Seisho Coast Research Articles

NUMERICAL SIMULATION OF GRAVEL NOURISHMENT TO THE SEISHO COASTLINE IN JAPAN

It is feared that as a consequence of the potential future intensification of tropical cyclones due to ongoing climate change the patterns of coastal erosion around the world could change. In this study, the mesoscale meteorological model WRF (Weather Research and Forecasting), the third-generation stand-alone wave model SWAN (Simulating WAves Nearshore), and the coastal deformation model XBeach (eXtreme Beach behavior) are combined to calculate the short-term coastal deformation caused by high waves during the approach of Typhoon No.21 (Paolo) in 2017 to the Seisho Coast and Niigata Coast in Japan (Nishida and Shibayama, 2020). The model is then applied to predict future patterns of coastal erosion. Potential countermeasures to prevent future coastal erosion are then examined. The construction of detached breakwaters, which are currently used as a countermeasure against coastal erosion in many parts of the world, is often opposed by local residents due to the changes they can produce on the fishing environment and landscape. Therefore, it is often desirable to stabilize the beach without constructing structures offshore. Thus, gravel nourishment is proposed as a countermeasure against the short-term erosion that can be caused high waves, and its benefits and impacts were verified through numerical simulations.

Extreme wave runup at the Seisho Coast during Typhoons Faxai and Hagibis in 2019

Two successive extreme typhoons, Faxai and Hagibis, hit Eastern Japan and left devastating damage along its long coastline due to high waves. During the two typhoons, the characteristics of extreme waves at Seisho Coast, the most affected coast, were investigated. The investigations were based on wave observation data, post-event surveys, and numerical simulations. The observation data confirmed that extreme wave fields were generated on the coast during both typhoons and that Hagibis produced higher waves than Faxai. The maximum spectral significant wave height of incident short waves exceeded 8 m at the depth of 13 m and that of infragravity waves reached 2 m in shallower water during Hagibis. The post-event surveys revealed that the typhoons produced different variations of runup heights over nonuniform bathymetry along the coast. Wave hindcast results suggested that the overall variability of runup heights was primarily attributed to alongshore variations of the beach slope rather than offshore wave conditions. Further numerical investigations of nearshore waves using SWASH were conducted to reveal the mechanisms of local extreme runup, which could not be explained solely by cross-shore wave processes. The model results suggested the significant involvement of two-dimensional wave processes in the observed variations. Further detailed analyses revealed that local high runup was caused by concentrations of incident short waves during Faxai. Conversely, infragravity waves trapped on the coastal bathymetry contributed to extreme runup during Hagibis. This study highlights the importance of considering nonlinear wave processes over a broad coastal area for predicting local wave impacts under extreme wave conditions.

Infragravity wave dynamics on Seisho Coast during Typhoon Lan in 2017

ABSTRACT The extreme typhoon Lan caused devastating damage to the Seisho Coast of Japan in 2017. This study presents nearshore wave measurements conducted at different locations along the shore and numerical hindcasts to elucidate the extreme wave field during the typhoon focusing on infragravity wave dynamics, which was comparable in magnitude to incident short waves in shallow water. During the peak of the typhoon, there was a significant wave height of ~8 m, and the infragravity wave height exceeded 2 m at a 14 m depth. The measurement data suggest that infragravity waves developed as bound waves from incident wave groups and partially dissipated during the typhoon peak in the surf and swash zones through their energy transfer to high-frequency waves. Numerical wave hindcasting models successfully reproduced the measured extreme wave field and indicated that the wave refraction over Oiso Spur caused an incident wave concentration in the severely damaged area where high waves collapsed a seawall during the typhoon. Moreover, infragravity waves also exhibited significant alongshore variation, which was due to the different bottom slopes and incident short-wave properties. This study identifies several key features that may have caused the localization of significant damage along the Seisho Coast.

Coastal impacts of super typhoon Hagibis on Greater Tokyo and Shizuoka areas, Japan

ABSTRACT A post-event survey of the coastal impacts of Typhoon Hagibis was conducted in the Greater Tokyo and Shizuoka areas. The typhoon’s intensive landfall generated the highest-level storm surges recorded in the study area with a maximum sea-level anomaly of nearly 2 m at the head of Tokyo Bay. Highly populated coastal areas in the inner sheltered part of Tokyo Bay were subject to only minor damage partially because the peak storm surge occurred during a low spring tide. High water levels along the coastline facing the Pacific Ocean mostly measured 4–8 m above the mean sea level, the exception being the Seisho coast, where extreme wave runup reached over 10 m above the mean sea level. The high water levels predominantly caused by energetic swells were comparable with the crest heights of defense structures in a broad range of the study area. The intensive typhoon did not trigger catastrophic flood damage, but caused minor flooding due to wave overtopping, and there was substantial damage to coastal infrastructures along the coast. This paper presents both an overview of and detailed information regarding the coastal impacts of the intensive typhoon along the highly altered coastline with complex geometry.

