Dg. Mclean et M. Church, Sediment transport along lower Fraser River - 2. Estimates based on the long-term gravel budget, WATER RES R, 35(8), 1999, pp. 2549-2559
The long-term rate of gravel (bed material) transfer is established for a 4
5 km gravel-bed reach of lower Fraser River by comparing river morphology o
n the basis of complete channel surveys conducted 32 years apart. The reach
is aggradational, but the sediment budget reveals substantial local variat
ions in transport within the reach. Methods for estimating sediment transpo
rt on the basis of observed morphological changes are compared with the sed
iment budget and with measurements of bed load transport made at the upstre
am end of the study reach. All of the methods indicate that the average gra
vel load entering the study reach is about 200 x 10(3) tonnes/year. Certain
computational formulae estimate similar amounts, but there is no obvious w
ay to decide beforehand which formula might be reliable. Lessons from this
work include the observation of substantial within-reach variations in tran
sport, so that the results from a single cross section are not necessarily
representative, and the observation that, most of the time, the river is tr
ansporting an amount that is far less than its "hydraulic capacity." A summ
ary discussion compares the various methods for estimating bed material tra
nsport in terms of reliability and applicability. Estimates based on observ
ed morphological change appear to be most cost-effective and can best take
advantage of historical information about the river. However, potential sou
rces of bias must be carefully evaluated.