IV. Water Resource Management of Kobe City

WATER SOURCE SUPPLY:

Though Kobe earlier developed fresh water supplies within its own municipal area, today only about 25 percent of the water is from those sources. The rest is drawn primarily from Lake Biwa, Japan's largest lake, and from the Yodo River, downstream from the lake. These serve as water supplies not only for Kobe but also for the nearby major cities of Osaka and Kyoto. Lake Biwa supplies water to 14 million residents, of whom only 1 million live near the lake itself.

This water supply, about 30km away from Kobe, comes to the city via two tunnels through the Rokko mountains.

Lake Biwa

Kobe purchases water from the Hanshin Water Supply Authority, comprised of Kobe, Osaka, Ashiya, Nishinomiya and Amagasaki cities. The authority has expanded over the years to keep up with increasing population and with economic growth. In recent years the authority has also focused on earthquake resistant facilities and on advanced water purification processes. Currently, Kobe pays 61.96 yen per cubic meter of water for this supply.

Lake Biwa is one of the ancient lakes of the world. It is the largest lake in Japan, both in terms of surface area (670 square kilometers) and volume. It is also the biggest freshwater resource in Japan. Its deepest point is 100.3 meters and the average depth is 41 meters. Its watershed area is a relatively small 8,240 square kilometers and serves one of the most extensively developed and urbanized regions in the world.


The lake is fed by 120 major rivers and about 340 small streams. It has, though, just one outlet. Water replenishment mainly comes from melting snow, the June-July rainy season, and during the typhoon season in early autumn.

The 460 rivers and streams create an inflow to the lake of about 100 cubic meters per second. The outflow from the lake is about 150 cubic meters per second. The difference is made up by direct precipitation into the lake.

Outflow from the lake is controlled at its single outlet point. The original structure was the Nango Weir, built in 1906. The Seta Weir was built in 1964, and Shin-Seta Weir in 1992. (A weir is a type of dam.)

A number of factors in the watershed area directly affect the lake itself. These include the many paddy fields around the banks, and the highly urbanized southern watershed area. The population of Shiga Prefecture, where the lake is located, has also been steadily increasing.

Kobe City (shown in yellow) sources of Water and Supply line.

Between 1965 and 1994, there has been a development in urbanization, industrialization, and recreation in the watershed area. Now, after rain, the runoff inflow into the lake occurs in much less time. This is also due to the many concrete surfaces everywhere. Contaminants also rush in faster.

The biggest problem the lake faces is maintaining the water quality. The transparency of the water has decreased since World War II. Various indicators of water quality, such as nitrogen, phosphorus, and phytoplanktons, are all down, though the nitrogen level is stabilizing. A recent change seen in the lake is an increase in biomass during the winter. Also, cyanobacteria levels have recently increased in winter.

An improved water quality has been recorded in the lake's southern basin. One indicator of this, submerged macrophytes, was found in 11 percent of the lake's area in 1994, and in 52 percent in 2000. This is also related to the 1994 drought, during which an increase in light penetration to lower depths stimulated the germination of seeds at the lake bottom.

The control of discharge from land-based sources of pollution is crucial for the improvement of lake water quality. Officials have been sharing research with residents to help enlist their cooperation in reducing pollution and to help improve biodiversity in the lake. They are also
working to reclaim lagoons that had been previously reclaimed for agriculture, a novel case of re-reclamation.

In recent years the frequency of drought has been increasing, and precipitation has been on a declining trend since the 1940's. To meet the water demands of Osaka, Kyoto, and Kobe, the water resource capacity had to be greatly increased. This was the reason for the Lake Biwa Comprehensive Development Project, developed between 1972 and 1997. This plan allowed for more water to be released, greater flood control, and greater regional infrastructure development.

One of the major facilities in controlling water flow near Lake Biwa is the Hiyoshi Dam, which was completed five years ago. It is a gravity concrete dam, 67.4m high and 438m wide. It holds 66 million cubic feet of water, and has a very high capacity for its height, relative to other dams.

Hiyoshi Dam

An abbreviated timeline of the dam history is as follows:

1961 - Concept first originated.
1993 - Start of main dam construction.
1994 - Beginning of concrete pouring.
1997 - First test of dam.
1998 - Dam opening.

The cost of the Hiyoshi Dam was 183.6 billion yen, including the cost of a local road replacement.

The purpose of the Hiyoshi Dam is to provide water during a period of low rainfall or drought, provide flood control, and maintain the river environment for transportation and recreation.

The dam is equipped with a water-purity control facility in the reservoir to supply water of good quality. It consists of a bottom layer aeration system that prevents oxygen starvation and algae growth, a surface layer water circulation system that prevents algae growth, and, as mentioned, selective water withdrawal equipment.

The sediment capacity (the amount of sediment that the dam stores in 100 years) is 2.5 million cubic meters.

The dam has electrical generating capacity of 850kw. The power generated is used only for dam operations. Any surplus power remaining is sold to the power company.

Making the dam accessible, especially to the local community, is a key objective. Each year a marathon race and festival is held at the dam. There is also an annual tricycle festival. The dam includes a museum, a community center, and an information gallery, which is actually inside the dam itself. There are also parks, campsites, festivals and tree planting events, all in the dam area.