Causal relationship between water-use of the Tedori River and coastal erosion of Chirihama Nagisa Driveway, Ishikawa, Japan

Abstract

Chirihama Nagisa Driveway, a natural sandy beach located in Ishikawa Prefecture, is known as the only beach in Japan where cars and even large tourist buses can drive along the shoreline. It is a popular tourist destination that should be reliably preserved for future generations; however, it is also recognized as a beach facing serious coastal erosion and requiring measures to address sand loss. A major landslide has occurred in the upstream area of the Tedori River, a primary source of sediment supply to Chirihama, resulting in turbid water flowing downstream. This study demonstrates that Chirihama is composed of fine sand ranging from 0.11 to 0.25 mm in diameter, characterized by a bumpy surface, and investigates where the fine sand contained in the turbid water was deposited. The investigation revealed that the fine sand accumulated in the irrigation canals and rice paddy fields on the alluvial fan, as well as along the coast near the river mouth. The findings suggest that the reduction in sand supply to Chirihama is a result of water use in the Tedori River, such as water intake at the downstream headworks for irrigation and water storage at upstream dams.