The development of a risk of failure evaluation tool for small dams in Mzingwane Catchment, Zimbabwe

Small dams in Mzingwane Catchment in southern Zimbabwe are mostly in poor physical condition mainly due to lack of resources for repair and maintenance. Most of these dams are likely to fail thereby adversely affecting water availability and livelihoods in the area. To assist those involved in maintenance, repair and rehabilitation of small dams in resource poor and data sparse areas such as Mzingwane Catchment, a non-probabilistic but numerical risk of failure evaluation tool was developed. The tool helps to systematically, and objectively classify risk of failure of small dams, hence assist in the ranking of dams to prioritise and attend to first. This is important where resources are limited. The tool makes use of factors such as seepage, erosion and others that are traditionally used to assess condition of dams. In the development of the tool, an assessment of the physical condition of 44 (1 medium sized and 43 small dams) dams was done and the factors were identified and listed according to guidelines for design and maintenance of small dams. The description of the extent to which the factors affect the physical condition of small dams was then standardised. This was mainly guided by standard based and risk-based approaches to dam safety evaluation. Cause-effect diagrams were used to determine the stage at which each factor is involved in contributing to dam failure. Weights were then allocated to each factor depending on its stage or level in the process of causing dam failure. Scores were allocated to each factor based on its description and weight. Small dams design and maintenance guidelines were also used to guide the ranking and weighting of the factors. The tool was used to classify 10 dams. The risk of failure was low for one dam, moderate for one, high for four and very high for four dams, two of which had already failed. It was concluded that the tool could be used to rank the risk of failure of small dams in semi-arid areas. The tool needs to be tested in a wider geographical area to improve its usefulness.