Obstructions in HECRas 1

[dwolford]

When using obstructions in HECRas modeling, do obstructions entered on adjacent cross-sections extend between the sections, or do the obstructions only apply at the cross-section location? It seems obvious that obstructions (on adjacent sections)that are not in-line with each other would not project diagonally between the sections, but if used to model seperate piers that are in-line with each other, would the model consider them to act as a wall between sections or would it assume no obstructions between the two sections? Does anyone know of a reference that addresses this. The HECRas manuals stop short of explaining this to that detail.

 

[gbam]

dwolford;
You can easily answer your own question by modeling it both ways to determine the profile through your sections. My first inclination is that obstructions only impact the cross section in which you code the obstruction. From HEC2 days, I recall blocking out a house in the overbank and then tracing the 3:1 expansion on the downstream to determine the shadow affect from the obstruction.

 

[dwolford]

That may provide a reasonable comfort level, but would not necessarily account for all the things that are going on in the model. I would prefer to see documentation.

 

[froude]

From Chapter 4 of the HEC-RAS Hydraulic Reference:

"Ineffective flow areas, levees, and blocked obstructions are only interpolated if both of the user-entered cross sections have these features turned on."

My interpretation would be that if you have 3 cross sections (1, 2, and 3) and you only have a B.O. in XS 2, then the program will not interpolate blocked obstruction to XS 1 or XS 3.