|Shields Jr, Fletcher|
Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2006
Publication Date: 7/1/2007
Citation: Doyle, M.W., Shields Jr, F.D., Boyd, K.F., Skidmore, P.B., Dominick, D. 2007. Channel-forming discharge selection in river restoration design. Journal of Hydraulic Engineering 133(7)831-837. DOI:10.1061(ASCE)0733-9429(2007)133:7(831). Interpretive Summary: Currently there are three main ways to size channels so that they do not erode or fill with sediment, and there is much debate regarding which way is most appropriate. The bankfull discharge method is based on study of the shape of a stream channel, its bank vegetation and its sediments to estimate the water surface elevation for "channel-forming" events. The return interval method uses statistics to select a discharge that occurs, on average, every one to two years. The effective discharge method is a computational procedure designed to find the discharge that, over a long time period, moves the most sediment down the channel. All three procedures were applied to four channels with widely varying hydrologic and geologic conditions, and results were compared with published findings of other researchers. In general, the effective discharge method is preferred for alluvial channels, particularly those in disturbed watersheds with flashy hydrology. These findings will be useful to engineers seeking to design and restore streams with low maintenance requirements.
Technical Abstract: The concept of dominant or channel-forming discharge is important in river channel restoration design and in other aspects of hydraulic engineering and fluvial geomorphology. Three measures of channel-forming discharge are common: effective discharge, bankfull discharge, and a discharge of a certain recurrence interval. The latter two measures have become popular in some channel restoration design procedures, often to the exclusion of effective discharge analyses. However, the relationship among the three measures varies considerably at the regional scale due to differences in hydrology and geomorphology. The three measures are most similar for coarse-bedded, nonincised alluvial channels with snowmelt hydrology. Channel incision produces bankfull discharges far greater than the effective discharge, and flashy hydrology is associated with generally larger, briefer, and more frequent effective discharges. Regional mean or median values for the relative magnitudes of the three measures tend to be tightly constrained, but site to site variation is quite large. The construction of a cumulative sediment discharge curve and associated determination of Qeff allows quantification of the sediment budget of a channel for a given hydrologic regime, which provides insight into drivers of current and future destabilization. Reliance on only return-interval or bankfull discharge for channel design is often risky and unwise.