What Is Stormwater Detention?
Stormwater detention temporarily stores runoff from paved and roofed areas and releases it slowly, so a developed site does not send more peak flow downstream than before. Most local codes require it for new development above a size threshold.
Why it is required
Roofs and pavement turn rain that once soaked in into fast runoff. Without control, peak flows increase, causing downstream flooding, erosion and pollution. Local stormwater ordinances and state rules, together with the federal NPDES program, require new and redevelopment projects to manage runoff quantity and quality.
Requirements usually apply above a threshold of new impervious area or land disturbance set by the local ordinance. Many jurisdictions also require water quality treatment and, increasingly, runoff volume reduction through infiltration or reuse.
Types of facilities
Dry detention ponds that drain between storms; wet retention ponds with a permanent pool; underground pipe or chamber systems under parking lots; and green infrastructure such as bioretention, permeable pavement and infiltration trenches that reduce runoff volume.
An outlet structure with orifices and weirs controls the release rate for different design storms, and an emergency spillway safely passes larger storms.
How it is designed
A civil engineer computes pre-development and post-development runoff using methods such as the rational method or NRCS TR-55 and TR-20 hydrographs, sizes the storage to meet the allowable release rates, and routes storms through the facility. The design also addresses water quality, maintenance access and safety.
The engineer prepares a stormwater management report with calculations and drawings for agency review, and often a maintenance plan. After construction, many jurisdictions require an as-built survey and engineer's certification that the facility was built as designed.
Frequently asked questions
How big does a detention pond need to be?
It depends on site size, imperviousness, local rainfall and release rules. A preliminary estimate can be made with the modified rational method.
Who maintains a detention pond?
Usually the property owner or HOA, under a maintenance agreement with the local government.
Is detention needed for a single house?
Usually not, unless local rules set a low threshold; some jurisdictions require small on-lot measures.
Sources
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