Artificial Coastline Research Articles

BEACH EROSION EXTENDING BETWEEN SHINKAWA AND MAKI FISHING PORTS ON NIIGATA COAST AND FUTURE PERSPECTIVE

On the Niigata coast facing the Sea of Japan, westward longshore sand transport dominates. In recent years, sand supply by this longshore transport is exhausted and severe erosion with forming the scarp is widening. Beach erosion is especially severe west of Shinkawa fishing port located in the middle of this coastline. The present authors warned this extension of beach erosion in 1999 and pointed out that effective measures should be taken urgently. The fundamental reason of the erosion in this area is due to the sectorby-sector works of related coastal management offices. In December 2001, the authors conductedanother field observation and found the situation became much worse than that in 1999: the coastline length with the formation of scarp further expanded. Drastic measures are needed including the reform of social system among the related organizations to prevent the extension of artificial coastline.

STUDY ON FACTORS OF RAPID CHANGE TOWARD ARTIFICIAL COASTLINE-THE EXAMPLE OF SETOHAMA BEACH IN SOTOBO AREA IN CHIBA PREFECTURE

Factors of rapid change from natural sandy beach to artificial coastline covered by concrete armor units were investigated at the Setohama Beach in Sotobo area in Chiba Prefecture. The Setohama Beach was famous for oceanic recreation in the past, and along the coastline a national road runs. With the shoreline recession, wave overtopping became severe, and land reclamation over the sandy beach was carried out as well as the construction of gently sloping revetment in front as the measure at the sacrifice of natural sandy beach. A part of local people welcomed land reclamation because of widening of parking lot and the others claimed loss of nature. This process of change was investigated by field observation and comparison of past aerial photographs.

FIEID OBSERVATION OF SEACLIFF AROUND POINT LOMA AND DEL MAR IN SAN DIEGO IN CALIFORNIA

Cliff erosion was investigated at the coasts around Point Loma in San Diego and Del Mar areas in California. On these coasts, installation of countermeasures against cliff erosion is rare in contrast to the cliff coasts in Japan. On Japanese coasts, concrete armor units are usually installed to protect coastal cliff and this causes decrease in sediment supply from the cliff, which induces beach erosion on the down coast. This gives one of the reasons for extension of artificial coastline in Japan. Overall stability of the sandy beach including cliffy coast as a sand supply source should be deeply considered.

RAPID DISAPPEARANCE OF NATURAL SANDY BEACH CAUSED BY LACK OF ADJUSTMENT BETWEEN PROTECTION OF COASTAL FOREST AND SHORE PROTECTION

The social mechanism of rapid disappearance of natural sandy beach caused by lack of adjustment between protection of coastal forest and shore protection is investigated through the case study on the Niigata coast facing the Sea of Japan. This coast has been eroded for a long time because of decrease in sand supply from the Shinano River, which is the main supply source to this coast, and subsidence of the ground level due to the excess pumping-up of natural gas. Until now, most severe erosion expanded west of Shinkawa fishing harbor. Coastal forest extends along this coastline and in order to protect this coastal forest, seawall has been constructed without sufficient understanding of causes and mechanisms of shoreline recession, and this brings a monotonous extension of artificial coastline covered with seawall. In order to change this situation, the overall cooperation between the works of shore and coastal forest protections is needed as well as the comprehensive consideration of stabilization of all the Niigata coast.

FIELD OBSERVATION OF CORAL COASTS BY OBLIQUE AERIAL PHOTOGRAPHS

Aerial photographs were taken to investigate beach changes on coral coasts and sand movement around reef gap at Sanur and Nusa Dua Beaches on Bali Island by using a ultra light plane. At Sanur dominant beach changes can be observed around groins because of obstruction of longshore sand transport. On both beaches, beach erosion has become serious problem and various measures were taken, but lack of overall comprehensive plan reduces natural sandy beach to artificial coastline protected by many kinds of structures.

Cause of Increase in Artificial Coastline Associated with Extension of Harbor Breakwater and its Measures

Causes of increase in artificial coastline associated with the extension of harbor breakwater were studied through the case study of Ohtsu Fishery Habor and Oharai Port both located in Ibaraki Prefecture. Circurating currents with longshore sediment transport toward the lee side of the harbor breakwater were induced after the extension of the breakwater in these areas. Measures were taken after the severe beach erosion to reduce their effect and the natural sandy coastline were transformed into the artificial one. To improve this condition, it is neccesary for related organaizations responsible for coastal protection works to take measures cooperatively.