How best to model a bridge running parallel with river over a canal/ditch

[godzilla]

Modeling a major river with a canal/ditch entering at 90 degrees. This ditch has a huge culvert under a road running parallel for a way down the river side. A lateral weir with culvert would work there. If I were going to model a bridge to replace that culvert, what is the best lateral structure to use? And as you can see in the attached image, the low area (reds and yellow/green) the canal leads to is also the low area along the downstream river. For the low areas, 2D flow area or just use xsecs? You might can see where the culvert crossing is just before the river turns 90 degress and heads southwest.

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