1. Design of Infiltration Practices
Center for Watershed Protection
Infiltration practices are those that capture and temporarily store the water quality volume before allowing it to infiltrate into the soil over approximately a two day period. These practices are an excellent technique for meeting recharge requirements and can also provide stormwater detention and channel protection storage in certain limited cases. Design variations include the infiltration trench and porous pavement. Infiltration basins are also sometimes used; however, they tend to clog very easily, and therefore rely heavily on the site-specific conditions for success. The dry well, which is a infiltration trench draining a rooftop downspout, is also an infiltration practice used by many practitioners. Since it is difficult to provide effective pretreatment for the dry well, it tends to clog easily, and is therefore not widely recommended for application.
This slide show provides a presentation on basic design guidance for stormwater infiltration systems, which include infiltration trenches, shallow infiltration basins and porous pavement.
Copyright 2000, CWP

2. 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
Infiltration systems have some of the best pollutant removal capabilities of any stormwater treatment practice; however, soluble pollutants tend to pass through the practice with limited treatment. Caution should be used with these practices in groundwater recharge and karst areas due to the risk of contamination.
Copyright 2000, CWP

3. 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
General design considerations for infiltration systems are provided on this slide. Consideration can be given to relaxing depth to groundwater table criteria in coastal areas for non hotspot land uses.
Copyright 2000, CWP

4. Infiltration Systems Conveyance 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
Conveyance to an infiltration practice is an important design consideration to avoid potential erosion and scouring. Where feasible, designs should be off-line.
Copyright 2000, CWP

5. Infiltration Systems Pretreatment 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
Pretreatment is a critical component of infiltration systems and will have a significant role in the longevity of the practice. Without adequate and redundant pretreatment, infiltration practices can be expected to clog easily and quickly. This and the following slide provide some guidance on recommended pretreatment criteria.
Copyright 2000, CWP

6. Infiltration Systems Pretreatment 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 = (WQV) ft2 for impervious cover  75%
As = (WQV) ft2 for impervious cover > 75%
Exit velocities from pretreatment shall be non-erosive for 2-yr design storm
The equation above is used to size pretreatment settling basin surface area. It was derived by the Washington State Department of Ecology from the Camp-Hazen.
As = -(Qo/w) ( Ln(1-E)
where:
As = Sedimentation basin surface area (ft2)
E = Trap efficiency; which is the target removal efficiency of suspended solids (set equal to 90%)
w = Particle settling velocity; for target particle size (silt) use settling velocity = ft/sec ( ft/sec for I > 75%, where I is percentage impervious area)
Qo = rate of outflow from the basin; which is equal to the water quality volume (WQv) divided by the detention time (td); use 24 hours.
The equations simplify to:
As = ( (WQV) ft2 for I < 75%
As = (WQV) ft for I >= 75%
Copyright 2000, CWP

7. Infiltration Systems Treatment 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
Infiltration systems should be sized to treatment the water quality volume minus the pretreatment volume. This volume should be infiltrated within 48 hours.
Copyright 2000, CWP

8. Infiltration Systems Landscaping 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
Clogging is the biggest concern with infiltration practices. Many fail as a result of construction site runoff entering the system prior to site stabilization. Great care must be exercised to avoid fouling the system prematurely.
Copyright 2000, CWP

9. Infiltration Systems Maintenance 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
One source of failure is from compacted soils below infiltration practice that occurs during construction of the practice itself. Provisions need to be incorporated at the design stage to ensure that contractors know that the must excavate the practice without compacting subsoils.
Copyright 2000, CWP

10. Infiltration Systems Three Design Variations
Infiltration trench
Infiltration basin
Porous pavement
The three main design variations of infiltrations systems are presented here. The dry well is basically a form of the infiltration trench. The following slides will provide specific design considerations for these three alternatives.
Copyright 2000, CWP

11. An infiltration trench is a rock-filled trench with no outlet that receives stormwater runoff. Stormwater runoff passes through some combination of pretreatment measures, such as a swale and detention basin, and into the trench. There, runoff is stored in the voids of the stones, and infiltrates through the bottom and into the soil matrix. The primary pollutant removal mechanism of this practice is filtering through the soil.
Copyright 2000, CWP

