Theory of Stable Sandy Beach Formation and Its Applicability

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1) Theory of stable sandy beach formation : Under the assumption of narrow sandy beach compared with the surf zone width, by applying the so-called boundary layr method, the equation for longshore currents was derived from the equations of nearshore currents. Introducing the law of longshore sediment transport into it gave the general equation of longshore sediment transport. By use of the equations of longshore sediment transport and continuity for shoreline change the equation of stable sandy beach was derived. The theoretical solution was obtained by the up- and down-coast boundary conditions. This concludes that eroding sandy beach can be stabilized by forming a stable sandy beach with two boundary conditions. Comparison of the theoretical plan configurations of dynamic stable beach with those of Amanohashidate coast confirmed good applicability of the theory. 3D-model for bottom topography change also confirmed the formation process of stable sandy beach.2) Applications to beach erosion control : Since the formation theory was established, a methodology for beach erosion control was examined by the formation of a seried of stable sandy beaches.a) In relation to the sandy bypassing, dynamic stable sandy beaches are successfully formed between groins at Amanohashidate coast. Practical criteria for stabilizing sandy bezch were obtained.b) In order to staabilize eroding sandy beaches, large scale coastal behavior must first be investugated, and stable sandy beaches can be formed. Its practical application was proposed to Joetsu-Ogata coast. Its aspplicability was shown experimentally.c) Conclusion : On the basis of the basic studies of the formation of stable sandy beaches, a methodology for beach erosin control using headlands was proposed. Its applications were made to Amanohashidate and Joetsu-Ogata coasts as well as Niigata coast and coral-sand beaches in Bali.