Stormwater Infiltration

Stormwater that is filtered into the ground is called Stormwater Infiltration. When this excess stormwater infiltrates into the ground, the water passes or moves through the soil where it is either absorbed by plant roots or enters into the groundwater system. Many designers designate specific areas of a site as Stormwater Infiltration areas in order to reduce the peak flow to acceptable rates. However, much thought should go into the designs of these structures and their use will require regular maintenance and observation.

	BMPs Best Management Practices (BMPs) are an essential part to any stormwater management plan
	Final Stabilization In order to terminate NPDES coverage the site must achieve 70% permanent vegetative cover..
	Bioretention (B-R) B-R are infiltration areas designed to mimic natural, predevelopment hydrology and to address water quality.

Any land-disturbance projects seeking coverage under the NPDES General Construction Permit and that are incorporating additional infiltration practices to meet regulatory requirements for either water quality or quantity, must comply with a stringent list of limitations that may prohibit the use of infiltration practices for a specific site.

In addition to the list of limitations that may prohibit infiltration use, there are also some design criteria for infiltration structures that must be addressed during the design phase of the project. The required design criteria and limitations are listed below. Additional information is provided in our Stormwater Best Management Practices Handbook.

Infiltration Design Criteria

• Permanent infiltration practices, when used, shall be designed to accept, at a minimum, the first inch of runoff from all impervious areas.
• Soil borings must be provided from the selected infiltration areas to check for soil permeability, porosity, depth to seasonally high water table, and depth to bedrock.
• Permanent infiltration practices must completely drain between the timeframe of 24 and 72 hours.
• All infiltration areas should be protected from heavy traffic and sediment-laden stormwater runoff prior to and during construction.
• Pretreatment, such as sediment forebays, are highly recommended to treat runoff from all contributing areas for the infilration practice.
• Rigourous maintenance schedules and a maintenance agreement must be provided for any proposed infiltration practices.

Infiltration Limitations

Infiltration practices have certain limitations on their use on certain sites. These limitations include the following items:

1. Areas draining to these practices must be stabilized and vegetative filters established prior to runoff entering the system. Infiltration practices shall not be used if a suspended solids filter system does not accompany the practice. If vegetation is the intended filter, there shall be, at least a 20 foot length of vegetative filter prior to stormwater runoff entering the infiltration practice;
2. The bottom of the infiltration practice shall be at least 0.5 feet above the seasonal high water table, whether perched or regional, determined by direct piezometer measurements which can be demonstrated to be representative of the maximum height of the water table on an annual basis during years of normal precipitation, or by the depth in the soil at which mottling first occurs;
3. The infiltration practice shall be designed to completely drain of water within 72 hours;
4. Soils must have adequate permeability to allow water to infiltrate. Infiltration practices are limited to soils having an infiltration rate of least 0.30 inches per hour. Initial consideration will be based on a review of the appropriate soil survey, and the survey may serve as a basis for rejection. On‑site soil borings and textural classifications must be accomplished to verify the actual site and seasonal high water table conditions when infiltration is to be utilized;
5. Infiltration practices greater than three feet deep shall be located at least 10 feet from basement walls;
6. Infiltration practices designed to handle runoff from impervious parking areas shall be a minimum of 150 feet from any public or private water supply well;
7. The design of an infiltration practice shall provide an overflow system with measures to provide a non‑erosive velocity of flow along its length and at the outfall;
8. The slope of the bottom of the infiltration practice shall not exceed five percent. Also, the practice shall not be installed in fill material as piping along the fill/natural ground interface may cause slope failure;
9. An infiltration practice shall not be installed on or atop a slope whose natural angle of incline exceeds 20 percent.
10. Clean outs will be provided at a minimum, every 100 feet along the infiltration practice to allow for access and maintenance.