Bioretention Designs to Meet Different Goals

Bioretention Designs to Meet Different Goals

Jay Dorsey & John MathewsODNR-DSWRApril 16, 2014

Goals for Presentation Sizing Requirements for WQv

New Development Redevelopment

Base DesignDesign Modifications to Address Location

Specific Conditions/Limitations or Meet Watershed Specific Goals

Basic Design Example

Sizing Requirements for WQv- New Development -

From NPDES Construction Stormwater Permit

Sizing Requirements for WQv- New Development -

Target Drawdown Time, Td = 24 hr

Design Drawdown Assumption - Kfs of settled filter bed media

(planting soil) is between 0.5 to 2.0 in/hr[Maintenance required when Kfs < 0.5/in/hr]

Td = dWQv /Kfs = (12 in)/(0.5 in/hr) = 24 hr

Where: Td – drawdown time dWQv – equivalent depth of WQv Kfs – saturated hydraulic conductivity

Filter Bed Sizing Requirement If impervious area exceeds 25% of

contributing drainage area, filter bed area shall be a minimum 5% of contributing impervious area.

Filter Bed Sizing Requirement Example 1

Total contributing drainage area = 0.82 Ac Impervious percent = 45% (>25%) Contributing impervious area = (0.82 Ac)(0.45)

= 0.37 Ac = 16,073 ft2

Minimum filter bed area = (16,073 ft2)(0.05) = 803 ft2

Filter Bed Sizing Requirement If impervious area exceeds 25% of

contributing drainage area, filter bed area shall be a minimum 5% of contributing impervious area.

If impervious area makes up less than 25% of contributing drainage area, filter bed area shall be at least equal to the WQv divided by the one foot maximum ponding depth.

Filter Bed Sizing Requirement Example 2

Total contributing drainage area = 0.82 Ac Impervious percent = 15% (<25%) For 15% impervious,

C = (0.858)(0.15)3 – (0.78)(0.15)2 + (0.774)(0.15) + 0.04= 0.141

WQv = C*P*A = (0.141)(0.75 in)(0.82 Ac)(1 ft/12 in)= 0.007 Ac-ft = 315 ft3

Minimum filter bed area = (315 ft3)(1 ft) = 315 ft2

Filter Bed Sizing Requirement If impervious area exceeds 25% of

contributing drainage area, filter bed area shall be a minimum 5% of contributing impervious area.

If impervious area makes up less than 25% of contributing drainage area, filter bed area shall be at least equal to the WQv divided by the one foot maximum ponding depth.

Assumption - sediment storage requirement (20% of WQv) will be met with excess bowl volume

Filter Bed Area(%

)

Filter Bed Area

What about Redevelopment? For redevelopment projects, the full WQv

must be captured for all new/additional impervious area, but for existing impervious area the volume that must be captured is 20% of the WQv.

What about Redevelopment? A rule of thumb based on research shows an

optimal 10:1 to 20:1 ratio for contributing impervious drainage area to bioretention filter bed area (i.e. hydrologic loading ratio). If all best practices are used (pretreatment, energy dissipation, construction, etc.) a hydrologic loading ratio of 25:1 is probably okay for most sites.The filter bed area of the bioretention cell

should not be less than 4% of the contributing impervious area.

Redevelopment BRC Options For straight redevelopment (no new

impervious), capture and treat the full WQvfrom 20% of the site

For mixed redevelopment and new development, match the size of your bioretention cell to your contributing impervious area

Build a bioretention practice capable of capturing the full WQv from the entire site, and use the rest as credit toward reduction of stormwater fees or as mitigation

Bioretention Cell Components

Bioretention Decisions

Nitrogen Treatment

HSG D Soils(depending on

limitations)

Temperature

Base Design30-36” depth;

IWS LayerDepth Limitations

(e.g., Shallow Outlet, High Water Table)

