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<br />erodible bank is limited by the angle of repose of the material. The rate of <br />width change depends on the rate at which sediment material is removed or <br />deposited along the banks. For the same sediment rate, width adjustment at <br />a tall bank is not as rapid as that at a low bank. The rates of width <br />adjustment for cases of width increase and decrease are somewhat different <br />as described below separately. <br /> <br />An increase in width at a channel section depends on sediment removal <br />along the banks. The maximum rate of widening occurs when sediment inflow <br />from the upstream section does not reach the banks of this section while <br />bank material at this section is being removed. River banks have different <br />degrees of resistance to erosion; therefore, the rate of sediment removal <br />along a bank needs to be modified by a coefficient. For this purpose, bank <br />erodibili ty factor is introduced as an index for the erosion of bank <br />material and the four bank types reflecting the variation in erodibility are <br />classified as follows <br />(1) Non-erodible banks, <br />(2) erosion-resistant banks, characterized by highly cohesive material or <br />substantial vegetation, or both, <br />(3) moderately erodible banks having medium bank cohesion, and <br />(4) easily erodible banks with noncohesive material. <br />Values of bank erodibility factor varies from 0 for the first type to 1 for <br />the last type of banks. The values of 0.2 and 0.5 have been empirically <br />determined for the second and third types, respectively, based upon test and <br />calibration of the model using field data from rivers in the western U. S. <br />However, bank erodibility factor should still be calibrated whenever data on <br />width changes are available. <br /> <br />A decrease in channel width is accomplished by sediment deposition <br />along the banks or a decrease in stage, or both. For practical reasons, <br />deposition does not exceed the stage in the model. The maximum amount of <br />width reduction at a section occurs when sediment inflow from the upstream <br />section is spread out at this section and the sediment removal from the bank <br />areas at this section is zero. <br /> <br />Within the limit of width adjustment, changes in width are made at all <br />18 <br />