Geomorphic,Aquatic,and Engineering Design Considerations for Low-Water Crossings

Abstract

A wide variety of low-water stream crossings have been built on low-volume roads across the United States over the past 50 years with the primary objectives of traffic access and cost savings. They are generally a rigid structure in a dynamic stream system, so most have worked to varying degrees from a road use standpoint. Many have required maintenance, repairs, or design modifications to function as they exist today. In addition, most structures have caused problems with regard to stream sediment transport, promoting upstream aggradation or downstream scour and creating barriers or limitations to the passage of aquatic organisms. Three fundamental types of fords both maintain good stream function and provide for traffic use: simple at-grade fords, low-water bridges, and vented fords with a large waterway opening area, preferably with a natural stream channel bottom. Each has a large waterway open area that does not constrict most flow. Ideally the width is at least equal to that of the natural bankfull channel. This paper discusses the problems observed and the requirements for a ford to meet stream function needs, pass sediment appropriately, provide for the passage of fish and other aquatic organisms, and meet engineering design needs. Some fords are designed specifically to function in extreme dynamic conditions, such as in debris torrent or alluvial fan channels, in desert or other “flashy” hydrologic regimes, or where debris jams and overbank flooding are frequent. Fitting the structure to the site requires experience and sound geomorphic, hydraulic, and engineering design. The task should be undertaken with an interdisciplinary team approach.