36+” media depth; IWS Layer (>18”);48” depth to drain

Underdrain w/ 3” of cover & 3” of

bedding

36” media depth; IWS layer (>18”), outlet raised >6”

into planting media

24” mediadepth

High Water Table, Karst, Shallow

Bedrock or High Pollution Loads

Impermeable liner

HSG A Soils

If Kfs > 1 in/hr, may not require

underdrain, aggregate, filter

Base Bioretention Configuration

Base Bioretention Configuration

30”-36” Planting Soil

6” Filter

12” Aggregate

Base Bioretention Configuration

24” Planting Soil above Invert

6” (min) Planting Soil in IWS

Special Designs Pollutant Load Reduction Goals

Temperature Mitigation Nitrogen Removal Phosphorus Mitigation

Site Conditions or Limitations High Permeability Soils (> 1 in/hr) Very Low Permeability Soils (<0.05 in/hr) Depth Limitations Groundwater Pollution Potential

Bioretention Decisions

Nitrogen Treatment

HSG D Soils(depending on

limitations)

Temperature

Base Design30-36” depth;

IWS LayerDepth Limitations

(e.g., Shallow Outlet, High Water Table)

36+” media depth; IWS Layer (>18”);48” depth to drain

Underdrain w/ 3” of cover & 3” of

bedding

36” media depth; IWS layer (>18”), outlet raised >6”

into planting media

24” mediadepth

High Water Table, Karst, Shallow

Bedrock or High Pollution Loads

Impermeable liner

HSG A Soils

If Kfs > 1 in/hr, may not require

underdrain, aggregate, filter

Bioretention Decisions

Nitrogen Treatment

HSG D Soils(depending on

limitations)

Temperature

Base Design30-36” depth;

IWS LayerDepth Limitations

(e.g., Shallow Outlet, High Water Table)

36+” media depth; IWS Layer (>18”);48” depth to drain

Underdrain w/ 3” of cover & 3” of

bedding

36” media depth; IWS layer (>18”), outlet raised >6”

into planting media

24” mediadepth

High Water Table, Karst, Shallow

Bedrock or High Pollution Loads

Impermeable liner

HSG A Soils

If Kfs > 1 in/hr, may not require

underdrain, aggregate, filter

High Permeability Soils

If measured subgrade infiltration rate exceeds 1.0 in/hr, the underdrain, and aggregate and filter layers, can be eliminated

High Permeability Soils

Bioretention Decisions

Nitrogen Treatment

HSG D Soils(depending on

limitations)

Temperature

Base Design30-36” depth;

IWS LayerDepth Limitations

(e.g., Shallow Outlet, High Water Table)

36+” media depth; IWS Layer (>18”);48” depth to drain

Underdrain w/ 3” of cover & 3” of

bedding

36” media depth; IWS layer (>18”), outlet raised >6”

into planting media

24” mediadepth

High Water Table, Karst, Shallow

Bedrock or High Pollution Loads

Impermeable liner

HSG A Soils

If Kfs > 1 in/hr, may not require

underdrain, aggregate, filter

Source: Bill Hunt, NCSU-BAE

Temperature Mitigation

Planting soil media depth - minimum 36”Underdrain/outlet configuration

minimum 48” depth to drain; more is better upturned elbow with internal water storage

(IWS) layer, minimum 18” sump

Temperature Mitigation

Planting Soil36” Minimum

18” IWS Min.48” DrainDepth Min.

Bioretention Decisions

Nitrogen Treatment

HSG D Soils(depending on

limitations)

Temperature

Base Design30-36” depth;

IWS LayerDepth Limitations

(e.g., Shallow Outlet, High Water Table)

36+” media depth; IWS Layer (>18”);48” depth to drain

Underdrain w/ 3” of cover & 3” of

bedding

36” media depth; IWS layer (>18”), outlet raised >6”

into planting media

24” mediadepth

High Water Table, Karst, Shallow

Bedrock or High Pollution Loads

Impermeable liner

HSG A Soils

If Kfs > 1 in/hr, may not require

underdrain, aggregate, filter

Nitrogen Removal

Planting soil media depth - minimum 36”Underdrain/outlet configuration

upturned elbow with internal water storage (IWS) layer, minimum 18” sump with at least 6” IWS in planting media

if necessary, orifice on drain outlet to control discharge rate

Nitrogen Removal

Planting Soil36” Minimum

18” IWS Min.

