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Miyahara L.,Coastal Engineering Laboratory Co. | Uda T.,Public Works Research Center | Serizawa M.,Coastal Engineering Laboratory Co.
Proceedings of the Coastal Engineering Conference | Year: 2014

When waves are incident to a sandy beach from two opposite directions, a cuspate foreland or a land-tied island may develop. A typical land-tied island can be seen offshore of Shodoshima Island in the Seto Inland Sea. Another example is Chiringashima Island located in the south part of Satsuma Peninsula, Kyushu. In both cases, the island and land are connected by an extremely slender sand bar that has been stably maintained for a long time, suggesting that it is stable against wave action from both sides of the sand bar. We developed a numerical model for predicting the elongation of a sand bar of a land-tied island using the BG model (a three-dimensional model for predicting beach changes based on Bagnold's concept).


Serizawa M.,Coastal Engineering Laboratory Co. | Uda T.,Public Works Research Center | Miyahara S.,Coastal Engineering Laboratory Co.
Proceedings of the Coastal Engineering Conference | Year: 2014

On a flat shallow seabed, sand spits and cuspate forelands with rhythmic shapes may develop and a barrier island can elongate alongshore, which significantly differs from the beach changes on coasts facing a deep ocean. In this study, the interaction between two circular sandy islands on a flat shallow seabed owing to waves was investigated by numerical simulation, focusing on the wave-sheltering effect of the islands themselves. The topographic changes caused by the interaction between two sandy islands were predicted using the BG model (a three-dimensional model for predicting beach changes based on Bagnold's concept).


Serizawa M.,Coastal Engineering Laboratory Co. | Uda T.,Public Works Research Center | Miyahara S.,Coastal Engineering Laboratory Co.
Procedia Engineering | Year: 2015

In a slender water body with a large aspect ratio, the angle of wind waves relative to the direction normal to the shoreline may exceed 45°, resulting in the emergence of cuspate forelands and the subdivision of a lake, because the fetch distance along the principal axis becomes large. The BG model (a three-dimensional model for predicting beach changes based on Bagnold's concept) was used to predict the segmentation of a rectangular lake by wind waves under the conditions with/without the construction of offshore breakwaters that reduce the fetch distance, together with the numerical simulation of the deformation of a circular lake when a straight seawall cutting off a part of the lake was constructed. © 2015 Published by Elsevier Ltd.


Uda T.,Public Works Research Center | Onaka S.,Chiyoda Corporation | Serizawa M.,Coastal Engineering Laboratory Co.
Procedia Engineering | Year: 2015

Beach changes near Pengambengan fishing port in the western part of Bali Island, Indonesia, were investigated by the analysis of satellite images and field observations. In this area, northwestward longshore sand transport prevails because of the oblique wave incidence from the Indian Ocean, and this longshore sand transport has been blocked by a fishing port breakwater, resulting in severe downcoast erosion. As a measure, a seawall has been constructed, instead of maintaining the continuity of the natural longshore sand transport. The adoption of this method caused further downcoast erosion. The sand bypassing method should be adopted to mitigate such erosion. © 2015 Published by Elsevier Ltd.


Miyahara S.,Coastal Engineering Laboratory Co. | Uda T.,Public Works Research Center | Serizawa M.,Coastal Engineering Laboratory Co. | San-Nami T.,Coastal Engineering Laboratory Co.
Procedia Engineering | Year: 2015

Topographic changes around the tip of the Mihono-matsubara sand spit with successive sand deposition into a deep sea were investigated using bathymetric survey data. Then, numerical simulation of the elongation of the sand spit on a sloping seabed with different water depths and slopes was carried out using the BG model (a three-dimensional model for predicting beach changes based on Bagnold's concept), and the relationships among the sand volume comprising the sand spit, the planar area of the sand spit, and the length were shown. © 2015 Published by Elsevier Ltd.


