Journal of water resources and protection, 2014; 6: 1017-1028
Small surface reservoirs play an important role of providing ready and convenient source of water
for various uses in semi-arid areas which are characterized by erratic and low rainfall. Lack of
current data on reservoir capacity loss due to sedimentation is one of the challenges to the sus-
tainable management of surface reservoirs. The study investigated the capacity loss due to sedi-
mentation from 2000-2012, and estimated the trap efficiency of the Mutangi reservoir which is
located in semi-arid Chivi, Southern of Zimbabwe. Hydrographic surveys, grab sampling and water
depth-capacity methods were used to determine the capacity of the dam as of 2012. To compute
capacity loss from 2000 to 2012, the 2000 and 2012 dam capacities were compared whilst the trap
efficiency of the reservoir was determined using a set of empirical models that relates trap effi-
ciency to the capacity-watershed area ratio and capacity-inflow ratio. The results show that Mu-
tangi reservoir has a trap efficiency of 95% - 98% (av = 96.4%) and has lost 37% of its capacity
due to sedimentation in 12 years (2000 and 2012). Rates of sedimentation were 8539 t·yr −1 , 9110
t·yr −1 and 8265 t·yr −1 for the hydrographic survey, grab sampling and water depth-capacity me-
thod respectively, and the little difference in these figures demonstrates that any method can be
used to determine sedimentation rates. The area specific sediment yield (ASY) ranged from 14 -
15.5 t·ha −1 ·yr −1 (av = 14.956 t·ha −1 ·yr −1 ). At the current rate of sedimentation the projected dead
level of the reservoir will be lost to sedimentation in 8 years while the useful life of the reservoir is
estimated to be 30 years. Capacity loss due to sedimentation is further complicating the already
strained water scarcity situation in semi-arid areas and management decisions should be made
based on the current sedimentation rates estimated by different methods. These results imply that management practices that reduce erosion, hence sedimentation in these small reservoirs
should be practiced in order to prolong their lifespan.