STORMWATER BEST MANAGEMENT PRACTICE BMP

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STORMWATER BEST MANAGEMENT PRACTICE (BMP)

• CONSTRUCTED STORMWATER WETLANDS FOR STORMWATER
  TREATMENT

JEREMY FINCH
RHETT BUTLER
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OVERVIEW OF PRESENTATION

• Definition Functions
• Characteristics of Stormwater Wetlands
• Natural Wetlands vs. Stormwater Wetlands
• Advantages Disadvantages
• Design of Stormwater Wetlands
• Practical Design Example

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DEFINITIONS FUNCTIONS OF STORMWATER WETLANDS

• Definition
• - Wetland Systems that are specifically
  designed to treat/manage
• the impact of development/urbanization on
  stormwater quality
• and quantity
• Functions
• - Improve Water Quality
• (i.e. pollutant removal)
• - Minimize flooding/flood control
• - Establish/create wildlife habitat

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CHARACTERISTICS OF ALL STORMWATER WETLANDS

• Inundated with surface or groundwater for most of
  the year
• Contains vegetation that is adapted to saturated
  conditions
• Primary source of water is runoff from urban
  areas
• Contains forebays, deep pools, shallow pools,
  outlet control structures
• High pollutant removal efficiency

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POLLUTANT REMOVAL IN STORMWATER WETLANDS
Ref Bill Hunt
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POLLUTANT REMOVAL EFFICIENCY
Ref US EPA
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NATURAL VS. STORMWATER WETLANDS

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

Ref NCDENR
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ADVANTAGES DISADVANTAGES

• ADVANTAGES
• - Removes multiple pollutants from stormwater
  runoff
• - Improves overall water quality
• - If designed constructed properly, can be
  aesthetically pleasing
• - Reduce flooding potential in downstream areas
  due to
• development upstream
• DISADVANTAGES
• - Can occupy large areas of developable land
• - Can dry out and become nuisance if drainage
  area is too small
• - Mosquito/Snake habitat
• - May be difficult to establish native wetland
  plants

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DESIGN OF STORMWATER WETLANDS

• Optimal Location
• - Where water availability is
• high
• - Flat topography
• - Areas where seasonal high
• water table is approx. 6
• above bottom of wetland
• - Areas where soil studies
• show that the underlying
• soils have an low infiltration
• rate to maintain a
• permanent 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
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DESIGN OF STORMWATER WETLANDS (continued)

• DESIGN REQUIREMENTS
• - Must have a permanent pool
• of water (for stormwater
• wetlands, we use 3 feet)
• - Must meet required surface
• area to drainage area ratio

Schematic Ref Bill Hunt
Ref NCDENR
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DESIGN OF STORMWATER WETLANDS (CONTINUED)

• - Must detain volume of runoff resulting from
  the 1 storm (i.e.
• first flush runoff) above the permanent
  pool. This volume is
• calculated using the SCS Curve Number method
  shown below

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

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FOREBAY

• Placed where runoff enters the wetland (i.e.
  storm drainage)
• Serves as a preliminary screening device to
  prevent degradation of primary wetland functions
  ? traps sediments, larges pieces of debris, etc.
• Deepest part of wetland ? usually 2-2.5 deep
• Must allow access to forebay for maintenance
  purposes (sediment cleanout, etc.)
• Designed to occupy approximately 10 of total
  wetland area

Forebay Ref Bill Hunt
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SHALLOW POOLS

• Typically 0-9 deep
• Area where primary wetland functions occur (i.e.
  denitrification, sedimentation, filtration,
  adsorption, etc.)
• Water velocity decreases dramatically here, thus
  causing pollutants to settle out of stormwater
• Area where majority of wetland vegetation grows
• Designed to be a long, winding flowpath occupying
  approximately 35 of total wetland area
• During low flow conditions, this is path the
  water takes through the wetland

Shallow Pool Ref Bill Hunt
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DEEP POOLS

• Typically 9-18 deep
• Designed to occupy approximately 35 of total
  wetland area
• Contains water in times of drought
• Where animal habitats (i.e. fish) are located
• Area with least amount of vegetation

Deep Pool Ref Bill Hunt
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SHALLOW LAND

• Designed to be dry except during storm events
• 0-12 above normal pool
• Supports a wide variety of vegetation
• Designed to occupy approximately 30 of the total
  wetland area
• Supports various types of wildlife

Shallow Land Ref Bill Hunt
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STORMWATER WETLAND OUTLET STRUCTURE

