50
Design Criteria and Secrets to Success, Part 2 T. Andrew Earles, Ph.D., P.E., D.WRE Wright Water Engineers, Inc.
Design Criteria and Secrets to Success, Part 2
T. Andrew Earles, Ph.D., P.E., D.WRE Wright Water Engineers, Inc.
Filtration Practices
Sand/Media Filters
Sand/Media Filters—Key Design Criteria
» Provide WQv as storage above sand bed. » If space allows, channel protection volume can also be integrated
into filter design. » Calculate Minimum Filter Area, A = 2∙V/9
• A = Minimum filter area (flat surface area) (ft2) • V = Design volume (ft3).
» Provide minimum filter material (sand for most applications) thickness of 18 inches.
» Side slopes of 4H:1V or milder are recommended if side slopes will be vegetated. Use vertical walls where side slopes are steeper than 3H:1V.
Sand/Media Filters—Key Design Criteria » Determine full infiltration, partial
infiltration or no infiltration: – Use no infiltration when proximate to
buildings, “hot spots,” expansive soil and/or bedrock
– Assure seasonally high groundwater is at least several feet below bottom of filter for full and partial infiltration sections.
» Provide energy dissipation at all inlets to filter.
– Typically Type VL or L riprap underlain with geotextile separator fabric.
– Fill void in rock with filter material.
Sand/Media Filters—Key Design Criteria
Profile and Underdrain Section
Sand/Media Filters—Installation » Do not put a filter sock on the underdrain. This is not necessary and can cause
the BMP to clog. » Install cleanouts:
– Cleanouts can be used for inspection (by camera) immediately following construction to ensure that the underdrain pipe was not crushed during construction.
– They can also be used to for ongoing maintenance practices. – Consider locating cleanouts in the side slopes of the basin and above the depth of
ponding. » Provide vegetated side slopes to pre-treat runoff by filtering (straining) to reduce
the frequency of maintenance. » Protect area from excessive sediment loading during construction. » When using an impermeable liner, ensure enough slack in the liner to allow for
backfill, compaction, and settling without tearing the liner.
Sand/Media Filters—Maintenance » Inspect at least twice annually
following precipitation events to assure filter is infiltrating runoff. – Check for and repair minor erosion. – Remove debris and litter from filter
surface and overflow structure. » Every 2 to 5 years (or more
frequently as needed) scarify top 2 inches of sand on the surface.
» After two or three scarification cycles, remove and replace top few inches of sand.
Detention Practices
Wet Detention/Retention Ponds
Wet Ponds—Key Design Criteria » Unless permanent pool is fed by groundwater,
baseflows exceeding losses (infiltration, evapotranspiration) must be present.
» When pond is above groundwater table, liner is recommended.
» Provide WQv as storage above permanent pool.
» Provide permanent pool with volume of at least 1.2∙WQv.
» If space allows, channel protection volume and/or flood control volumes can also be integrated.
» Maximize distance between inlets and outlet. Aim for Length: Width of 2:1 to 3:1.
Wet Ponds—Key Design Criteria » Provide two depth zones for permanent pool:
– Safety wetland bench around perimeter--6 to 12 inches deep with minimum width of 4 feet.
– Open water zone—for sedimentation & nutrient uptake, limit depth to 12 feet to avoid oxygen depletion at bottom.
» Side slopes: – No steeper than 4H:1V above safety bench (pond banks). – Bench should be relative flat (10H:1V or milder Typ.). – 3H:1V below safety bench.
» Provide forebay at inlets to drop out sediment. VForebay≥ 0.03∙WQv or larger if significant sediment loads are anticipated.
» Size outlet to drain WQv in 12 to 24 hours for best treatment.
Wet Ponds—Key Design Criteria » Provide a trash rack. » Provide for overflows for larger events. » Provide drain for pond (if feasible) to aid in
maintenance/inspection. » Provide maintenance access—pond will
require periodic sediment removal: – Keep grades ≤ 10% for haul road surfaces. – Keep grades ≤ 20% for backhoes and skid-
loaders. – Provide solid driving surface (gravel,
concrete, articulated concrete block or concrete grid pavement).
Trash Rack Sizing
Wet Ponds—Installation & Maintenance » Provide pretreatment upstream of permanent pool. » Provide maintenance access to forebay(s) and outlet. » Oversize and deepen permanent pool to deter algae growth. Baseflows are
essential for refreshing pond and controlling algae. » Inspect at least annually—note sediment in forebay and debris/trash at
outlet and remove as needed. » Complete harvesting of vegetation (pulling out roots) generally not
recommended due to resuspension of materials and erosion caused by harvesting. – If cattails are a problem, harvest annually in late summer by cutting off at base
of plant just below waterline.
Wet Ponds—Installation & Maintenance » Mosquito control may be necessary if the BMP
is located in proximity to outdoor amenities. » Remove sediment from the forebay before it
becomes a significant source of pollutants for the remainder of the pond: – Inspect annually and remove as necessary. – Dry forebays likely will require sediment
removal annually, wet forebays, less frequently. » Monitor sediment accumulation in pond bottom
annually (“sludge judge”) and remove when accumulated volume is approximately 20% of WQv. Removal cycle likely 10 years + unless there is a heavy sediment load.
