Copyright 2000, CWP Design of Infiltration Practices Center for Watershed Protection.

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

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, 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


• 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


• 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, 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.

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

Copyright 2000, CWP Design of Infiltration Practices Center for Watershed Protection.

Documents

Transcript of Copyright 2000, CWP Design of Infiltration Practices Center for Watershed Protection.