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The purpose of this study is to develop a numerical model based
on multimode characteristics method for fully coupled
simulation of water and sediment movement in mobile-bed
alluvial channels with nonuniform bed materials. According to
multimode characteristics method, the characteristics equations
and tota- -lly differential compatible equations of the
numerical model could be obtained by consolidating water
continuity and momen- -tum equations, sediment continuity
equation and bed material sorting equtaions. The model contains
explicit scheme, spatial reachback scheme, temporl reachback
scheme and implicit scheme for solving flow depth, velocity,
bed elevation, and bed-mate- -rial size fraction at each time
step. The multimode scheme has the advantages of releasing
Courant constraint and of effectively dealing with the problem
of large differences in celerities among water-surface waves,
bed-mate- -rial sorting waves. The multimode scheme is based on
Lagrangian concept. The unknows could be computed from one time
step to another by solving the compatible equations along the
corres- ponding characteristics curves. When there are more
then three governing equations, the eignvalues can only be
solved numeri- -cally. The concept of buffer reach was adopted
in this study to improve the stabilty and accuracy in numerical
simulation affected by the upstream boundary condition. The
simulation results for aggradation based on experimen- -tal
data of Soni et al.(1980) and Yen et al.(1987) show that the
proposed model can reasonably predict the evolution of bed
elevation. Through the numerical simulation, the effect of
boun- -dary condition on simulation results was examined.
Furthermore, an assessment of model applicability was made
through the analy- -sis of models stabilty, accuracy, and
sensitivity.
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