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
Prominence and Diversity of Coastal Wetland Habitats
Prominence and Diversity of Coastal Wetland Habitats
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
(From the NCDENR Stormwater BMP Manual)
• 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)
(From the NCDENR Stormwater BMP Manual)
Basic Wetland System Components
(From the NCDENR Stormwater BMP Manual and the NCSU Stormwater Group)
Basic Wetland System Components
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
(From the NCDENR Stormwater BMP Manual)
Recommended Plants
• Deep pool Floating aquatics
Submerged aquatics
• Shallow poolHerbaceous plants
• Upland soil characteristicsHerbaceous plants
Shrubs
• No cattails!
• Only native species
(From the NCDENR Stormwater BMP Manual)
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
(From the NCDENR Stormwater BMP Manual)
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
(From the NCDENR Stormwater BMP Manual)
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
(From the NCDENR Stormwater BMP Manual)
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