STORMWATER BEST MANAGEMENT PRACTICE (BMP) CONSTRUCTED STORMWATER WETLANDS FOR STORMWATER TREATMENT JEREMY FINCH RHETT BUTLER.

STORMWATER BEST MANAGEMENT STORMWATER BEST MANAGEMENT PRACTICE (BMP)PRACTICE (BMP)

CONSTRUCTED STORMWATER CONSTRUCTED STORMWATER WETLANDS FOR STORMWATER WETLANDS FOR STORMWATER

TREATMENTTREATMENT

JEREMY FINCH

RHETT BUTLER

OVERVIEW OF PRESENTATIONOVERVIEW OF PRESENTATION

Definition & FunctionsDefinition & Functions Characteristics of Characteristics of

Stormwater WetlandsStormwater Wetlands Natural Wetlands vs. Natural Wetlands vs.

Stormwater WetlandsStormwater Wetlands Advantages & Advantages &

DisadvantagesDisadvantages Design of Stormwater Design of Stormwater

WetlandsWetlands Practical Design Practical Design

ExampleExample

DEFINITIONS & FUNCTIONS OF DEFINITIONS & FUNCTIONS OF STORMWATER WETLANDSSTORMWATER WETLANDS

DefinitionDefinition- - Wetland Systems that are specifically designed to treat/manage Wetland Systems that are specifically designed to treat/manage the impact of development/urbanization on stormwater quality the impact of development/urbanization on stormwater quality

and quantityand quantity

FunctionsFunctions- - Improve Water Quality Improve Water Quality

(i.e. pollutant removal)(i.e. pollutant removal)

- Minimize flooding/flood control- Minimize flooding/flood control

- Establish/create wildlife habitat- Establish/create wildlife habitat

CHARACTERISTICS OF ALL CHARACTERISTICS OF ALL STORMWATER WETLANDSSTORMWATER WETLANDS

Inundated with surface or Inundated with surface or groundwater for most of groundwater for most of the yearthe year

Contains vegetation that is Contains vegetation that is adapted to saturated adapted to saturated conditionsconditions

Primary source of water is Primary source of water is runoff from urban areasrunoff from urban areas

Contains forebays, deep Contains forebays, deep pools, shallow pools, outlet pools, shallow pools, outlet control structurescontrol structures

High pollutant removal High pollutant removal efficiencyefficiency

POLLUTANT REMOVAL IN POLLUTANT REMOVAL IN STORMWATER WETLANDSSTORMWATER WETLANDS

REMOVAL PROCESSREMOVAL PROCESS POLLUTANTS REMOVEDPOLLUTANTS REMOVED

Plant UptakePlant Uptake Nutrients including nitrogen and Nutrients including nitrogen and phophorusphophorus

Microbial ProcessesMicrobial Processes Nitrogen, pathogens, organicsNitrogen, pathogens, organics

Sedimentation & FiltrationSedimentation & Filtration TSS, floating debris, trash, TSS, floating debris, trash, sediment-attached phosphorus, sediment-attached phosphorus,

pathogenspathogens

AdsorptionAdsorption Dissolved metals, soluble Dissolved metals, soluble phosphorusphosphorus

Ref: Bill Hunt

POLLUTANT REMOVAL EFFICIENCYPOLLUTANT REMOVAL EFFICIENCY

POLLUTANTPOLLUTANT REMOVAL EFFICIENCYREMOVAL EFFICIENCYTSSTSS 67%67%

Total PhosphorusTotal Phosphorus 49%49%

Total NitrogenTotal Nitrogen 28%28%

Organic CarbonOrganic Carbon 34%34%

Petroleum HydrocarbonsPetroleum Hydrocarbons 87%87%

CadmiumCadmium 36%36%

CopperCopper 41%41%

LeadLead 62%62%

ZincZinc 45%45%

BacteriaBacteria 77%77%

Ref: US EPA

NATURAL VS. STORMWATER NATURAL VS. STORMWATER WETLANDSWETLANDS

Stormwater wetlands do not perform the same functions as natural wetlandsStormwater wetlands do not perform the same functions as natural wetlands

