報告書番号

380025

タイトル（和文）

海浜流の発生機構に関する実験的研究

タイトル（英文）

EXPERIMENTAL STUDY ON THE MECHANISM OF NEARSHORE CURRENT GENERATION

概要 （図表や脚注は「報告書全文」に掲載しております）

本研究は，波浪の卓越する遠浅海岸で温排水拡散範囲に影響を及ぼすと考えられる海浜流自体の特性や発生機構を実験的に把握する目的で実施した。主な研究成果は以下の通りである。１．砕波帯内平均水位上昇の発生過程波発生初期岸方向への水粒子輸送流量は保存されるが，砕波後，波は輸送流量を保存するため変形し段波形状に近づく。その際，段波の平均水位は静水位よりも高くなるが，これが平均水位上昇の発生である。２．海浜流の発生要因沿岸流の発生は，主として平均水位上昇の汀線方向変動による圧力勾配に起因するが，離岸流の発生は，平均水位分布のみならず，砕波波高分布とも強い相関を有する。３．底面摩擦の効果摩擦力は海浜流速のほぼ２乗に比例し，モルタル仕上げ模型海底上の無次元摩擦係数は約０．０２である。海浜流の形成に対し底面摩擦は重要な役割を果たす。

概要 （英文）

ON AN OPEN COAST HAVING A GENTLY SLOPING BOTTOM THE SURF ZONE IS RELATIVELY WIDE,HENCE PHYSICAL PHENOMENA SUCH AS BREAKING OF WAVES,NEARSHORE CURRENTS,ETC. MAY HAVE SOME EFFECTS ON THE DIFFUSION OF WARMED WATER DISCHARGED FROM POWER STATIONS.FROM THIS STANDPOINT,THE PRESENT STUDY AIMS AT MAKING CLEAR THE MECHANISM OF NEARSHORECURRENTS IN THE SURF ZONE ON THE BASIS OF A HYDRAULIC EXPERIMENT. THE FOLLOWING INFORMATION WAS OBTAINED FROM THE EXPERIMENTAL STUDY. 1)PROCESS OF WAVE SET-UP GENERATION: THE SHOREWARD FLOW RATE DUE TO MASS TRANSPORT IS PRESERVED IN THE SURF ZONE AT THE BEGINNING OF WAVE ACTION.IN ORDER TO PRESERVE THE SHOREWARD FLOW RATE AFTER BREAKING WAVES CHANGE THEIR FORM INTO BORES.THEY CAN TRANSPORT WATER PARTICLES EFFICIENTLY EVENAT A PLACE WITH SMALL DEPTH AND THE EXISTENCE OF THE BORE-SHAPE WAVE HAVING SHORT AND SMALL TROUGH RESULTS IN THE GENERATION OF THE WAVE SET-UP. 2)MAIN CAUSE OF NEARSHORE CURRENT GENERATION: LONGSHORE CURRENTS ARE GENERATED MAINLY BY A DIFFERENCE IN THE WAVE SET-UP IN THE LONGSHORE DIRECTION.ON THE OTHER HAND,THE GENERATION OF RIP CURRENTS HAS A STRONGCONNECTION WITH BOTH DISTRIBUTIONS OF BREAKING WAVE HEIGHT AND THE MEAN WATER LEVEL 3)EFFECT OF BOTTOM FRICTION: BOTTOM FRICTION FORCE IS ALMOST PROPORTIONAL TO THE SQUARE OF NEARSHORE CURRENT VELOCITY AND THE NON-DIMENTIONAL FRICTION COEFFICIENT FOR THE HYDRAULIC MODEL COVERED WITHMORTAR IS ABOUT 0.02. THE BOTTOM FRICTION PLAYS AN IMPORTANT ROLE IN THE PROCESS OF NEARSHORE CURRENT FORMATION.

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