Stormwater Management (SWM) programs for the purpose of controlling increased urban runoff generated by development are a top priority in urban planning. More frequent flooding, increased rates and volumes of runoff, increased stream channel erosion and degradation, increased sedimentation and increased water pollution are all problems intensified by development. Stormwater control measures (SCMs) such as detention ponds, open channels, and storm drains (for a full list refer to DCM 1.2.4), have proven to significantly reduce downstream flooding, reduce sediment and pollutant loads, and provide debris removal which can benefit water quality.
The basic concept of SCM for peak rates of runoff is to provide for a temporary storage of stormwater runoff. Runoff is then released at a controlled rate which cannot exceed the capacities of the existing downstream drainage systems, or the predeveloped peak runoff rate of the site, whichever is less.
The solid lined hydrograph shown in Figure 8-1 in Appendix D of this manual represents a storm runoff event without SCM, while the dashed line hydrograph depicts the same event with SCM. The peak flow of the undetained hydrograph could exceed the capacity of the downstream conveyance system thereby cause surcharging and flooding problems. With the introduction of the SCM, the solid lined hydrograph is spread over a longer time period and its peak is reduced. The area between the two curves to the left of their intersection represents the volume of runoff which is temporarily stored or detained in the SCM.
The City of Austin approaches the control of excess flows through the application of both on-site/off-site and regional SWM detention facilities. Essentially, the distinction between the two approaches is that on-site or off-site is generally limited to site specific criteria, while regional incorporates a basin wide hydrologic analysis.