Maximum Allowable Velocity Prediction for Vegetated Waterways

Abstract

ABSTRACT PHYSICALLY based equations were developed to estimate the maximum allowable velocity and the permissible shear stress for vegetative channels. Two alternative equations were provided. The input variables required to make predictions include fraction of vegetative cover at the soil surface, slope of channel, and maximum allowable velocity or the maximum allowable shear stress for bare soil conditions. The physically measurable variable, fraction of vegetative cover at the soil surface, is the only grass characteristic used to estimate the maximum allowable velocity or largest permissable shear stress. Predictions from the equations agree well with values given in the Soil Conservation Service Handbook of Channel Design for Soil and Water Conservation. An R^ value of 0.97 was obtained which was significant at the 99.9% level. Based on the derived equation, fraction of vegetative cover near the soil surface was determined to be an appropriate variable to assess the quality of vegetation for waterway design. The derived equation was modified to estimate the maximum allowable velocity in waterways downstream from infrequently used emergency spillways. Finally, design limitations due to low flow or trickle flow conditions were discussed.