Natural and Engineered Wetlands for Stormwater Management · General Stormwater Wetland Design Criteria in GA • Minimum drainage area = 25 acres • Minimum dry weather flow path

Natural and Engineered Wetlands

for Stormwater Management

Dan Hitchcock, Ph.D., P. E.

Baruch Institute of Coastal Ecology and Forest Science

Clemson University

Georgetown, SC

Prominence and Diversity of Coastal Wetland Habitats

Clemson University

Baruch Institute



Wetland Biogeochemistry

• Highly productive ecosystems

• Partially versus fully inundated

systems

• Aerobic vs. anaerobic conditions

• Nutrient cycling – redox chemistry

• Carbon sequestration

• Water quality benefits

HYDROLOGY

VEGETATION SOILS

WETLAND SYSTEM DYNAMICS

Landscape parameters, their interactive complex processes & ecosystem services = GREEN INFRASTRUCTURE(Modified from Ge Sun, Southern Global Climate Program, USDA Forest Service)

HYDROLOGY

VEGETATION SOILS

PLANT SUCCESSIONPRODUCTIVITY

WATER/NUTRIENT UPTAKE

ORGANIC LOADINGTEMPERATURE REGULATION

SOIL STABILIZATION

SERVICES:FLOOD CONTROL

STORM PROTECTIONAIR/SOIL/WATER QUALITY

NATURAL RESOURCE PRODUCTSCLIMATE REGULATIONHABITAT/CORRIDORSNUTRIENT CYCLINGCARBON STORAGE

BIODIVERSITYRECREATIONAESTHETICS

HEALTH

CommercialConservationPublic/Semi-publicPrivate Recreational

Public RecreationalHigh Density ResidentialLow Density ResidentialMedium Density Residential

Zoning for Future Land Uses?

Map courtesy of

Georgetown County, SC

Waccamaw Neck,

Coastal Georgetown County, SC

Land Use Map

Bannockburn Plantation

Surface and Groundwater Interaction(from Buol,

1978)

Weather StationPiezometerSample well / rain gage / soil moisture sensorStream monitoringPond monitoring From Dr. Tom Williams, Clemson

Upper Debidue Creek (UDC)

PIEZOMETERPIEZOMETER

MET STATION

STREAM STATION

2008 2009 2010

Upper Debidue Creek – Rainfall and Stage

2009: P = 1031 mm2010: P = 1247 mm

Seasonal Runoff Variability

Epps, T. H., D. R. Hitchcock, A. D. Jayakaran, D. R. Loflin, T. M. Williams, and D. M. Amatya, 2013. Characterization of Storm Flow Dynamics of Headwater Streams in Lower Coastal Plain South Carolina. Journal of American Water Resources Association 49:76-89.

UDC

WS80

What drives the water table??

Wetland Water Budget

Modified from

Kadlec and Knight, 1996

EVAPOTRANSPIRATION

PRECIPITATIONINFLOW

BANK RUNOFF INFLOW

OUTFLOW

GROUNDWATER

Upper Debidue Creek – Stand Inventory

Poorly-drained sandy loam

Somewhat poorly drained clayWell-drained sandy clay clay loam

Poorly-drained sandy clay loam

Well-drained sandy loamVery poorly-drained sand

Excessively well-drained sand

Poorly-drained sand

Private protected lands

Major roadsParcelsOpen water

State protected lands

N

5 mi.

Online Community Resource Inventory:www.cri-sc.org

The Systems Approach

HYDROLOGY

VEGETATION SOILS

IMPERVIOUS SURFACE

FOREST AND WETLAND COVER

Water Quantity and Quality

QUANTITY:

• Flood control

• Conveyance

• Storage

• Salt water intrusion

• Sea level rise / climate

change

QUALITY:• Harmful bacteria and viruses• Nuisance algal blooms• Low dissolved oxygen (DO)• Fish kills• Sediment and chemicals• Invasive species• Downstream ecological health

Wetlands for Stormwater Management!!!

