Advanced Design Workshop: Bioretention Basin

…to discuss the process of designing a Bioretention Basin

Why Are We Here?

Bioretention BasinShallow stormwater basin/landscaped area that uses engineered soils, vegetation and an underdrain to treat runoff

Purpose– Reduce stormwater pollution through,

filtration, biological uptake and microbial activity, using vegetation, engineered soil mix and an underdrain

Considerations– Most applicable adjacent to roadway

shoulder and filter strip along rural sections

Bioretention Basin with Open Underdrain

Vegetative Conveyance

Filtration

Settling

Infiltration

TSS Removal = 85%Detention

Runoff Reduction = 50%

Shallow stormwater basin/landscaped area that uses engineered soils, vegetation and an underdrain to treat runoff

• Design Variations– Upturned Underdrain

Bioretention Basin

Purpose– Underdrain modified to promote

infiltration where soils allow

– Reduce stormwater pollution through filtration, biological uptake and microbial activity, using vegetation, engineered soil mix and an underdrain

Considerations– Forebay required if used with curb

and gutter sections.

Vegetative Conveyance

Filtration

Settling

Infiltration

TSS Removal = 85%Detention

Runoff Reduction = 75%

Bioretention Basin with Upturned Underdrain

Shallow stormwater basin/landscaped area that uses engineered soils, vegetation and an underdrain to treat runoff

• Design Variations– Capped Underdrain

Bioretention Basin

Purpose– Underdrain capped for maximum

infiltration

– Reduce stormwater pollution through infiltration, filtration, biological uptake and microbial activity, using vegetation, engineered soil mix.

Considerations– Native soils must have an infiltration

rate of at least 0.5 in/hr

– Forebay required if used with curb and gutter sections.

Vegetative Conveyance

Filtration

Settling

Infiltration

TSS Removal = 100%Detention

Runoff Reduction = 100%

Bioretention Basin with Capped Underdrain

Advantages– Effective in treating TSS,

fecal coliform, nitrogen, & heavy metals

– Low land requirement– Pleasing aesthetics– Can achieve runoff

reduction where soils allow

Bioretention Basin

Disadvantages– High capital cost– Medium maintenance

burden– Generally limited to 5

ac drainage area – Not intended for

discharge attenuation

Bioretention Basin

BMP RRv TSS Total Phosphorus

Total Nitrogen

Fecal Coliform

Metals

Filter Strip 25% 60 % 20 % 20 % ---------- 40 %

Grass Channel 10-25% 50 % 25 % 20 % ---------- 30 %

Enhanced Dry Swale 50% 80% 50% 50% ---------- 40%

Enhanced Dry Swale(w/ capped Underdrain

100% 100 % 100 % 100 % ---------- 100 %

Enhanced Wet Swale 0% 80 % 25 % 40 % ---------- 20 %

Infiltration Trench 100% 100 % 100 % 100 % 100 % 100 %

Sand Filter 0% 80 % 50 % 25 % 40 % 50 %

Dry Detention Basin 0% 60 % 10 % 30 % ---------- 50 %

Wet Detention Pond 0% 80 % 50 % 30 % 70 % 50 %

Stormwater Wetland – Level I 0% 80 % 40 % 30 % 70 % 50 %

Stormwater Wetland – Level II 0% 85 % 75 % 55 % 85 % 60 %

Bioslope 25-50% 85 % 60 % 25 % 60 % 75 %

OGFC 0% 50 % ---------- ---------- ---------- ----------

Bioretention with open underdrain 50 % 85 % 80 % 60 % 90% 95%

Bioretention with upturned underdrain 75 % 85 % 80 % 60 % 90% 95%

Bioretention with capped underdrain 100 % 100 % 100 % 100 % 100% 100 %

Bioretention Basin

Size to detain & treat WQV (or RRV, if applicable)

Footprint: ~ 3 to 6% of contributing area

Drainage area < 5 acres

Pretreatment to prevent clogging

Engineered soil media: fines, sand, organic matter

Max ponding depth: 12 inches (9 inches preferred)

Drain time: 24 hours

Landscaping plan, no trees

Design Considerations

Bioretention Basin Components

Forebay

Underdrain

Outlet Control Structure

Emergency Spillway

Engineered Media

Vegetation

Provides pretreatment for filtrating/infiltrating BMPs

Removes suspended solids and dissipates energy

Slows down the velocity of the inflow

Reduces large particulates and trash entering the BMPSizing:

