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6.~ Stabl• Alluvial Channel Deslan - Method of Maximum Permissible Velocity <br />6.x.1 General proosdur• <br />Tro major variables effecting channel design end sediment transport ace <br />velocity and shear stress. In reslity, determining shear stress is usually <br />difficult. Sfiarafora, velocity is often accepted a• the moat S~ortant factor <br />Then designing stable alluvial channels using the static equilibrium approach. <br />The procedure is based on the condition that SC the adopted mean velocity is <br />Loves than uxiwm permissible velocity (or the nonerodible velocity), the <br />channel i• aaa~aed to be stable (Fortier and Scobey, 1926). <br />Appreciable work has bean devoted to developing the permissible velocity <br />approach. Many limits have been suggested for eha permissible velocity under <br />given conditions; horewr, experience has identified discrepancies in these <br />values. For example, channels carrying sediment may ba stable at velocities <br />higher than the given liaiting velocity. Consequently Fortier and Scobey <br />(1926) introduced a certain increase in their listed values o[ maximum <br />permissible valoeitias rhan rater was transporting colloidal silt. The <br />authors emphasi:ad the importance of exercising judgment on each particular <br />problea. Subsequently Lhasa Baits rer• recommended by a Spacial Committee on <br />• Itrigatios Aaseareh, ASCE. Sines then many designs have been based on their <br />suggested permissible velocities. <br />Tabl• 6.1a •uasarises the permissible w Loeities given by Fortier and <br />Scobey. Other tabular listings o! permissible velocity are given in Tables <br />6.1b, 6.1c and 6.7d. <br />Th• design procedure for a trapewidal channel acing the maximum per- <br />sisslbla velocity consists of the following steps (Chw, 1959): <br />• <br />