The project team is currently coordinating with the ASD and Real Estate Services, and

obtaining the necessary easements is expected to be feasible. The approximate proposed footprint of the bioswale

is shown in the conceptual plan and profile drawings in Appendix D.

Long‐term Facility Maintenance. The long‐term maintenance of this alternative is expected to be fairly low. Bioswale

maintenance generally includes activities such as periodic trash removal, vegetation maintenance, sediment

removal, and repair of any eroded areas. In this case, the bioretention area is expected to be utilized very

infrequently, which significantly reduces the frequency of required maintenance. Vegetation maintenance may be

needed more frequently in the first few growing seasons before the vegetation is fully established. Over time, native

vegetation such as alders is expected to grow in the bioswale. Native vegetation growth is not expected to be

problematic for facility function, and ongoing maintenance to prevent native vegetation growth is not needed.

Additional maintenance requirements will be established if this alternative progresses to design.

Public Input. Preliminary public input indicates that this alternative differs from what was originally envisioned by

area stakeholders, but stakeholders are generally supportive of the alternative concept because it serves their

interests in a different way. The community has provided valuable suggestions for the design such as utilizing

vegetation that will discourage pedestrian or ATV use of the bioswale and ensuring that the vegetation is not treated

with fertilizers and pesticides. This section will be updated after public input on the draft DSR is received.

Cost and Fundability. The estimated total project cost of Alternative 3 is $2.1 million, as shown in Table 4 below.

This is equivalent to the current project funding, and construction of this alternative is not expected to require

additional funds. Supporting information is provided in Appendix C.



Table 4: Alternative 3 Bioswale ‐ Estimated Project Cost



B Utilities $0

C Easement Acquisition $50,000


E Administration and Overhead $600,000

F Design $350,000

G Project Total D+E+F $2,100,000 Notes: 1. Construction estimate includes a 30% Contingency. 2. Numbers shown in this table are rounded.

Other Considerations. A bioswale in this location will need to be carefully designed to avoid becoming a publicly

used route for pedestrians or ATVs. As mentioned above, bioswale vegetation can be selected to discourage foot

or ATV travel. This may include plants that are thorny and/or branchy and are difficult to traverse. Other options

include posting informational signs or providing fencing to discourage access.

Additional Work to Advance this Alternative. Advancement of this alternative will require design survey of the


coordination for necessary permits and approvals, and acquisition of a drainage easement, as discussed in this

section.

Permitting for this alternative is expected to be fairly straightforward. Confirmation of wetland locations and an

associated permit form the Corps of Engineers may be required for work immediately adjacent to the pond. A

municipal ROW permit will be needed for work inside the ROW, and a temporary construction permit will be needed

for work that impacts the ASD property.



Figure 11: Bioswale Schematic



Figure 12: Bioswale Typical Section



5.5. Hydraulic Performance Summary

A summary of the modeling results for the baseline conditions as well as for Alternatives 1, 2, and 3 are shown in

the tables below. Table 5 presents the alternatives performance for the 10‐year event, and Table 6 presents the

alternatives performance for the 100‐year event. Colors correspond to the levels of service depicted on Figure 8.



Table 5: Modeling Results Summary 10‐year Event

Location

Condition and Level of Service

Notes Baseline Condition

Extended Bypass

Shortened Bypass

Bioswale

Tyre Circle A A A

No Change from Baseline

Park West Circle A A A

Tarsus Drive B B B

Katahdin Drive B B B

Lucy Street C C C

Gate Creek Drive B B B

Yukon Charlie Loop B B B

Kenai Fjords Loop B B B

Westpark Drive (General) B A B

Grand Teton Loop North C B C

Grand Teton Loop South C B C

Big Bend Loop North B B B

Big Bend Loop South B B B

Westpark Drive (sag) C C C Based on estimated elevation of

future homes.

Dry Creek Loop North C A B Based on estimated elevation of

future road and homes.

Dry Creek Loop South B A B

Chalk Hill Loop North B A A Based on estimated elevation of

future roads and homes.