Natural WetlandsNatural Wetlands Stormwater WetlandsStormwater WetlandsWater balance dominated by Water balance dominated by

groundwatergroundwaterWater Balance dominated by Water Balance dominated by

surface runoff from urban areassurface runoff from urban areas

Amount of standing water varies Amount of standing water varies seasonallyseasonally

Standing water year-roundStanding water year-round

Wetland boundaries can shift Wetland boundaries can shift depending on seasonal depending on seasonal groundwater changesgroundwater changes

Wetland boundaries are clearly Wetland boundaries are clearly defineddefined

Complex topographyComplex topography Simple topographySimple topography

Self-maintainingSelf-maintaining Requires year-round maintenanceRequires year-round maintenance

High Wildlife potentialHigh Wildlife potential Low to Moderate Wildlife PotentialLow to Moderate Wildlife Potential

Occurs naturallyOccurs naturally Constructed & Designed specifically Constructed & Designed specifically for urban stormwater runofffor urban stormwater runoff

Ref: NCDENR

ADVANTAGES & DISADVANTAGESADVANTAGES & DISADVANTAGES

ADVANTAGESADVANTAGES- Removes multiple pollutants from stormwater runoff- Removes multiple pollutants from stormwater runoff

- Improves overall water quality- Improves overall water quality

- If designed & constructed properly, can be aesthetically pleasing- If designed & constructed properly, can be aesthetically pleasing

- Reduce flooding potential in downstream areas due to - Reduce flooding potential in downstream areas due to

development upstreamdevelopment upstream

DISADVANTAGESDISADVANTAGES- Can occupy large areas of developable land- Can occupy large areas of developable land

- Can dry out and become nuisance if drainage area is too small- Can dry out and become nuisance if drainage area is too small

- Mosquito/Snake habitat- Mosquito/Snake habitat

- May be difficult to establish native wetland plants- May be difficult to establish native wetland plants

DESIGN OF STORMWATER DESIGN OF STORMWATER WETLANDSWETLANDS

Optimal LocationOptimal Location- - Where water availability is Where water availability is

highhigh

- Flat topography- Flat topography

- Areas where seasonal high - Areas where seasonal high

water table is approx. 6” water table is approx. 6”

above bottom of wetlandabove bottom of wetland

- Areas where soil studies - Areas where soil studies

show that the underlying show that the underlying

soils have an low infiltration soils have an low infiltration

rate to maintain a rate to maintain a

permanent pool of waterpermanent pool of water

NOTE: THE FOLLOWING DESIGN PROCEDURE VARIES REGIONALLY. THIS IS ONE DESIGN PROCEDURE SUMMARIZED FROM THE SOURCES SHOWN IN THE REFERENCES AT THE END OF THIS PRESENTATION. IT IS IMPORTANT TO NOTE THAT THIS IS ONE OF MANY POSSIBLE DESIGN PROCEDURES FOR STORMWATER WETLANDS

DESIGN OF STORMWATER WETLANDSDESIGN OF STORMWATER WETLANDS (continued)(continued)

DESIGN REQUIREMENTSDESIGN REQUIREMENTS- Must have a p- Must have a permanent pool ermanent pool

of water (for stormwater of water (for stormwater wetlands, we use 3 feet)wetlands, we use 3 feet)

- Must meet required surface - Must meet required surface area to drainage area ratioarea to drainage area ratio

Schematic Ref: Bill Hunt

Ref: NCDENR

DESIGN OF STORMWATER WETLANDSDESIGN OF STORMWATER WETLANDS (CONTINUED)(CONTINUED)

- Must detain volume of runoff resulting from the 1” storm (i.e. - Must detain volume of runoff resulting from the 1” storm (i.e. first flush runoff) above the permanent pool. This volume is first flush runoff) above the permanent pool. This volume is

calculated using the SCS Curve Number method shown below:calculated using the SCS Curve Number method shown below:

)8.0(

)2.0( 2

SP

SPmeRunoffVolu

Where: P = Precipitation (in) 1” storm

S = Ultimate Storage Capacity (in/in)

