9.3.1 Surface Drainage
Surface water allowed to flow down a slope or to pond on benches of a slope can infiltrate into the ground along discontinuities and thereby cause an increase in the driving forces on an unstable area through a buildup in pore pressure. Grading and shaping are major considerations in the control of surface water. Surface water can be controlled through a combination of topographic shaping and runoff control structures (Glover et al. 1978). Topographic shaping is used to control the rate and direction of surface water flow by manipulating the gradient, length, and shape of the slope. Grading benches to divert water away from the slope face and off the bench. Flatten the gradient of the slope to encourage sheet runoff as apposed to channel flow.
Surface runoff is usually collected in permanent facilities such as V- or U- shaped concrete lined or semicircular corrugated steel pipe channels and diverted away from the slide mass (figure 12).
In climates experiencing intense rainfall that can rapidly saturate the slope and cause surface erosion, it is beneficial to construct drains both behind the crest and on benches on the face to intercept the water for stability (Government of Hong Kong, 2000). These drains are lined with masonry or concrete to prevent the collected water from infiltrating the slope and are dimensioned to carry the expected peak design flows. The drains are also interconnected so that the water is discharged to the storm drain system or nearby water courses. Where the drains are on steep gradients, it is sometimes necessary to incorporate energy dissipation protrusions in the base of the drain to limit flow velocities. In climates with high rainfall there is usually rapid vegetation growth, and periodic maintenance is required to keep the drains clear.
Figure 12: Network of ditches which converge to carry surface waters away from the instable slope