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Copyright 2000, CWP
Design of Infiltration Practices
Center for Watershed Protection
Copyright 2000, CWP
Infiltration Systems • Primarily a water quality BMP• Helps meet groundwater recharge• Moderate to high pollutant removal
– assumed, but seldom measured– not good for soluble pollutants (nitrate & chloride)– possible risks of groundwater contamination
• Best used in conjunction with other treatment practices
• Longevity is less than 5 years without multiple pretreatment practices
• Cannot be used if contributing drainage is a hotspot
Copyright 2000, CWP
Infiltration Systems Feasibility Criteria
• Soil infiltration rate, fc, > 0.5 inches/hour (requires geotechnical tests)
• Soil clay content < 20% and silt/clay content < 40%
• Cannot be located on slopes > 15% or within fill soils
• Runoff from hotspots must be pretreated before infiltration
• May be prohibited within karst regions
• Bottom should be > 4 feet above high water table or bedrock layer (requires soil borings)
• 100 feet horizontal setback from water supply well
• Maximum contributing area < 5 acres
• Horizontal Setback 25 feet down-gradient from structures
Copyright 2000, CWP
Infiltration SystemsConveyance Criteria
• Conveyance system to and from facility– Design to deliver and pass excess water at non-
erosive velocities– Off-line design should be used if flow is delivered
by storm drain pipe or along main conveyance system
Copyright 2000, CWP
Infiltration SystemsPretreatment Criteria
• > 25% of WQv must be pretreated before entry (> 50% if soil infiltration rate is > 2 inches/hour) using the following (or equivalent):– sedimentation basin– stilling basin– sump pit
• Use redundant pretreatment methods– grass channel– grass filter strip– bottom sand layer– upper sand layer– washed bank run gravel as aggregate
Copyright 2000, CWP
Infiltration SystemsPretreatment Criteria Cont’d
• Camp-Hazen equation may also be used to calculate pretreatment requirements
– Pretreatment area based on WQV
– Camp-Hazen equation: As = -(Qo/W)*Ln(1-E), where As is surface area of sedimentation basin
– As = 0.066 (WQV) ft2 for impervious cover 75%
– As = 0.0081 (WQV) ft2 for impervious cover > 75%
• Exit velocities from pretreatment shall be non-erosive
for 2-yr design storm
Copyright 2000, CWP
Infiltration SystemsTreatment Criteria
• Design to exfiltrate the WQv less pretreatment volume
• Design storage reservoir to de-water WQv within 48 hours after storm
• Assume a porosity value (i.e., volume of voids/total volume) of 0.4 used for stone reservoirs
• Downstream detention often needed for Cpv and Qp
Copyright 2000, CWP
Infiltration SystemsLandscaping Criteria
• Dense vegetative cover should be established over the contributing pervious drainage areas
• Infiltration systems should not be constructed until all of the contributing drainage area has been completely stabilized
• Infiltration basins should establish dense vegetation on the basin side slopes and floor to prevent erosion and sloughing
Copyright 2000, CWP
Infiltration SystemsMaintenance Criteria
• Observation well should be installed in infiltration trench• Extreme care should be taken during construction stage
– no coverage with impermeable surface– avoid sediment entry– do not use as sediment control device– avoid compacting subsoils during construction
• OSHA standards for trench excavation• Underdrain pipe system is recommended for de-watering
where marginal soils exist • Direct access for maintenance and rehabilitation should be
provided
Copyright 2000, CWP
Infiltration Systems Three Design Variations
• Infiltration trench• Infiltration basin• Porous pavement
Copyright 2000, CWP
Copyright 2000, CWPCopyright 2000, CWP
Copyright 2000, CWP
Infiltration TrenchDesign Notes
• Field verification of soil permeability essential
– fc > 0.5 in/hr
• Require infiltration tests and test pit/soil boring at location of proposed facility
– 1 test pit/soil boring per 50 ft of trench to depth of 4 ft below bottom of proposed facility bottom
• Geotextile fabric should interface between the trench sidewalls and stone reservoir and top gravel filter
• A 6-inch sand filter layer should be placed on the bottom of the infiltration trench
Copyright 2000, CWP
Infiltration TrenchDesign Notes Cont’d
• Maximum trench depth is determined by:
dmax = f Ts/n
Where:
dmax = maximum depth
f = final infiltration rate (in/hr)
Ts = maximum allowable storage time (hr)
n = porosity
Copyright 2000, CWP
Infiltration TrenchDesign Notes Cont’d
• Surface area of trench is determined by:
At = Vw / (ndt + fT)Where:
At = surface area of trench
Vw = design volume entering trench (e.g., WQv)n = porosity
dt = trench depth based on the depth required above seasonal groundwater table or a depth less than dmax, whichever is smaller
f = infiltration rate of trenchT = time to fill trench (generally assumed to be less
than 2 hours)
Copyright 2000, CWPCopyright 2000, CWP
Copyright 2000, CWP
Copyright 2000, CWP
Copyright 2000, CWP
Copyright 2000, CWPCopyright 2000, CWP
Copyright 2000, CWPCopyright 2000, CWP
Copyright 2000, CWPCopyright 2000, CWP
Copyright 2000, CWP
Copyright 2000, CWP
Infiltration BasinDesign Notes
• Failure rates of 25 to 100% recorded in the field• Pretreatment with settling basin is recommended • Partially excavated basins should not be used as
sedimentation basins during construction• Final excavation should be deferred until all
contributing areas have been stabilized or protected
• A 6 to 12 inch layer of filter material (e.g., coarse sand) is recommended to prevent the builup of impervious deposits on the soil surface. This layer can be replaced when it becomes clogged.