Case study on impact of storm waves on inundation characteristics

Modeling interactive features of storm waves and surges may be one of important but challenging tasks toward more accurate estimations of coastal hazards under severe storms. This paper discusses how storm waves and interactions of waves and surges affect amplification of local inundation characteristics through overviews of several case studies of recent coastal disasters. The paper first describes witnessed inundation characteristics caused by Super Typhoon Haiyan along the coast of San Pedro Bay and also the east coast of Eastern Samar in the Philippines. Along the west coast of San Pedro Bay, neither predicted storm surge nor storm wave can reasonably represent the following observed features: inundation around the mouth of San Pedro Bay started about an hour prior to the time when the inundation was witnessed at the inner part of the bay and the water level quickly descended after the peak around the bay mouth while it was kept high for an hour in the inner part of the bay. Along the east coast of Eastern Samar, deformation of wind wave components strongly depends on bathymetry of fringing coral reefs and refraction, diffraction and wave-wave interactions appear to have significant influence on observed locally varying damage levels and inundation characteristics. Similar locally concentrated inundation was also observed along the west coast of Batan island, the Philippines when the super typhoon Meranti passed north of the island. Significant influence of infra-gravity waves on the locally concentrated inundation characteristics was also observed in the disaster along Seisho coast on the Pacific coast of Japan when the typhoon, Fitow, hit the coast in 2007. Based on these findings, finally, this paper discusses how the numerical model should be extended for better estimations of coastal hazard with brief example of model application to South Pacific islands.

IMAGE-BASED FIELD OBSERVATION OF INFRAGRAVITY WAVES ALONG THE SWASH ZONE

This study develops an image-based monitoring techniques for observations of surf zone hydrodynamics especially focusing on evolution and propagations of infragravity waves along the coast. Laboratory experiment was first performed to investigate the relationship between image-captured shoreline fluctuations and recorded actual water surface fluctuations inside the surf zone. Frequency components of infragravity waves were extracted respectively from observed shoreline changes and recorded water surface fluctuations. Both components showed good agreement in terms of their alongshore distributions of phases and amplitudes. The same image-based technique was applied to the field, the Seisho coast when the typhoon 1112 approached the site and extracted infragravity wave components were found to be consistent with offshore wave data and showed characteristics of edge waves.

低天端突堤による砂礫海岸の漂砂制御の評価

A numerical simulation of the beach changes on the Seisho coast was carried out using a contour-line-change model considering changes in the grain size. The applicability of the simulation model to a gravel-sand beach with protruding rocks was verified by comparing the calculated and observed changes after storm waves generated by T0709. The sand transport controls achieved by different types of coastal facilities were estimated and compared under an imaginary wave condition that could occur in future. A newly-designed groin with a low crown height was found to be most effective in preserving a straight coastline, if a sufficient amount mount of sand will be supplied by beach nourishment.

QUANTITATIVE PREDICTION OF SAND DISCHARGE INTO SUBMARINE CANYON OFF MORITO RIVER ON SEISHO COAST, JAPAN

On the Seisho coast, submarine canyons have developed very close to the shoreline and the discharge of fluvial sediment of the Sakawa River into the submarine canyons has been reported, resulting in the net loss of sand into the offshore zone. The beach topography under dynamically equilibrium conditions owing to the sediment supply from the river and the sand loss into the submarine canyons during several thousand years was reproduced using the contour-line-change model considering the grain size composition. Long-term beach changes around the submarine canyons were accurately predicted and the effect of beach nourishment using a mixture of fine and coarse materials was investigated.

Field Measurements of Deposition of Gravel on Foreshore in Oiso Area on Seisho Coast

Field Measurements of Deposition of Gravel on Foreshore in Oiso Area on Seisho Coast

Quantitative Prediction of Sand Discharge into Submarine Canyons - Application to Kozu Coast

On the Seisho coast, submarine canyons have developed very close to the coastline and the discharge of fluvial sand of the Sakawa River into the submarine canyons has been reported, resulting in the net loss of sand into the offshore zone. The beach topography under the dynamically equilibrium conditions owing to the sediment supply from the river and the sand loss into the submarine canyons during several thousand years was reproduced using the contourline-change model considering the grain size composition. Long-term beach changes around the submarine canyons were accurately predicted and the effect of beach nourishment using a mixture of fine and coarse materials was investigated.

