Interpolating Cross Sections in Expansion/Contraction reach

[Anonymous]

I have a question regarding interpolating cross sections around bridges. I calculated the contraction and expansion reach lengths using the guidance provided within the HEC-RAS Manual, which worked out to be about 190 and 330 feet, respectively. In addition, the bridge is situated at the outer extent of a large meander, so the channel and floodplain undergo significant changes in the area of the bridge.

The HEC-RAS Manual suggests that cross sections should be interpolated in between large contraction/expansions reaches so that the losses are not overestimated. However, the interpolated sections are not accurately estimating the changes in topo/geometry.

Is it detrimental to the accuracy of the model if I sample intermediate cross sections within the exp/contr reaches, instead of using the interpolation tool? Obviously this would decrease the effective expansion and contraction reach lengths. However, the cross section and reach geometry would be setup more accurately.

Thanks for any advice.
Kyle

[cameron]

You can have cross-sections in expansion/contraction reaches, you just have to make sure the ineffective areas are added to them correctly.

I generally only use interpolated cross-sections in unsteady models to help with stability. Real data if available should be used over interpolated cross-sections almost always.

[Anonymous]

Thanks for your reply.

If I feel that the expansion/contraction reaches are under represented due to the large reach lengths, would it be detrimental to cut real sections through that area, thus decreasing the effective exp/contr reach lengths?

[cameron]

It would make sense to include them if they impact the wse. You would need to make sure to include ineffective flow areas in those sections following the expansion/contraction guidelines.

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