Hydrologic and hydraulic design for low water stream crossings
A low water stream crossing (LWSC) is a structure that provides roadway access over a waterway. It is designed to be periodically overtopped with high stream flow and therefore closed to traffic during those flood events. There are three types of LWSCs: unvented fords, vented fords (with pipes), and low water bridges. In hydrologic design, daily flow data used to develop flow-duration curves is more suitable than conventional methods using annual flow data since the acceptable number of days for road closure during a given year can be considered. In hydraulic design, overtopping flow depth on a raised unvented ford is influenced by LWSC height and the design process can be simplified by setting LWSC height to a maximum level. For vented fords, inlet control design for culverts results with larger pipe size than outlet control design in most cases. Therefore, culvert sizing is simplified because outlet control does not have to be checked. Structure position and height is important for low water bridge design and placement should be planned so that channel disturbance is avoided. The result of this study is a systematic approach for hydrologic and hydraulic design for LWSCs. Hydrologic and hydraulic design guidelines and procedures are provided so that analyses and design computations can be completed by LWSC designers and planners.