Ishikawa T.,Public Works Research Center | Uda T.,Public Works Research Center | San-Nami T.,Coastal Engineering Laboratory Co.
Procedia Engineering | Year: 2015

On a coral reef coast near Point Kin located in central Okinawa, a large-scale excavation of the sandy beach was carried out by the American armed forces after WWII and sandy beach disappeared. However, after the excavation, the sandy beach was rapidly recovered. In this study, the rate of sand supply from the reef was estimated on the basis of change in foreshore area after the excavation of the shoreline area by using aerial photographs. It was concluded that sand supply of 0.5-0.8 m3/m/yr could be anticipated in the long term. © 2015 Published by Elsevier Ltd.


Uda T.,Public Works Research Center | Serizawa M.,Coastal Engineering Laboratory Co. | Kumada T.,Laboratory of Aquatic Science Consultant Co. | Sakai K.,Public Works Research Center
Coastal Engineering | Year: 2010

The empirical bay shape model proposed by Hsu and Evans in 1989 for predicting the static planform of a pocket beach is expanded to enable the calculation of three-dimensional beach changes on a pocket beach with a seawall. The original formulation was developed on the basis of a second-order regression analysis. Unlike the one-line model of shoreline changes, the model of Hsu and Evans does not require repeated calculations of the wave field and shoreline position, because it was derived on the assumption of null sediment movement within a pocket beach in static equilibrium, hence without the need of applying the continuity condition of total sand volume in the calculation. The expanded model proposed by the present authors satisfies the total sand budget on a pocket beach, by taking into account the concept of depth change due to longshore sand transport. Model tests were carried out and the new model was further applied to the beach changes at Kemigawa on the northeast of Tokyo Bay in Chiba Prefecture, as well as at Oarai in Ibaraki Prefecture, Japan. On both locations, seawall has been installed as countermeasures against beach erosion, where wave sheltering effect of the main breakwater and beach changes in front of the seawall has also been observed. With this expansion, the present model can be applied to predict the three-dimensional beach changes on a coast with seawall on a pocket beach. © 2009 Elsevier B.V. All rights reserved.


Ishikawa T.,Public Works Research Center | Uda T.,Public Works Research Center | Miyahara S.,Coastal Engineering Laboratory Co.
Proceedings of the Coastal Engineering Conference | Year: 2012

The concept of the moving gravel body (MGB) method, in which topographic changes are controlled by beach nourishment using gravel, was introduced, using the contour-line-change model considering changes in grain size. The model was applied to the Fuji coast, assuming that coarse materials were nourished on a straight coast wherein a predominant longshore sand transport develops. The grain size that is effective for the recovery and maintenance of the shoreline and that does not cause further downcoast erosion was d, which is 5-10 times larger than that of the original sand, d0.


Uda T.,Public Works Research Center | Serizawa M.,Coastal Engineering Laboratory Co. | Miyahara S.,Coastal Engineering Laboratory Co.
Proceedings of the Coastal Engineering Conference | Year: 2012

In a slender water body with a large aspect ratio, the angle between the direction normal to the shoreline and the wave direction exceeds 45°, resulting in the emergence of cuspate forelands and the subdivision of a lake, because wind fetch along the principal axis becomes long. In this study, the BG model (a three-dimensional model for predicting beach changes based on Bagnold's concept) was applied to this problem. The 3-D subdivision process of a long slender water body was predicted.


Serizawa M.,Coastal Engineering Laboratory Co. | Uda T.,Public Works Research Center | Miyahara S.,Coastal Engineering Laboratory Co.
Proceedings of the Coastal Engineering Conference | Year: 2012

The BG model (a three-dimensional model for predicting beach changes based on Bagnold's concept) was used to simulate the shoreline evolution caused by the high-angle wave instability discussed by Ashton et al. Three calculations were carried out: the wave direction was assumed to be obliquely incident from 60° counterclockwise (Case 1) or from the directions of ±60° with probabilities of 0.5:0.5 (Case 2) and 0.65:0.35 (Case 3), while determining the incident wave direction from the probability distribution at each step. The three-dimensional development of multiple sand spits and cuspate forelands with rhythmic shapes was successfully explained using the BG model. The results of the previous study conducted by Ashton et al. were reconfirmed and reinforced.

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