I have a model rec’d from FEMA that represents a stream in NC. The stream is approximately 2.5 miles long and is represented by approximately 78 cross sections. There are several culverts in the area I’m working in. The area where I’m working is on the outskirts of the small town best described as suburban.
Our client insists on putting a structure in the floodway against our recommendations. The original model was approved by FEMA in 2007 with a revision in 2010 (LOMR). Since that time there have been several changes on the ground that are significant including the removal of some buildings and the addition of others. The model also has several large ineffective areas not associated with a culvert or a bridge. I have modified the FEMA model to reflect these changes on the ground by adding and removing the buildings using blocked obstructions in the appropriate XS’s. I have not modified any of the ineffective areas nor changed any “n” values. I haven’t changed any of the culverts either. In one instance, I added a 75 ft and a 60 ft bldg. (blocked obstruction) in a cross section that is 600 +/- feet in width.
The results don’t seem reasonable to me. They show that the water surface elevation doesn’t rise but that the velocity and E.G both increase in the XS’s affected. I would think that blocking out significant portions of any cross section would result in some rise in elevation. Right now the results show 0.00. I have not yet run an encroachment analysis.
Any thoughts?
Assuming the study reach is experiencing subcritical flow, it is typical for a constriction to cause a decrease in computed water surface elevation and an increase in velocity at the location of the constriction. However, you will often see an increase in WSEL further upstream of the constriction.