This case study provides an example of a multi-purpose reservoir receiving a heavy sediment load. The reservoir was re-constructed by sub-dividing it into a main compartment and a sediment bypass compartment used to sluice floods from a sediment-laden tributary around the main storage pool.
Wonogiri is a multipurpose reservoir located on the Upper Solo River basin in Central Java Province, Indonesia, as shown in Figure 1, which began impounding in 1980. The project provides flood control, irrigation, water supply and hydropower. The reservoir is impounded by a 40 m tall rockfill dam with an 830 m crest length. The 12.4 MW power plant generates 55,000 MWh annually and provides irrigation water for 29,330 hectares of farmland where triple or double cropping is being practiced. Most flow used for power generation is diverted further downstream at the Colo Intake Weir for irrigation. The 730 Mm3 reservoir includes provides 220 million m3 of flood control capacity.
Hydrology and sediment
Wonogiri reservoir regulates the upper catchment of the Bengawan Solo River, the largest watershed on Java. Ten different tributary rivers drain into the reservoir. The watershed above the reservoir extends from 140 to 2000 m, with most of the catchment receiving an average rainfall of 1900 - 2100 mm/year. Flow seasonality is shown in Figure 2.
The original Master Plan for the Bengawan Solo River Basin Development Project, which includes Wonogiri reservoir, predicted an average sedimentation rate of 1.2 million m3/year. Project development was accompanied by the construction of 46 check dam and reforestation around the reservoir (green belt) to reduce sediment inflow. Nevertheless, bathymetric measurements in 2005 confirmed that the reservoir was losing capacity at a rate exceeding 4.5 Mm3/year (JICA, 2007). Approximately 38 percent of the sediment load was estimated to come from the Keduang Watershed (Table 1). The Keduang River discharges near Wonogiri dam and drains 31 percent of Wonogiri reservoir’s 1,350 km2 catchment and rises to an elevation of 2,000 m. The high sediment yield is attributed to the high erosion rate from the catchment due to poor land use; intensive farming on highly erosive and steep-sloped uplands, as the dense settlement pattern.
Design studies had assigned all sedimentation to the dead pool and estimated it would become filled in 100 years. However, the sedimentation rate was far higher than anticipated. After only 25 years of operation, 40 percent of the dead storage and 20 percent loss of the conservation pool capacity had been lost to sedimentation, though there was little sedimentation in the normally dry flood pool (Figure 3). Based on the 730 Mm3 total capacity, the average annual rate of capacity loss has been ~ 0.64 percent per year. Sediment deposition and floating debris also seriously affected the power intake, and for approximately 20 days each rainy season the blockage of intake trash racks is so severe it causes the power plant to shut down. The turbine runners have not been affected by abrasion, indicating that only fine sediments, without sand, have been passing through the turbines.
To decrease sedimentation and extend reservoir life, the main strategy adopted was to divide the reservoir into two compartments by constructing an internal dam and a new Low Level Outlet (LLO) for sediment release. The general layout of sediment management elements is shown in Figure 4.
The sediment-laden Keduang tributary was separated from the main body of the reservoir by an internal dam, thereby creating two separate compartments or pools: the main reservoir pool and the Keduang sediment management pool. The small “Keduang” pool was constructed as a compartment within the main reservoir to allow drawdown of the smaller pool and sluicing of sediment-laden floods inflows from the Keduang watershed. That watershed was highlighted in Figure 1.
The new low level outlet allows sediment management operations within the Keduang pool, without affecting the water level in the main reservoir. Additionally, a separate ungated spillway on the internal dam allows water from the Keduang sediment storage reservoir to overflow into the main reservoir. This way, the operator has the option of allowing the Keduang compartment to fill and overflow into the main pool, thereby acting as a sediment trap for water diverted into the main pool. This arrangement also allows extreme floods from the Keduang watershed to overflow the internal weir, enter the main flood control pool, and exit via the main flood spillway.
The operating rule was modified to minimize sedimentation. The new Keduang LLO is opened at the beginning of a flood and continues releasing sediment-laden water up to a discharge of 400 m3/s, which is the original target flood release rate for the entire reservoir. Sediment discharging is effective given the shorter detention period, shallow water depth, and higher gradient in the Keduang pool. However, when the water level in the main reservoir reaches the top of the design flood pool, the Keduang LLO is closed and thereafter the flood is controlled via the higher-capacity main spillway.
Additionally, periodic maintenance dredging was initiated at the intake, and more aggressive watershed conservation was implemented in the Keduang River basin. Watershed conservation in other tributaries are proposed to be applied continuously in the mid- and long-term. The combination of these measures would ultimately reduce the rate of reservoir storage loss and prevent sediment from entering the intake while reducing environment and social impacts.
A new sediment balance was computed. The two urgent countermeasures, compartmentalization of the reservoir and watershed management, are expected to produce a reduction of 22 percent and 13 percent respectively, in the rate of storage loss. As seen in Figure 5, these measures will produce a sediment balance across the Keduang sediment management pool, and additional measures will be undertaken in the mid-term to further reduce the sedimentation rate in the main storage pool.
In this case the ability to route sediments downstream was enhanced by modifying the internal geometry of the reservoir. The pool was sub-divided to create a hydraulic short-circuit between the source of the highest sediment load and the dam, thereby reducing the sediment load entering the main storage pool. It demonstrates that modifying the internal geometry of a shallow reservoir can represent an important element in a comprehensive sediment management plan.
Government of Indonesia. 2007. Study on Countermeasures for Sedimentation in the Wonogiri Multipurpose Dam Reservoir.
T. Joko. 2016. Analysis of Sedimentation In Wonogiri Reservoir. Journal of the Civil Engineering Forum
Wulandari, Dyah; Legono, Djoko; Darsono, Suseno. (2014). Reservoir Operation to Minimize Sedimentation. International Journal of Science and Engineering. 6. https://doi.org/10.12777/ijse.6.1.16-23
Jayadi, Rachmad; Istiarto, Istiarto; Pradipta, Ansita. (2018). Impact of Sedimentation Counter Measure on the Performance of Flood Control: A Case Study of Wonogiri Reservoir. Applied Mechanics and Materials. 881. 78-85. 10.4028/www.scientific.net/AMM.881.78.