Chalk Hill Loop South A A C Based on estimated elevation of

future roads and homes. Note: Italicized text indicates a road that has not been constructed yet.

Legend

A ‐ No above ground flow/ponding.

B ‐ Flow/ponding is above ground, but within the ROW.

C ‐ Flow/ponding is above ground, & outside the ROW, but not impacting homes.

D ‐ Flow/ponding is above ground, outside the ROW, and may impact homes.



Table 6: Modeling Results Summary 100‐year Event

Location

Level of Service

Notes Baseline Condition

Extended Bypass

Shortened Bypass

Bioswale

Tyre Circle C C C

No Change from Baseline

Park West Circle C C C

Tarsus Drive B B B

Katahdin Drive B B B

Lucy Street C C C

Gate Creek Drive B B B

Yukon Charlie Loop C C C

Kenai Fjords Loop C C C

Westpark Drive (General) B B B

Grand Teton Loop North C C C

Grand Teton Loop South D C D

Big Bend Loop North B B B

Big Bend Loop South C B B

Westpark Drive (sag) C C D Based on estimated elevation of

future homes.

Dry Creek Loop North D A D Based on estimated elevation of the future road and homes.

Dry Creek Loop South B B B

Chalk Hill Loop North B A A Based on estimated elevation of the future road and homes.

Chalk Hill Loop South D C D Based on estimated elevation of the future road and homes.

Note: Italicized text indicates a road that has not been constructed yet.

Legend

A ‐ No above ground flow/ponding.

B ‐ Flow/ponding is above ground, but within the ROW.

C ‐ Flow/ponding is above ground, & outside the ROW, but not impacting homes.

D ‐ Flow/ponding is above ground, outside the ROW, and may impact homes.



5.6. Other Alternatives Considered

Several additional alternatives were considered for disconnecting the existing emergency overflow or providing

treatment for stormwater. These were determined to be infeasible or impractical as discussed below.

Alternate Bypass Alignments and Pipe Sizes. Several different combinations of bypass pipe alignments and sizes

were considered in addition to those presented in Alternatives 1 and 2, but were determined to be less

advantageous than the ones presented. Other sizes and/or alignments resulted in issues such as moving ponding

from one location to another, conflicting with proposed development timelines, or generally being more cost‐

prohibitive than the options discussed in this document.

Second Bore at Dimond Boulevard. This option considered installation of a second pipe next to the existing one

that was bored across Dimond Boulevard, at the lower portion of the basin. This option was not hydraulically

feasible as a stand‐alone alternative. The cost of a second bored pipe alone was estimated at $3 to 4 million, and it

is more cost prohibitive when combined with other necessary upgrades.

Incorporating Open Channels in the Bypass. Open channels were considered in lieu of subsurface pipes along the

bypass route for both Alternatives 1 and 2. Open channels were considered along the school property adjacent to

Westpark Drive and in the green space behind the houses in the Sonoma Glen area. Utilizing open channels is not

hydraulically feasible as a stand‐alone option, and did not provide significant cost savings when combined with

other upgrades. Additionally, community stakeholders were concerned that open channels could cause safety and

maintenance issues, and area developers were concerned about impacts to the subdivision aesthetics.

Detention at the School Site. Construction of a large detention facility on the school was considered as both a

stand‐alone option and in conjunction with other system upgrades. However, the ASD plans to utilize the school

property for future construction of both an elementary school and a middle school, and a detention facility on the

site would adversely impact future school‐related development. Additionally, this alternative is not hydraulically

feasible as a stand‐alone option, and is cost‐prohibitive when combined with other upgrades.

Detention in Sonoma Glen. This option considered a subsurface detention facility (chamber system) located behind

both existing and future houses in the Sonoma Glen subdivision. This alternative is not hydraulically feasible as a

stand‐alone option, and is cost‐prohibitive when combined with other upgrades. Additionally, it is not known if the

privately‐owned area would be available for municipal drainage facilities.

Installing Rain Barrels. This option looked at installation of rain barrels throughout the project drainage basin to

capture roof runoff from each home. The addition of rain barrels was determined to provide no substantial benefit.

