The Garang River with a catchment area of 204 km2 originates in Mt. Ungaran (el. 2,050 m) and flows through Semarang City, finally pouring into the Java Sea. There exists a large-scale water intake named the Simongan Weir about 5.3 km upstream from the river mouth, and the river channel below the Weir changes its name to West Floodway from the Garang River.

The river is one of the important water sources for Semarang City. The sustainable water supply from the river is, however, hardly secured due to the extremely low flow discharge during the dry season. As a result, Semarang City and its vicinities currently suffer from chronic and severe shortage of the municipal water supply, while flooding is recognized as another major problem.

Although several projects and plans have been proposed in order to cope with all those problems, legal reforms on the river administration hampered the implementation of those projects. A new study was conducted by the government of Indonesia with a loan by JBIC to achieve the following objectives:

• To mitigate flood and inundation damages along the West Floodway/Garang River, to develop water resources for municipal and industrial water in Semarang and to improve the drainage system in the central area of Semarang City,
• To improve the environmental conditions along the river and the urban area, and
• To upgrade the living conditions of inhabitants in the project area.