Stream design curves are created by analyzing the morphology of a statistically significant number of stable streams and plotting the data. The data gathered, typically bankfull width, bankfull height, and sinuosity, is then plotted against drainage area to demonstrate the correlation between the various channel characteristics and the hydrology of the region. The trend line can then be used by designers and managers to provide initial channel measurements for a stream restoration project.
The development and use of regional curves by fluvial geomorphologists began in the 1950’s by Luna Leopold. This work indicated that it is common for there to be strong correlations between channel shapes, discharges and drainage areas. After Leopold published design curves for the San Francisco Bay area in 1978, it became clear that this region needed more data at the sub-regional level. A number of hydrogeomorphologists in the bay area have since worked to improve the accuracy and precision at the sub-regional scale.
The user of a regional curve, when used appropriately, can acquire meaningful initial estimates of the average channel shapes for streams in that region based on drainage area. This is useful to the restoration designer or public works official who wants to know if a stream reach of concern has a cross-sectional area which is close to, or far off from the the regional average. This comparison can provide insight to the stability of the reach. “Stability” in this context means that the channel is not excessively eroding or depositing sediment but has dynamic meandering, sediment transport and erosion to build pools, riffles, step-pools and other instream complexity. The regional curves can be very helpful in determining “equilibrium” channel shapes for stream reaches for which there are not nearby reference reaches in good condition to help inform a stream channel restoration .
If these design curves are used to inform a channel design, it is necessary to incorporate additional information into determining an equilibrium channel shape. These design tools can include the more complex dimensionless rating curves, and computations of effective discharges which form the channels. Some stream channel types are not conducive to being represented by regional curves such as streams located in alluvial fans, channels just upstream of marshes, or channels in steep areas of a watershed.[toggle_box]
[toggle_item title=”Background” active=”false”]In 1978 a regional curve was published for the Bay Area by Dunne and Leopold and included information from most of the Bay Area counties. Because the Bay Area has so many different micro climates, it is necessary to develop curves which represent these different sub-regional environments to ensure designers have the most applicable data available. For this reason the EPA and SFEP are supporting the development of regional curves which represent the conditions found in the different sub-regions of the Bay. Developed urban environments that have not experienced much land use change in the past few decades can benefit from developing curves representing a built out urban equilibrium. Areas in process of undergoing land use changes are not good candidates for developing the curves.[/toggle_item]
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Design PrimerbyAnne Riley(San Francisco Bay Water Quality Control Board)