A circular culvert feeds into box culvert - how to evaluate in HEC-RAS 2

[CanSwat99]

I am evaluating an existing CSP under a highway that ties into a concrete box culvert (50m of CSP while 55m of Conc. box). The suggestion is to line the existing CSP (which is in a pretty bad condition) and provide a manhole at the tie-up to improve the hydraulics. If the lininng affects the culvert efficiency (diameter to downsize from 2m to 1.7m)then a seperate CSP will be put in place along the existing culvert. Although I can utilise other softwares however, I am utilising HEC-RAS as we have to be careful about the upstream water surface levels too. I am thinking of a cross-section at the junction of CSP and box culverts. Could someone help me out with this strange and complex situation?

Thanks in advance and your help will be higly apprecited.

 

[civilperson]

Lining the bottom one fourth of a corrugated steel pipe diameter with smooth troweled concrete will INCREASE the flow characteristics, (Manning's coefficient is smaller for the smooth invert). Parallel lines can require a splitter box to assure sufficient flow depths for minimum velocities.

 

[CanSwat99]

civilperson, thanks for your reply. I understand that but my problem is evaluating it in HEC-RAS. I am not sure how to model and evaluate the two different material culverts (upstream CSP and downstrean concrete box) in HEC-RAS. I am thinking of taking a cross-section at the junction of the two but not sure whether hydraulically it will be OK.

Thanks.

 

[gbam]

Can you establish the hydraulic charateristics of the junction and CSP externally usiong standard hydraulics? Use the computed headwater as your starting condition for your HECRAS model from the inlet upstream.

 

[Bayou]

HEC-RAS does not have the capability to properly model this type of crossing - where the pipe shape & size changes. The best method is perform the culvert calculations outside of HEC-RAS, using the downstream data provided by the model. Then manually input the crossing's headloss into HEC-RAS for the upstream wsel calculations.