6” Min. inPlanting Soil

Phosphorus Removal

Planting soil media depth - minimum 36” Planting soil phosphorus content – 15-40

mg/kg P by Mehlich3Recommend adding water treatment

residuals (WTR) or other iron or aluminum rich amendment

Bioretention Decisions

Nitrogen Treatment

HSG D Soils(depending on

limitations)

Temperature

Base Design30-36” depth;

IWS LayerDepth Limitations

(e.g., Shallow Outlet, High Water Table)

36+” media depth; IWS Layer (>18”);48” depth to drain

Underdrain w/ 3” of cover & 3” of

bedding

36” media depth; IWS layer (>18”), outlet raised >6”

into planting media

24” mediadepth

High Water Table, Karst, Shallow

Bedrock or High Pollution Loads

Impermeable liner

HSG A Soils

If Kfs > 1 in/hr, may not require

underdrain, aggregate, filter

Depth Limitations

Bioretention Decisions

Nitrogen Treatment

HSG D Soils(depending on

limitations)

Temperature

Base Design30-36” depth;

IWS LayerDepth Limitations

(e.g., Shallow Outlet, High Water Table)

36+” media depth; IWS Layer (>18”);48” depth to drain

Underdrain w/ 3” of cover & 3” of

bedding

36” media depth; IWS layer (>18”), outlet raised >6”

into planting media

24” mediadepth

High Water Table, Karst, Shallow

Bedrock or High Pollution Loads

Impermeable liner

HSG A Soils

If Kfs > 1 in/hr, may not require

underdrain, aggregate, filter

Low Permeability Soils or Impediments to Infiltration

If subgrade infiltration rate is less than 0.05 in/hr, or if shallow bedrock or seasonal high water table is present, there may be limited benefits and potential issues from the IWS; a level drain with 3” sump allows limited exfiltration

Low Permeability Soils or Impediments to Infiltration

Bioretention Decisions

Nitrogen Treatment

HSG D Soils(depending on

limitations)

Temperature

Base Design30-36” depth;

IWS LayerDepth Limitations

(e.g., Shallow Outlet, High Water Table)

36+” media depth; IWS Layer (>18”);48” depth to drain

Underdrain w/ 3” of cover & 3” of

bedding

36” media depth; IWS layer (>18”), outlet raised >6”

into planting media

24” mediadepth

High Water Table, Karst, Shallow

Bedrock or High Pollution Loads

Impermeable liner

HSG A Soils

If Kfs > 1 in/hr, may not require

underdrain, aggregate, filter

High Groundwater Pollution Potential

In Karst areas or areas with shallow groundwater aquifers, water supplies are susceptible to contamination – use an impermeable liner

In sites with contaminated soils or pollution hot spots, bioretention cells should use an impermeable liner

Alternative configurations can still be used to mitigate temperature and nutrient impacts

GW Pollution Potential – Add Liner

Bioretention Decisions

Nitrogen Treatment

HSG D Soils(depending on

limitations)

Temperature

Base Design30-36” depth;

IWS LayerDepth Limitations

(e.g., Shallow Outlet, High Water Table)

36+” media depth; IWS Layer (>18”);48” depth to drain

Underdrain w/ 3” of cover & 3” of

bedding

36” media depth; IWS layer (>18”), outlet raised >6”