Model for Predicting Sand Discharge into Submarine Canyon

A submarine canyon develops offshore of the Morito River mouth on the Seisho coast in Sagami Bay. Previous studies revealed that littoral sand originally supplied from the Sakawa River located 3 km west of the submarine canyon has been lost in this submarine canyon. To study the mechanism of sand discharge into the submarine canyon, beach changes around the canyon were investigated using bathymetric survey data. Then, a model for predicting sand discharge into the submarine canyon was developed using the BG model proposed by Uda et al. (2008). The major causes of offshore sand loss are the presence of very steep slope of 1/6 of the submarine canyon and the change in shoreline orientation at the submarine canyon.

Numerical Simulation of Rapid Erosion of Seisho Coast Triggered by Storm Waves during Typhoon 0709

On September 6, 2007, the Seisho Bypass extending along the Seisho coast was severely damaged over a length of 1.1 km owing to storm waves associated with Typhoon 0709, resulting in roads being closed to traffic for urgent repairs. During the typhoon, severe beach erosion occurred owing to both strong offshore sand transport and westward longshore sand transport by storm waves, and the foreshore was rapidly narrowed. However, the foreshore was recovered except the damaged area of the highway as a result of the effect of normal waves. These beach changes were predicted using the contour-line-change model incorporating Fukuhama et al.'s concept. The predicted results were in good agreement with the measured beach changes.

Damage to Seisho Bypass due to Storm Waves During Typhoon 0709

Some 1.1 km of the Seisho Bypass along the Seisho coast was severely damaged on September 7, 2007, due to Typhoon 0709 storm waves, closing the road to traffic for emergency repairs. Damage to the highway, caused primarily through wave impact and secondarily by long-term shoreline recession due to decreased fluvial sand supply from the Sakawa River and sand loss into a submarine canyon, was investigated through field observations. A factor contributing further to shoreline recession was the obstruction of eastward longshore sand transport by the Ninomiya fishing port breakwater. Storm waves hit the coastline counterclockwise to normal to the shoreline, causing offshore and westward longshore sand transport with part of the sand transported by the longshore sand transport flowing into the submarine canyon, causing a net sand loss.

Locally Concentrated Damages and Erosions on Seisho Coast Caused by Typhoon T0709

This paper explores physical mechanisms of locally concentrated damages and erosions observed on Seisho coast caused by typhoon T0709. Field survey was first conducted to capture alongshore distributions of hydrodynamic forces during the storm. The measured water mark elevations showed certain periodic undulation along the coast and their peak was located at the most damaged area. Time-series of surface water levels and velocities recorded at Hiratsuka Experiment Station were then analyzed and it was found that both free and bounded long waves grew as T0709 approached to the damaged site and short wave heights increased. Numerical analysis is finally carried out to investigate how free long waves and nearshore currents affect observed locally concentrated damages.

西湘バイパスの被災原因の検討

The Seisho-bypass highway runs along the Seisho coast facing the Sagami Bay. On September 6, Typhoon 0709 attacked this coast with rough waves, and the road of this highway was severely damaged over 1.1km length, resulting in full traffic stoppage. During this typhoon, maximum significant wave height reached 6m with a wave period of 14s. Damages of the road were investigated through the field observation. Strong waves attacked the coastline from the Southeast, which is in contrast to the ordinary wave direction of the Southwest. Due to this oblique wave incidence, westward longshore sand transport was developed. Along this coast, the submarine canyons develop at several locations, and part of sand transported by longshore sand transport is considered to be flowed out of the submarine canyons, causing net loss of sand.

海底谷への細粒土砂の選択的損失の予測モデル

Selective movement of sand into a submarine canyon depending on the grain size was investigated using the contour-line-change model. Seabed topography and grain size distribution were measured around the submarine canyon off the Morito River on the Seisho coast. The seabed slope of this submarine canyon is as steep as 1/6 in the depth shallower than 10m and 1/3 in the depth deeper than 10m. Assuming that the coarse (fine) material has a large (small) equilibrium slope, and that if seabed slope is greater than the equilibrium slope, offshore sand movement is occurred, a model for predicting offshore discharge of sand was developed.