I’m working on a 1D hydraulic model of an open channel with a downstream culvert. The culvert is represented as an open orthogonal section. I’m trying to understand why the upstream water surface elevation increases in the channel even though I widened the cross-section upstream of the culvert entrance. My current assumption is that the increase in water level is caused by contraction losses associated with the transition from a wider channel section into the narrower culvert. What are the recommended practices for modelling these transitions in a 1D environment?