into planting media

24” mediadepth

High Water Table, Karst, Shallow

Bedrock or High Pollution Loads

Impermeable liner

HSG A Soils

If Kfs > 1 in/hr, may not require

underdrain, aggregate, filter

Base Bioretention Configuration

Design Example – Holden Arboretum

North WshedDA = 0.67 Ac%Imp (est) = 58%ImpArea = 0.39 AcABRC = 0.0195 Ac

= 850 sq ft~ 23 x 40 ft

South WshedDA = 0.48 Ac%Imp (est) = 59%ImpArea = 0.28 AcABRC = 0.014 Ac

= 610 sq ft~ 20 x 30 ft

Watersheds

North WshedDA = 0.67 Ac%Imp (est) = 58%ImpArea = 0.39 AcABRC = 0.0195 Ac

= 850 sq ft~ 23 x 40 ft

South WshedDA = 0.48 Ac%Imp (est) = 59%ImpArea = 0.28 AcABRC = 0.014 Ac

= 610 sq ft~ 20 x 30 ft

Watersheds

proposed bioretentionlocations

Design Example – Holden ArboretumNorth Bioretention Cell

Drainage Area = 0.67 AcImperviousness = 58%Impervious Area = 0.39 AcABRC = 0.05*0.39 Ac = 0.0195 Ac = 850 sq ftC = 0.394WQv = C*P*A = 0.394*(0.75 in)*(0.39 Ac)

= 0.016 Ac-ft = 719 ft3

Soil Map

PlateaHSG-D

PierpontHSG-C

proposed bioretentionlocations – measure infiltration rate at proposed depth of excavation ~48-54”

other potential samplinglocations – sample at ground surface

Infiltration Tests

Measured Kfs (in/hr)BRC1(N): 0.02, 0.02BRC2(S): 0.02, 0.08

Target Bioretention Configuration

36” Planting Soil

6” Filter

12” Aggregate

Bioretention Cell 1(N) - Section

Not to Scale

existing pavement

Lowest Pavement = 99.3’

All Elevations are Relative, Not MSL

Outlet Invert = 94.0’

existing 15”outlet

12” clean gravel (#57)

3” filter - clean gravel (#8)

3” filter – clean concrete sand

~36” bioretention soil

Bioretention Cell 1(N) - Section

Not to Scale

existing pavement

Lowest Pavement = 99.3’

All Elevations are Relative, Not MSL

freeboard = 0.5’

max ponding depth = 1.0’

Outlet Invert = 94.0’

existing 15”outlet

drain

drainoutfall

12” clean gravel (#57)

3” filter - clean gravel (#8)

3” filter – clean concrete sand

~36” bioretention soil

Bioretention Cell 1(N) - Section

Not to Scale

existing pavement

Lowest Pavement = 99.3’

All Elevations are Relative, Not MSL

freeboard = 0.5’

max ponding depth = 1.0’

Outlet Invert = 94.0’

existing 15”outlet

drain

drainoutfall

Bottom of Excavation = 93.3’

Proposed Overflow = 98.8’

Filter Bed Surface = 97.8’

Sand/Gravel Filter = 94.3’

Filter Bed Bottom = 94.8’

Drain Outfall = 95.1’

Scarifying Bottom of Cell

Underdrain w/Upturned ElbowCreating 21” Internal Water Storage (IWS) Zone or Sump

Waterproof Connection

Hydraulic Cement

12” #57 gravel

Water Table Monitoring Well

3” #8 gravel filter

3” clean C-33 sand filter

36” bioretention planting soil

Holden Bioretention Configuration

36” Planting Soil

6” Filter

12” Aggregate

Holden North Cell Drawdown Data

Drawdown Begin Date/Time

Drawdown End Date/Time

Beginning Stage (ft)

Ending Stage (ft)

Delta Stage (ft)

Delta time (days)

Drawdown Rate (ft/day)

Drawdown Rate (in/hr)

Infiltrated Volume (ft3)

