Regime consequences of the Assiniboine River diversion

Abstract

Using Blench's empirically deduced modified regime theory equations, an attempt is made to quantify predicted changes in the reach of the Assiniboine River downstream of the diversion at Portage la Prairie. These empirical equations have been used because the conditions of the channels that provided data for the regime equations are similar to the conditions found in the Assiniboine River channel downstream of the Portage Diversion and because Blench's system appeared to provide the widest choice of relationships to be utilized in an attempted quantitative prediction of this kind. It is estimated the ultimate depth of degradation immediately downstream of the diversion site will be about three feet in a period of about 30 years and the downstream extent of degradation will be approxiamtely 20 miles. Also, immediately downstream of the diversion site, the mean channel width is anticipated to increase up to 20%, the mean depth is expected to decrease about 5%, and the mean velocity is expected to decrease about 15%. However, the quantitative estimates of increased channel width, decreased depth, and decreased velocity are subject to doubt due to their computation being extremely sensitive to error propagation. It should be noted that these results are theoretical maximums because in the analysis, no acccount has been taken of the sediment supplied to the downstream reach by the 13-foot conduit under the dam. Unforturnately, no information regarding the effect of this conduit on sediment supply was available at the time of the study.