Hydraulic evaluations are generally focused on scour and clear opening capacity, but the effects on overall hydraulic performance can be overlooked, or only peripherally addressed. Be sure to optimize your hydraulic evaluation by checking the following important details.

Low Steel. This dictates the allowable high water level. However, low steel is not always the fascia girder. Many older structures have a shallower fascia girder because they were designed before codes were changed, or are located under sidewalks. Check all beam elevations to determine the correct high water design elevation.

Scour. Entrance and exit openings are frequently, but not always, prime scour locations. Conduct a thorough evaluation at critical sections within the bridge opening to identify other potential scour locations.

Bridge Skew. A change in bridge skew can improve flow performance and minimize the potential for scour and bank erosion by better aligning a waterway upstream and downstream of a bridge. A bridge replacement should not be initiated without investigating alternative skew angles and optimizing hydraulic performance along the affected stream channel.

Approach Features. Identify drainage pipes, meanders in the stream approach, utilities, and similar features located in an influence area. Check channel geometry and the sizes of upstream and downstream structure and weirs. An oversized opening for a channel that is naturally narrower will silt in and not perform as modeled. The new structure should not be enlarged based on the site study without considering the impacts on the next downstream structure. Are there problems downstream? Will a new structure aggravate the situation?

Local Input. Often, the best input is local. Talk to residents adjacent to the site, the local highway department, or town and village officials. Get a sense of the history of the site, discover flood history and high water elevations, and learn the concerns over flooding from residents. Do basements or backyards flood? Just how high did the water get during Hurricane Agnes? This goes a long way towards determining the appropriate design approach and will smooth the approval process by involving the residents in the project.