10/7/2013 17:22 10/16/2013 0:30 2.099 1.17 0.929 8.30 0.112 0.056 26110/17/2013 6:42 10/17/2013 15:38 2.085 1.97 0.115 0.37 0.309 0.154 3210/18/2013 2:48 10/19/2013 12:20 2.084 1.721 0.363 1.40 0.260 0.130 10210/20/2013 12:12 10/21/2013 20:30 2.052 1.624 0.428 1.35 0.318 0.159 12010/22/2013 14:16 10/23/2013 7:02 2.07 1.783 0.287 0.70 0.411 0.205 8110/26/2013 18:36 10/26/2013 21:12 1.923 1.894 0.029 0.11 0.268 0.134 810/27/2013 12:56 10/31/2013 4:00 1.892 1.352 0.54 3.63 0.149 0.074 15111/2/2013 3:48 11/2/2013 9:22 1.883 1.815 0.068 0.23 0.293 0.147 1911/4/2013 1:30 11/6/2013 17:18 1.847 1.344 0.503 2.66 0.189 0.095 14111/9/2013 10:00 11/11/2013 17:46 1.851 1.355 0.496 2.32 0.213 0.107 13911/15/2013 7:16 11/17/2013 18:46 1.794 1.491 0.303 2.48 0.122 0.061 8511/19/2013 4:14 11/21/2013 21:28 1.789 1.279 0.51 2.72 0.188 0.094 14311/23/2013 21:28 12/9/2013 9:06 1.811 1.165 0.646 15.48 0.042 0.021 181

Avg drawdown rate: 0.125 ft/day TotalExfiltrated Volume: 1463Avg drawdown rate: 0.062 in/hrStandard Deviation: 0.0507

North Cell Well Drawdown Rates

Holden North Cell Drawdown Data

Drawdown Begin Date/Time

Drawdown End Date/Time

Beginning Stage (ft)

Ending Stage (ft)

Delta Stage (ft)

Delta time (days)

Drawdown Rate (ft/day)

Drawdown Rate (in/hr)

Infiltrated Volume (ft3)

10/7/2013 17:22 10/16/2013 0:30 2.099 1.17 0.929 8.30 0.112 0.056 26110/17/2013 6:42 10/17/2013 15:38 2.085 1.97 0.115 0.37 0.309 0.154 3210/18/2013 2:48 10/19/2013 12:20 2.084 1.721 0.363 1.40 0.260 0.130 10210/20/2013 12:12 10/21/2013 20:30 2.052 1.624 0.428 1.35 0.318 0.159 12010/22/2013 14:16 10/23/2013 7:02 2.07 1.783 0.287 0.70 0.411 0.205 8110/26/2013 18:36 10/26/2013 21:12 1.923 1.894 0.029 0.11 0.268 0.134 810/27/2013 12:56 10/31/2013 4:00 1.892 1.352 0.54 3.63 0.149 0.074 15111/2/2013 3:48 11/2/2013 9:22 1.883 1.815 0.068 0.23 0.293 0.147 1911/4/2013 1:30 11/6/2013 17:18 1.847 1.344 0.503 2.66 0.189 0.095 14111/9/2013 10:00 11/11/2013 17:46 1.851 1.355 0.496 2.32 0.213 0.107 13911/15/2013 7:16 11/17/2013 18:46 1.794 1.491 0.303 2.48 0.122 0.061 8511/19/2013 4:14 11/21/2013 21:28 1.789 1.279 0.51 2.72 0.188 0.094 14311/23/2013 21:28 12/9/2013 9:06 1.811 1.165 0.646 15.48 0.042 0.021 181

Avg drawdown rate: 0.125 ft/day TotalExfiltrated Volume: 1463Avg drawdown rate: 0.062 in/hrStandard Deviation: 0.0507

North Cell Well Drawdown Rates

Holden North Cell Drawdown Data

References ODNR. Rainwater and Land Development Manual. NCDENR Stormwater Manual. 2009. Hunt, Davis, and Traver. 2012. Meeting Hydrologic and

Water Quality Goals through Targeted BioretentionDesign. J. Env. Eng. 138(6): 698-707.

Wardynski and Hunt. 2012. Are Bioretention Cells Being Installed per Design Standards in North Carolina? A Field Assessment. J. Env. Eng. 138(12): 1210-1217.

Brown, Hunt, and Kennedy. 2009. Designing Bioretention with an Internal Water Storage (IWS) Layer. NCSU-CE.

CWP. 2012. West Virginia Stormwater Management and Design Guidance Manual.

Questions:

Jay DorseyWater Resources EngineerODNR, Soil & Water Resources(614) [email protected]