One way to reduce the risk of floods is the implementation of a retention volume above an affected area. The artificially created local retention measure, from natural structures of different sizes, offers great advantages.
Retention basins are used to collect surface runoff and to improve the quality of water by natural processes such as sedimentation, decomposition, solar disinfection and soil filtration. The retention areas can be countercyclical emptied after heavy stream inflow or allowed to infiltrate the collected water. The mode of action depends on situation and design intent.
The exploration of a potential retention area in catchments or floodplains is carried out using a digital terrain model (DTM) or based on topographic maps (contour). The retention effect is due to the variations of flow time intervals within the modeled terrain.
Several factors affect the retention of a surface. They vary from slope gradient, groundwater fluctuations, soil structure to vegetation type. It is important to note that the efficiency is greater with lower terrain slope.
For an effective mitigation of flood waves a targeted management strategy is most of the time necessary. In some cases an extensive dyke system with controlled outflow and inflow structures are essential. At the beginning of the flood wave, the retention area must allow sufficient retention volume, and the system should be coordinated and programmed properly for an effective peak reduction.
Retention area effect on the hydrograph