• Must drawdown the 1” storm over a period of 2 to 5 days

• Should contain a sediment forebay for initial settling

• To function properly as a wetland, 35% of the total wetland area should have a depth 0-9”

• To function properly as a wetland, 35% of the total wetland area should have a depth 9-18”

• To function property as a wetland, 30% of the total wetland area should be “shallow land”

FOREBAYFOREBAY Placed where runoff enters the wetland (i.e. storm drainage)Placed where runoff enters the wetland (i.e. storm drainage) Serves as a preliminary “screening” device to prevent Serves as a preliminary “screening” device to prevent

degradation of primary wetland functions degradation of primary wetland functions traps sediments, traps sediments, larges pieces of debris, etc.larges pieces of debris, etc.

Deepest part of wetland Deepest part of wetland usually 2-2.5’ deep usually 2-2.5’ deep Must allow access to forebay for maintenance purposes Must allow access to forebay for maintenance purposes

(sediment cleanout, etc.)(sediment cleanout, etc.) Designed to occupy approximately 10% of total wetland areaDesigned to occupy approximately 10% of total wetland area

Forebay Ref: Bill Hunt

SHALLOW POOLSSHALLOW POOLS Typically 0-9” deepTypically 0-9” deep Area where primary wetland functions occur (i.e. Area where primary wetland functions occur (i.e.

denitrification, sedimentation, filtration, adsorption, etc.)denitrification, sedimentation, filtration, adsorption, etc.) Water velocity decreases dramatically here, thus causing Water velocity decreases dramatically here, thus causing

pollutants to settle out of stormwaterpollutants to settle out of stormwater Area where majority of wetland vegetation growsArea where majority of wetland vegetation grows Designed to be a long, winding flowpath occupying Designed to be a long, winding flowpath occupying

approximately 35% of total wetland areaapproximately 35% of total wetland area During low flow conditions, this is path the water takes During low flow conditions, this is path the water takes

through the wetlandthrough the wetland

Shallow Pool Ref: Bill Hunt

DEEP POOLSDEEP POOLS Typically 9-18” deepTypically 9-18” deep Designed to occupy approximately 35% of total wetland Designed to occupy approximately 35% of total wetland

areaarea Contains water in times of droughtContains water in times of drought Where animal habitats (i.e. fish) are locatedWhere animal habitats (i.e. fish) are located Area with least amount of vegetationArea with least amount of vegetation

Deep Pool Ref: Bill Hunt

SHALLOW LANDSHALLOW LAND Designed to be dry except during storm eventsDesigned to be dry except during storm events 0-12” above normal pool0-12” above normal pool Supports a wide variety of vegetationSupports a wide variety of vegetation Designed to occupy approximately 30% of the total Designed to occupy approximately 30% of the total

wetland areawetland area Supports various types of wildlifeSupports various types of wildlife

Shallow Land Ref: Bill Hunt

STORMWATER WETLAND OUTLET STORMWATER WETLAND OUTLET STRUCTURESTRUCTURE

Designed to store the 1” Designed to store the 1” storm runoff volumestorm runoff volume

Contains siphon/drawdown Contains siphon/drawdown device designed to device designed to drawdown the 1” storm drawdown the 1” storm volume over 2 to 5 daysvolume over 2 to 5 days

Passes higher flows (i.e. Passes higher flows (i.e. 100-year storm) during 100-year storm) during extreme storm events extreme storm events safely through the wetlandsafely through the wetland

Typically riser-barrels or Typically riser-barrels or weir overflow structuresweir overflow structures

STORMWATER WETLAND VEGETATION STORMWATER WETLAND VEGETATION GUIDELINESGUIDELINES

Avoid non-native species or aggressive plantsAvoid non-native species or aggressive plants Select plants that can adapt in saturated Select plants that can adapt in saturated

conditions and withstand long term inundationconditions and withstand long term inundation Include species that are evergreen (meaning not Include species that are evergreen (meaning not

dormant during winter time)dormant during winter time) Can use donor soils (if necessary) from natural Can use donor soils (if necessary) from natural

wetlands to establish vegetationwetlands to establish vegetation

BLACK WILLOW

RICE CUT GRASS

SMARTWEED

Ref: US EPA & Bill Hunt

MAINTENANCE OF STORMWATER WETLANDSMAINTENANCE OF STORMWATER WETLANDS Stormwater wetlands should be Stormwater wetlands should be

inspected after each rainfall inspected after each rainfall eventevent

Check for clogging of the outlet Check for clogging of the outlet structure, or too rapid of a structure, or too rapid of a drawdown drawdown

