Vegetation scour velocities. 1

[dkDan]

I am looking for assistance in determining when and if vegetation will washout during a flood event. The determining factors that I see are: Water Velocity, Water Depth, Vegetation Type, and Soil Type.
I would like to use this information for adjusting Manning's (N) values during the 100yr storm event, because of loss of flow restriction from vegetation being washed out.

Thank You,
Daniel Wagner, EIT

 

[bpattengale]

Determining Manning's value for various vegetative covers is an itterative process. HEC-15 goes through the design process in detail. You can download it here

http://www.fhwa.dot.gov/engineering/hydraulics/library_listing.cfm?archived=false

Now to determine if the vegetation will washout you need to determine the sheer stress that the vegetation will be exposed to and see if it greater than the permissable sheer stress for the class of vegetation. The permissable sheer stress for various class of vegetation are listed in HEC-15 along with how to calculate the max sheer stress.

Basically the deeper the water over the vegetation and the steeper the channel slope the higher the shear stress is going to be. So to calculate if the vegetation will washout determine the permissable sheer stress of the vegtation and back calculate the depth of the water. If your 100-year high water level exceeds this depth then the vegetaion will washout.