Research Areas > Stormwater > Effect of Increases in Peak Flows and Imperviousness on the Morphology of Southern California Streams

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

Background and Objective

Although the effects of increasing watershed impervious cover (i.e., hydromodification) on stream flow have been documented, the majority of past studies have focused on perennial streams. Until recently, few comparable studies have evaluated urbanization impacts on ephemeral or intermittent streams in arid or semi-arid climates. This lack of knowledge precluded effective management of stormwater impacts on southern California’s streams.

The objective of this study was to assess the relationship between urbanization and stream channel erosion in southern California streams. Relationships between stream channel type and resistance to erosion were investigated, to allow prediction of channel response with changing impervious cover conditions. Specifically, the study aimed to:

• Establish a stream channel classification system for southern California streams
• Assess stream channel response to change in the watershed
• Define deterministic or predictive relationships, if any, between changes in impervious cover and stream channel enlargement
• Build a conceptual model of stream channel behavior to form the basis for future development of  numeric (computational) models

Status

This project was completed in 2005.

Methods

The effects of hydromodification on southern California streams were assessed by evaluating changes in total basin impervious cover (TIMP), and then comparing them to changes in stream channel configuration over time. Data was collected from 11 sites throughout southern California, so as to provide a regional representation. Data collection occurred in two phases. In the first phase, historic information was gathered for each site. In the second phase, detailed field data was collected. The following data were collected for both historical and contemporary time periods: 1) characteristics of the catchment draining to the study site, 2) rainfall, 3) streamflow, and 4) physical condition of the stream channel. Land use records and aerial photographs were used to estimate TIMP. Precipitation records were evaluated to gain an understanding of when the wet and dry periods occurred in the study regions. Stream flow records provided data for statistical analyses of peak flow frequency, and served as secondary evidence (with precipitation) to determine typical climatic conditions during the period of urbanization.

The combined data set of historic and contemporary information was then used to develop predictive relationships between changes in impervious cover and channel form. Evaluation of channel enlargement over time required multiple data points, including 1) the predevelopment condition (as the baseline or beginning point in time), 2) one or more “historical” data points, and 3) the current condition . In many cases the current condition did not represent the ultimate stream adjustment response (the final, fully adjusted form of the stream). Therefore, the ultimate condition needed to be estimated. These data were then used to create an "enlargement curve,"  relating changes in TIMP to the stream channel response over time.

Findings

Main results of this study were:

• 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 appeared to be more sensitive to changes in TIMP than streams in other areas.

• Control (undeveloped) sites showed a drop in elevation over time due to downcutting, although the channel shape, size, and direction did not change appreciably. In contrast, sites surrounded by developed watersheds did show changing channel morphology over time. Downcutting was likewise observed in the developed sites, at a higher rate than in the control sites.

• Stream channels were influenced by changes in both peak discharge amount and duration of discharge. Ephemeral and intermittent streams appeared to have a low resistance to erosion, and a high susceptibility to channel enlargement with increases flows.

The predictive relationships found in this study can be used to evaluate the potential effects of proposed development projects on the stability of natural streams. Though there are a variety of strategies that can be used to help minimize these effects, the selection of a management strategy is dependent upon the extent to which a stream channel has been impacted, the nature of the stream channel reach, and the anticipated future watershed conditions (i.e., expected increases in TIMP).

Partners

This project was conducted in coordination with the Southern California Stormwater Monitoring Coalition (SMC). Technical leads on this project included Earth Tech, Inc. and Aquafor.