Erosion/scour of wetland Erosion/scour of wetland embankmentsembankments

Erosion in areas where the Erosion in areas where the stormwater runoff enters the stormwater runoff enters the wetland (i.e. storm drainage wetland (i.e. storm drainage pipes, etc.)pipes, etc.)

Sediment accumulation in the Sediment accumulation in the forebay forebay

Condition of outlet structureCondition of outlet structure Prevention of woody vegetation Prevention of woody vegetation

on the wetland bermon the wetland berm Any additional Any additional

fertilizing/seeding to maintain fertilizing/seeding to maintain healthy vegetation healthy vegetation Ref: NCDENR

QUESTIONS?QUESTIONS?

PRACTICAL STORMWATER WETLANDS PRACTICAL STORMWATER WETLANDS DESIGN EXAMPLEDESIGN EXAMPLE

Development RequirementsDevelopment Requirements Example Project: Residential development consisting of mult-Example Project: Residential development consisting of mult-

family townhomes, associated streets, parking, and sidewalksfamily townhomes, associated streets, parking, and sidewalks

Detention of 2- & 10-year post-development peak flowrates back Detention of 2- & 10-year post-development peak flowrates back to pre-development levelsto pre-development levels

Reduction of nitrogen loading rate down to at least 6 lbs/ac/yr Reduction of nitrogen loading rate down to at least 6 lbs/ac/yr (must then buydown to 3.6 lbs/ac/yr)(must then buydown to 3.6 lbs/ac/yr)

Removal of 85% of the total suspended solids from the stormwater Removal of 85% of the total suspended solids from the stormwater runoff resulting from the developmentrunoff resulting from the development

Safe passage of the 100-year storm event through the wetlandSafe passage of the 100-year storm event through the wetland

EXAMPLE STORMWATER WETLANDS EXAMPLE STORMWATER WETLANDS DESIGN DESIGN (CONTINUED)(CONTINUED)

STEP #1STEP #1- Determine the stormwater wetlands drainage area- Determine the stormwater wetlands drainage area

EXAMPLE STORMWATER WETLANDS EXAMPLE STORMWATER WETLANDS DESIGN DESIGN (CONTINUED)(CONTINUED)

Watershed CharacteristicsWatershed CharacteristicsPRE-DEVELOPMENTPRE-DEVELOPMENT

- Total Drainage Area = 12.27 acres- Total Drainage Area = 12.27 acres

- SCS Curve Number = 72 (Hydrologic soil - SCS Curve Number = 72 (Hydrologic soil

group C)group C)

- Pre-development time of concentration = - Pre-development time of concentration =

10.5 minutes10.5 minutes

- 2-Year pre-development peak flowrate = 20 cfs - 2-Year pre-development peak flowrate = 20 cfs

- 10-Year pre-development peak flowrate = 39 cfs- 10-Year pre-development peak flowrate = 39 cfs

POST-DEVELOPMENTPOST-DEVELOPMENT

- Total Drainage Area = 11.22 acres- Total Drainage Area = 11.22 acres

- SCS Curve Number = 78 (Hydrologic soil group C)- SCS Curve Number = 78 (Hydrologic soil group C)

- Post-development time of concentration = 5 - Post-development time of concentration = 5

minutesminutes

EXAMPLE STORMWATER WETLANDS EXAMPLE STORMWATER WETLANDS DESIGN DESIGN (CONTINUED)(CONTINUED)

CALCULATION OF 1” STORM RUNOFF VOLUMECALCULATION OF 1” STORM RUNOFF VOLUME-- Precipitation amount = 1”Precipitation amount = 1”- Directly connected impervious area = 5.67 acres (CN = 98)Directly connected impervious area = 5.67 acres (CN = 98)

