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~~ Memorandum ~ • <br />` August 27, 1980 <br />Page Two <br />3. Road cuts have several hydrologic effects. First, surface runoff <br />is disrupted, sheet-type overland flow becomes faster-moving, con- <br />centrated ditch water. Erosion could occur not only in the ditch <br />itself if designed improperly, but also at the ditch's discharge <br />point. Utah International assumes that if all this runoff is directed <br />along roads to natural channels, erosion and geomorphic disruption <br />will be less. <br />This is no[ necessarily [rue: <br />A. The transformation of slower-moving sheet flow to ditch flow <br />which discharges into a channel will increase peak flow in <br />that channel in most cases, with the resultant erosion and <br />geomorphic changes. This is especially true in the case of <br />interflow (subsurface flow) which might not normally appear <br />in the channel for a much longer time than if it flows into <br />a roadcut and runs directly to a stream. The interflow com- <br />ponent is very important in areas where snowmelt is thr. cause <br />of highest peak flows. <br />B. Ditching runoff long distances along a haul road or across a <br />catchment boundary could decrease [he net runoff in one area <br />and increase it in another which could alter streamflow <br />regimen and even vegetation. <br />C. In areas of steep slopes, road cuts frequently slump into drain- <br />age ditches. If runoff were not culverted across the haul road <br />and dumped on slopes that provide some vegetative sediment <br />trapping, [he slumped material would be transported directly <br />into [he stream. <br />In summary, while Utah International is correct that the culvert spacings <br />in [he regulations may have little relevance to necessary function, culverts <br />beneath haul roads serve other purposes than preventing erosion of the road <br />and its associated ditches. Three purposes for culverts "that are periodically <br />spaced to simply move water from one side of the road to the other when there <br />is no natural drainage to recieve the water" have been mentioned: <br />Keeping peak flows closer to the natural. <br />2. Conserving the hydrologic relationship between two parts of a stream <br />or two different catchments. <br />3. Spreading sediment over the slopes rather than dumping i[ in a <br />s[ream.~ <br />