Slope Drainage

In this picture, the hazards of mass movement are mitigated with drainage lines within the cliff face, field drains, gravel trenches and by intercepting overland flow. Shallow surface drains are used to intercept the overland flow while vertical drainage is employed to remove the water from the cliff face as well as the body of the cliff.

Some of the mass movement processes operating on cohesive materials occur over very long time spans. One of the most widespread of these processes is soil creep. Soil creep involves the movement of slope sediments in a series of numerous cyclical. The cyclical effects of temperature fluctuations, variations in moisture, and gravity on inclined soil sediments often cause this process.

Soil creep can only occur on a slope. It occurs when some mechanism causes the surface soil layer to expand and contract. Mechanisms that can cause this expansion and contraction include cyclical changes in soil temperature and moisture. Expansion moves soil particles upward and perpendicular to the angle of the slope. When contraction occurs the soil particles move downward under the influence of gravity. As depicted in the animation, the angle of this movement differs from the angle of the movement due to expansion. The net result is a slow downslope movement of the slope's materials. Thus, the drainage prevents this from occurring by removing water from the cliff face.

The drainage is especially useful in clay cliffs which are susceptible to slumping and rotational slides which are triggered by high water content in the clay. The method deals with the main factor of cliff-face failures – the action of water – and therefore it is one of the most effective way of strengthening the cliff.

However if successful, changes in the cliff hydrology can have impacts on the ecology and land uses of the cliff top. Successful drainage schemes may also result in subsidence of cliff-top land as the cliff dries out. Implementation of a comprehensive drainage network sometimes requires a considerable level of technical know-how and expertise.

Done By : Jonathan Chua, 2AD2