Peak Flow Impacts


Effect of Increases in Peak Flows and Imperviousness on the Morphology of Southern California Streams


Peak Flow Impacts, as the Phase 1 of the Hydromodification Study, was completed by the SMC in April 2005.

Urbanization in southern California has resulted in direct and indirect effects on natural stream courses that have altered their physical and biological character by altering watershed hydrology. It can result in a higher peak discharge rates for floods for an equivalent rainfall than they were prior to development. This process has been termed hydromodification.

Hydromodification can result in adverse effects to stream habitat, surface water quality, and water supply. Although the effects of increased impervious cover on stream flow have been well documented, the majority of past studies have focused on perennial streams. Until recently, few comparable studies have evaluated the impacts of urbanization on ephemeral or intermittent streams of arid or semi-arid climates. This had made it difficult to effectively manage stormwater impacts on southern California’s natural streams.

The SMC conducted this study to assess the relationship between stream erosion and urbanization and allow the prediction of channel response under changed conditions associated with increased impervious cover. The specific study objectives were to:

  1. Establish a stream channel classification system for southern California streams
  2. Assess stream channel response to watershed change, and attempt to develop deterministic or predictive relationships between changes in impervious cover and stream channel enlargement
  3. Provide a conceptual model of stream channel behavior that will form the basis for future development of a numeric model

Ten sites, located in Ventura, Los Angeles, and Orange Counties were examined stream geomorphology, and these results were compared to historical geomorphologic data and changes in land use with in respective catchments. This analysis resulted in four main conclusions regarding the relationship between impervious cover and stream channel form for ephemeral streams in southern California:

  1. Southern California streams exhibit deterministic relationships between bankfull discharge and measures of channel geometry such as cross section area
  2. The ephemeral/intermittent streams in southern California appear to be more sensitive to changes in total impervious cover than streams in other areas
  3. There is a natural background level of channel degradation that is occurring in all stream channels studied, even in the absence of development within the drainage area
  4. Streams are sensitive to both peak discharge and duration of discharge

Three general management strategies were identified to stabilize at risk channels:

  1. Limit impervious area
  2. Control runoff
  3. Stream channel movement – allow the greatest freedom possible for “natural stream channel” activity

This project represented the first step in a multi-year, multi-targeted research program and provided a good overview of potential impacts and categorized the types of channels most susceptible to stream bed and bank erosion.  It laid a good foundation for further hydromodification studies to provide specific numerical guidance and to validate results in all types of watersheds found in southern California.