12. A variation of the infiltration trench is the dry well which can be effectively used to accept rooftop runoff, as indicated in this schematic. Dry wells can be effective practices at reducing total runoff volume from a site. Note the inherent problem of no pretreatment, depending on the roof surface, this can be either a minor problem or a potential for premature failure (i.e., asphalt shingles contain a sizable solid load that will eventually clog the practice).
Copyright 2000, CWP
Copyright 2000, CWP

13. Infiltration Trench Design 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
The next 2 slides provide specific design and sizing guidance for infiltration trenches.
Copyright 2000, CWP

14. Infiltration Trench Design 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
Maximum trench depth is determined by the above equation.
Copyright 2000, CWP

15. Infiltration Trench Design 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 trench
T = time to fill trench (generally assumed to be less than 2 hours)
Trench surface area is determined by the above equation.
Copyright 2000, CWP

16. This slide shows an infiltration trench receiving runoff from a parking lot. Note the PVC observation well and clean out in the background. This particular example lacks adequate pretreatment (a common problem with many practices installed in the field).
Copyright 2000, CWP
Copyright 2000, CWP

17. This slide shows the inflow to the infiltration trench and the make shift pretreatment berm that is present in the parking lot to try to drop out much of the larger sediment that would otherwise clog the filter surface.
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18. This infiltration trench has good pretreatment in the form of a vegetated swale. The actually infiltration chamber is below the pea gravel on the surface. A high permeability filter fabric is below the pea gravel. This technique acts as pretreatment for the main infiltration chamber and as a sacrificial failure zone. If the pea gravel clogs, it can be replaced without digging up the entire practice.
Copyright 2000, CWP

19. This is another example of an infiltration trench receiving runoff from a parking lot. Limited pretreatment is provided by the grass strip between the pavement and the trench surface. The good thing about this design is that the runoff does not concentrate and enters the practice as sheet flow, and therefore loads the trench surface in a fairly uniform manner.
Copyright 2000, CWP 1

20. This slide shows a good example of a vegetated swale being used for pretreatment. The infiltration trench is located behind the street lamp in the background.
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Copyright 2000, CWP 1

21. This slide shows how the larger particulate solids settle out at this site even before entering the pretreatment swale. Periodic sweeping of the parking lot will help minimize resuspension of this sediment either by water or air.
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22. Inadequate pretreatment, poor construction techniques, inadequate soil testing, or poor design can all lead to infiltration trench failure. Extreme caution and extra care is needed to ensure successful practice implementation.
Copyright 2000, CWP
Copyright 2000, CWP 1

23. An infiltration basin is a shallow impoundment which is designed to infiltrate stormwater into the ground. This practice is believed to have a high pollutant removal efficiency, and can also help recharge the groundwater, thus restoring low flows to stream systems. Infiltration basins can be challenging to apply on many sites, however, because of soil requirements. In addition, some studies have relatively high failure rates compared with other management practices.
Copyright 2000, CWP

24. Infiltration Basin Design 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.
Specific considerations associated with infiltration basins are listed in this slide.
Copyright 2000, CWP

25. Infiltration Basin Design 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)
An infiltration basin maximum depth is based on the same principles as an infiltration trench. However, because the basin uses an open area or shallow depression for storage, the maximum allowable depth is a function of maximum allowable ponding time instead of storage time.
Copyright 2000, CWP

26. Infiltration Basin Design 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 smaller
P = design storm rainfall depth
f = infiltration rate of basin
T = time to fill basin (generally assumed to be less than 2 hours)
Basin bottom surface area is determined by the above equation.
Copyright 2000, CWP

27. Infiltration Basin Design 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 basin
P = design rainfall event
Note: The basin top length and width should be greater than 2Zdb
Basin top length is determined by the above equation.
Copyright 2000, CWP

28. This is a photo of an infiltration basin with ponded water, obviously not infiltrating very well. This is a fairly common occurrence with infiltration basins. Some common reasons for failure include, surface soils clog easily with incoming sediments, soils are often overly compacted during construction or the basin is excavated down into the water table.
Copyright 2000, CWP
Copyright 2000, CWP

