I am running a steady flow analysis of a creek flowing under a roadway through a simple bridge - one opening, no piers. I am adjusting the width of the bridge to evaluate the affect on the amount of freeboard and am finding that the wider the bridge gets, the greater the depth of flow gets. Also, the depth of flow through the bridge does not seem to be changing proportionally to the change in width (0.22 ft change in depth for 12 foot change in width. Any insight as to what may be causing this result would be appreciated.
Lot of things could be going on here. But whats the DS boundary condition? Whats the profile look like? As a bridge deck widens the ineffective flow areas should adjust as well to compensate for the cross section being closer the the edge of the now wider bridge deck.