Infiltration basins are depressions that collect and store stormwater until it can infiltrate into the subsoil. Sediment settles out in the device, and nutrients, metals, and organic material are adsorbed by the soil as the water infiltrates. Infiltration basins may also be designed to reduce peak flows from a site if the storage capacity of the device is increased and a stable outlet structure is included in the design.

Infiltration basins are appropriate on sites with highly permeable soils and drainage areas of less than 15 acres. Infiltration basins should not be used near foundations, basements, or roads or on sites with high water tables, steep slopes, or clay soils. In addition, these devices must not be used on sites with large concentrations of soluble pollutants, as groundwater contamination may result.

While these structures effectively treat the runoff volume from small storms, larger storm events quickly overwhelm the capacity of the device and render it ineffective. Basins are also susceptible to clogging from sediments and, as a result, they must be used in conjunction with other management practices, such as pretreatment for sediment removal.

Advantages

• Increases discharge to the groundwater
• Preserves base flow in streams
• Removes sediment, nutrients, and organic material from stormwater
• May be designed to reduce peak flows
• Reduces thermal impacts of runoff

Disadvantages

• Limited functionality with frozen ground
• May cause ground water pollution if not sited properly
• Susceptible to clogging
• Requires frequent maintenance
• Not applicable on sites with high sediment loads or sites with large concentration of hydrocarbons

Infiltration basins are depressions that collect and temporarily store runoff and should be designed to drain within 24 hours. To prevent erosion of the basin and to increase the infiltration capacity of the practice, they should be lined with vegetation that is tolerant to frequent inundation (refer to Seeding or Native Vegetation). The effective infiltration area must receive runoff that has been pretreated.

Stormwater must be delivered to the basin from pretreatment devices at non-erosive velocities to prevent erosion of the structure. The pretreatment device must provide TSS reduction of 60% for residential development and 80% for commercial, industrial or institutional development. Pretreatment for oil and grease separation may be necessary depending on tributary source areas. Basins receiving runoff from rooftops only do not require pretreatment.

The depth of the basin is dependent upon the infiltration rate of the soil and outlet design, but may not exceed 2 feet. The bottom of the basin must be at least 3 feet above the seasonally high water table to prevent groundwater contamination. Side slopes must be 3:1 or flatter to promote uniform infiltration and safety while making maintenance tasks, such as mowing, easier. A drawdown device must be included to provide winter pass through, and allow for timely maintenance.

In order to prevent channelized flow and extended localized ponding, large basins should be divided into multiple cells. Level spreaders that distribute the runoff over the effective infiltration area of each cell should be utilized. A drawdown device must be included for each cell in the basin.

An emergency spillway should be incorporated into the design of the structure to safely pass flows that exceed the design capacity of the basin. These structures prevent large flows from overwhelming the capacity of the structure without causing damage to the basin or downstream structures by discharging to stable outlets (refer to Stone Outlet Protection or Lined Waterway or Outlet).

• Construction of the basin should not commence until the entire site has been stabilized to prevent sediment slogging.
• Care should be taken during all phases of construction to prevent the compaction of soils in and around the practice
• Stabilize infiltration basins immediately after construction is completed

• Accumulated sediment in pretreatment devices must be removed as needed
• Infiltration basins need to be inspected for signs of erosion and bare spots after all storm events until vegetation has become well established - all necessary repairs shall be performed immediately

WinSLAMM, RECARGA or other approved model must be used to determine the performance of this practice. Additional information on the modeling of infiltration practices is found on the Infiltration Guidance page.

Infiltration Basin Guidance for Owners

DNR Infiltration Basin Technical Standard 1003