HECRas modeling / piers in an overbank

[dwolford]

Several rows of circular columns support a house above the base flood within the overbank of a large river. I have established cross-sections through each set of piers, and include natural sections just upstream and downstream of the piers. Each pier is modeled as an obstruction. My question is: should additional cross-sections be established between the rows of piers to account for open areas between the piers, or do the obstructions not project (extend) between cross-sections? If there is a reference that addresses this specifically that would be helpful.

 

[francesca]

http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_516.pdf

It seems like over-kill to have multiple cross-sections through one house. You could model the house as a bridge and the piers as actual piers. That way you can calculate your scour directly.

 

[dwolford]

The house is to be higher than the base flood. Only the piers will be in flow area. I'm mainly wanting to know if anyone has a reference that indicates whether obstructions set in adjacent cross-sections are interpolated between the sections. In this case, does HECRas see adjacent piers as a wall between piers or is the area between the cross-sections treated as open area by the model?

 

[francesca]

You should read the Hydraulic Reference Manual. HEC-RAS is a 1-dimensional model. The model doesn't "see" anything, it calculates a friction loss at the point of the entered cross-section and determines the HGL accordingly.

You should also read HEC-18 and HEC-20 to see how scour occurs at round piers and how the spacing between them affects the scour.