Copyright 2000, CWP
Infiltration BasinDesign Notes Cont’d
• Maximum basin depth is determined by:
dmax = f Tp
Where:
dmax = maximum depth
f = final infiltration rate (in/hr)
Tp = maximum allowable ponding time (hr)
Copyright 2000, CWP
Infiltration BasinDesign Notes Cont’d
• Bottom surface area of basin is determined by:
Ab = [2Vw - At db] / [db - 2P + 2fT]Where:
Ab = bottom surface area of basin
Vw = design volume entering basin (e.g., WQv)
At = top surface area of basin
db = basin depth based on the depth required above seasonal groundwater table or a depth less than dmax, whichever is
smallerP = design storm rainfall depthf = infiltration rate of basinT = time to fill basin (generally assumed to be less
than 2 hours)
Copyright 2000, CWP
Infiltration BasinDesign Notes Cont’d
• Top length of basin (assuming a rectilinear shape) is determined by:
Lt = [Vw + Zdb (Wt – 2 Zdb)] / [Wt (db – P) – Zdb2]
Where:
Lt = top length of basin
Vw = design volume entering basin (e.g., WQv)Z = side slope ratio of basin (h:v)
db = basin depth based on the depth required above seasonal groundwater table or a depth less than dmax, whichever is smaller
Wt= top width of basinP = design rainfall event
Note: The basin top length and width should be greater than 2Zdb
Copyright 2000, CWPCopyright 2000, CWP
Copyright 2000, CWPCopyright 2000, CWP
Porous Pavement
Copyright 2000, CWP
Copyright 2000, CWPCopyright 2000, CWP
Copyright 2000, CWP
Porous PavementDesign Notes
• Soils need to have a permeability between 0.5 and 3.0 inches per hour.
• The bottom of the stone reservoir should be completely flat so that infiltrated runoff will be able to infiltrate through the entire surface.
• Porous pavement should be sited at least 2 to 5 feet above the seasonally high groundwater table, and at least 100 feet away from drinking water wells.
• Porous pavement should be sited on low traffic or overflow parking areas, which are not sanded for snow removal.
Copyright 2000, CWP
Porous PavementDesign Notes Cont’d
• Asphalt, concrete or concrete-grid can be used• Vacuum sweeping needed• Construction stage sediment control is critical• Overflow inlets are recommended to convey
larger storms and as a relief for sealed pavement surfaces
• Needs an informed owner and long term education commitment
• Winter plowing/sanding can be a problem
Copyright 2000, CWP
Porous PavementDesign Notes Cont’d
• Proper maintenance of porous pavement should include the use of a carefully worded maintenance agreement that provides specific guidance, including how to conduct routine maintenance, and how the surface should be repaved. Signs should be posted on the site identifying porous pavement areas.
Photo Copyright 1999, Center for Watershed ProtectionCopyright 2000, CWP
Photo Copyright 1999, Center for Watershed ProtectionCopyright 2000, CWP
Copyright 2000, CWPCopyright 2000, CWP
Copyright 2000, CWP
Infiltration SystemsLongevity
• 3 studies in Maryland indicate poor to moderate longevity of first generation infiltration systems
• Infiltration basins had poorest longevity (100% failure)• Infiltration trenches
– about 20% failed initially– additional 30% failed within 5 years of construction– most trenches were never maintained– grass filter strips, alone, are inadequate pretreatment– sump pit pretreatment appeared useful
Copyright 2000, CWP
Infiltration SystemsLongevity (cont’d)
• Porous pavement - significant evidence of failure• initial failure
• clogging over time
• resurfacing
• Longevity can be improved with:– appropriate application (e.g., overflow parking only)– better soil testing– careful construction techniques– redundant pretreatment– regular maintenance