Investigating Impacts of Sedimentation on Water Availability in Small Dams: Case Study of Chamakala II Small Earth Dam in Malawi

Abstract

The contribution of multipurpose small earth dams in ensuring equitable and sustainable water supply especially to rural communities cannot be emphasized. Water from small dams is basically meant to improve rural livelihoods of communities dependent on reservoir water. However, there are some issues of interest that need to be paid enough attention if small dam development projects are to achieve their intended goals. In Malawi small earth dams are normally not subjected to detailed sediment transport data assessment despite the fact that currently the Malawi Government has embarked on the construction of about 350 new small dams throughout the country. Reports from some parts of the country show that sedimentation problems of dams have reached levels of great concern leading to anxieties over the availability of useable water resources and examples are given of Nyakamba and Masambanjati Small Earth Dams which were actually silted up. The Ministry of Irrigation and Water Development admits that the problem has reached levels which require serious considerations regarding catchment management issues. This study was therefore carried out at Chamakala II Small Earth Dam in order to assess the impact of sedimentation on water availability for the communities that are dependent on the reservoir water. Field reconnaissance surveys were carried out to collect catchment data on activities carried out by the local communities. Official data was referred to in many cases and interviews mostly unstructured with key informants were conducted. Currently in Chamakala Catchment there are about 8200 people, 1400 cattle, 1100 goats and 200 goats that are said to be direct beneficiaries from the reservoir water especially during dry seasons. The study looked at catchment characteristics and some activities such as cultivation practices, deforestation, land cover, soil characteristics, and grazing practices. Several studies have shown that these parameters are major causes of sedimentation of the small dams in some instances depending on other factors. The dam at Chamakala II had been desilted twice since its rehabilitation works in 2002 but continues to show signs of increasing sediment yield at an alarming average rate of 2250 m3/yr. In addition to the methods described above, a hydrographic survey was conducted at the dam in order to determine impacts of sedimentation on water availability in the dam. In this study a small boat, a staff and a rope marked at known intervals of 10 and 20 metres, leveling instrument and many others were used The study found that reservoir capacity was reduced by 38.7% and its life expectancy reduced by 50%. From the study findings the current rate of sedimentation in the dam is at 2250 m3/yr and 517 Tkm-2y-1 of sediment is transported from the catchment and get deposited in the dam. The assessment shows that if no appropriate measures would be put in place to arrest the current rate of sediment deposition, the dam would lose its capacity by the year 2015 thereby leaving a growing human population at 8.2 percent per annum plus the increasing population of livestock with no water for the most part of the year. The findings from the study show that there is an inverse relationship between sedimentation and reservoir yield which indicates that sedimentation may lead to water scarcity if not controlled. The study sees the need for reconsideration on the need for proper sediment transport assessment if the availability of water resources in small dams is to be sustainable.