Interceptor Ditches

Figure 3-13.-Types of culverts. a deep V-bottom ditch, the side slope is 3:1, 2:1, or 1:1. Culverts The side slope for a trapezoidal ditch is 3:1,2:1, or 1:1. The flat bottom is generally 2 feet wide but can range from 1 foot to 6 feet or more. Interceptor Ditches The volume of water draining into a roadway ditch can be decreased by the use of shallow ditches that extend around the top of the cut and intercept the water draining from the original ground toward the roadway. An interceptor ditch shown in figure 3-12 is dug 2 or 3 feet behind the backslope limits. Its size depends on the original ground slope, runoff area, type of soil and vegetation, and other factors related to runoff volume. Diversion Ditches As it leaves the cut, water from the roadway ditches cannot be allowed to pond in the ditches or against the roadway fill. Therefore, diversion ditches are dug to carry the water away from the roadway to natural drains. These drains can be rivers, streams, gullies, sinkholes, natural depressions, or hollows. Sometimes it is necessary to have the water flow from one side of the road to the other or have the road cross a small stream. You do this with cross drains. They are called culverts if they are 10 feet or less in width. Over 10 feet wide, they are called bridges. Culverts are made of many materials, such as corrugated metal, reinforced concrete, concrete pipe, timber, logs, and even open-ended oil drums. The type of material selected is dependent upon various factors including, in part, the type and life expectancy of the road. For permanent roads and highways with concrete or asphalt paving, the most durable of materials, such as reinforced concrete or concrete pipe, should be used. Concrete pipe is one of the strongest and most durable materials used in making culverts. The shell thickness and length depend on the pipe diameter. (The larger the diameter, the thicker the shell and longer the section.) Pipe diameters are nominal inside dimensions. For semipermanent and temporary roads, the design engineer may choose to use materials such as those shown in figure 3-13. 3-13