What is the role of the limiting Froude numbers for channel and overland flow?
A. The limiting Froude numbers for channel, street and overland flows will essentially reduce the velocity by increasing the roughness. When a limiting Froude number is exceeded, the roughness n-value is increased by 0.001 for the next timestep. This continues until the maximum Froude number is no longer exceeded. For certain physical environments, such as alluvial fans with sand-bed surfaces, there is practical maximum Froude that should not be exceeded. Generally, on alluvial fans supercritical flow (Froude No. = 1) does not occur because more sand will be entrained in the flow reducing the flow energy. Unless bedrock is encountered, it is reasonable to assume that flow on alluvial fans will be subcritical. A practical limiting Froude number on steep slope alluvial fans is 0.95. For most river channels at bankfull, the limiting Froude number can be calculated and will generally range from 0.4 to 0.6.
