10.2 BMP 710 - Infiltration Basins
Infiltration basins are earthen impoundments used for the collection, temporary storage and infiltration of incoming stormwater.
10.2.2 Design Criteria Specific to Infiltration Basins
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
The slope of the basin bottom should not exceed 3% in any direction.
A minimum of one foot of freeboard is recommended when establishing the design ponded water depth. Freeboard is measured from the rim of the infiltration facility to the maximum ponding level or from the rim down to the overflow point if overflow or a spillway is included.
Maximum ponding depth shall be between 2 and 6 feet.
Provide erosion protection for all inlets per A400 - Outfall Systems.
Provide a primary overflow structure (either a Type 2 catch basin with a grated opening or a maintenance hole with a beehive grate). The overflow structure shall be designed to bypass the stormwater above the water quality design volume (if the basin is designed for treatment only) or designed to bypass stormwater above the flow control or LID Performance Standard Requirements.
In addition to the primary overflow, ponds must have an emergency overflow spillway. Design the Emergency Overflow Spillway per A600 - Emergency Overflow Spillway.
Lining material – Basins can be open or covered with a 6 to 12-inch layer of filter material such as coarse sand, or a suitable filter fabric to help prevent the buildup of impervious deposits on the soil surface. A nonwoven geotextile should be selected that will function sufficiently without plugging (The non-woven geotextile shall conform to A2000 - Geotextile Specifications, Geotextile for Underground Drainage - Low Survivability, Class C.) The filter layer can be replaced or cleaned when/if it becomes clogged.
Sod or seed exposed earth on the pond bottom and interior side slopes with an appropriate seed mixture. Plant all remaining areas around the wetpond with grass or landscape and mulch with a 3" cover of shredded wood mulch. The mulch should be free of garbage and weeds and should not contain excessive resin, tannin, or other material detrimental to plant growth. Do not use construction materials, wood debris, or wood treated with preservatives for producing wood mulch.
Provide maintenance access road(s) to the inlet and outlet. Access roads and ramps shall conform to A3000 - Access Ramps and Roads.
An access ramp is required for removal of sediment. For small, shallow ponds, an access ramp may not be required if the trackhoe can load a truck parked at the pond edge. Extend the access ramp to the bottom of the first cell unless all portions of the cell can be reached and sediment loaded from the top of the pond.
The internal berm may be used as access only if all the following apply:
The internal berm is no more than 4' above the first wetpool cell.
The first wetpool cell is less than 1,500 square feet (measured without the ramp)
The internal berm is designed to support a loaded truck, 80,000 pounds minimum, considering the berm is normally submerged and saturated.
Provide a stormwater facility sign. Sign shall conform to A800 - Signage.