Effects of Weir with an Opening on Bed Configuration and Flow Structure

Stream-crossing structures such as dams and weirs tend to interrupt the continuity of natural streams, destroy the diversity of the physical and biological environments and reduce natural disturbances. In the United States, more than 500 weirs and dams have already been removed to remove deteriorated dams including weirs and improve river environments (Heinz Center,2002). In 2011, the dam on the Elwha River in the state of Washington in the western United States was removed. It has been reported (Amy E.East etc.,2015) that the ecosystem of the river has been recovering to a sound state as can be seen, for example, from the native species recovery. In a gravel-bed river, sediment, if supplied in an appropriate manner, penetrates the spaces between gravel particles so as to stabilize the river bed. Sediment flowing into pools in a river section where bed slope is steep and water flows through a series of riffles and pools transforms the pool bed surface and thereby facilitates the establishment of new epiphytic algae (M.W.,Doyle et al.,2005; K. M.Kibler et al.2011).

The Arase Dam is a hydroelectric power generation dam located in the town of Sakamoto, Yatsushiro City, about 20 km upstream from the estuary of the Kuma River flowing through the southern part of Kumamoto Prefecture in Japan. It was decided to remove the dam by the phased removal method starting in fiscal year 2012. The Enterprise Bureau of the Kumamoto Prefectural Government, therefore, has been conducting a study on the effect of dam removal on river bed processes and the river environment. On March 31, 2010, the gates of the Arase Dam were opened. Since the height of the dam to its crest is 11.3 m, it is essentially categorized as weir removal.

Scouring or depositing downstream of submerged weirs with an opening is a sediment phenomena resulting from the interaction of the three-dimensional turbulent flow field around the structure and the moveable sand bed. This paper presents the experimental study on the downstream channel bed due to weir with an opening, paying attention to the effects of relative overflow depth on local scouring around the structure, sand bars and three-dimensional flow patterns. The experimental results show that local scouring and sand bar development downstream of the submerged weirs decrease with relative overflow depth and turn out strongly paired cellular secondary currents.