Capacity of 0.1 inch runoff/impervious acre

Sediment Forebay

Filter Media – min 24” of permeable soil

Place on top of aggregate blanket (see Specification 169)

3-4" hardwood mulch (non-floating)

2 to 4 ft/day infiltration rate

Soil media to contain a high level of organic material to promote pollutant removal

Engineered Soil Media

Pipe: 8” diameter polyethylene (typ.) in an aggregate layer: No. 57 stone, 12” depth

Rock filter bed: 2-3”, No. 8 or 89 stone

Place engineered media min. 24” above water table

Discharge to storm drainage or stable outlet

Underdrain

No. 57

No. 8

Underdrain Installation

Underdrain System

10 ft max.

10 ft max.0.5% min. slope

100 ft max.

Underdrain Cleanout 5 ft max.

- refer to details

Underdrain System

10 ft max.

0.5% min. slope

- refer to details

Underdrain System

NOTE: DO NOT MOW!

Outlet Control Structure: Open Underdrain

Perforated Pipe

EmergencySpillway

WeirGeotextile

Outlet Control Structure

Engineered Soil Media24 to 48 in

Hardwood Mulch3 to 4 in

No. 8 or 89 Aggregate2 to 3 in

No. 57Aggregate12 in

Outlet Control Structure: Open Underdrain

Perforated Pipe

WeirGeotextile

Outlet Control Structure

Engineered Soil Media24 to 48 in

Hardwood Mulch3 to 4 in

No. 8 or 89 Aggregate2 to 3 in

No. 57Aggregate12 in

EmergencySpillway

Outlet Control Structure: Open Underdrain

Perforated Pipe

WeirGeotextile

Outlet Control Structure

Engineered Soil Media24 to 48 in

Hardwood Mulch3 to 4 in

No. 8 or 89 Aggregate2 to 3 in

No. 57Aggregate12 in

EmergencySpillway

Outlet Control Structure: Upturned Underdrain

WeirGeotextile

Outlet Control Structure

Engineered Soil Media24 to 48 in

Hardwood Mulch3 to 4 in

No. 8 or 89 Aggregate2 to 3 in

No. 57Aggregate12 in

Perforated Pipe

EmergencySpillway

Outlet Control Structure: Upturned Underdrain

WeirGeotextile

Outlet Control Structure

Engineered Soil Media24 to 48 in

Hardwood Mulch3 to 4 in

No. 8 or 89 Aggregate2 to 3 in

No. 57Aggregate12 in

Perforated Pipe

EmergencySpillway

Outlet Control Structure: Upturned Underdrain

WeirGeotextile

Outlet Control Structure

Engineered Soil Media24 to 48 in

Hardwood Mulch3 to 4 in

No. 8 or 89 Aggregate2 to 3 in

No. 57Aggregate12 in

Perforated Pipe

EmergencySpillway

Outlet Control Structure: Closed Underdrain

WeirGeotextile

Outlet Control Structure

Engineered Soil Media24 to 48 in

Hardwood Mulch3 to 4 in

No. 8 or 89 Aggregate2 to 3 in

No. 57Aggregate12 in

Perforated Pipe

EmergencySpillway

Outlet Control Structure: Closed Underdrain

WeirGeotextile

Outlet Control Structure

Engineered Soil Media24 to 48 in

Hardwood Mulch3 to 4 in

No. 8 or 89 Aggregate2 to 3 in

No. 57Aggregate12 in

Perforated Pipe

EmergencySpillway

Outlet Control Structure: Closed Underdrain

WeirGeotextile

Outlet Control Structure

Engineered Soil Media24 to 48 in

Hardwood Mulch3 to 4 in

No. 8 or 89 Aggregate2 to 3 in

No. 57Aggregate12 in

Perforated Pipe

EmergencySpillway

Mulch Layer

Dry media zone

≥ 3 inches deep

Double shredded hardwood mulch

Floating mulch clogs outlet control structure

Three zones present:

• highest elevation planted with species adapted to dry conditions

• middle level – plants able to withstand the fluctuating water levels & tolerant to dryer conditions

