Design Variants

Infiltration Basin

An infiltration basin is a natural or constructed impoundment that captures, temporarily stores and infiltrates the design volume of water over several days. In the case of a constructed basin, the impoundment is created by excavation or embankment. Infiltration basins are commonly used for drainage areas of 5 to 50 acres with land slopes that are less than 20 percent. Typical depths range from 2 to 12 feet, including bounce in the basin. An infiltrates basin construction detail is located in the Computer-aided design and drafting (CAD/CADD) drawings section.

Infiltration Trench and infiltration basin - Construction inspection checklists.
Link to this table
To access an Excel version of form (for field use), click here.

Site Status:
Construction Sequence Satisfactory / Unsatisfactory Comments
1. Pre-Construction
Pre-construction meeting
Runoff diverted
Soil permeability verified
Groundwater / bedrock verified
Project benchmark established
Facility location staked out
Temporary erosion and sediment control established
2. Excavation
Size and location per plans
Side slopes stable
Depth adjusted to soil layer with specified soil type and permeability
Sub-soil not adjacent to excavation area and stabilized with vegetation and/ or silt fence
Stockpile location not adjacent to excavation area and stabilized with vegetation and/ or silt fence
3. Filter Fabric Placement
Fabric per specifications
Fabric per specifications
Placed per plan location
4. Aggregate Material
Size as specified
Clean / washed material
Placed properly
5. Observation Well
Pipe size per plans
Under-drain installed per plans
Inlet installed per plans
Pre-treatment devices installed per plans
6. Vegetation
Complies with planting specifications
Topsoil complies with composition and placement in specifications
Permanent erosion control measures in place
7. Final Inspection
Dimensions per plans
Check dams operational
Inlet / outlet operational
Effective stand of vegetation and stabilization
Contributing watershed stabilized before flow is routed to the facility
Actions to be taken:

Infiltration Trench (a.k.a. infiltration gallery)

An infiltration trench is a shallow excavated trench, typically 3 to 12 feet deep, that is backfilled with a coarse stone aggregate allowing for the temporary storage of runoff in the void space of the material. Discharge of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. Trenches are commonly used for drainage areas less than 5 acres in size.

Dry Wells (a.k.a. infiltration tubes, french drains, soak-away pits or soak holes)

A dry well or soak away pit is a smaller variation of an infiltration trench. It is a subsurface storage facility (a structural chamber or an excavated pit backfilled with a coarse stone aggregate) that receives and temporarily stores stormwater runoff. Discharge of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. Due to their size, dry wells are typically designed to handle stormwater runoff from smaller drainage areas, less than one acre in size (e.g. roof tops).

Underground Infiltration Systems

Several underground infiltration systems, including pre-manufactured pipes, vaults, and modular structures, have been developed as alternatives to infiltration basins and trenches for space-limited sites and stormwater retrofit applications. These systems are similar to infiltration basins and trenches in that they are designed to capture, temporarily store and infiltrate the design volume of stormwater over several days. Underground infiltration systems are generally applicable to small development sites (typically less than 10 acres) and should be installed in areas that are easily accessible to routine and non-routine maintenance. These systems should not be located in areas or below structures that cannot be excavated in the event that the system needs to be replaced.

Retrofit Suitability

The narrow and versatile shape of infiltration trenches and dry wells makes them well suited for retrofit projects. For example, infiltration practices can be situated along the margin or perimeter of a developed site or roadway in many cases. They are particularly desirable as retrofit practices in watersheds or catchments that are targeting volume reduction practices to help minimize channel erosion.

Warning: Use of infiltration practices is PROHIBITED in the CGP for treatment of runoff from industrial areas with exposed significant materials or from vehicle fueling and maintenance areas.

Generally, infiltration should not be used to treat runoff from manufacturing or industrial sites or other areas with high pollutant concentrations unless correspondingly high levels of pretreatment are provided.

Special Receiving Waters Suitability

The BMP design restrictions for special watersheds table below provides guidance regarding the use of infiltration practices in areas upstream of special receiving waters. This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated. The corresponding information about other BMPs is presented in the respective sections of this Manual.

BMP design restrictions for special watersheds

Cold Climate Suitability

Various options for use of infiltration are available for treating snowmelt runoff. Some of the installations are built below the frost line (trenches, sub-grade proprietary chambers) and do not need further adaptation for the cold. However, some special consideration as described in the Cold climate impact on runoff management section is HIGHLY RECOMMENDED for surface systems. The problem with infiltration in cold weather is the ice that forms both over the top of the facility and within the soil interstices. To avoid these problems to the extent possible, it is HIGHLY RECOMMENDED that the facility be actively managed to keep it dry before it freezes in the late fall. This can be done by various methods, including limiting inflow, under-drainage, and surface disking.

Even if the infiltration properties of an infiltration practice are marginal for snowmelt runoff, the storage available in the facility will provide some storage if it is dry entering the melt season. Routing the first highly-soluble portions of snowmelt to an infiltration facility provides the opportunity for soil treatment (such as filtration, adsorption, microbial activity) of these solubles. Again, however, flow originating in an industrial area, a high traffic area where large amounts of salt are added, or another PSH should be diverted away from infiltration systems.

