Abstract
On the Shonan coast facing Sagami Bay, part of the nourishment sand was considered to be transported offshore, devaluing the effect of beach nourishment. Beach changes after the beach nourishment were investigated using the Narrow Multi-Beam survey data, which have been collected since 2002. Analysis of the sand volume and grain size composition in the subareas showed that the effect of the beach nourishment on the deposition of fine sediment in the offshore zone was negligible, and that the fine material in the offshore zone was mainly brought from the erosion of the river mouth terrace.
Highlights
On the Shonan coast facing Sagami Bay, Japan, the beach has been eroded owing to the rapid decrease in sand supply from the Sagami River, the primary source of sediment, after the construction of the Sagami Dam, and the riverbed mining to obtain aggregates
In the analysis of the topographic changes, the study area was subdivided into six subareas, as shown in Fig. 6, where area R is the Sagami River mouth and E is the offshore area between Hirashima Rocks and Eboshi Rock offshore of the Chigasaki fishing port
The sand deposition west of these structures suggests that predominant eastward longshore sand transport was blocked by the structures, and part of the longshore sand transport discharged into the offshore zone while turning around the tip of the structures
Summary
On the Shonan coast facing Sagami Bay, Japan, the beach has been eroded owing to the rapid decrease in sand supply from the Sagami River, the primary source of sediment, after the construction of the Sagami Dam, and the riverbed mining to obtain aggregates. The Chigasaki fishing port breakwater was extended on the coast, which obstructed eastward longshore sand transport, resulting in downcoast erosion. As a measure against beach erosion, beach nourishment using materials composed of sand of different grain sizes and dredged from the reservoir upstream of the dam has been carried out since 2005 at the Yanagishima and Chigasaki-naka areas east of the river mouth (Ishikawa et al 2009; 2013). Since the Chigasaki fishing port is located downcoast of the nourishment area, nourishment sand carried by prevailing eastward longshore sand transport was blocked by breakwaters, resulting in shoreline advance upcoast. Apart from the sand deposition near the shoreline, part of the nourishment sand was considered to be transported offshore, devaluing the effect of beach nourishment. The grain size composition of the seabed material was analyzed
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