4.12.4 Artificial Detention 



An alternative to control storm sewer systems that is often feasible 

 is to collect stormwater in dispersed detention facilities and delay its 

 release to approximate the predevelopment rate of flow as closely as 

 possible. Detention facilities should be of sufficient capacity to 

 absorb the maximum amount expected and, where possible, stormwater should 

 be introduced into the soil for reconditioning. 



There are many design techniques for detaining and returning rainfall 

 to the soil, based on collection of water and seepage through gravel into 

 the groundwater. Runoff can be held on-site in any combination of the 

 following: 



1. Runoff detention ponds 



2. Gravel -filled ditches 



3. Gravel -filled seepage pits. 



A combination of runoff detention ponds with seepage facilities is often 

 successful . 



Detention basins (Figure 37), impoundments, or ponding areas make up 

 for the reduction in natural storage capacity caused by devegetation, 

 paving, and buildings [116]. They also allow for sediments and absorbed 

 pollutants to settle out. (Standard urban storm drainage systems are 

 often designed to handle a 25-year storm.) 



M.^ure 37. Large, well -engineered detention pond and sediment 

 basin dam (Source: Reference 70). 



I ' freeboard - 



Crest of emergency spiHwair 



Cr«8t of principal spiHway 



Provide tra»h rack 

 and protective fence 



Sed iment 

 a) location 



Z , + Z, 5 5 



Principal spillway - minimum outflow of 20% of peak inflow 



151 



