Analysis of Flow through Vegetation

The Manning n value is quantitatively evaluated for flow in heavily vegetated channels. Momentum analysis shows that the composite n value as a function of flow depth is dependent on the bottom roughness and the density of vegetation. In some cases, the vegetation density can be evaluated directly from physical measurements of the vegetation. However, in most situations, it must be evaluated indirectly from limited field data. The results show that the n value increases in proportion to two-thirds power of the hydraulic radius for conditions where the vegetation density is a constant over the flow depth. Field application of the flow resistance model are described for projects concerned with flood routing, backwater computations, extension of rating curves, channel improvement work, and erosion control.