• lowest elevation - plants that can withstand standing & fluctuating water levels

Landscaping plan is required

List & quantity of each plant species

Spacing between plants

Vegetation

Roots enhance soil & microbial activity

Vegetation uptakes nutrients

Native species preferred

Avoid invasive species

Goal: 90% vegetative cover within 2 years

Landscaping Plan

University of Idaho

Landscaping PlanShrubs HerbaceousFothergilla Broomsedge

Chokeberry Blue Flag IrisFlorida Leucothoe

Purple Coneflower

Virginia Willow Pink Turtlehead

Possumhaw Black Eyed Susan

Winterberry Wild Blue Indigo

Dwarf Palmetto Lurid Sedge

Spicebush WoolgrassWax Myrtle Marsh Milkweed*selection is based on Georgia plant hardiness areas and planting zone within bioretention basin per the GDOT Planting Schedule Special Construction Detail

Bioretention Basin Planting Zones

Construct after site stabilization

Avoid using heavy equipment in basin area

– Maintain hydraulic conductivity– Prevent damage of underdrains

Construction Considerations

Provide safe access to BMP, outlet structure, forebay, etc. (See figure on next slide)

Provide adequate right-of-way, safely exit & enter of highway

Provide cleanouts for the underdrain system

Maintenance Considerations

Maintenance Considerations

OUTLET

FOREBAY

Determine the goals and primary function of the bioretention basin – RR or WQ

Determine if the basin will be on-line or off-line

RRv

– Calculate the Stormwater Runoff Reduction Target Volume.– Determine the storage volume of the practice and the pretreatment volume– Verify total volume provided by the practice is at least equal to the RRv– Verify that the bioretention basin will drain in the specified timeframes.

WQv

– Calculate the Target Water Quality– Determine the footprint of the bioretention basin and the pretreatment volume required

volume

Design outlet control structure and emergency overflow

Prepare a vegetation and landscaping plan

Design Steps

Example 1

Given:

– New roadway project located in Savannah, Georgia.• 1,100 feet of roadway (in length)• Two 12- foot lanes plus two 3-foot shoulders• Curb and gutter• Runoff exits the roadway through 18” RCP outlet• Soils allow for infiltration

Detention is not needed

Removal of phosphorus, metals, and other pollutants is more important than nitrogen removal

Example 1: Post-development

• Impervious area = 0.76 ac (38.5%)

Total area = 1.97 ac

1,100’ new roadway 2 x 12’ lane + 3’ shoulder

Proposed Bioretention

Area 50’ x 50’

18” RCP

1. Determine whether the bioretention area is intended to meet the Runoff Reduction target

or Water Quality target.

2. Since Soils are good - Try Runoff Reduction Volume first:

𝑅𝑅𝑣𝑣 = Volumetric runoff coefficient = 0.05 + 0.009 𝐼𝐼

𝑅𝑅𝑣𝑣 = 0.05 + 0.009 38.5 = 0.3965

𝑅𝑅𝑅𝑅𝑣𝑣 = Runoff Reduction Volume =1 𝑖𝑖𝑖𝑖 × 𝑅𝑅𝑣𝑣 × 𝐴𝐴 × 43560 𝑓𝑓𝑓𝑓2

𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎12 𝑖𝑖𝑖𝑖𝑓𝑓𝑓𝑓

𝑅𝑅𝑅𝑅𝑣𝑣 =1 × 0.3965 × 1.97 × 43560

12= 2,835 𝑓𝑓𝑓𝑓3

Example 1: Runoff Reduction (Cont’d)

4. Determine the Storage Volume of the practice.

• Space Available: 50 feet wide x 50 feet long = 2,500 ft2

• Forebay: Sized to contain 0.1 inches per impervious acre of contributing drainage• Impervious area: 33,000 ft2 = 0.76 acres• Forebay storage:

0.76 acres x 0.1 in = 0.076 acre-in = 275 ft3

Example 1: Storage Volume

4. Determine the Storage Volume of the practice (continued)

The actual volume provided is: 𝑉𝑉𝑃𝑃 = 𝑃𝑃𝑉𝑉 + 𝑉𝑉𝐸𝐸𝑆𝑆(𝑁𝑁𝐸𝐸𝑆𝑆) + 𝑉𝑉𝐴𝐴(𝑁𝑁𝐴𝐴) Where:

