Problems with erosion in unlined spillways of dams, have caused
environmental and safety hazards, and necessitate the development
of reliable scientific methods to assess the erodibility of the
In an attempt to analyze the causes and effects of different
geological, geotechnical, hydraulic, hydrological and engineering
design parameters on erosion of unlined spillways, a number of
dam spillways in the R.S.A. were studied. All the parameters
considered important for the erosion resistance of the rock were
collected and correlated with hydraulic factors and observed
extent of erosion.
Rock type, strength, weathering and uniformity of the geological
conditions are important geotechnical considerations while
velocity and energy of the water flow are the main hydraulic
parameters. The main consideration of the study was the effect
of the discontinuities. All the properties of the jointing of
the rock mass, such as number of joint sets, RQD, joint spacing,
joint separation, joint orientation, roughness and filling
material, were surveyed. Joints. weaken the rock mass and induce
removal of the rock blocks if the joint properties are
unfavourable to stability. The jointed bedrock is much less
resistant to flowing or falling water, the result being an
extensive and quickly formed scour.
Various rock mass classification systems have been applied and a reasonable correlation between rock class and extent of erosion
was established by means of slightly modified Kirsten (1982) and
Weaver (1975) rippability classifications.
A number of methods for the prevention and repair of erosion
damage have been proposed.