Even if every house in the project drainage basin incorporated eight rain barrels, this still did not provide a

substantial reduction of water entering the pond during the larger rain events.

Lining the Pond. Installation of an impermeable liner along the pond bottom was considered as a way to isolate the

pond. This option presented multiple issues. The pond is expected to be approximately 30 feet deep, and installing

and stabilizing a liner at that depth is expected to be difficult and potentially infeasible. Additionally, this option

would create stagnant water which tends to cause problems with insects and unpleasant smells. The pond would

eventually fill, so it would require periodic pumping which can be costly.



Lift Stations. Installation of lift stations was considered in two separate locations to move stormwater away from

the project drainage basin and into an adjacent area, such as the stormwater basin in the Skyhills subdivision.

However, there is not an adjacent area with a stormwater facility large enough to safely accept the larger storm

events from the project drainage basin. This option also created multiple maintenance and practicality concerns,

and is expected to be highly cost prohibitive.

Pressurizing Flow in the Existing Dimond Bore Pipe. This option considered the feasibility of using a pump station

to pressurize flow in the bored pipe under Dimond Boulevard to provide additional capacity of that pipe. This option

was not hydraulically feasible.

Changes to Road Grades. Modifying the elevation and slope of select existing roads was considered as a way to

mitigate adverse drainage impacts, but was determined to be impractical and provide no substantial benefit.

Other Pipe Upgrades. Upgrading select pipes in the Westpark Subdivision was considered as a way to mitigate

adverse drainage impacts, but was determined to provide no substantial benefit.

6. Public Involvement Summary

The project team implemented a variety of forms of community outreach to inform, consult, and involve project

stakeholders including creating a project website, attending community council meetings, distributing email

newsletters, working with a Stakeholder Working Group, and holding a public meeting. A summary of public

involvement actives is presented here and supporting information is included in Appendix E.

Sand Lake Community Council (SLCC). The project team attended and made presentations at three Sand Lake

Community Council Meetings, described below.

October 9, 2018. Project staff attended to kick‐off the project, provide a brief project description, outline

the project scope of work and public involvement activities, and communicate the project schedule. The

team answered initial questions about the project and listened to comments, concerns, and feedback.

Lastly, the team asked the SLCC for help finalizing the participants in the Stakeholder Working Group (SWG)

by nominating representatives from the SLCC and the Westpark Home Owners Association (HOA) to

participate.

January 14, 2019. Project staff attended to provide a brief update on the project schedule, progress of the

SWG, and to answer questions.

June 10, 2019. Project staff attended this special meeting of the SLCC after a very thorough analysis of

possible project alternatives that were feasible and within the available construction funds. The intent was

to present the alternatives that would likely be illustrated in the Draft DSR and to receive initial comments

and feedback from SLCC members. The team communicated the project schedule and discussed that the

community should expect the Draft DSR to be released in late August along with an opportunity to comment

and talk with project staff at an August public meeting.



Public Meetings. This section will be updated for the final DSR, after completion of the upcoming public meeting.

Stakeholder Working Group. In order to engage and consult with the community on a higher level, a SWG was

formed. The SWG included eight community members. Four were representing the SLCC, two were from the

Westpark HOA, one was from the Anchorage School District, and one was an at‐large member. The SWG met at the

three dates shown below. Complete SWG meeting summaries can be found in Appendix E.

SWG Meeting 1. November 13, 2018: Project Launch

SWG Meeting 2. April 3, 2019: Present Preliminary Project Alternatives

SWG Meeting 3. June 6, 2019: Present Refined Project Alternatives

Project Website. A project website was created to reach a broad audience. The website describes the project

background, purpose, timeline, and budget. It also allows visitors to sign up for email updates, download project

documents and meeting summaries, provides the project team’s contact information, and summarizes public

involvement efforts to date. The website is www.westparkstormwater.com.

Summary of Public Comments. This section will be updated for the final DSR.

