CGRG Bibliography of Canadian Geomorphology
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Author : Church, M.
Date : 1999.
Title : Sedimentation and flood hazard in the gravel reach of Fraser River: Progress report.
Publication : Prepared for District of Chillliwack, 8550 Young Road, Chilliwack, British Columbia, V2P 4P1, under District of Chilliwack OP01220.
Issue :
Page(s) : 19 p.
Abstract
Year 1 activities of the Fraser River gravel-bed reach study were primarily to recover historical data of river morphology and sedimentation. Analysis of these data has clarified the relation between sedimentation and flood hazard in the reach. Bedload sediment transport measurements undertaken by the Water Survey of Canada between 1967 and 1986 established the mean annual transport of gravel past the Rosedale-Agassiz bridge as 227 000 tonnes. There is no significant gravel transport past Mission. Deposited evenly over the 40 km reach between Agassiz and Sumas Mountain, the influx would create 4 m/year rise of the river bed. About 10 times as much material is moved within the reach in any one year as the amount entering it, yielding a false impression of much larger influx. Comparison of channel surveys undertaken in 1952 and 1984 indicates that the bed material influx at Agassiz in the intervening years was 203 000 tonnes/year. This is essentially the same result as reported above, when account is taken of the different reporting period. The surveys permit aggradation to be calculated along the river in 29 local “cells”, revealing bed elevation changes between +1 metre and -1 metre (with an extreme of +2.39 m at Hog Island, where a channel filled up). These findings show that sedimentation can substantially reduce the margin of protection from flooding provided by the dykes, but that the effect is local. The locations of major aggradation change over time, but significant deposition persists at a particular site for some years -- hence, there is some forecasting capacity to identify sites of concern. Quantitative information of aggradation in the reach presently extends upstream only to Agassiz, and only up to 1984. A new survey of the complete reach is required to update information. During the 32-year intersurvey period, the actual mean aggradation in the Agassiz-Mission reach was 2 mm/year. The difference with the figure based on sediment transport measurements arises because documented gravel removals from the channel during the period reduced the aggradation by at least 38% (23%, if the major removal at Minto Channel is not included). Possible measures to maintain or reinforce the level of flood protection include social/institutional arrangements, raising and/or reconstruction of the dykes, and removal of gravel from the river. The first measure is unlikely to be socially acceptable by itself. Dyke improvement is expensive, although it could be undertaken over many years, concentrating in a short term on currently aggrading subreaches. Gravel removal appears attractive. However, it may have significant impacts on river morphology and processes, thence upon the economically valuable and socially valued riverine ecosystem. In rivers where major volumes of gravel have been removed, there have been significant observed changes of river morphology and of the aquatic ecosystem. There is insufficent information at present to determine what level of sustained gravel removal (if any) would have negligible effects in Fraser River. Modest levels of gravel removal over more than 30 years have not produced superficially obvious effects on the river or its ecosystem. But no study has been conducted to identify such effects and it is known that they can remain difficult to detect for substantial periods. It would not be prudent, with present knowledge, to assume that removal of gravel is a viable means to maintain flood protection without significantly affecting the river ecosystem.
Bibliography of Canadian Geomorphology