2C.Research on Conservation and Formation of Sandy Beaches

Background and Objectives

  • The post-war economic development created a number of problems in regard to sandy beaches and tidal flats, such as beach erosion caused by reduced sediment supply from rivers, as well as sedimentation in waterways and basins. The former problem leads to loss of scenic beauty, while the latter impairs port functions. These problems may be aggravated by the rising sea level, large typhoons and other effects of global warming. However, much remains unclear about sediment movement that causes changes in the morphology of sandy beaches and tidal flats, while the forecasting accuracy of numerical models remains unsatisfactory.
  • This research analyzes field data in order to investigate long- and short-term variations in nearshore current, sediment movement, and morphological change, and to develop numerical simulation models. Additionally, using the results of field data analysis and numerical simulations, we propose effective methods for beach management for the long-term conservation of beautiful, sandy beaches under the threat of global warming.

Research topics

 Research comprises the following two subthemes with respect to the wide-area, long-term change of sandy beaches.

  1. Field Measurements on morphological changes
     By analyzing the field data collected at the Hazaki Oceanographical Research Station and other locations, we are exploring long- and short-term variations of nearshore current, sediment movement, and morphological change. We are also developing numerical simulation models for beach morphological changes based on the analysis of field data in order to improve the accuracy of forecasting sandy beach deformation, thus providing more reliable evaluation of the effects of measures against beach erosion and sedimentation in waterways and basins.
  2. Numerical Investigations on Morphological Changes
     Using the results of field measurements and numerical simulations, we are proposing effective methods for beach management in terms of both erosion and sedimentation. These methods combine "hard" countermeasures (groins, detached breakwaters, and other structures) with "soft" countermeasures (beach nourishment), thus aiming to mitigate the effects of global warming.

Activities in FY 2014

  • We analyzed the influence of surface waves on the mud on the seabed about 20 m below the surface based on field measurements of the bottom velocity and proposed a method for evaluating shear stress introducing a probabilistic representation considering the variation in the bottom velocity. Using this method, we confirmed that the conventional method (e.g.,evaluation of period averages and maximum values) evaluates the shear stress too small or too large. We also conducted field monitoring around Kumamoto Port in Ariake Bay and revealed that sediment is resuspended due to wave force (of wind waves and ship waves) and suspended sediments are significantly moved toward the coast due to tidal flow. Also, we further clarified the mechanism of SS flux by analyzing various on-site data. Regarding the analysis of the topographic data of tidal flats, we evaluated the topographic change at the mouth of the Shirakawa River by comparing the aerial survey data between February 2013 and February 2014. These results were used to establish simulation models for predicting the movement of sediment in wide areas such as inner bays as well as the spatial and temporal change in the sediment characteristics. The models are expected to be utilized as a tool for the management of coastal areas including environmental assessment.

Figure 1.1.1.11 Spatial distribution of water content in bottom sediment at the mouth of Tama River in Tokyo Bay

  • Regarding the sand waves that make waterways locally shallow at the Bisan Seto Waterways and their peripheral sea areas,we modified the model of the development of sand wave topography at the Bisan Seto Waterways and confirmed that the modified model can precisely estimate the development of sand waves at the waterways. The results of this research were used to predict sand waves at the Kanmon Waterway and are planned to be used to predict sand waves near waterways throughout the Seto Island Sea of Japan.

Figure 1.1.1.12 Comparison of calculation result (thick solid line marked by ○) with the actual development of sand waves (solid line marked by △)

 

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