Figure 3-11.—Types of ditchesFigure 3-12.—Drainage systems.the bottom of the ditch to slow down the water. Ingeneral, a moderate velocity is desirable because itprevents excessive erosion and can offset the pendingeffect of slower moving water.One factor involving the volume of water thatcannot be controlled is the rainfall itself. The moreintense the rainfall and the longer the duration, thegreater the volume of water the ditch has to carry.Talking to local residents and observing high-watermarks along streams are helpful to the engineer indetermining the heaviest rainfall to expect in a particulararea.The engineer must consider not only the factorsinvolving the volume of water but also the design of theditch itself. Two common types of ditches are theV-bottom and the flat bottom, or trapezoidal, ditch.Examples of these ditches are shown in figure 3-11.Under similar conditions, water flows faster in aV-bottom ditch than in a trapezoidal ditch. The sideslope for a shallow V-bottom ditch is 4:1 or greater. For3-12
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