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An Evaluation of Reservoir Sedimentation: The Case of Macagtas Water Impounding Project, Catarman, Northern Samar
Abstract:
The rate of sedimentation of the Macagtas reservoir was observed for one year. Stream velocity, cross-sectional area of stream flow, and sediment concentration of the water were monitored during rainfall greater than 18 mm.
The relationship between water discharge and sediment inflow for the two streams draining the watershed was analyzed using the simple linear regression model. A direct relationship between these variables was shown; a correlation coefficient value of0.99 was obtained for both streams.
For the entire period of the study, the computed sediment discharge in the reservoir was about I, 702 tons. This value was higher than previously computed rates which were based on the estimated annual soil erosion loss rates using the Modified Universal Soil Loss Equation Model. Soil erosion loss rates were estimated based on three land-use maps representing the periods before project construction and during operation phases. Land-use trends indicated various stages of watershed transformation.
The estimated soil erosion loss during the period of study was 2,475 t/yr. With a reservoir sediment inflow of 1,702 t/yr, the sediment delivery ratio (SDR) was about 69 percent. Sedimentation rates in the past and future were computed based on the above SDR value and an assumed reservoir trap efficiency of 93 percent. It is predicted that if the present rate will remain constant, the dead storage volume capacity will be filled up with sediments after 25 years (since 1976) of continuous water impoundment.
The sediment inflows for the projected 30-year period under the scenarios with and without watershed management conditions are 35,348 tons (27,191 m3) and 47,672 tons (36,666 m3, respectively).
The damage cost of sedimentation was determined by comparing the benefits that can be derived from the with and without watershed management options. However, benefits and costs which were difficult to quantify were not included in the analysis. At a 10 percent discount rate, the economic analysis showed that the net present value for the with watershed management was P29,378,845.00 and only P2,717,81 0.81 for without watershed management.
The net present value of the watershed management project was tested using a sensitivity analysis and found to be relatively insensitive to the various assumptions made in the analysis.