One current limitation in HEC-RAS 5 is that when you put a culvert or gate in a 2D Area Connection, flow through the culvert or gate can only go from a cell adjacent to the 2D Area Connection line on the upstream side to a cell adjacent to the 2D Area Connection on the downstream side. If your cell size is relatively small and your culvert is relatively long, then your culvert may in reality span many cells between its upstream side and downstream side. But the current limitation doesn't allow this.
But we're in luck! Mr. Con Katsoulas, a senior water resources engineering from SMEC in Sydney Australia came up with a method to get around this limitation. He first discussed this in The RAS Solution forum. And my friend and colleague, Mr. Krey Price of Surface Water Solutions wrote a detailed explanation on how to use this technique. Please read his blog post on wormhole culverts for the full description. Good luck! Give it a try and let me know how it works for you.