• VP = volume provided (temporary storage) • PV = ponding volume • VES = volume of engineered soils • NES = porosity of engineered soil (use 0.25) • VA = volume of aggregate • NA = porosity of aggregate (use 0.4)

Software program and/or BMP sizing calculator spreadsheet

recommended

Example 1: Storage volume (Cont’d)

4. Determine the Storage Volume of the practice - Starting points/assumptions:

Example 1: Storage Volume (Cont’d)

3

1

1

1

50’

14"

24"

12"3"

4. Determine the Storage Volume of the practice - Starting points/assumptions:

14"

24"

38.5’

42.5’

50’

36.2’

3"44’ 12"

Example 1: Storage Volume (Cont’d)

4. Determine the Storage Volume of the practice (continued)

• PV = ponding volume– Top surface = 50 ft (length) x 50 ft (width)= 2,500 ft2

– Bottom surface = 44 ft (length) x 44 ft (width) = 1,936 ft2

– VES = (2,500 ft2 + 1,936 ft2)/2 x 1 ft(depth) = 2,218 ft3

• VES = volume of engineered soils – Top surface = 42.5 ft (length) x 42.5 ft (width) = 1,806 ft2

– Bottom surface = 38.5 ft (length) x 38.5 ft (width) = 1,482 ft2

– VES = (1,806 ft2 + 1,482 ft2)/2 x 2 ft(depth) = 3,288 ft3

• NES = porosity of engineered soil (use 0.25)

Example 1: Storage Volume (Cont’d)

4. Determine the Storage Volume of the practice (continued)• VA = volume of aggregate

– Top surface = 38.5 ft (length) x 38.5 ft (width) = 1,482 ft2

– Bottom surface = 36.2 ft (length) x 36.2 ft (width) = 1,310 ft2

– VA = (1,482 ft2 + 1,310 ft2)/2 x 1.167 ft (depth) = 1,629 ft3

• NA = porosity of aggregate (use 0.4)

𝑉𝑉𝑃𝑃 = 𝑃𝑃𝑉𝑉 + 𝑉𝑉𝐸𝐸𝑆𝑆(𝑁𝑁𝐸𝐸𝑆𝑆) + 𝑉𝑉𝐴𝐴(𝑁𝑁𝐴𝐴) 𝑉𝑉𝑃𝑃 = 2,218 + 3,288(0.25) + 1,629(0.4) = 3,691 𝑓𝑓𝑓𝑓3

𝑅𝑅𝑅𝑅𝑅𝑅 = 2,835 𝑓𝑓𝑓𝑓3 < VP

Example 1: Storage Volume (Cont’d)

5. Verify that the bioretention basin will drain in the specified timeframes.

• Ponded volume should drain within 24 hours

𝑓𝑓𝑓𝑓 =𝑃𝑃𝑉𝑉(𝑑𝑑𝑓𝑓)

𝑘𝑘(ℎ𝑓𝑓 + 𝑑𝑑𝑓𝑓)𝐴𝐴𝑓𝑓Af = top surface area of filter media (1,806 ft2)PV = ponding volume (2,218 ft3)df = filter media depth (2 ft)k = hydraulic conductivity (4 ft/day)hf = average water depth (0.5 ft)tf = drain time (days)

Example 1: Drain Time

5. Verify that the bioretention basin will drain in the specified timeframes.

• Ponded volume should drain within 24 hours

𝑓𝑓𝑓𝑓 =𝑃𝑃𝑃𝑃(𝑑𝑑𝑓𝑓)

𝑘𝑘(ℎ𝑓𝑓+𝑑𝑑𝑓𝑓)𝐴𝐴𝑓𝑓= 2218 24 0.5+2 1806

= 0.24 𝑑𝑑𝑎𝑎𝑑𝑑𝑎𝑎

Example 1: Drain Time (Cont’d)

5. Verify that the bioretention basin will drain in the specified timeframes.

• Bioretention basin should drain within 72 hours

𝑓𝑓𝑓𝑓 =𝑉𝑉𝑃𝑃

(𝑘𝑘𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑)𝐴𝐴𝑎𝑎VP = total volume provided (3,691 ft3)kdesign = hydraulic conductivity

