29.95 million m3
Miwa is a multipurpose reservoir operated for purposes of flood control, irrigation water supply and hydropower generation. It is located on the Mibu River, a tributary of the Tenryu River in Japan, under the management of the Ministry of Land, Infrastructure and Transport. The scheme is composed of a 69 m high gravity concrete dam and 29.95 million m3 gross storage volume of reservoir.
Since completion of the dam, the safety degree of flood control along the river improved dramatically and a stable supply of irrigation water was achieved, resulting in a remarkable increase in the agricultural water supply area from 1,200 ha to 2,500 ha. Electric power production at the Miwa and Haruchika hydropower plants plays an important role, amounting to 40 percent of the total power output under the prefecture/region.
The reservoir has experienced high sediment deposition facilitated by heavy floods since right before its completion in August 1959, due to accelerated sediment yield within the basin. Until 1998, approximately 30 percent of accumulated sediment was removed by dredging. Following the completion of the sediment bypass facilities in 2004, up to 60 percent of inflowing fine sediment is diverted through the bypass tunnel  while the bed load and coarse sediment are trapped behind the check dam and diversion weir at the upstream end of the tunnel intake and consequently removed and transported for construction materials.
The Miwa reservoir intercepts water from two thirds of Mibu River’s catchment from an area of 311.1 km2. The Mibu River is the largest tributary of the Tenryu River, nicknamed the “Wild Tenryu,” and one of Japan’s largest rivers. The Mibu River has its source in Mt. Senjogatake in the Southern Japanese Alps and joins the Tenryu River in Ina City after flowing 60 Km through a bed slope of 1/100 making its stream faster than that of the Tenryu River.
According to the master plan of the redevelopment project carried out for Miwa dam since 1981, the estimated annual sediment inflows into the reservoir amounted to 685,000 m3. Of the inflowing sediment, about three quarters (77 percent) is wash load of fine particles smaller than 74 μm, the rest being bed load and suspended sediment. The sediment yields of Japanese rivers are high in comparison with other countries due to the topographical, geological and hydrological conditions. The flood control plan at the time of constructing Miwa dam assumed the design flood discharge for a 100-year return period to be 1200 m3/s with a 300 m3/s maximum outflow from the dam.
The reservoir has been challenged by high sediment deposition due to a series of flood events that mobilise high sediment loads into the reservoir. At the time of construction of the dam, it was estimated that 6.6 million m3 of sediment will fill the reservoir after 40 years. However just before completion of construction in August 1959 a large flood, equivalent to the design flood, deposited about 6.8 million m3 of sediment in the reservoir only 3 years after commissioning, exceeding the allocated 40-year volume of deposited sediment. Of the above volume, 4.4 million m3 was deposited in the active reservoir storage, thereby compromising the flood control capability of the reservoir.
Similar extreme runoff events occurred in 1959, 1961, 1982 and 1983 with associated high sediment yield increasing the rate of storage loss due to reservoir sedimentation. The 1982 flood was the largest recorded flood in the Mibu River with a discharge of 1321 m3/s, exceeding the design flood discharge at Miwa dam. This resulted in about 4.3 million m3 of deposited sediment within that year . The next flood of 1983 delivered about 1.6 million m3 of deposited sediment. After 40 years of operation repeated floods conveyed enormous quantities of sediment into the Miwa reservoir amounting to 20 million m3.
Works to evacuate sediment from the reservoir commenced in 1965. However, operations were not as successful as desired and only 27 percent of the volume of deposited sediment was removed over a period of 33 years, requiring development of more sustainable techniques for removing sediment to preserve the reservoir’s capacity.
Sediment removal from Miwa reservoir has been executed since 1966, for which an estimated 27 percent (5.32 million m3) of the total accumulated sediment was removed by dredging over 33 years up to 1998 . With more than two thirds sediment accumulation in the reservoir, a 15 m high upstream check dam was completed in 1994 as a tentative facility to trap large volumes of sediment before reaching the reservoir, with a capacity of up to 200,000 m3. Large volumes of the trapped sediment have since been evacuated since 2000.
In 2004, a sediment bypass system was completed as a sustainable solution to preserve the reservoir’s storage capacity. The sediment bypass system comprises a 20.5 m high diversion weir and a 4.3 Km long bypass tunnel with a maximum discharging capacity of 300 m3/s. The sediment bypass tunnel removes an average of 399,000 m3 per year.
An overview of the sediment management facilities of Miwa dam is shown in Figure 3 and Figure 4 .
The bypass system was designed to discharge mainly fine suspended sediment since about three quarters of the sediment deposited in the reservoir is wash load smaller than 74 μm. The bed load and coarse suspended load that flow in at an annual average of 106,000 m3 are trapped at the check dam and thereafter excavated for construction materials. In cases when large floods occur and the check dam is filled to capacity, the diversion weir located downstream, having a capacity of about 500 000 m3, prevents the inflow of sand and gravel into Miwa dam to the maximum possible extent. Consequently, the wash load reaching the diversion weir consists only of fine particles, of which an annual average of 399,000 m3 (76 percent of the estimated average wash load) is routed directly downstream through the sediment bypass tunnel. During 12 years of operation of the bypass tunnel since 2006, a total of 14 releases occurred, passing about 1.74 million m3 of the inflowing sediment downstream preventing sediment deposition in the reservoir.
Several parameters such as rainfall, fish, benthic animals, suspended sediment concentration in the upstream river, the reservoir and the downstream areas are monitored in order to optimize the operation of the bypass system. This monitoring data is used change operational modes (see Figure 6). Generally, inflow discharge is used to guide decisions for switching modes determining the bypassing efficiency. No major ecological impacts downstream have been reported.
A sediment bypass facility comprising a check dam, diversion weir and sediment bypass tunnel has been collectively utilised at Miwa dam since 2006 and has effectively reduced accumulation of sediment in the reservoir. This case study shows that sediment bypass is suitable for medium size reservoirs with steep riverbed slopes and can be effectively operated to manage reservoir sedimentation. Additional sediment management measures increasing sediment bypass efficiency are underway at Miwa dam. This will include guiding dredged sediment to the sediment bypass tunnel from where it can be flushed downstream during flood events.
1. Intl. Energy Agency. Case Study 04-02: Reservoir Sedimentation—Miwa Dam, Japan. 2006. Available online: https://www.ieahydro.org/media/b7f7e732/Annex_VIII_CaseStudy0402_Miwa_Japan.pdf (accessed on 7 July 2020).
2. Sawagashira, Y., Suzuki, A. and Fukumoto, A., 2017. Sedimentation control effect and environmental impact of sediment bypass in Miwa Dam Redevelopment Project. In Proceedings of the 2nd International Workshop on Sediment Bypass Tunnels (pp. 1-12). Kyoto University. http://hdl.handle.net/2433/245513
3. Sumi, T. and Kantoush, S.A., 2011. Comprehensive sediment management strategies in Japan: Sediment bypass tunnels. In Proceedings of the 34th World Congress of the International Association for Hydro-Environment Research and Engineering.
4. Sumi, T., Kantoush, S.A. and Suzuki, S., 2012, June. Performance of Miwa Dam sediment bypass tunnel: Evaluation of upstream and downstream state and bypassing efficiency. In 24th ICOLD Congress (pp. 576-596).