(Hydraulic Engineering Circular No. 25) The purpose of this manual is to provide guidance on hydraulic modeling for bridges over tidal waterways. This document includes descriptions of: (1) common physical features that affect transportation projects in coastal areas, (2) tide causing astronomical and hydrologic processes, (3) approaches for determining hydraulic conditions for bridges in tidal waterways, (4) applying the hydraulic analysis results to provide scour estimates. By using the methods in this manual, better predictions of bridge hydraulics and scour in tidal waterways will result. In many cases, simplified tidal hydraulic methods will provide adequate results. However, when the simplified methods yield overly conservative results, use of the recommended modeling approaches will provide more realistic predictions and hydraulic variables and scour.