Settling basin

What a settling basin is

A settling basin, chinsachi in Japanese, is a basin in which water taken from a river into a channel is slowed so that the sediment it carries sinks to the bottom and can be removed. The channel is widened or deepened to cut the velocity, and sand and gravel drop out. Such basins are built below the intakes of hydroelectric headraces, water-supply systems and irrigation channels.

In a hydroelectric channel, sediment damages turbine blades if it reaches them and reduces the flow if it builds up in the channel. The settling basin prevents both, and the accumulated sand is flushed back to the river by opening a sluice. The record for the Endogawa settling basin of the Heisakubara power station in Okayama notes that it was built on the upper reaches of a fast stream carrying much sediment and has a flushing sluice in its north wall, cleaning the water for power generation.

Other structures along a headrace regulate the water in different ways. A head tank just before the power station, which evens out the flow and feeds the penstock, is one example, and its role differs from that of a settling basin. When reading records, the name and position of each structure show which stage of the system it belongs to.

Reading the records and the site

Records for registered structures often give only the construction and area in the structure line, such as reinforced concrete, 85 square metres. The commentary describes the plan and any measures against sediment. The record for the headrace settling basin at Heisakubara describes an almost rectangular basin with reinforced-concrete slabs projecting over its west and south sides, where the hillside presses close, to keep soil from falling in. It is an example of a basin designed not only to settle sediment carried in with the water but also to keep out material from its surroundings.

On site, look for a place where a channel widens into a rectangle. Still water with sand on the bottom points to a settling basin. A gate in the side wall, or a short channel leading back to the river, is the flushing sluice.