Supporting Information. The following public involvement items can be found in Appendix E. The final DSR will

include the noted additional items documenting the public meeting and public comments.

1. SWG Meeting 1 Meeting Summary & Documents 2. SWG Meeting 2 Meeting Summary & Documents 3. SWG Meeting 3 Meeting Summary & Documents 4. SLCC Meeting Documents: June 10, 2019 5. Public Meeting Notice Post Card & E‐Newsletter [Will add for final DSR] 6. Public Meeting Summary Documents [Will add for final DSR] 7. Public Comments & Responses [Will update for final DSR]

7. Recommendations

The recommended alternative for disconnecting the existing stormwater emergency overflow is Alternative 3 ‐

Bioswale. This option would leave the existing overflow in place but re‐route discharge from that overflow to a

bioswale. The bioswale would provide stormwater treatment via filtration, sedimentation, infiltration, and

evapotranspiration. This option eliminates a direct connection between the overflow and the pond, and does not

cause adverse drainage impacts to the surrounding area. It is also the only alternative that is currently fundable and

could be designed and constructed in the near future.

If this alternative is selected for construction, survey and geotechnical investigations could be completed in the late

summer and fall of 2019 with design completed over the winter of 2019‐2020. With construction funding already

in place, this would allow for bioswale construction in the summer of 2020.

8. Summary

This project analyzed multiple options for disconnecting an existing stormwater emergency overflow that

discharges stormwater into a local pond during heavy rain events. The analyses focused on how to disconnect the



overflow safely without causing adverse drainage impacts to the adjacent areas. Hydrologic and hydraulic analyses

and modeling were completed to establish the baseline performance of the drainage system in the project area for

the 10‐year and 100‐year rainfall events. Baseline performance was quantified by applying a level of service rating

to various locations throughout the project drainage basin. Alternatives were then analyzed by updating the

baseline analyses as needed to reflect the proposed changes. Three primary alternatives were identified.

1. Alternative 1 – Extended Bypass. This alternative involves removing the existing emergency overflow and

constructing a stormwater bypass route that would provide additional conveyance capacity. This alternative

was developed to ensure no increase in surface water ponding or flow along the streets at any location

throughout the project drainage basin. This alternative includes approximately 4,900 feet of new storm

drain pipe as well a detention basin. Acquisition of six undeveloped future parcels is expected to be needed

for construction of the detention basin. This alternative would maintain or improve the baseline level of

service at all locations throughout the project area drainage basin. The estimated total project cost of

Alternative 1 is $10 million.

2. Alternative 2 – Shortened Bypass. This alternative was developed to reflect a minimal bypass option with a

reduced pipe length and no detention facilities. It includes 3,350 feet of new storm drain pipe and is

expected to require acquisition of one undeveloped future parcel. This alternative would worsen the

baseline level of service in two locations in the project drainage area, resulting in additional stormwater

being directed onto privately owned land, which is not an acceptable option. The estimated total project

cost of Alternative 2 is $4.7 million.

3. Alternative 3 – Bioswale. This alternative would leave the existing emergency overflow in place but re‐direct

discharge from the overflow into a constructed bioswale. The bioswale would provide treatment for

stormwater before it discharges to the pond. Treatment would be provided via a combination of filtration,

sedimentation, infiltration, and evapotranspiration. The bioswale would be separated from the pond by a

vegetated buffer strip such that any swale overflow during flood events would flow through the vegetation

prior to entering the pond. Because the bioswale does not send additional water into downstream receiving

pipes, this alternative does not change the levels of service from baseline conditions. The estimated total

project cost of Alternative 3 is $2.1 million.

Available funding for this project is currently $2.1 million. The feasibility of obtaining additional funding through

bonds or state grants is uncertain due to limited available funding and a significant number of competing capital

projects.

The recommended alternative for disconnecting the existing overflow is Alternative 3 – Bioswale. This option

eliminates a direct connection between the overflow and the pond, and does not cause adverse drainage impacts

to the surrounding area. It is also the only alternative that is currently fundable and could be designed and

constructed in the near future.