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Copyright 2000, CWP Design of Stormwater Ponds and Wetlands Center for Watershed Protection.

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Presentation on theme: "Copyright 2000, CWP Design of Stormwater Ponds and Wetlands Center for Watershed Protection."— Presentation transcript:

1 Copyright 2000, CWP Design of Stormwater Ponds and Wetlands Center for Watershed Protection

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6 Features of the Standard Pond System Adequate Water Quality Treatment Volume Multiple Treatment Pathways Minimum Pond Geometry Sediment Forebay Non-clogging Low Flow Orifice Riser in Embankment Adjustable Gate Valve Pond Drain Principal Spillway Emergency Spillway

7 Copyright 2000, CWP Features of the Standard Pond System (cont.) Embankment Specifications Inlet Protection Adequate Outfall Protection Pond Benches Safety Features Pondscaping Plan Wetland Elements Pond Buffers and Setbacks Maintenance Measures Maintenance Access

8 Copyright 2000, CWP #1 Adequate Water Quality Treatment Volume Provide water quality treatment storage for the 90 th percentile storm depth per impervious acre For extended detention (ED) ponds, permanent pool should be equal to at least one half the water quality treatment volume Forebay should be sized to provide volume equal to at least 0.1 inch per contributing impervious acre for pretreatment Water quality storage can be provided in multiple cells Provide water quality treatment off-line when topography, head and space permit

9 Estimated Sediment Deposition Rate Copyright 2000, CWP

10 Pollutant Removal Performance of Stormwater Ponds and Wetlands: By Pond Type Copyright 2000, CWP

11 Irreducible Concentrations in Wastewater Wetlands and Urban Best Management Practices Water quality parameter (mg/l) Wastewater (Kadlec and Knight, 1996) Wastewater (Reed, 1995) Stormwater BMPs (CWP, 2000)

12 Copyright 2000, CWP #2 Multiple Treatment Pathways Provide multiple or redundant treatment Use multiple cells, longer flowpaths, high surface area to volume ratios, and complex microtopography

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15 #3 Minimum Pond Geometry Ponds should be wedge-shaped, narrowest at the inlet, and widest at the outlet Minimum length to width ratio should be 1.5:1 (i.e., length equal to 1.5 x width). Greater flowpaths are recommended Maximum depth of the permanent pool should not exceed eight feet (average of 4 to 6 feet)

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19 #4 Sediment Forebay - I Each pond shall have a sediment forebay, consisting of a separate cell Size forebay to contain 0.1 inches per impervious acre. Maintain forebay at 3 to 6 feet deep Exit velocities from the forebay shall not be erosive

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21 #4 Sediment Forebay - II Direct maintenance access by heavy equipment should be provided to the forebay Harden forebay bottom to make sediment removal easier Provide a vertical sediment depth marker in the forebay, to measure sediment deposition over time

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23 #5 Non-clogging Low Flow Orifice Provide a submerged, reverse-slope pipe, extending downward from the riser to a release point one foot below the normal pool elevation Low flow orifice shall have a minimum internal diameter of 3 inches Broad crested weirs, protected by a half-round CMP, extending at least 18 inches below the normal pool can also be used

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28 #6 Riser in Embankment Locate riser within the embankment for purpose of maintenance, access, safety and aesthetics Access to the riser by manholes with lockable nuts The riser can be “fenced” with pipe or rebar at 6-8 inch intervals for safety purposes and to prevent large trash and debris from entering the riser

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31 #7 Adjustable Gate Valve Both the ED pipe and pond drain pipe should be equipped with an adjustable gate valve Size one pipe schedule higher than the calculated design diameter Locate valves inside of the riser, where they will remain dry and can be operated safely and conveniently Lock the handwheel with a chain to a ringbolt, manhole step, or other fixed object

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34 #8 Pond Drain Each pond shall have a ductile iron drain pipe to completely or partially drain the pond Drain pipe should have an inverted elbow within the pond to prevent sediment clogging The diameter of the pipe shall be sufficient to drain the pond within 24 hours Exercise care during pond drawdowns to prevent downstream discharge of sediments or anoxic water

