The required depth and cross sectional area of a roadside ditch is determined by the slope of the ditch, area to be drained, estimated intensity and volume of run-off, and the amount of sediment that can be expected to be deposited in the ditch during periods of low flow. Triangular or trapezoidal-shaped ditches may be utilized, whichever is appropriate. The ditch cross section is designed so that it will produce the desired water velocity for a given discharge. Minimum full capacity flow velocities should be 0.76 to 0.91 meters/second (2.5 to 3 feet/second) to permit sediment transport. It is best to remember that, in shaping a ditch, given equal grade and capacity, a wide, shallow cross section will generate lower water velocities with correspondingly lower erosion potential than will a narrow, deep cross section. Maximum permissible velocities for unlined ditches of a given soil type are listed in Table 32.
Beaver barriers, trash racks or debris interceptors should not be used because of the potential to block adult salmon without robust and regular maintenance. Bollards at least one bankfull width upstream of the culvert may be used if needed to trap debris or move beaver dam building upstream. However, we have found that these are not usually needed when the culvert is designed with sufficient headroom for large woody debris to pass at the 100-year flood. In addition, our experience has been that beavers are not attracted to entrances as long as there is no increase in velocity and sound over the natural stream conditions.
Velocity Trap download torrent
2ff7e9595c
Comments