• Designed to store the 1 storm runoff volume
• Contains siphon/drawdown device designed to
  drawdown the 1 storm volume over 2 to 5 days
• Passes higher flows (i.e. 100-year storm) during
  extreme storm events safely through the wetland
• Typically riser-barrels or weir overflow
  structures

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STORMWATER WETLAND VEGETATION GUIDELINES

• Avoid non-native species or aggressive plants
• Select plants that can adapt in saturated
• conditions and withstand long term inundation
• Include species that are evergreen (meaning not
  dormant during winter time)
• Can use donor soils (if necessary) from natural
  wetlands to establish vegetation

BLACK WILLOW
RICE CUT GRASS
SMARTWEED
Ref US EPA Bill Hunt
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MAINTENANCE OF STORMWATER WETLANDS

• Stormwater wetlands should be inspected after
  each rainfall event
• Check for clogging of the outlet structure, or
  too rapid of a drawdown
• Erosion/scour of wetland embankments
• Erosion in areas where the stormwater runoff
  enters the wetland (i.e. storm drainage pipes,
  etc.)
• Sediment accumulation in the forebay
• Condition of outlet structure
• Prevention of woody vegetation on the wetland
  berm
• Any additional fertilizing/seeding to maintain
  healthy vegetation

Ref NCDENR
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QUESTIONS?
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PRACTICAL STORMWATER WETLANDS DESIGN EXAMPLE

• Development Requirements
• Example Project Residential development
  consisting of mult-family townhomes, associated
  streets, parking, and sidewalks
• Detention of 2- 10-year post-development peak
  flowrates back to pre-development levels
• Reduction of nitrogen loading rate down to at
  least 6 lbs/ac/yr (must then buydown to 3.6
  lbs/ac/yr)
• Removal of 85 of the total suspended solids from
  the stormwater runoff resulting from the
  development
• Safe passage of the 100-year storm event through
  the wetland

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EXAMPLE STORMWATER WETLANDS DESIGN (CONTINUED)

• STEP 1
• - Determine the stormwater wetlands drainage area

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EXAMPLE STORMWATER WETLANDS DESIGN (CONTINUED)

• Watershed Characteristics
• PRE-DEVELOPMENT
• - Total Drainage Area 12.27 acres
• - SCS Curve Number 72 (Hydrologic soil
• group C)
• - Pre-development time of concentration
• 10.5 minutes
• - 2-Year pre-development peak flowrate 20 cfs
• - 10-Year pre-development peak flowrate 39 cfs
• POST-DEVELOPMENT
• - Total Drainage Area 11.22 acres
• - SCS Curve Number 78 (Hydrologic soil group
  C)
• - Post-development time of concentration 5
• minutes

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EXAMPLE STORMWATER WETLANDS DESIGN (CONTINUED)

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

0.79 inches 16,279 cubic feet
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EXAMPLE STORMWATER WETLANDS DESIGN (CONTINUED)

• Other non-connected area 5.55 acres (CN74)

0.023 inches 467 cubic feet
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EXAMPLE STORMWATER WETLANDS DESIGN (CONTINUED)

• Total Runoff from 1 storm 16,279 cubic feet
  467 cubic feet
• Total Runoff from 1 storm 16,746 cubic feet
• This volume must be stored on top of the
  permanent pool, and should be drawn down over a
  period of 2 to 5 days

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EXAMPLE STORMWATER WETLANDS DESIGN (CONTINUED)

• CALCULATION OF WETLANDS SURFACE AREA
• Total Impervious Area 5.67 acres
• Total Drainage Area 11.22 acres
• Impervious 51
• Assumed average depth 3.00 feet
• Using table below, required wetlands surface area
  10,157 square feet (interpolation required)

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EXAMPLE STORMWATER WETLANDS DESIGN (CONTINUED)

• PREPARE STORMWATER WETLANDS GRADING PLAN

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EXAMPLE STORMWATER WETLANDS DESIGN (CONTINUED)

• Specify stormwater wetlands vegetation planting
  plan

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REFERENCES

• Doll, Barbara A Hunt, William F. Urban
  Waterways Designing Stormwater Wetlands for
  Small Watersheds.
• North Carolina Cooperative Extension Service.
• USEPA. EPA Stormwater Technology Fact Sheet
  Stormwater Wetlands. EPA 832-F-99-025, September
  1999.
• Harris County Texas Stormwater BMP Manual.
  Constructed Wetlands for Stormwater Treatment.
  2001 Edition.
• NCDENR Division of Water Quality. Stormwater
  Best Management Practices. April 1999 Edition.