Tojiro Ishihara

On the Investigation of Beach Erosion along the North Coast of Akashi Strait

SynopsisSandy beaches are defensive zones created by nature, protecting the land against aggressive oceanic forces. They are very flexible and their configulations are varying daily, seasonally and annually, depending on many factors, such as wave characteristics, beach materials, shore structures and the others. The investigation of beach erosion is one of the most important and difficult subjects in coastal engineering.In this paper, a method for investigating the mechanism of beach erosion is presented, concerning the investigation practice of beach erosion along the north coast of the Akashi Strait, and the authors opinion of the following three subjects to be investigated is described:(a) Distribution of sand drift along the coast,(b) Distribution of sand drift along the beach profile, and(c) General characteristics of sand drift, suCh-as the type of sediment transport, the direction of drift and the others.

STABILITY OF BEACHES USING GROINS

When, under the strong impulse of Prince Henry, Portuguese maritime discoveries began, in the dawn of the XVth Century, methods of navigation were still those of coastal and estimated navigatxon and resulted mostly from the progress made in the Xlllth Century. During that period the knowledge of geometry of ancient Greece had spread widely and the mariners compass had been adopted on board the Mediterranean ships. These innovations had the following consequences: a) Rutters: to the former indication of the principal characteristics of the coasts, particularly as regards to ports and bars, and of the distance between the most remarkable accidents, was from now on added the indication of the magnetic azimuths between these accidents. b) Nautical charts: the nautical chart, nonexistent m Europe, was created at that time.Casual observations over a period of years of a long sand beach in the southern end of Monterey Bay, California, suggested that the sand elevation, while varying noticeably from one time to another, does not display the well-defined seasonal alternation between build-up in the summer and erosion in the winter that is now widely recognized on the exposed beaches of California. Accordingly, a program was established to measure the beach-profile changes by means of serial observations and to attempt to relate the changes to wave, tide, and beach conditions prevailing during the observation period. The results of the study, covering nine months, are presented herein.Sea tests of motion and mooring force were conducted on an LST (Landing Ship Tank) of about 44O0 long tons displacement. The LST was spread-moored by six 2-1/16 inch and one 1-1/4 inch (port breast) stud-link chains in simple catenary configuration in about 45 feet of water in the open Gulf of Mexico about 65 air miles south of New Orleans, Louisiana. Water-level variations at a single location, ship rotations and accelerations, mooring force, and wind were measured in sea states of 2 and 4. Three recordings of 38, 62, 67 minutes duration were analyzed, using timeseries techniques to provide apparent amplitude-response operators for all of the ships motions and seven mooring chains. Theoretical prediction of the operators using long crested regular waves was made also. In longitudinal plane, theory predicts motions 1/3 to 4 times and chain tensions 1/4 to 9 times those measured. The most probable maximum-motion amplitude responses in sea state 4 are found to be 1.7, 1.1, and 1.7 feet, respectively, in surge, sway and heave, and 3.4 and 0.5 degrees, respectively in pitch and yaw. Roll was measured only in sea-state 2 with a corresponding maximum of 2.1 degrees. Maximum wave-induced chain tensions in kips were: 85.1 and 48.0 in port and starboard bow chains respectively; 10.6 (sea state 2) and 19.7 in port and starboard breast chains; 13.9 and 4.3 in port and starboard quarter chains (sea state 2) and 9.7 in stem chain. Total tension in port bow chain was 116.1 kips (85.1 plus initial tension of 31.0 kips). Chain response operators vary directly with initial tension, whicl complicates design. It is concluded that: (i) moor was unbalanced, i.e., port bow chain took most of load; (ii) chains loaded lightly, e.g., maximum wave induced tension was 116 kips compared to new proof load of 300 kips for the particular chain, the port bow; (iii) water level should be measured at more than one point; (iv) discouragement over differences is balanced by encouragement over agreements between measurements and theoretical prediction of motion and chain tension; (v) toward improvement: Theory needs extension to include short crested waves and barge types; (vi) initial tension unique to problem of mooring design; (vii) propulsion devices may be needed toward maintaining design initial tension, especially in storm; (viii) if directional spectra had been measured and if theory involving short crested waves had been available and used, then discrepancies between observation and theory likely would have been less.At the moderate velocity of the pure water which lies on the quiet sal£ water stable internal waves appear at the interfac in the stratified flow, and these waves will break and violated surface will arise if the velocity of the pure water may be increased. In this phase of phenomena the shear stress at the interface has the most important part. However observed aalues of this shear stress have not been reported in the systematic style. Experiments have been conducted in our laboratory since i960. Some theoretical considerations could be served to get an empirical equation on the mterfacial shear using experimental results and data presented by other researchers.

SOME CONTRIBUTIONS TO HTDRAULIC MODEL EXPERIMENTS IN COASTAL ENGINEERING

INTRODUCTION For the design of a coastal structure, the height of its crown must be determined rationally and economically, taking into consideration the water level of the sea and its occurence probability. The water level in the sea is mainly referred to the astronomical tide, the meteorological effect and the short period wave. If component height according to these elements are given as; 3c, ; the tidal level, xr ; the level rise caused by meteorological origin, and x, ; the half height of wave, the level of the wave crest X at a certain tidal condition is shown by following equation under several assumption: I a i + X, + X, (1)