29. This is another photo of an infiltration basin that is not performing as designed due to a clogged surface.
Copyright 2000, CWP
Copyright 2000, CWP

30. Porous Pavement
Porous pavement is a permeable pavement surface with an underlying stone reservoir to temporarily store surface runoff before it infiltrates into the subsoil. This porous surface replaces traditional pavement, allowing parking lot stormwater to infiltrate directly and receive water quality treatment. There are a few porous pavement options, including porous asphalt, pervious concrete, and grass pavers. Porous asphalt and pervious concrete look the same as traditional pavement from the surface, but are manufactured without the “fine” particle sizes, and incorporate void spaces to allow infiltration. Grass pavers are concrete interlocking blocks or synthetic fibrous gridded systems with open areas designed to allow grass to grow within the void areas. Other alternative paving surfaces can help reduce the runoff from paved areas but do not incorporate the stone trench for temporary storage below the pavement. While porous pavement has the potential to be a highly effective treatment practice, maintenance has been a concern in past applications of the practice.
The ideal application for porous pavement is to treat a low traffic or overflow parking area. Porous pavement may also have some application on highways, where it is currently used as a surface material to reduce hydroplaning. Caution needs to be exercised in colder climates where winter sanding can clog the porous surface.
Copyright 2000, CWP

31. A typical profile of porous pavement might look like this schematic
A typical profile of porous pavement might look like this schematic. Some basic features should be incorporated into all porous pavement practices, including: pretreatment, treatment, and conveyance.
Pretreatment
In porous pavement designs, the pavement itself acts as pretreatment to the stone reservoir below. Because the surface serves this purpose, frequent maintenance of the surface is critical to prevent clogging. Another pretreatment item can be the incorporation of a fine gravel layer above the coarse gravel treatment reservoir. Both of these pretreatment measures are marginal, which is one reason that these systems have a high failure rate.
Treatment
The stone reservoir below the pavement surface should be composed of layers of small stone directly below the pavement surface, and The stone bed below the permeable surface should be sized to attenuate storm flows for the storm event to be treated. Typically, porous pavement is sized to treat a small event, such as the water quality storm (i.e., the storm that will be treated for pollutant removal) which can range from 0.5" to 1.5". Like infiltration trenches, water can only be stored in the void spaces of the stone reservoir.
Conveyance
Water is conveyed to the stone reservoir through the surface of the pavement, and infiltrates into the ground through the bottom of this stone reservoir. A geosynthetic liner and sand layer should be placed below the stone reservoir to prevent preferential flow paths and to maintain a flat bottom. Designs also need some method to convey larger storms to the storm drain system. One option is to use storm drain inlets set slightly above the elevation of the pavement. This would allow for some ponding above the surface, but bypass flows that are too large to be treated by the system, or if the surface clogs.
Copyright 2000, CWP
Copyright 2000, CWP

32. Porous Pavement Design 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.
Porous pavement has the same siting considerations as with other infiltration practices (see Infiltration Trenches). The site should meet the above criteria.
Copyright 2000, CWP

33. Porous Pavement Design 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
Other design considerations include the following.
Copyright 2000, CWP

34. Porous Pavement Design 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.
Other design considerations include the following.
Copyright 2000, CWP

35. This slide shows a parking lot with traditional asphalt in the foreground and porous asphalt in the background. Note how water is ponding in the foreground, but not in the background.
Photo Copyright 1999, Center for Watershed Protection
Copyright 2000, CWP 1

36. This lot uses grass pavers which allow water to infiltrate through the spaces provided.
Photo Copyright 1999, Center for Watershed Protection
Copyright 2000, CWP 1

37. This slide illustrates what grass pavers look like just after a storm event. The temporary ponding of water makes porous pavement more appropriate for low traffic volume parking areas.
Copyright 2000, CWP
Copyright 2000, CWP

38. Infiltration Systems Longevity
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
Infiltration practices have a tendency to clog and fail in short periods of time. This is usually the result of inadequate pretreatment and limited or no maintenance. Before implementation of infiltration practices, designers and reviewers should be aware of some of the pitfalls associated with the practices, as outlined in these final two slides.
Copyright 2000, CWP

39. Infiltration Systems Longevity (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