Floodway Encroachment- Method 4

[mreme]

I am trying to map a flood way encroachment for a stream using encroachment method 4 by specifying a target water surface elevation of 0.2 feet. When I look at the results the difference between the normal calculated water surface elevation and the encraochment water surface elevation is more like 0.3 feet. The state will only let us show a difference of the 0.2 feet. Why is there this difference if I specify a target increase of only 0.2 feet.

Thanks!!

Mark Reme
Birdsall Engineering, Belmar, NJ

HEC-RAS simply is doing a "target" rise in water surface elevation. You can put in several target water surface elevations for a rise with the floodway like 0, 0.05, 0.1, 0.15, and 0.2'. You could also change the target water surface elevation increase in the area where it is resulting in the 0.3' rise.

I also suspect that when you submit your model for review that you will use Method 1 (at least all our models use Method 1 when we submit for FEMA review).

Hope this helps!
Nancy Gambill

Some additional information about Method 4 is found in HEC-RAS 3.0 from January 2001 in the Applications Guide see 6-8 to 6-19. It really explains how HEC-RAS is determining the water surface elevation.

Nancy Gambill