TITLE:
Estimation of Small Reservoir Sedimentation in Semi-Arid Southern Zimbabwe
AUTHORS:
Tavengwa Chitata, Francis Themba Mugabe, Japhet Joel Kashaigili
KEYWORDS:
Hydrographic Survey, Sedimentation, Semi-Arid Area, Small Reservoir, Water Resources
JOURNAL NAME:
Journal of Water Resource and Protection,
Vol.6 No.11,
August
20,
2014
ABSTRACT:
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 sustainable management of
surface reservoirs. The study investigated the capacity loss due to
sedimentation 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 efficiency to the capacity-watershed area ratio and
capacity-inflow ratio. The results show that Mutangi 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,
9110t·yr-1 and 8265 t·yr-1 for the hydrographic survey,
grab sampling and water depth-capacity method 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.