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36 #9 Principal Spillway - I Principal spillways shall be designed in accordance with local embankment specifications. NRCS Pond Specifications Code are often available locally and serve as good guidance Provide capacity to accommodate the design storm with adequate height to the crest of the emergency spillway Crest elevation of the principal spillway shall be no less than one foot below the emergency spillway crest

37 Copyright 2000, CWP #9 Principal Spillway - II Design riser to go from weir flow control to barrel flow control without going into orifice control Reinforced concrete pipe and cast-in-place reinforced concrete box culverts are recommended to increase longevity Equip principal spillway with a removable trash rack

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39 #10 Emergency Spillway Provide emergency spillway for large flood flows. Design in accordance with NRCS Pond Code, National Engineering Handbook, or local equivalent Earthen spillways shall be trapezoidal and located in undisturbed earth (i.e., not in fill) Side-slopes should be no greater than 2:1 (h:v) Provide an inlet channel, level section, and an exit channel (8 feet minimum width with non- erosive velocities through the control section and exit channel)

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42 #11 Embankment Specifications - I Design and construct in a manner to prevent dam breach or seepage (exact criteria depend on local requirements or NEH dam safety criteria) Minimum top width of 10 feet (or 16’ to 26’ if it is a roadway embankment) Increase dam height to account for settlement (often 5 to 10 %) Provide cutoff trench and impervious core located along centerline of dam

43 Copyright 2000, CWP #11 Embankment Specifications - II Combined upstream and downstream side slopes of embankment shall not be steeper than 5:1 (h:v) with neither slope exceeding 2:1 Provide freeboard, depending on dam classification (usually 1 to 2 feet) Provide anti-seep collars or seepage diaphragms for all conduit pipes through the embankment greater than six inches in diameter

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45 #12 Inlet Protection Inlets to ponds should be adequately protected with rip-rap to avoid erosion and scouring. Flared end section pipes help reduce the erosion potential Inlet pipes to the pond can be partially submerged in warmer climates Provide a forebay at each inlet (unless it provides less than 10% of the total design storm inflow rate to the pond)

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49 #13 Adequate Outfall Protection - I Provide flared end pipe sections that discharge at or near the stream invert Minimize tree clearing along the downstream channel, and reestablish a forested riparian zone in the shortest possible distance

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51 #13 Adequate Outfall Protection - II Provide a pilot channel underdrain pipe (located 2 to 3 feet below the rip-rap) to prevent excessive warming of dry weather flows. Protect the channel with shade trees Modify the channel immediately below the pond outfall to conform to natural dimensions, stabilize with large rip-rap place over filter cloth Provide a stilling basin to reduce flow velocities from the principal spillway, when necessary

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53 #14 Pond Benches Provide two benches for all deep pool areas (four feet or greater in depth) Safety Bench- extends feet outward from the shoreline to the top of the pond sideslope (5% maximum cross-slope) Aquatic Bench- extends 15 feet, on average, inward from normal shoreline, maximum depth of 18 inches

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57 #15 Safety Features - I Fencing of ponds is not generally desirable, but may be required by local regulations Safety can be provided by managing contours of ponds to eliminate drop-offs and other hazards Sideslopes to the pond shall not exceed 3:1 (h:v), and shall terminate on a safety bench

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61 #15 Safety Features - II Landscape both the safety bench and the aquatic bench to prevent access Limit the principle spillway opening to prevent access by small children Provide warning signs prohibiting swimming and skating

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64 #16 Pondscaping Plan Prepare a pondscaping plan that indicates how aquatic and terrestrial areas will be stabilized with vegetation Consult local guidance documents or NRCS extension office for appropriate plant lists Depending on the pond or wetland design, there may be as many as 6 pondscaping zones

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71 #17 Wetland Elements Encourage wetland plants whenever possible in the design (along the aquatic bench, the safety bench and sideslopes, or within shallow areas of the pool itself) Establish wetland plants, either through transplantation or volunteer colonization, within six inches (plus or minus) of the normal pool