101000

CN

ityorageCapacUltimateSt

1098

1000ityorageCapacUltimateSt

204.0ityorageCapacUltimateSt

)8.0(

)2.0( 2

SP

SPmeRunoffVolu

)204.0*8.0"1(

)204.0*2.0"1( 2

meRunoffVolu

meRunoffVolu 0.79 inches = 16,279 cubic feet

EXAMPLE STORMWATER WETLANDS EXAMPLE STORMWATER WETLANDS DESIGN DESIGN (CONTINUED)(CONTINUED)

Other non-connected area = 5.55 acres (CN=74)Other non-connected area = 5.55 acres (CN=74)

101000

CN

ityorageCapacUltimateSt

1074

1000ityorageCapacUltimateSt

51.3ityorageCapacUltimateSt

)8.0(

)2.0( 2

SP

SPmeRunoffVolu

)51.3*8.0"1(

)51.3*2.0"1( 2

meRunoffVolu

meRunoffVolu 0.023 inches = 467 cubic feet

EXAMPLE STORMWATER WETLANDS EXAMPLE STORMWATER WETLANDS DESIGN DESIGN (CONTINUED)(CONTINUED)

Total Runoff from 1” storm = 16,279 cubic Total Runoff from 1” storm = 16,279 cubic feet + 467 cubic feetfeet + 467 cubic feet

Total Runoff from 1” storm = 16,746 cubic Total Runoff from 1” storm = 16,746 cubic feetfeet

This volume must be stored on top of the This volume must be stored on top of the permanent pool, and should be drawn permanent pool, and should be drawn down over a period of 2 to 5 daysdown over a period of 2 to 5 days

EXAMPLE STORMWATER WETLANDS EXAMPLE STORMWATER WETLANDS DESIGN DESIGN (CONTINUED)(CONTINUED)

CALCULATION OF WETLANDS SURFACE AREACALCULATION OF WETLANDS SURFACE AREA Total Impervious Area = 5.67 acresTotal Impervious Area = 5.67 acres Total Drainage Area = 11.22 acresTotal Drainage Area = 11.22 acres % Impervious = 51%% Impervious = 51% Assumed average depth = 3.00 feetAssumed average depth = 3.00 feet Using table below, required wetlands surface area = 10,157 Using table below, required wetlands surface area = 10,157

square feet (interpolation required)square feet (interpolation required)

EXAMPLE STORMWATER WETLANDS EXAMPLE STORMWATER WETLANDS DESIGN DESIGN (CONTINUED)(CONTINUED)

PREPARE STORMWATER WETLANDS GRADING PLANPREPARE STORMWATER WETLANDS GRADING PLAN

EXAMPLE STORMWATER WETLANDS EXAMPLE STORMWATER WETLANDS DESIGN DESIGN (CONTINUED)(CONTINUED)

Specify stormwater wetlands vegetation planting planSpecify stormwater wetlands vegetation planting plan

REFERENCESREFERENCES

Doll, Barbara A; Hunt, William F. Doll, Barbara A; Hunt, William F. Urban Waterways: Urban Waterways: Designing Stormwater Wetlands for Small Watersheds.Designing Stormwater Wetlands for Small Watersheds.

North Carolina Cooperative Extension Service.North Carolina Cooperative Extension Service. USEPA.USEPA. EPA Stormwater Technology Fact Sheet: EPA Stormwater Technology Fact Sheet:

Stormwater WetlandsStormwater Wetlands. EPA 832-F-99-025, September 1999.. EPA 832-F-99-025, September 1999. Harris County Texas Stormwater BMP Manual. Harris County Texas Stormwater BMP Manual.

“Constructed Wetlands for Stormwater Treatment”. 2001 “Constructed Wetlands for Stormwater Treatment”. 2001 Edition.Edition.

NCDENR Division of Water Quality. NCDENR Division of Water Quality. Stormwater Best Stormwater Best Management Practices.Management Practices. April 1999 Edition. April 1999 Edition.