• Total N = 28-39%

• Total P = 42-53%

• Metals = 14-72%

• Copper = 29-50%

• Zinc = 32-52%

• Lead = 62-76%

• Hydrocarbons = 80%

• Pathogens = 58-78%

• Total Suspended Solids = 66-78%

Stormwater Wetland Pollutant Removal

(SCDHEC BMP Manual and based on data from the

Center for Watershed Protection)

• Total Suspended Solids = 85%

• Total N = 40%

• Total P = 40%

Pollutant Removal

(for NC Regulatory Credits)

(From the NCDENR Stormwater BMP Manual)

General Stormwater Wetland

Design Criteria in GA

• Minimum drainage area = 25 acres

• Minimum dry weather flow path of 2:1 (L:W)

• Minimum of 35% area has depth of <= 6 inches

• Minimum of 10-20% area has deep pool (1.5 -6 ft)

• All wetland designs should include a forebay

(From the Georgia Stormwater Management Manual)

Basic H&H Considerations

• Base flow – critical for plant survival

• Stormwater flow prediction from drainage area

• Working with given topography

• Retrofitting stormwater infrastructure

• Consider groundwater!

Engineered Wetlands in CoastalLandscapes…

…the right practice for the right place?

Wetland Design – A Six-Step Process

1. Understand basic layout concepts

2. Determine what volume of water to treat

3. Determine surface area and depth of each wetland

zone

4. Select the soil media type

5. Select the appropriate outlet structure

6. Select plants


• Inlet – swale, pipe, diverter, sheet flow, must be stabilized

• Deep pool – forebay (required) + non-forebay pools (PPE)

• Shallow water – depth = 3-6”

• Shallow land – only wet after a rain event, top of land is top of the temporary pool elevation

• Upland – natural amenity with access for maintenance

• Outlet – weir overflow + drawdown + access + stabilized

Basic Wetland System Components




(From the NCDENR Stormwater BMP Manual and the NCSU Stormwater Group)


Major Design Elements - Sizing

• Wetland sized to treat the first flush (1”-1.5”) of design

storm

• Minimum treatment volume shall be 3,630 ft3

• Minimum length-width ratio of 1.5:1, but 3:1 is

preferred

• Max shallow land depth (TPE) shall be 1 ft.

• Vegetated side slopes no steeper than 3:1

• Drawdown must occur in 2-5 days


Recommended Plants

• Deep pool Floating aquatics

Submerged aquatics

• Shallow poolHerbaceous plants

• Upland soil characteristicsHerbaceous plants

Shrubs

• No cattails!

• Only native species


Plant Selection and Planting Strategies

• Pondscaping – have a plan with planting zones

• Minimum of 10 different species with at least 5 of these as

emergent plants

• Shallow water area: minimum of 50 herbaceous plants of 4

cubic inch

container per 200 ft2, planted max. of 2 ft on center

OR 8 shrubs in 1 gal. container, max. 5 ft on center

OR 1 tree at least 3 gal. container and 40 grass-like plants of

4 cubic inch


Plant Selection and Planting Strategies

• Shallow land area: minimum of 50 herbaceous plants of 4

cubic inch container per 200 ft2, planted minimum of 2 ft on

center

• Centipede grass as buffer

• Specific supplementary plantings

for any losses

• Water plants frequently until

established

• At least 4 in. of topsoil must be added to liner for plant growth


Maintenance Requirements

• Have a clear and specific approved

maintenance agreement

• Monitor sediment (especially in

forebay) and selectively remove at

specified depth of accumulation(s)

• Inspect for erosion and repair and

stabilize as needed

• Inspect inlets and outlets for debris and

clean as needed

• Replace wetland vegetation to maintain

>= 70%, 90-95% desired

• Remove invasive vegetation


37

BMPs in Series or

“Treatment Train”

• Some BMPs are better than others

for certain pollutants

• Consider the connectivity of BMPs

• Front-end sediment collection;

i.e., forebays or traps

• Vegetated BMPs can be overloaded

so should be toward the end of train

Rain Garden/Bioretention

Buffer

Pond

(w/ Forebay)

Wetland

Rainwater

Harvesting

Receiving

Waters

For more info, please contact:

Dr. Dan Hitchcock, P.E.

[email protected]

??? Questions ???

Natural and Engineered Wetlands for Stormwater Management · General Stormwater Wetland Design Criteria in GA • Minimum drainage area = 25 acres • Minimum dry weather flow path

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