Pond and Wetland BMPs, Retrofitting Detention Basins

Report
Pond and Wetland BMPs,
Retrofitting Detention Basins
WET PONDS
Wet Pond Treatment Processes
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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
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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…
Soluble Nutrients….
Lead………………..
Zinc…………………
50-90%
30-90%
40-80%
70-80%
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
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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 volume
water quality
• Do Not Provide
aesthetics
wildlife habitat
Retrofitting Detention Basins Benefits
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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

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