=1.24 in/hr / Safety factor of 2 = 1.24 ft/dayAa = bottom surface area of aggregate (1,310 ft2)tf = drain time (days)

Example 1: Drain Time (Cont’d)

5. Verify that the bioretention basin will drain in the specified timeframes.

• Bioretention basin should drain within 72 hours

𝑓𝑓𝑓𝑓 = 𝑃𝑃𝑃𝑃(𝑘𝑘𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑)𝐴𝐴𝑎𝑎

= 3,6911.24 1,310

= 2.27 𝑑𝑑𝑎𝑎𝑑𝑑𝑎𝑎

Example 1: Drain Time (Cont’d)

Ex 2: Pre-development

Ex2: Post-development

BIO-RETENTION

BASINCULVERT

OUTLET

FOREBAY

Available BMP area: 2,675 ft2

Post: Open Space – Good: 1.20 ac Impervious: 1.15 ac

Ex2: Post-development

BIO-RETENTION

BASINCULVERT

OUTLET

FOREBAY

Pre: Open Space – Good: 2.10 ac Impervious: 0.25 ac

Determine WQv

𝑹𝑹𝒗𝒗 = 𝟎𝟎.𝟎𝟎𝟎𝟎 + 𝟎𝟎.𝟎𝟎𝟎𝟎𝟎𝟎 ∗ (% 𝑰𝑰𝑰𝑰𝑰𝑰. )

𝑅𝑅𝑣𝑣 𝑝𝑝𝑝𝑝𝑑𝑑 = 0.05 + 0.009 ∗0.252.35

∗ 100 = 𝟎𝟎.𝟏𝟏𝟏𝟏𝟏𝟏 𝑅𝑅𝑣𝑣 𝑝𝑝𝑝𝑝𝑑𝑑𝑝𝑝 = 0.05 + 0.009 ∗1.152.35

∗ 100 = 𝟎𝟎.𝟏𝟏𝟎𝟎𝟎𝟎

𝑹𝑹𝒗𝒗(𝒏𝒏𝒏𝒏𝒏𝒏) = 𝟎𝟎.𝟏𝟏𝟎𝟎𝟎𝟎 − 𝟎𝟎.𝟏𝟏𝟏𝟏𝟏𝟏 = 𝟎𝟎.𝟑𝟑𝟏𝟏𝟏𝟏

Example 2: WQv

𝑾𝑾𝑸𝑸𝒗𝒗 =𝟏𝟏.𝟐𝟐 𝒊𝒊𝒏𝒏 × 𝑹𝑹𝒗𝒗 × 𝑨𝑨 × 𝟏𝟏𝟑𝟑,𝟎𝟎𝟏𝟏𝟎𝟎 𝒇𝒇𝒏𝒏𝟐𝟐

𝒂𝒂𝒂𝒂𝒂𝒂𝒏𝒏𝟏𝟏𝟐𝟐 𝒊𝒊𝒏𝒏

𝒇𝒇𝒏𝒏

𝑾𝑾𝑸𝑸𝒗𝒗 =1.2 𝑖𝑖𝑖𝑖 × 0.344 × 2.35 × 43,560 𝑓𝑓𝑓𝑓2

𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎12 𝑖𝑖𝑖𝑖

𝑓𝑓𝑓𝑓= 𝟑𝟑,𝟎𝟎𝟐𝟐𝟏𝟏 𝒇𝒇𝒏𝒏𝟑𝟑

Size media surface area for WQv

Where, AB = surface area of bioretention basin (ft2)

WQv = water quality volume (ft3)df = filter media depth (ft) – 2.0 to 4.0 ftk = coefficient of permeability of media (ft/day) – typ. 2.0-4.0 ft/dayhf = average height of ponded water (ft) – ½ hmax, max. 1.0 fttf = design filter bed drain time (days) – 1 day max.

