| Summary: | Bengawan Solo often produces flooding and creates sufficient damage and
loss. One of flood control efforts in the upstream Bengawan Solo River Basin is
conducted through the operation of Wonogiri Reservoir during flood season.
Changes in land use and characteristics of high resolution rainfall in the
catchment area of Wonogiri Reservoir allow changes of inflow hydrograph.
Therefore, a study to evaluate the operational pattern of Wonogiri Reservoir and
to improve the performance of flood control is urgently required.
Two important analyses are carried out to clarify the flood control issue,
i.e. inflow flood hydrograph based on the current land use and the operating rule
of spillway gates for flood period. The current inflow flood hydrographs are
obtained by using a semi distributed rainfall-runoff model. For this purpose the 2,
60, 500 and 5500 (PMF) year return period of flood hydrographs are simulated
with input the calibrated watershed parameters and hourly design rainfall
distribution of the current condition. The hydrologic simulation results are then
used to simulate several scenarios of reservoir flood routings for obtaining the
optimum spillway gates operation. The optimum flood control is determined by
considering the controlled water level (+ 135.3 m MSL) and the extra flood water
level (+ 139.1 m MSL) as the critical boundaries of water level during flood
period.
The results of the study indicate that the watershed response to the peak
discharge changes considerably. The current land use produces flood inflow
hydrographs that higher than the design flood hydrographs, both peak discharge
and total runoff volume. For the spillway gates operation, it is found that the
operating rule for the PMF has to be modified. In order to avoid high risk of dam
over-topping, full opening gate is operated when the water level in the reservoir
reaches +136.6 m MSL instead of +138.2 m MSL. Besides, the spillway gates
operation time should be monitored every 15 minutes when the water level
reaches +136.4 m MSL and outflow discharge arrangement is a multiple of 50
m
3
/s. Generally, it is concluded that Wonogiri Reservoir is still able to control the
extreme floods with special consideration for the PMF. Real time flood
forecasting facility is necessary to improve the performance of reservoir
operation for flood control purpose.
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