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 Volume Channel Protection & Flood Control Detention Time (24-48 Hours) Water Quality Release Rate Channel Protection & Flood Control

Wet Pond – Other Design Criteria Treat Up to 75 Acres Include Forebay Depth: 4-6 Feet Geometry Affects Performance dead 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………… % Total Phosphorus…30-90% Soluble Nutrients….40-80% Lead……………… % Zinc…………………40-50%

Wet Pond Criteria 1.Minimum Contributory Area: 10 Acres 2.Pretreatment: Sediment Forebay Sized to Contain 0.1 inches per impervious area 3.Design to Treat Water Quality Volume 4.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 WQ v 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)

Retrofitting Detention Basins

Conventional Detention Basins Designed For Flood Control & Channel Protection large storm events (50 year…100 year) Do Not Control: runoff volume water quality Do Not Provide aesthetics wildlife habitat

Retrofitting Detention Basins - Benefits Pollutant Removal Water Quality Improvement Wildlife Habitat Creation Aesthetic Improvement Reduced Maintenance

Retrofitting Detention Basins - Modifications Modify Outfall Structure create two-stage release treat small storms for water quality Eliminate Paved Low-Flow Channels use meandering vegetated swales increase detention time for small storms

Retrofitting Detention Basins - Modificatons Use Stilling and Settling Basins at Inlets & Outlets remove sediment & attached pollutants plant with wetland plants Regrade to Provide Micro-Habitats Create Wetland - Pond Areas

Extended Detention Basin Conversion

Wet Pond Conversion

Proposed Conditions – Post Construction Construction