Proprietary, sub-grade infiltration systems provide an alternative to standard surface based systems. Essentially, these systems provide an insulated location for pre-treated snowmelt to be stored and slowly infiltrated, or simply filtered and drained away if ground-water sensitivity is an issue. The insulating value of these systems adds to their appeal as low land consumption alternatives to ponds and surface infiltration basins.

Water Quantity Treatment

The amount of stormwater volume infiltrated depends on the design variant selected. Smaller infiltration practices (e.g. infiltration trenches) should either be designed off-line using a flow diversion, or designed to safely pass large storm flows while still protecting the iinfiltration area. In limited cases (e.g. extremely permeable soils), these smaller infiltration practices can accommodate the channel protection volume, Vcp, in either an off- or on-line configuration.

In general, supplemental stormwater practices will be necessary to satisfy channel and flood protection requirements when smaller infiltration practices are used. However, these practices can help reduce detention requirements for a site through volume reduction.

Due to their size, the larger infiltration practices (e.g. infiltration basins and underground infiltration systems) have the potential to provide greater water quantity benefits. Surcharge storage above the practice bottom is available for detention. Outlet structures can be sized to partially or fully accommodate larger storm peak discharge control while allowing the volume below the outlet to infiltrate.

Water Quality Treatment

Infiltration practices can remove a wide variety of stormwater pollutants through chemical and bacterial degradation, sorption, and filtering. Surface water load reductions are also realized by virtue of the reduction in runoff volume.

There are few data available demonstrating the load reductions or outflow concentrations of larger-scale Infiltration practices such as Infiltration trenches. Similarly, few sampling programs collect infiltrating water that flows through an Infiltration system.

For properly designed, operated, and maintained Infiltration systems, all water routed into them should be “removed” from stormwater flow, resulting in 100 percent efficiency relative to volume and pollutant reduction. For this reason, any Infiltration BMP performance table should show all 100 percent entries. This logic assumes that stormwater is the beneficiary of any Infiltration system, but ignores the fact that pollution, if any remains after the internal workings of the infiltration BMP itself (see later discussion in this section), is being transferred into the shallow groundwater system. Good monitoring data on the groundwater impact of infiltrating stormwater are rare, but there are efforts underway today to document this, so future Manual revisions should be able to include some data updates.

Properly designed Infiltration systems discussed later in this section will accommodate a design volume based on the required water quality volume. Excess water must be by-passed and diverted to another BMP so that the design Infiltration occurs within 48 hours if under state regulation, or generally within 72 hours under certain local and watershed regulations. In no case should the by-passed volume be included in the pollutant removal calculation.

Data that are reported in performance literature for infiltration systems, unless reporting 100 percent effectiveness for surface water or documenting outflow water downward, are not accurately representing behavior, or are representing the excess flow (overflow) from a system. Design specifications in the following sections should prevent putting contaminated runoff and excess water beyond that which will infiltrate within the given timeframe. Any runoff containing toxic material or excess volume that cannot infiltrate should be diverted away from the infiltration system and reported as inflow to another treatment device.

Follow guidance and carefully use of infiltration BMPs to make sure they are not transporting highly loaded or toxic contaminants into the groundwater system. These sections address the pollution remediation processes at work in infiltration systems to reduce or totally remove pollutants that move through them. However, extreme caution must be exercised and serious planning undertaken to assure that no highly contaminating material is routed into these BMPs. Of particular concern are toxic organics (gasoline, solvents) and high levels of chloride.

The Computer-aided design and drafting (CAD/CADD) drawingssection contains details on how design and operations can either raise or lower the expected level of performance for infiltration BMPs.


The following general limitations should be recognized when considering installation of infiltration practices:

  • Limited monitoring data are available and field longevity is not well documented.
  • Failure can occur due to improper siting, design, construction and maintenance.
  • Systems are susceptible to clogging by sediment and organic debris
  • There is a risk of groundwater contamination depending on subsurface conditions, land use and aquifer susceptibility.
  • They are not ideal for stormwater runoff from land uses or activities with the potential for high sediment or pollutant loads.
  • They are not recommended for areas with steep slopes
  • Please not that even though there are potential pollution and physical clogging problems with infiltration, it is one of the most important elements in the stormwater runoff treatment train. Fear of the limitations should not prevent well designed systems from being used.

As noted in various sections, discussion of BMP selection, the benefits associated with infiltration BMPs should only be accrued based on the amount of water actually passing through the BMP. Excess runoff beyond that designed for the BMP should not be routed through the system because of the potential for hydraulic and particulate over-loading, both of which will adversely impact the life and operation of the BMP. .

For example, an infiltration device designed to treat the first 0.5 inch of runoff from a fully impervious surface will catch about 30 percent of the volume of runoff in the Twin Cities. This means that 70 percent of the runoff volume should be routed around the filtration system and will not be subject o the removals reflected in the above tables. Attributing removal to all runoff just because a BMP. is in place in a drainage system is not a legitimate claim.

Warning: It is REQUIRED that some form of pre-treatment, such as a plunge pool, sump pit, filter strip, sedimentation basin, grass channel, or a combination of these practices be installed upstream of the infiltration practice.