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HECRas modeling / piers in an overbank

HECRas modeling / piers in an overbank

HECRas modeling / piers in an overbank

(OP)
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.  

RE: HECRas modeling / piers in an overbank

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.  

RE: HECRas modeling / piers in an overbank

(OP)
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?  

RE: HECRas modeling / piers in an overbank

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.  

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