Having read the Paper in conjunction with the reportla on which it is based, I should like to comment on $5 49-53 and Table 10. 58. I am pleased that Richards' method" is one of the two 'most commonly applied methods of flood estimation for small catchments in Kenya'. Recently my colleagues and I found that Richards' method is used extensively by French hydrologists in North Africa and Iran. British consultants have also applied it to the semiarid and arid regions of South America, Africa and the Middle East.13-15 It has been used in the design of many water resources and hydro-power projects in the UK and recently a comparison of Richards' method and a new methodx6 yielded encouraging re~u1ts.l~ 59; Having established the fact that Richards' method has been applied in a variety of climatic conditions with a good degree of success, the Author's statement in 8 50 should not go unchallenged. I have read Richards' papers11*18.'9 and these show the Author's statement to be inaccurate and that the Author has misrepresented Richards' method.60. First, Richards' method does not assess the runoff factor on the basis of rainfall and catchment characteristics as the Author asserts.61. Second, the method can best be described as quasi-rational inasmuch as it makes the fundamental assumption that the peak flood flow from a catchment results from a rainfall of some average intensity extending to the whole of the catchment and having a duration at least equal to the time of concentration To.62. It is in the determination of T, that Richards' method differs significantly from other methods. Although most formulae specify To in terms of a function of catchment slope and length (e.g. references 2&22) Richards' method, by inclusion of rainfall parameters and especially by the innovative rainfall coefficient, attempts to make allowance for the theoretically correct concept that stream velocity is dependent on depth. The time of concentration is based on the velocity of overland flow which Richards visualized as a sheet of water, the properties of which conform to the Bazin equation. Despite this quasi-theoretical treatment of the progress of overland flow and some other drawbacks (e.g. subjective selection of the runoff factor K ) the method recognizes that velocity (and hence To) is basically a function of depth and catchment roughness.63. Third, Richards' method does not compute floods of specified return periods (say ten years). What it simply does is, depending on the chosen values of the rainfall coefficient, storm shape factor area, reduction factor and runoff factor, give a peak flood
The design of rock dumps comprising reactive rock should aim to limit the potential for the release of acid drainage to the environment. This requires the recognition of a number of contributing factors. Clean runoff should be kept separate from mine water, by diverting runoff upslope of the dump and lining surface drainage channels to be buried by the dump with benign rock. There is a need to selectively place benign and reactive rock to encapsulate the reactive rock. The rock dump hydrology needs to be understood, in particular the wetting-up of the dump by rainfall infiltration, in the context of the climatic setting, the surface hydrology, and the groundwater regime. Finally, a low percolation cover is required over the top of the dump, and side slopes should not be underlain by reactive rock and should be designed to minimise erosion. The paper describes the application of these principles to the design of a new rock dump being constructed at an existing open pit mine that has experienced acid drainage from a past rock dump. The design of the new rock dump to limit the potential for acid drainage required a change in the mind set for rock dump construction, and imposed a number of constraints on site preparation, construction of the rock dump, and the sequencing of rock dumping, which had to be overcome.
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