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Road Drainage 



Crossing Natural Drainaqeways 



There are three methods for crossing natural drainageways: fords, 



culverts, and bridges. Factors influencing the appropriate crossing 



include construction and maintenance cost, equipment and supplies 



available, debris potential, stream size, contemplated road use and 



life, foundation conditions, and vertical position of the road relative 



to the stream. 



1. Fords are attractive alternatives for crossing small streams, 

 particularly in areas where large amounts of rock, sediment, and 

 organic debris tend to plug bridges or culverts. Fords cause 

 minimal disturbance to the stream channel (when properly located 

 and designed), are inexpensive, and avoid many of the problems 

 associated with bridge and culvert installation. Fords require 

 stable channel bottoms able to support vehicles or channels that 

 can be protected by gabions or paving. 



2. Culverts (metal or wood) or bridges are required for channels where 

 fords are impractical. Availability of construction equipment 



and materials, size of stream, potential for debris, terrain 

 steepness, and reliability of the calculation for determining 

 culvert capacity are some of the points to consider when deciding 

 whether to use a culvert or a bridge at a given location. Bridges, 

 are preferable, particularly in areas with debris or excessive 

 sediment because the chances of failure are less. 

 Structures should be large enough to carry the flows to which they 

 are subjected within acceptable limits of risk. Costs increase 

 rapidly with size, so adequate local hydrological studies are 

 needed. 



