Hydrograph Modification

Occurrence of adverse ancillary impacts due to the implementation of traditional flood control approaches in highly urbanized areas has become increasingly evident. Traditional approaches have employed the practice of centralizing stormwater in relatively impervious regional basins, channels, and storm drains to remove the “nuisance” flooding as quickly as possible. This practice has led to depleted groundwater reserves, decimation of natural riparian corridors, and the disturbance of natural fluvial geomorphic processes that are necessary to maintain healthy riparian ecosystems. To help offset and mitigate development impacts to natural riparian drainage corridors, many regions - such as communities within Southern California - have adopted newer hydrograph modification management standards. Hydrograph modification management is intended to promote the beneficial uses of runoff by reasonably protecting natural drainage areas from the potential cumulative effects of future development in a watershed. Hydrograph modification is used to ensure post-project runoff peaks and durations do not exceed estimated pre-project peaks and durations. Any increase in stormwater runoff peaks or durations could cause erosion or other adverse impacts on natural drainage areas. HELM offers expertise in development of hydrograph modification management plans (HMPs) that can be implemented so that runoff from urbanization will not exceed flowrates or duration when compared to runoff patterns prior to developed conditions. HMPs provide a blue-print for strategic use of low impact development infrastructure that incorporates either best management practices or integrated management practices within urbanized watersheds.