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. Development typically increases impervious surfaces on formerly undeveloped (or less developed) landscapes and reduces the capacity of remaining pervious surfaces to capture and infiltrate rainfall. The result is that as a watershed develops, a larger percentage of rainfall becomes runoff during any given storm. In addition, runoff reaches the stream channel much more efficiently, so that the peak discharge rates for floods are higher for an equivalent rainfall than they were prior to development. This process has been termed hydromodification.
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:
- Establish a stream channel classification system for southern California streams
- Assess stream channel response to watershed change, and attempt to develop deterministic or predictive relationships between changes in impervious cover and stream channel enlargement
- Provide a conceptual model of stream channel behavior that will form the basis for future development of a numeric model
The study approach was to evaluate the changes in stream channel configuration over time and compare them to the changes in total basin impervious cover (TIMP) over the same time period. Data collection occurred in two phases. In the first phase background and historic information was gathered on each site and its contributing drainage area. In the second phase detailed field data was collected on the geomorphic condition of each study reach. The combinations of historic and contemporary data were used to develop predictive relationships between changes in impervious cover and channel form.
This study resulted in the following general conclusions regarding the relationship between impervious cover and stream channel form for ephemeral streams in southern California:
- Southern California streams exhibit deterministic relationships between bankfull discharge and measures of channel geometry such as cross section area
- The ephemeral/intermittent streams in southern California appear to be more sensitive to changes in TIMP than streams in other areas
- 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
- Streams are sensitive to both peak discharge and duration of discharge
Three general strategies were identified for the management of increases in peak flow:
- Limit impervious area
- Control runoff
- Stream channel movement – allow the greatest freedom possible for “natural stream channel” activity