Infiltration-Recharge Basins

From the MassHighway Storm Water Handbook for Highways and Bridges

Description

Recharge basins temporarily store runoff, but release at least a portion of that runoff by infiltrating the water into the ground. The recharge volume is stored and allowed to infiltrate into the underlying soils over a period of time following a storm event. The storage volume above this level may be released by an outlet structure designed to bypass all excess flows. Recharge Basins may be designed on-line or off-line. To maintain proper function of an infiltration-recharge basin, it is important to avoid soil compaction.

General Information
Applicable DEP Stormwater Management Policy Performance Standards	Standards #3 and #4.
TSS Removal	DEP Credit:	80% ( for infiltration basin)
	Estimated Range from Literature:	>90%
Relative Cost	Construction: Moderate to high; varies depending on types soils of contributing drainage area, soils at site of system, storage depth, and type of outlet structure Maintenance: Moderate to high
Potential Constraints to Use	Depth to groundwater Depth to bedrock or other impermeable stratum Suitability of site soils for infiltration
Other Considerations	Pre-treatment must be provided, to prevent clogging of soils surface by sediments in the influent storm water. Frequent maintenance may be required, to remove accumulated sediment and restore infiltrative capacity. Recharge systems can provide high levels of treatment of other pollutants, in addition to TSS removal.
Maintenance Requirements	Inspect a minimum of twice annually. Periodic mowing of embankments Removal of woody vegetation from embankments Removal of debris from outlet structures Clean forebay as required. Clean basin of accumulated sediment as required. Clean flow control structures at least once annually, or as indicated by inspection. Maintain vegetation in healthy condition.
Primary Design References	MA-DEP Stormwater Technical Handbook: http://www.mass.gov/dep/brp/stormwtr/stormpub.htm Schueler, 1987 Schueler, et. al., 1992 Young, et. al., 1996

DESIGN CRITERIA
Design Parameter	Criteria
Contributing Drainage Area	15 acres or less
Storm frequency for design flow	Varies with system. Recharge systems are sized for recharging an annual volume, not an event storm.
Annual Recharge Volume	Compute annual recharge volume using methodology specified in the MA-DEP Stormwater Technical Handbook, or use an alternative method conforming to accepted engineering practice.
Required Storage Volume	Compute storage volume using methodology specified in MA-DEP Stormwater Technical Handbook.
Design Recharge Rate	Determine recharge rate based on soil texture/hydrologic group as specified in the MA-DEP Stormwater Technical Handbook, confirmed by on-site field testing; or use an alternative method conforming to accepted engineering practice.
Design Safety Factor	Surface systems shall be sized with a safety factor of 1.0 times the design recharge rate.
Maximum Draw-down Time	The basin should be designed to drain the design storage volume in 48 hours or less, using the design recharge rate times the applicable safety factor.
Maximum Depth of System	Depth of system shall be equal to or less than the depth permitting draw-down in the required time.
Depth to Bedrock or Impermeable Stratum	Minimum 0.6 meters (2 feet) below bottom of system.
Depth to seasonal high groundwater	Small systems: Minimum 0.6 meters (2 feet) below bottom of system. Large systems: Groundwater mounding analysis may be required.
Pretreatment	Pre-treatment system required; provide 25% TSS removal prior to discharge to a surface recharge system. Use forebay or equivalent measure.
Velocity Dissipation at Inlet	Provide measures to dissipate velocity of flows into the recharge basin, to prevent erosion of basin interior.
Setback from slab foundation	3.0 meters (10 feet)
Setback from cellar foundation	6.1 meters (20 feet)
Setback from slope >15%	4.6 meters (15 feet) (top edge of system to top of slope), or as required for impoundment stability. Distance may need to be greater where potential for “break-out” and resulting slope instability may be a problem.
Setback from on-site sewage disposal system	15.2 meters (50 feet) (or greater, if required under 310 CMR 15.000 [Title 5])
Setback from private well	30.5 meters (100 feet)
Setback from groundwater supply	Zone I radius; additional setback may be required depending on hydro-geologic conditions
Setback from surface water supply	Zone A, and 30.5 meters (100 feet) from tributaries
Performance under frozen conditions.	Recharge basins should be designed to either: Provide for capture and recharge of required annual volume during the period April to December; or Provide for capture and recharge during the entire year, with provisions for introduction of recharged storm water into the ground under frozen conditions (e.g., through use of wick drains, leaching galleries, or chambers, or other measures).
Construction of infiltration surface	The infiltration surface shall be constructed to preserve and enhance the capability of the soil to pass flows from the basin into the groundwater. Consider measures such as: Minimizing trafficking by heavy construction equipment Use of a minimum thickness of topsoil required to establish plantings Using a planted surface, rather than crushed stone or sand surface
Protection During Construction	Runoff from disturbed areas shall not be discharged to the recharge basin. The contributing site shall be completely stabilized, prior to placing the recharge basin in service.
Access for maintenance, repair, and rehabilitation	Design shall consider accessibility to system, and capability to replace system components, to provide for eventual repair and rehabilitation of the system.
Other	Provide maintenance access, including access to basin interior Stabilize slopes as indicated for other impoundment-type BMPs Design embankment to meet applicable safety standards Provide emergency spillway as indicated for other impoundment-type BMPs Consider providing method to drain lowest stage in the event of clogging of infiltration surface, so that surface can be rehabilitated.

Several of the design criteria regarding setbacks from slopes, foundations, and other site features have been adapted from the requirements for on-site sewage disposal systems described in Massachusetts Title 5 (310 CMR 15.000). However, storm water quantities and flow durations differ markedly from the hydraulic loadings to septic systems. The design engineer should be aware of these differences, and may need to consider additional setbacks to provide for slope stability, protect structures, and provide for the satisfactory performance of the recharge system.

Example of Infiltration Basin

Illustration by Geosyntec Consultants, Inc.