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II rl I ~I ;1 I I IJ ., I) i) " I wi , I , . 'I 8; I . I !!lII I .. _>--+_~___,~________, :;::::.._..____ _v._~.._____..- . . F = Froude number .........,..-...,... ..-..--.-, v = velocity, feet per second g = acceleration of gravity, feet per second2 D = hydraulic depth, feet, defined as the cross- sectional area of the water normal to the flow divided by the width of the free surface. If F is less than unity, the flow is subcritical. In this stat~_the forces of gravity are dominant and the flow is tranquil and streaming. If F is greater than unity, the flow is super- critical. In the supercritical state, inertial forces are dominant and flows are torrential and of high velocity. The following criteria apply for the design of open channel systems: 1. Velocities Channels shall be designed to ensure subcritical flows at peak flow. Addition of drop structures as required to maintain subcritical flow may be necessary. Drop structures shall be designed to prevent drawdown at peak flow. Minimum velocities for all channels shall not be less than two feet per second for the five year storm event. Maximum velocities in unlined channels shall be limited to the values given in Table IV-2; for grass lined channels the values in Table IV-3 apply. CHANNELS permissable Velocity Lp.s. 1. 50* 1.75* 2.00* 2.00* 2.50 2.50 3.75 3.75 6.00 2.50 3.75 4.00 4.00 5.00 TABLE IV-2 PERMISSIBLE VELOCITIES - UNLINED Material Fine Sand, colloidal Sandy loam, noncolloidal Silt loam, noncolloidal Alluvial silts, noncolloidal Ordinary firm loam Volcanic ash Stiff clay, very colloidal Alluvial silts, colloidal Shales and hardpans Fine gravel Graded loam to cobbles when noncolloidal Graded silts to cobbles when colloidal Coarse gravel, noncolloidal Cobbles and shingles n 0.020 0.020 0.020 0.020 0.020 0.020 0.025 0.025 0.025 0.020 0.030 0.030 0.025 0.035 35