Effect of Sand Bypassing at Sakuma Dam in Tenryu River as a Measure Against Erosion of Tenryu River Delta Coast

Abstract

The long-term evolution of the Tenryu River delta associated with sand bypassing at several dams is investigated using the contour-line-change model considering the change in grain size. The effect of the increase in sediment yield from the river mouth on the nearby coasts is numerically predicted. When the sediment yield is artificially increased, the supplied sediment is mainly deposited around the river mouth, resulting in a large shoreline advance, but it takes a longer time for a sandy beach far from the river mouth to recover. Another measure, such as sand bypassing of the coast, is required to recover sandy beaches downcoast.

Prediction of Beach Erosion Caused by Reduction of Fluvial Sand Supply Due to Excess Sand Mining and Beach Recovery After Prohibition of Mining

“…A similar result was observed when the sand supply from the Tenryu River was artificially increased (Miyahara et al, 2010). Thus, it was concluded that the changes in the fluvial sand supply from the Abe River, which is 8 km from the most severely eroded site on the Shimizu coast, are independent of the beach changes at the most severely eroded site, and local beach nourishment is required near the eroded site owing to the fact that many detached breakwaters have already been constructed.…”

Field Investigation of Topographic Response to Floods and Waves Around Tenryu River Mouth

“…The sand volume in the study area decreased by 1.66×10 6 m 3 between 2006 and 2011 at a rate of 3.1×10 5 m 3 /yr. Assuming that the same rate of sand supply of 1.5×10 5 m 3 /yr is expected during the observation period, as described by Miyahara et al (2010), the sand volume around the Tenryu River is decreasing at a rate of 4.6×10 5 m 3 /yr by longshore sand transport in both directions, because fluvial sand is assumed to be supplied at a rate of 1.5×10 5 m 3 /yr. Under this condition, beach erosion around the Tenryu River mouth will be intensified in the near future.…”

“…7. Miyahara et al (2010) estimated that the fluvial sediment supply from this river was 6.0×10 5 m 3 /yr under the natural condition before the construction of dams and extensive riverbed mining, and the amount of sand supplied from the river was divided into two, 3.0×10 5 m 3 /yr, which constituted westward and eastward longshore sand transport. Thus, the decrease rate of sand volume in the examination area is comparable to the eastward and westward longshore sand transport rate estimated from the long-term evolution of the shoreline.…”

“…However, large dams including the Sakuma Dam have been constructed and extensive riverbed mining has been carried out to obtain construction materials since the 1950s, and sand supply to the river mouth has markedly decreased while losing the balance between sand supply from the river and longshore sand transport, resulting in the shoreline recession around the river mouth (Nagashima et al, 2005). To mitigate the shoreline recession around the river mouth by increasing sand discharge of the river, the bypassing of sand deposited in the reservoir upstream of the Sakuma Dam has been investigated, and its effect has already been predicted by using the contour-line-change model (Miyahara et al, 2010;2011). In the realization of the plan, the investigation of the topographic changes around the river mouth in recent years after the fluvial sand supply had markedly decreased is important.…”

Introductory Chapter: Morphodynamic Model for Predicting Beach Changes Based on Bagnold’s Concept and Its Applications

“…This model then was improved to predict the temporal and spatial changes in the grain size of bed material [41][42][43]. The authors applied this model to many coasts in Japan to work out the countermeasures against beach erosion [2,[44][45][46][47][48][49][50][51]. However, this model has weak points.…”