7.Research on Comprehensive Coastal Management
Background and Objectives
Since Japanese coastlines are suffering from erosion of approximately 160 ha/year, comprehensive coastal management is required to prevent massive beach erosion. The objectives of this research are to develop numerical models for predicting morphological changes in the nearshore region on the basis of results of field measurements and to propose effective coastal zone management systems.
Research topics
- Field measurement
On the basis of field data of wave, current, sediment size and bathymetry obtained at Hazaki Oceanographical Research Station (HORS) located on the Hazaki coast of Japan, beach profile changes during 15- to 20-year periods, and temporal and spatial changes of bathymetry and sediment size near the shoreline are investigated. Morphological responses to the constructions of detached and submerged breakwaters on other coasts are also examined. - Numerical model
In order to predict bathymetry and beach profile changes in the nearshore region during severe and mild wave conditions, one- to three-dimensional models are developed. - Coastal management
An effective sand bypassing system for reducing erosion in beaches and sedimentation in channels and basins is tested at various sites. Furthermore, a probabilistic approach employing a numerical model of which the validity is confirmed by field data is utilized to maintain beaches with appropriate widths.
Activities in FY 2010
In this research we applied the multiple regression model to the shoreline position data observed at the Hazaki coast in Ibaraki prefecture for a period of 22 years, and investigated the effect of bar and tide level fluctuation on shoreline fluctuation and the possibility of introducing new parameters
to the model for shoreline fluctuation. The results revealed that shoreline fluctuation is affected by offshore wave energy flux, shoreline position, shore-side bar height, offshore bar height, highest tide level and highest rising tide velocity. The results of the reproduction of the shoreline position of the model that includes the shore-side bar height, offshore bar height, highest tide level, and highest rising tide velocity (Chart 1, No.1) clearly showed a regression tendency around 1993 (the bar was undeveloped on the offshore side) and a progression tendency from around 1996 to 1998 (the bar had been developed on the offshore side), compared with the model that does not include the aforementioned parameters (Chart 1,No.2). Such a model that includes these parameters was never considered in past studies. The respective coefficients show that the amount of shoreline progression increases as the bar is more developed, and that the bar prevents erosion. They also indicate that as the highest tide level rises, the shoreline regression amount increases. This is interpreted to indicate a phenomenon in which energy flux that reaches near the shoreline decreases due to the wave breaking accompanying the development of the bar. Also, it suggests that wave breaking occurs less and the waves hitting near the shoreline become stronger at higher tide levels.
Chart 1. Results of reproduction using the measurements of shoreline positions and the shoreline fluctuation model (1986 to 2007)
To study the correlation between the geomorphological changes of the foreshore near the shoreline and the geomorphological changes in the offshore bar formation region, and the relation between these geomorphological changes and external forces, we studied the correlations among the below-mentioned variables by using long-term data obtained at Hazaki Oceanographical Research Station. The variables among which we studied the correlations with are the bar movement vate Ψ1/dt, shoreline position ys, shoreline position fluctuation vate dys/dt, offshore wave energy flux Ef and the annual average of PDO (Pacific Decadal Oscillation) index. As for the correlation between the geomorphological changes of the foreshore near the shoreline and the geomorphological changes in the offshore bar formation region, the correlation coefficient between the bar movement vate Ψ1/dt and the shoreline position ys was 0.31, but the two showed no correlation at a significance level of 5% (Chart 2). Similarly, no correlation was noted between the bar movement vate and the shoreline position fluctuation vate dys/dt, or between the shoreline position and the shoreline position fluctuation vate. Regarding the correlation between the geomorphological changes and external force, the shoreline position fluctuation vate had a negative correlation with offshore wave energy flux at a significance level of 1%. The long-term fluctuation of shoreline position, which is the integral value of shoreline position fluctuation vate, had a higher correlation coefficient with the PDO index than wave energy flux, and showed a negative correlation with the PDO index at a significance level of 5%.
Chart 2. Correlation coefficients among respective variables
