Pond and Wetland BMPs, Retrofitting Detention Basins

WET PONDS

Wet Pond Treatment Processes

• Settling

• Plant Uptake and Adsorption

• Infiltration

• Evaporation

Wet Ponds - Criteria

• Provide Quantity and Quality Control(not volume control)

• Drainage Area: Minimum of 10 Acres (>25 Acres Preferred)

• Treat Water Quality Volume• Retrofit Opportunities• Maintenance

sediment removal

Inlet & outlet

vegetation

Wet Ponds – Primary Design Parameters

• Storage VolumeChannel Protection & Flood Control

• Detention Time (24-48 Hours)Water Quality

• Release RateChannel Protection & Flood Control

Wet Pond – Other Design Criteria

• Treat Up to 75 Acres

• Include Forebay

• Depth: 4-6 Feet

• Geometry Affects Performancedead storage

short circuiting

Extended Detention Pond

Wet Pond

Multiple Pond System

Pocket Pond

Wet Pond – Hydraulic Design

• Design Pond Using Acceptable Methodology– NRCS…TR55…TR20– Penn State Runoff Model (PSRM)– Modified Rational Method– Other

• Hydrograph Routing Is Necessary

Wet Pond – Area Ratio

Area Ratio = Drainage Area

Pond Area

Should Be Less Than 100

For Maximum Efficiency

Wet Ponds – Pollutant Removal

Sediment………….. 50-90%

Total Phosphorus… 30-90%

Soluble Nutrients….40-80%

Lead……………….. 70-80%

Zinc…………………40-50%

Wet Pond Criteria

1. Minimum Contributory Area: 10 Acres2. Pretreatment: Sediment Forebay

Sized to Contain 0.1 inches per impervious area

3. Design to Treat Water Quality Volume4. Multiple Treatment Pathways Increase

Performance…Multiple Cells…Pond-Wetland Train

Wet Pond Criteria

5. Pond Benches

A. Safety Bench – 6%Maximum Slope

B. Aquatic Bench – Maximum Depth 18” Below Normal Pool

6. Provide for Maintenance Access

Types of Wetlands

• Shallow Wetland

• Extended Detention Wetland

• Pond-Wetland System

• Pocket Wetland

Component Design

Outlet Structure

Wetland Treatment Criteria

1. Surface Area: At least 1% of total Drainage Area

2. 25% of WQv Should Be In Deep Water Zones (4’)

3. Minimum of 35% of Area: Depth of 6” or less

4. At least 65% of Area: Shallower than 18”

Existing Conditions – (Pre-Construction)

Proposed Conditions (Post-construction)

Proposed Conditions (Post-construction)

Retrofitting Detention Basins

Conventional Detention Basins

• Designed For Flood Control & Channel Protection

large storm events (50 year…100 year)

• Do Not Control: runoff volumewater quality

• Do Not Provideaestheticswildlife habitat

Retrofitting Detention Basins - Benefits

• Pollutant Removal

• Water Quality Improvement

• Wildlife Habitat Creation

• Aesthetic Improvement

• Reduced Maintenance

Retrofitting Detention Basins - Modifications

• Modify Outfall Structurecreate two-stage release

treat small storms for water quality

• Eliminate Paved Low-Flow Channelsuse meandering vegetated swales

increase detention time for small storms

Retrofitting Detention Basins - Modificatons

• Use Stilling and Settling Basins at Inlets & Outlets

remove sediment & attached pollutantsplant with wetland plants

• Regrade to Provide Micro-Habitats

• Create Wetland - Pond Areas

Extended Detention Basin Conversion

Wet Pond Conversion

Proposed Conditions – Post Construction Construction