=3,521 𝑓𝑓𝑓𝑓3 ∗ 4.0 𝑓𝑓𝑓𝑓

3.0𝑓𝑓𝑓𝑓𝑑𝑑 ∗ 0.5 𝑓𝑓𝑓𝑓 + 4.0 𝑓𝑓𝑓𝑓 ∗ 1 𝑑𝑑𝑎𝑎𝑑𝑑= 1,043 𝑓𝑓𝑓𝑓2𝐴𝐴𝐵𝐵 =

𝑊𝑊𝑄𝑄𝑣𝑣 ∗ 𝑑𝑑𝑓𝑓𝑘𝑘 ∗ ℎ𝑓𝑓 + 𝑑𝑑𝑓𝑓 ∗ 𝑓𝑓𝑓𝑓

Req’d AB< Avail. AB = OK

Example 2: Size Media (Cont’d)

Determine CPv

• Given:– Pre-Development CNW: 65– Post-Development CNW: 79– Tc: 6 min.– P1-yr, 24-hr: 3.56 in– Valdosta, GA → SCS Type II rainfall distribution

Example 2: CPv

Determine CNW

Pre-Development Area (ac) CN

Open space – Good condition (HSG B) 2.10 61

Impervious 0.25 98

TOTAL 2.35 65

Post-Development Area (ac) CN

Open space – Good condition (HSG B) 1.20 61

Impervious 1.15 98

TOTAL 2.35 79

𝑪𝑪𝑪𝑪𝑾𝑾 =𝟐𝟐.𝟏𝟏𝟎𝟎 𝟏𝟏𝟏𝟏 + 𝟎𝟎.𝟐𝟐𝟎𝟎 𝟎𝟎𝟗𝟗

𝟐𝟐.𝟑𝟑𝟎𝟎= 𝟏𝟏𝟎𝟎 𝑪𝑪𝑪𝑪𝑾𝑾 =

𝟏𝟏.𝟐𝟐𝟎𝟎 𝟏𝟏𝟏𝟏 + 𝟏𝟏.𝟏𝟏𝟎𝟎 𝟎𝟎𝟗𝟗𝟐𝟐.𝟑𝟑𝟎𝟎

= 𝟕𝟕𝟎𝟎

Example 2: CPv (Cont’d)

𝑆𝑆 = 1000𝐶𝐶𝐶𝐶

− 10 = 100079

− 10 = 2.66 𝑖𝑖𝑖𝑖.

𝐼𝐼𝑎𝑎 = 0.2 ∗ 𝑆𝑆 = 0.2 ∗ 2.66 = 0.53 𝑖𝑖𝑖𝑖.

Determine CPv: Direct Runoff

Initial Abstraction

𝐼𝐼𝑎𝑎𝑃𝑃

=0.53 𝑖𝑖𝑖𝑖.3.56 𝑖𝑖𝑖𝑖.

= 0.15 𝑖𝑖𝑖𝑖.

𝑄𝑄 =𝑃𝑃 − 0.2𝑆𝑆 2

𝑃𝑃 + 0.8𝑆𝑆=

3.56 − 0.2 ∗ 2.66 2

3.56 + 0.8 ∗ 2.66= 1.62 𝑖𝑖𝑖𝑖.

Direct Runoff

Example 2: CPv (Cont’d)

Determine qu

– Tc: 6 min.(given)– qu = 990 csm/in

SCS Type II Distribution

Determine qo/qi

– qu = 990 csm/in– qo/qi = 0.01

qo/qi Ratio

𝑉𝑉𝑆𝑆𝑉𝑉𝑅𝑅

= 0.682 − 1.43𝑞𝑞𝑝𝑝𝑞𝑞𝑑𝑑

+ 1.64𝑞𝑞𝑝𝑝𝑞𝑞𝑑𝑑

2

− 0.804𝑞𝑞𝑝𝑝𝑞𝑞𝑑𝑑

3

= 0.682 − 1.43 0.01 + 1.64 0.01 2 − 0.804 0.01 3 = 0.67

Determine CPv: Volume Ratio

Where, 𝑃𝑃𝑑𝑑𝑃𝑃𝑅𝑅

= volume storage to release ratio 𝑞𝑞𝑜𝑜𝑞𝑞𝑑𝑑

= ratio of outflow to inflow

Example 2: CPv (Cont’d)

𝑉𝑉𝑆𝑆 =𝑉𝑉𝑑𝑑𝑉𝑉𝑅𝑅

∗ 𝑄𝑄 ∗ 𝐴𝐴 ∗ 3,630

Determine CPv

Where, VS = storage volume – CPV (ft3)𝑃𝑃𝑑𝑑𝑃𝑃𝑅𝑅

= volume storage to release ratio

Q = direct runoff (in)A = drainage area (ac)

