Local Road Management Manual 444 The longitude gradient for ditches should usually be 2% but may be flatter over short lengths. In very flat areas this may be difficult to achieve and the discharge of water after heavy rainfall may be slow. The maximum gradient for unlined ditches should be 5%. A grass cover will assist in protecting against erosion (IBRD, 2005). Alternatively, the slope of the ditch may be made less steep by building a series of steps in the base of the ditch, using check dams (Figure 5). The check dams are usually constructed of stone, although other materials such as wooden stakes may also be used.