Problem Statement

Sedimentation and Flood Hazard in the Gravel Reach of Fraser River: Progress Report prepared for District of Chillliwack 8550 Young Road Chilliwack, British Columbia, V2P 4P1 under District of Chilliwack OP01220 by Michael Church Department of Geography The University of British Columbia Vancouver, British Columbia, V6T 1Z2 15 April, 1999 Table of contents Executive Summary.................................................................................................................... 1 Problem statement....................................................................................................................... 2 The gross sediment budget of the gravel-bed reach ................................................................... 4 The distribution of the deposited gravel ..................................................................................... 7 Aggradation of the river bed..................................................................................................... 12 Discussion................................................................................................................................. 13 Conclusions............................................................................................................................... 16 References................................................................................................................................. 18 Table of Tables Table 1 Annual bed load transport at Agassiz, from measurements (103 tonnes a-1) and river flows........................................................................................................................................ 5 Table 2 Summary of Sediment Budget Calculations...................................................................... 6 Table 3 Sediment volume changes and aggradation in individual subreaches between Agassiz and Mission........................................................................................................................... 10 Table of Figures Figure 1. Fraser River in the Lower Mainland. Numbers along the river are kilometres upstream from Sand Heads. The study reach lies between km 85 and km 150. .................................... 4 Figure 2 Relations between annual flow volume/maximum daily flow (Qmax) and gravel transport at Agassiz (1967-1986). 1 dam3 (cubic decametre) = 1000 m3............................... 6 Figure 3 Map of Fraser River between Agassiz (km 130) and Mission (km 85) showing the computing cells used to determine the gravel budget from 1952 and 1984 surveys. ............. 8 Figure 4 (a) Distribution of stored gravel volume changes along Fraser River between 1952 and 1984......................................................................................................................................... 9 ii Executive Summary 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 mm/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. 1 Problem statement Fraser River presents a significant potential flood hazard to human settlement within the Lower Mainland of British Columbia. With annual minimum flows on the order of 1000 m3s-1, and flood flows of order 10 000 m3s-1, the annual range of flow is about 10x. This is normal for a large river. In the natural state, this range in flows was sufficient to flood extensive areas of the restricted floodplain of the river within the Lower Mainland (cf. North and Teversham, 1984). After the great flood of 1894, efforts commenced to protect developing settlements from the river, efforts which have now continued for a century. The river follows a steep, confined course through the mountains where it picks up sedimentary material from the banks and from tributaries. Within the Lower Mainland, the gradient of the river declines quickly as it approaches the sea. Hence, it cannot continue to move all of the material and a significant portion of its sedimentary load is deposited. The largest material is abandoned first, so that the river between Hope and Sumas Mountain flows over its own gravel deposits. These deposits form a confined alluvial fan -- a sediment wedge restricted within the confines of the relatively narrow valley northeast of Sumas Mountain. An alluvial fan is a deposit of river-transported sediment, dumped where the river encounters a sharply reduced gradient. Such fans are common at mountain fronts. A characteristic of alluvial fans is that they continue to accumulate sediment so long as the river delivers more material than can be transported across the fan and beyond. This is the situation in Fraser River, so the river in the eastern Lower Mainland is aggrading -- raising its bed -- as additional gravel and sand are deposited there year by year. This process slowly raises water levels, hence flood levels. Because of aggradation, rivers on alluvial fans are laterally unstable. They tend to shift their course relatively frequently because the deposited sediments fill the current channel bed, creating an obstruction to the conveyance of water downstream and elevating the bed above the level of the adjacent fan surface. The water then finds a new course around the deposits. How unstable a river is depends upon the size of the sediment load deposited annually in comparison with the size of the river. On Fraser River, the deposits are modest and the river is not highly unstable. Nevertheless, throughout this century a program of river dyking and bank protection has been pursued in order to protect adjacent land from flooding, and in order to reduce or eliminate erosion of those increasingly valuable lands. A result of this program has been to hold the river within a channel zone that is more narrowly confined than it originally was. The confinement is not extreme; the chief effects have been cutoff of sidechannels and elimination of floodwater storage areas. In general, confinement of the river raises flood water levels beyond those they otherwise would reach, and increases the rate of rise of the riverbed because sediment deposition occurs only within the restricted channel zone. Contemporary management

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