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74 Pollutant Removal Pathways within Stormwater Wetlands Sedimentation Adsorption to sediments/vegetation/detritus Physical filtration of runoff Microbial uptake/transformation Uptake by wetland plants Uptake by algae Extra detention and/or retention

75 Copyright 2000, CWP Creating Effective and Diverse Stormwater Wetlands Design to Maintain a Constant Pool Elevation and Baseflow—Maintain Water Balance Need Ability to Regulate Water Levels for Planting and Maintenance Forebay to Trap Sediments, Dissipate Velocity and Diffuse Flows

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77 Creating Effective and Diverse Stormwater Wetlands Provide Microtopography within Wetland Minimize Open Grass Areas (Geese Loafing) Deeper Water Areas for Greater Retention, Mosquito Control and Outlet Protection

78 Copyright 2000, CWP Complex Wetland Microtopography

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82 Pondscaping Zones 1.Deepwater-1.5 to -6.0 feet 2.Shallow Marsh-1.5 to -0.0 feet 3.Shoreline Fringe0.0 to 1.0 feet 4.Riparian Fringe1.0 to 3.0 feet 5.Floodplain Terrace 3 to 6 feet 6.Upland Areas 6 feet +

83 Pondscaping Zones in a Stormwater Wetland (Plan view) Copyright 2000, CWP

84 Preparing the Wetland Bed - Seven Steps Prepare grading plan Grade to interim elevations Add topsoil and/or mulch amendments Grade to final elevations (provide microtopography) Allow wetland to fill for a few months to verify planting depths Measure and stake planting depths De-water wetland prior to planting period

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87 Creating Effective and Diverse Stormwater Wetlands Select 2-3 Dense Growing, Aggressive Species, add 5 other Wetland Species to Promote Diversity (Avoid Cattails, Phragmites, and Loose Strife) Where Possible, Utilize Wetland Mulch/Seedbank to Promote Diversity

88 Copyright 2000, CWP Seedbanks for Wetland Establishment Wetland mulch - upper six inches of wetland soils Up to 20 species can emerge from seedbank (uncertainty about ultimate community) Donor sites are highly restricted due to 404 regulations Spread mulch 3-6 inches deep in hi marsh

89 Copyright 2000, CWP Creating Effective and Diverse Stormwater Wetlands When Planting Wetland Nursery Stock: Plant in Single Species Clumps, 18” On Center Initial Plantings Should be at least 50% of Wetland Surface Area Follow-up after First Growing Season with Reinforcement Planting Transplanting Window and Post-Nursery Care is Very Important

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94 #18 Pond Buffers and Setbacks Provide buffer that extends 25 feet outward from the maximum water surface elevation of the pond Provide an additional 15 foot setback to permanent structures Preserve trees in the buffer area during construction Plant trees, shrubs, and native groundcovers in the buffer Mow only maintained rights-of-way and the embankment within the buffer Manage remaining buffer as a meadow or forest

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97 #19 Maintenance Measures Vest maintenance responsibility for the pond and buffer with a responsible authority using a legally binding and enforceable maintenance agreement Inspect pond annually in wet-weather conditions Remove sediment in the forebay every 5 to 7 years, or after one foot of sediment deposition has been recorded in the forebay Provide a suitable on-site sediment disposal area

98 Copyright 2000, CWP #20 Maintenance Access Provide a 25 foot wide maintenance right-of-way easement, extending to the pond from a public or private road Maintain a maximum slope of less than or equal to 15%, stabilized to withstand heavy equipment Extend access to the forebay, safety bench and riser. Design to allow vehicles to turn around Provide lockable manhole covers and manhole steps

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100 Miscellaneous Considerations Adjust design to minimize stream impacts caused by STPs (e.g., temperature increases, bank erosion, fish barriers) Re-establish natural stream conditions below the practice in as short a distance as possible Comply with applicable federal/state/local permit requirements


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