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Situation:
Advances in HEC-RAS now
allow for improved simulation of overbanks using 2D Flow Areas. Coupled 1D/2D
models where 1D cross-sections represent the bank-to-bank cross-section data
and 2D flow areas represent overbank areas has greatly improved the accuracy
and robustness of HEC-RAS models. This is particularly the case in flat, urban
areas with significant overbank flow paths.
These coupled 1D/2D models
have highlighted several simplifications and shortcomings of the traditional HEC-RAS
Junction methodology. Note that the standard method is still required in 1D
steady flow modeling. The following
points suggest an alternative Junction method is worth considering when
building a coupled 1D/2D model.
- 1D to 2D offline flow transfer over the junction length is not possible in HEC-RAS. In other words, a lateral structure cannot span across a junction. In complex confluences, this transfer region may be critical. Simply reducing the distance between bounding cross-sections to minimize this region may not be an option depending on the channel and bank alignments (see Figure 1).
- The volume of water within the bounding cross-sections of junctions is not accounted for, whether forcing the water surface elevation to match the downstream bounding cross-section or using the Standard Step one dimensional Energy equation. Both solution techniques simplify the hydraulics of the region, and in particular, the main reach which conveys the greatest volume (see Figure 1).
Simplification of Junction hydraulics has been “accepted” as reasonable, as demonstrated by the following guidance from the USACE-HEC:
