The study area has been suffered from intensive retrogression of the coastline since early 1990's. Numerous coastal structures have been built and nourishments have been performed for protection and preservation of the coastline, however, it has not become a fundamental solution so far. It is very important to evaluate effects of countermeasure works of coastal erosion. We examined the effect of these structures on the basis of changes on beach topography and also facies change of sediments around these coastal structures at before and after a storm. Examination of airphotos during last 13 years are also studied and help our understanding of the erosion.

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Beach erosion at Oarai Beach caused by extraordinary waves and large sea level rise during a low pressure occurred on October 7, 2006 was investigated. On this beach, the shoreline has retreated since 1984 due to decrease in sediment supply from the Naka River. Also seawall was advanced seaward to expand the parking lot for Oarai Aquarium. These events triggered heavy damages of seawall and formation of high scarp invading the coastal park. The anthropogenic factors were studied in detail.

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Characteristics of scouring at the foot of the Flaring Shaped Seawall were investigated in this study. Twodimensional hydraulic experiment on a movable bed was conducted to obtain topographic changes in front of the seawall under some incident wave conditions. The scouring depth in front of the Flaring Shaped Seawall becomes large with increase of the incident wave height in longer waves, and this increase tendency is similar to that of the vertical seawall. In shorter wave conditions, the scouring depth shows almost constant even under the large incident wave height. Both scouring depth and scoured sand volume at the foot of the Flaring Shaped Seawall are smaller than those of the vertical seawall under the small incident wave height. On the other hand, those scouring values of the Flaring Shaped Seawall become slightly larger than that of the vertical seawall under the large incident wave height. The occurrence of scouring in front of the Flaring Shaped Seawall is closely related to the generation of the undertow at the foot of the seawall.

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Horizontally composite breakwaters, covered with wave-dissipating concrete blocks, are widely employed in Japan, However, a large amount of settlement failures of wave-dissipating blocks have occurred due to the scour under the rubble mound. Field experiment on the new countermeasures using small grain filter is conducted. Also, the design method is newly proposed.

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DRIM (Distorted Ripple Mat) blocks are placed in the flat to works as an unit which controls sediment transport. For practical design of DRIM, it is necessary to examine the stability of both block and unit. In this paper, three series of physical model test are conducted. Firstly, the stability of DRIM is examined and stability number, Ns, is shown. Secondary, both rotation of start block and local scour around the outer perimeter of the unit are key phenomenon to discuss the deformation of DRIM. Finally, it is confirmed that rubble blocks show high performance to increase the stability of both block and unit.

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A numerical model for 3-D beach deformation has been developed to evaluate the amount of sediment transport rate in the littoral drift system. The model includes the sediment transport rate formula under the coexistence of waves and currents, considering the effect of bottom slope on the sediment movement. In case of a longshore uniform beach, this model has a solution that total amount of littoral drift is proportional to the longshore component of wave energy flux, similar to existing longshore sediment transport rate formulas. Numerical calculations are conducted to estimate the trapping rates of longshore sediment transport due to a groin or detached breakwater. Furthermore, wave height distributions of irregular wave and bottom friction components in nearshore zone are calculated to evaluate the sediment transport rate vector and total longshore sediment discharge on a bar type beach with a longshore undulation.

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The construction of a tidal flat in shallow sea area is a difficult work, because the construction barge is hard to enter to the area where dry and wet condition repeatedly changes dependently on tides. Although the design condition of the tidal flat shape was managed to be constructed by careful works, the deformations by wave actions and the consolidations of the topography have been reported. According to this situation, new concept to form tidal flat in ‘self-design’ process, in which the shape of a tidal flat is allowed to deform by natural process due to waves, is introduced. In this study, the property of tidal flat deformation was studied by 2D wave flume model test. As for the flat shape of the initial topography, a berm was formed at the tip of tidal flat, moved to the shore, resulting in the formation of a tidal flat topography. This result shows that tidal flat can be formed by the self-design process under natural wave actions.

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The obstruction effect of littoral drift by an artificial headland was investigated using the contour-line-change model, taking the Kujukuri coast as an example. Numerical simulation was carried out assuming the seabed slopes of 1/50 and 1/100, while changing the length and the shape of the headland. The rate of the obstruction effect of littoral drift by a headland was quantitatively evaluated. It is given by the function of time after the construction. Only the construction of the headland is difficult to form a statically stable beach, and sand bypassing or sand recycling is required to prevent further beach erosion.

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It is very important for an environmental assessment of tidal flat ecosystem such as some benthic animals that clarifying formation mechanism of particle size distribution of sediment. The main aim of this research is the development of a method to evaluate the equilibrium particle size distribution in large river mouth. We considered a river with larger influences of waves than river currents and modeled the formation mechanism of sediment. It was assumed that the sand particles which correspond to the critical shear stress under waves deposit and form a particle size distribution depending on the frequency of disturbance of waves in this model. The particle size distributions which estimated based on a wave statistical data well coincided with field survey data.

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Long-term shoreline monitoring survey with more than three-years has been carried out after completion of beach nourishment in Sanur and Nusa Dua Beaches in Bali Island. The objective of the monitoring work is to know the beach behavior after initial beach filling and to prepare the future maintenance plan including the supplementary beach filling. It was observed that the beach reached in stable condition after half years although some unpredicted beach change appeared. After completion of project, some undesirable problems came into surfaced concerning maintenance and operation system.

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