Drainage systems reduce the water level inside a potentially unstable hillside, which leads to reduction in pore water pressures in the ground and an increase in the shear strength within the slope. The reduction in pore pressure by drainage can be achieved by shallow and/or deep drains, depending on hillside morphology, the kinematics of movement predicted and the depth of creep surfaces. Usually, shallow drainage is adopted where the potential hillside movement is shallow, affecting a depth of 5-6m. Where there are deeper slippage surfaces, deep drainage must be introduced, but shallow drainage systems may also be installed, with the aim of running off surface water.