Extended (Dry) Detention Basins (EDBs)
EDB—Key Design Criteria » Provide design volume = 1.2∙WQv. » Maximize distance between inlets and outlet.
Aim for Length: Width of 2:1 or greater. » Side slopes should be 3H:1V or milder. 4H:1V
or flatter is preferable for safe operation of maintenance equipment,
» Dissipate energy and provide forebays at inlet points.
» Size forebays from 1 to 3% of WQv depending on size of watershed, sediment loading and maintenance frequency. Separate forebay from main portion of EDB with berm with outlet.
» Determine forebay release flows.
EDB—Key Design Criteria » Trickle channels—Size for forebay release flow
– Concrete—drain pan or concrete channel with curbs for sides—slope between 0.4 to 1%.
– Soft Bottom—avoid meanders and maintain constant slope. » Micropool and outlet structure
– Locate outlet in embankment with micropool directly in front. – Concrete bottom is preferred. – Size outlet to drain WQv in 24 to 40 hours.
» Provide initial surcharge volume (at least 0.3% of WQv, Typ.) above micropool to contain frequent events.
» Provide trash rack. » Provide overflow for larger events.
A New Concept—Excess Urban Runoff Volume (EURV)
Extended Dry Detention with EURV
Wet Pond with EURV
Full Spectrum Detention
Constructed Wetland Ponds
Constructed Wetland Ponds
Constructed Wetland Ponds—Key Design Criteria » Unless permanent pool is fed by groundwater, baseflows exceeding losses
(infiltration, evapotranspiration) must be present. Conduct hydrologic budget analysis!
» When pond is above groundwater table, liner is recommended. » Provide WQv as storage above permanent pool. » Design for release of WQv over 24 hours. » To maintain healthy vegetation, limit WQv surcharge depth to 2 feet. » Maximize distance between inlets and outlet. Aim for Length: Width of 2:1 to
4:1. » Provide a trash rack.
» Provide permanent pool equivalent to at least 0.75∙WQv.
» Side slopes follow same criteria as wet ponds for bench and banks.
» Provide energy dissipation and forebay at inlets.
» Provide maintenance access.
Constructed Wetland Ponds—Key Design Criteria
Protect newly planted vegetation
Wetlands—Installation & Maintenance » Provide pretreatment upstream of wetland. » Provide maintenance access to forebay(s) and outlet. » Baseflows are essential for refreshing pond and controlling algae. » Inspect at least annually—note sediment in forebay and debris/trash at
outlet and remove as needed. » Complete harvesting of vegetation (pulling out roots) generally not
recommended due to resuspension of materials and erosion caused by harvesting. – If cattails are a problem, harvest annually in late summer by cutting off at base
of plant just below waterline.
Wet Ponds—Installation & Maintenance » Mosquito control may be necessary if the BMP
is located in proximity to outdoor amenities. » Remove sediment from the forebay before it
becomes a significant source of pollutants for the remainder of the pond: – Inspect annually and remove as necessary. – Dry forebays likely will require sediment
removal annually, wet forebays, less frequently. » Monitor sediment accumulation in open water
areas annually (“sludge judge”) and remove when accumulated volume is approximately 20% of WQv. Removal cycle likely 10 years + unless there is a heavy sediment load.
Additional Resources
T. Andrew Earles, Ph.D., P.E., D.WRE Wright Water Engineers, Inc.
Resources for Additional Information
http://www.intrans.iastate.edu/publications/posts/?publication =stormwater-management-manual
Iowa Stormwater
Management Manual
www.bmpdatabase.org
www.udfcd.org
Volume 1—Policy, Planning, Hydrology, Streets, Inlets and Storm Drains, Open Channels Volume 2—Storage, Hydraulic Structures, Culverts, Revegetation Volume 3—Water Quality and Best Management Practices
ADDITIONAL WEB RESOURCES »USEPA Low Impact Development Web Page: http://www.epa.gov/owow/nps/lid/
»UDFCD: http://www.udfcd.org/
»BMP Database: http://www.bmpdatabase.org/
»Low Impact Development Center: http://www.lowimpactdevelopment.org/
»University of New Hampshire Stormwater Center: http://www.unh.edu/erg/cstev/
»National LID Clearinghouse: http://www.lid-stormwater.net/clearinghouse/index.html
»North Carolina State University/North Carolina Cooperative Extension Stormwater Engineering Group: http://www.bae.ncsu.edu/stormwater/
»Prince George’s County, Maryland: http://www.goprincegeorgescounty.com/government/agencyindex/der/lid/bioretention.asp
Questions?
No one likes “ugly” stormwater management
facilities!
Andrew Earles Wright Water Engineers, Inc.
Ph: (303) 480-1700 Email: [email protected]
Bill Wenk
Wenk Associates Ph: (303) 628-0003
Email: [email protected]
Jim Sipes Sand County Studios Ph: (206) 755-6074
Email: [email protected]
