You are in charge of evaluating the safety (hazard) of a low water crossing. Tra

Question

You are in charge of evaluating the safety (hazard) of a low water crossing. Traffic surveys show that the "Aaverage"vehicle using the crossing is semi-rectangular in shape (10 ft long, 5 ft wide, and 4.0 ft high - not including the distance from the roadway to the floor of the vehicle, 1.5 ft), and weighs approximately 3,000 lbs. Assume the vehicle, including the engine compartment is completely sealed. Each tire has about 1 ft by 0.67 ft of tire tread in contact with the road surface. Assume that the specific weight of the water flowing is 62.4 lb/ft3 (likely a low estimate since the water is carrying sediment).If the coefficient of static friction (cs) is 0.35; the coefficient of drag is 2.1 (length of vehicle is perpendicular to flow); the velocity of flow, regardless of depth, is 5 ft/s; and the total vehicle weight is equally distributed to all tires. Given this

B.At what depth of flow does the vehicle begin to move?

Explanation / Answer

When deciding whether to use a low-water crossing and which low-water crossing type to select, it is important to evaluate the following: the site, costs, streamflow patterns, channel characteristics, and aquatic organism passage (AOP) needs. The various factors can be complicated and interrelated, but the selection process is simplified by a two-step process. First, evaluate whether a low-water crossing structure is appropriate and preferable to a culvert or bridge. Second, decide on the appropriate type of low-water crossing based upon the site characteristics and AOP needs. Each decision can be reached by considering these basic questions: ¦ Is the road a noncritical route or is there alternative access to the area? ¦ Is the traffic use low and are occasional traffic delays acceptable? ¦ Is the channel ephemeral or does it have relatively low baseflow? ¦ Does the watershed have large flow fluctuations or a “flashy” response? ¦ Does the channel carry a large amount of debris? ¦ Is the channel unentrenched to moderately entrenched (broad and shallow)? ¦ Is a low-water crossing the most cost effective or inexpensive structure? If the answer to most or all of these questions is YES, then the site is likely a good candidate for a ford or low-water crossing. ¦ Does the stream have low or zero baseflow and “flashy” high flows? If YES, first consider a simple (at-grade), unimproved ford. ¦ Are the channel bottom and streambank materials soft or erodable? If YES, consider an improved ford with a hardened driving surface. ¦ Is AOP or maintaining stream function an important issue at this crossing? If YES, consider (1) an unimproved ford with a natural bottom; (2) an improved at-grade ford with a roughened driving surface, (3) a low- water bridge, or (4) a high-VAR ford. Low-Water Crossings ¦ Is driving through water frequently prohibited or are long traffic delays unacceptable? If YES, consider only the vented structures and low-water bridges with an elevated driving surface. ¦ Is the channel incised or entrenched? If YES, consider a vented structure with boxes that match the channel’s shape. ¦ Is the channel very broad or does it carry a considerable baseflow with high peak flows? If YES, consider a relatively long span low-water bridge. ¦ Does the channel carry a lot of large woody debris? If YES, consider an unimproved or improved unvented ford. ¦ Does the drainage pass periodic debris torrents through an incised channel? If YES, consider rock-fill fords. Alternatively, massive concrete vented fords have been used with trash racks to pass the debris over the structure. ¦ Is a barrier needed to exclude exotic species? If YES, consider an improved, unvented ford with a raised platform or a raised vented ford with a perched outlet (consider, however, potential adverse channel effects). ¦ Is a grade control structure needed? If YES to promote aggradation, first consider an improved unvented ford with a raised platform (a low dam). A vented ford with perched vents may also work. If YES to stop headcutting, consider using a structure with a solid, stable bottom and downstream cutoff wall.

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