Cross Section Locations On A 90 Degree Bend

[HD]

Hi,
I am modelling a manmade trapezoidal channel with a 90 degree bend and require advice regarding the positioning of the cross sections through the bend. The attached sketch plus text below will hopefully explain my problem.

Immediately upstream of the 90 degree bend the watercourse passes through a culvert. Consequently the 4 no. typical cross sections associated with this structure are required.

Approx. 50m downstream of the bend the watercourse passes through a bridge. Again 4 no. cross sections are required.

In between the channel profile changes requiring 2 no. additional cross sections.

As you can see this 90 degree bend is becoming very congested. Furthermore if the cross sections are drawn perpendicular to the channel they all cross, which is not acceptable.

To add to the problem the site is in a coastal setting with very flat floodplains to the LHS and RHS. This means the exact overbank flow path is unclear and “glass walling” of the model is a distinct possibility should flows exceed the channel capacity.

How should the cross sections be surveyed to prevent crossing and ensure model accuracy?

Is a possible solution to use the plane of confluence technique applied to junctions?

Thanks for the assistance. Greatly appreciated.

[cameron]

Can you model the overbanks as 2D with a 1D channel?

[Anonymous]

There are bend losses (due to secondary circulation, etc.) and superelevation effects that are not included in the 1-D unsteady flow equations. There is some guidance for increasing the n-value to reflect these phenomena (e.g. Chow, “Handbook of Hydraulics”).

[V]

Has anyone added minor losses in HEC-RAS to account for bend losses rather than adjusting the n-values? Are there any references or guidance on how to apply minor losses at large sharp bends?

I know that the losses will be a function of the radius of curvature and the cross section top width. Any additional guidance or ideas would be appreciated.

[V]

[Author]

Posts