= 0.67 ∗ 1.62 𝑖𝑖𝑖𝑖.∗ 2.35 𝑎𝑎𝑎𝑎 ∗ 3,630 = 9,259 𝑓𝑓𝑓𝑓3

Example 2: CPv (Cont’d)

Design Example

Where, AB = surface area of bioretention basin (ft2)CPv = channel protection volume (ft3)df = filter media depth (ft) – 2.0 to 4.0 ftk = coefficient of permeability of media (ft/day) – typ. 2.0-4.0 ft/dayhf = average height of ponded water (ft) – ½ hmax, max. 1.0 fttf = design filter bed drain time (days) – 1 day max.

Size media surface area for CPv

=9,259 𝑓𝑓𝑓𝑓3 ∗ 4.0 𝑓𝑓𝑓𝑓

3.0𝑓𝑓𝑓𝑓𝑑𝑑 ∗ 0.5 𝑓𝑓𝑓𝑓 + 4.0 𝑓𝑓𝑓𝑓 ∗ 1 𝑑𝑑𝑎𝑎𝑑𝑑= 2,743 𝑓𝑓𝑓𝑓2𝐴𝐴𝐵𝐵 =

𝐶𝐶𝑃𝑃𝑃𝑃 ∗ 𝑑𝑑𝑓𝑓𝑘𝑘 ∗ ℎ𝑓𝑓 + 𝑑𝑑𝑓𝑓 ∗ 𝑓𝑓𝑓𝑓

Req’d AB> Avail. AB = RESIZE!

Example 2: Size Media (Cont’d)

Size media surface area for CPv

Where, AB = surface area of bioretention basin (ft2)CPv = channel protection volume (ft3)df = filter media depth (ft) – 2.0 to 4.0 ftk = coefficient of permeability of media (ft/day) – typ. 2.0-4.0 ft/dayhf = average height of ponded water (ft) – ½ hmax, max. 1.0 fttf = design filter bed drain time (days) – 1 day max.

=9,121 𝑓𝑓𝑓𝑓3 ∗ 𝟑𝟑.𝟎𝟎 𝒇𝒇𝒏𝒏

3.0𝑓𝑓𝑓𝑓𝑑𝑑 ∗ 0.5 𝑓𝑓𝑓𝑓 + 𝟑𝟑.𝟎𝟎 𝒇𝒇𝒏𝒏 ∗ 1 𝑑𝑑𝑎𝑎𝑑𝑑= 2,645 𝑓𝑓𝑓𝑓2𝐴𝐴𝐵𝐵 =

𝐶𝐶𝑃𝑃𝑃𝑃 ∗ 𝑑𝑑𝑓𝑓𝑘𝑘 ∗ ℎ𝑓𝑓 + 𝑑𝑑𝑓𝑓 ∗ 𝑓𝑓𝑓𝑓

Req’d AB< Avail. AB = OK

Example 2: Size Media (Cont’d)

Design Example

Design Example: Forebay

Where, VFB = Forebay Volume (ft3)Aimp = Impervious area (ac)

= 0.1 𝑖𝑖𝑖𝑖 ∗ 1.15 𝑎𝑎𝑎𝑎 ∗ 1 𝑓𝑓𝑝𝑝12 𝑑𝑑𝑑𝑑

∗ 43,560 𝑓𝑓𝑝𝑝2

1 𝑎𝑎𝑎𝑎= 417 𝑓𝑓𝑓𝑓3

𝑉𝑉𝐹𝐹𝐵𝐵 = 0.1 𝑖𝑖𝑖𝑖 ∗ 𝐴𝐴𝑑𝑑𝑖𝑖𝑝𝑝

Example 2: Forebay (Cont’d)

Major Elements• Bypass Structure• Forebay• Underdrain• Outlet Control Structure• Mulch • Vegetation Plan• Maintainable Slopes• Maintenance Access

Forebay

Underdrain

Outlet Control Structure

Bypass Structure

Example Large Established Bioretention Basin

Questions

Brad McManus, PE MS4 Program Manager

Office of Design Policy and Support

bmcmanus@dot.ga.gov

More GDOT Advanced Design Workshops can be found at https://learning.dot.ga.gov/