NCDENR Stormwater BMP Manual Chapter Revised: 4-10-14

Chapter 10: Wet Detention Pond

Description /
A Wet Detention Pond has a permanent pool of standing water that is maintained by the rise (the elevated outlet structure). Water in the permanent pool mixes with and dilutes the initial runoff from storm events. Wet detention basins fill with stormwater and release most of the mixed flow over a period of a few days, slowly returning the basin to its normal depth.
Siting & Use
Yes
Yes
Yes
Yes
Yes / High SHWT
Tight soils
Very steep sites
Highly impervious sites
Large drainage areas
Estimated Effectiveness Based on Design
BUA Status / The surface of the wet detention pond and surrounding side slopes shall be considered pervious.
Runoff Reduction / 20% reduction if MDC are implemented.
TSS removal / 85% reduction if MDC are implemented.
TN removal / 30% reduction if MDC and Nutrient DC are implemented.
25% reduction if only MDC are implemented.
TP Removal / 45% reduction if MDC and Nutrient DC are implemented.
40% reduction if only MDC are implemented.
Bacteria Removal / High if MDC and Bacteria DC are implemented.
Medium if only MDC are implemented.
Temperature Control / Medium if MDC and Temperature DC are implemented.
Low if only MDC are implemented.

Notes: Runoff reduction (used in LID crediting) is measured in terms of percentage reduction of runoff volume from the design storm.

TSS, TN and TP removal are measured in terms of percentage reduction in the annual mass loading of the pollutant.

Design Objective
A Wet Detention Pond shall be designed to capture the design storm and release it slowly over a period of two to five days. Stormwater shall have an adequate flow path to bring about removal of TSS. The pond shall be designed in a manner that protects the device, the areas around the device and the receiving stream from erosion.
Minimum Design Criteria (MDC), Other Design Criteria and Recommendations
MDC 1. / Criterion description (rule citation).
MDC 2. / Criterion description (rule citation).
MDC 3. / Criterion description (rule citation).
MDC 4. / Criterion description (rule citation).
MDC 5. / Criterion description (rule citation).
MDC 6. / Criterion description (rule citation).
MDC 7. / Criterion description (rule citation).
Nutrient DC 1. / Criterion description.
Nutrient DC 2. / Criterion description.
Bacteria DC 1. / Criterion description.
Temp DC 1. / Criterion description.
Recommendation 1. / Practice description.
Recommendation 2. / Practice description.
Procedure for Design Alternatives
Design professional may propose to meet the design objective for a wet detention pond in a manner that does not comply with every MDC listed above as well as the MDC for all BMPs. If so, then the design professional shall provide technical justification that the proposed design is equally or more protective of water quality (vague, anecdotal or isolated evidence is not acceptable). Stormwater review staff shall consider these requests on a case-by-case basis. Alternative designs may receive lower regulatory credits.

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Wet Detention Ponds 10-1 April 2014