Determining Channel-Forming Discharge for River Restoration and Stable Channel Design: A How-To/ How-Not-To Workshop

摘要

Stream erosion contributes a significant amount of sediment pollution to streams, rivers, lakes, and reservoirs. Stable channel design, in which the cross-sectional and plan-view dimensions of a stream are altered to a more stable configuration, has become an increasingly popular method to stabilize degrading streams. Channel-forming discharge is one of the most important parameters required for this type of stable channel design. Three popular methods for approximating channel-forming discharge are bankfull discharge, effective discharge, and the discharge of a given recurrence interval, such as the 2-year flow. Bankfull discharge, defined as the stream flow when the height of the water just reaches the top of the banks, is often used as the channel-forming discharge. Various methods for calculating bankfull discharge include resistance equations, gage station rating curves, and regional power curves. Studies performed on Kansas streams shows tremendous error with some methods for determining bankfull flow. Effective discharge, defined as the discharge that carries the most sediment over the medium to long term, includes information about the sediment transport capacity of a reach and the hydrologic record of the watershed. Effective discharge can be calculated using data from the internet, augmented by field-collected sediment data. Although this method is seen by many to be the most meaningful and correct approximation for channel-forming discharge, not many people know how to calculate it. The recurrence interval of channel-forming discharge varies greatly from region to region. Bankfull discharge on cohesive bed streams in Kansas, for example, was shown to be approximately half of the 2-year flow, while studies performed in other states showed bankfull discharge to more closely match the 10-year flow. A recurrence interval flow should not be used without correlating the return period with several local, stable, streams. This workshop will introduce participants to the most popular methods of channel-forming discharge determination. The benefits, limitations, and drawbacks of each method will be illustrated. Data from stable streams in Kansas will be used to show the applicability and variability of each method, as well as the error produced if a method is misapplied. Participants will then be shown in easy-to-duplicate steps how to calculate effective discharge, using data from the internet and from the field.