Onshore Wave Research Articles

Sedimentology of shallow, hurricane-affected lagoons : Grand Cayman, British West Indies

Frank Sound and Pease Bay are small narrow (~4 km long and <1 km wide) shallow water (1.5-2.0 m average depth) lagoons (< 0.5 m deep) located on the exposed-windward margin of south coast of Grand Cayman. Collectively, the Rubble and Knob, Bare Sand, and Thalassia and Sand zones form 95-97% of the substrates in these lagoons. The Bare Rock Zone, Coral Knolls, and Shoreline Zone are restricted to small areas in both lagoons. Between 1985 and 1992, the area covered by Thalassia expanded by colonizing the Bare Sand Zone. During fair-weather conditions, onshore wave energy is dampened by the reef. Nevertheless, waves that cross the reef are sufficient to maintain lagoonal circulation and normal marine conditions. Under these conditions, there is limited sediment production (e.g., Thalassia epibionts, bioerosion, green algae), active bioturbation, and expansion of the Thalassia banks. Sediment transportation is minimal. At the height of a hurricane, waves and currents pass over the reef and produce a turbulent sediment laden currents that cross the lagoons. As the current loses energy, deposition produces a sediment wedge that grades from boulders-cobbles near reef crest, to pebbles and coarse sand ~80 m from the reef crest, to fine sand 500-600 m from the reef crest. The sediment is poorly sorted and typically has a unimodal grain size distribution. Silt and clay sized sediment is rare. Skeletal constituents indicate that most sediment originates in the fore-reef and shelf environs. As the storm wanes, water that was piled in the lagoons during the storm starts to drain out through topographically controlled mega-rip currents. These high velocity currents destroy biota, uproot Thalassia, and strip sediment to reveal bare rock substrates.

Development of Armor Blocks to Use for Submerged Breakwaters in Large Tidal Zones

Submerged breakwaters are important coastal structures that protect seashore from waves. The transmission coefficient increases at high tide in large tidal zones.So, when the water depth is large over the submerged breakwater, effective armor blocks are developed. Eddies form between the ripple-type armor blocks during the onshore and offshore semiorbital wave motions. Incident waves reduce these eddies over the armor blocks, and transmission coefficient becomes small.

Recent and subrecent changes in the dispersal of amazon mud

The dispersal of Amazon mud reaches from the mouth of the Amazon river to the Orinoco delta and to the eastern Caribbean, a distance of more than 1500 km. The present dispersal system has been in existence for at least 10 3 years, but the deposition of mud from this system has not been constant during that period. A change from net erosion to net deposition along the Suriname coast between 1947 and 1981 was found to coincide with a shift of the trade winds from a dominantly NE to a more ENE direction from 1959 onward, accompanied by an increase in mean wind velocity. The change to net-deposition can be explained by enhanced longshore transport of suspended matter with simulataneous reduction of erosion by reduction of the onshore wave energy component. The sequence of a recent mud deposit 100–200 years old off the Amazon river mouth separated by a period of non-deposition from an older mud deposit less than 1000 years old, agrees well with indications for a wetter period in the Amazon basin and in the Colombian Andes since about 200 years BP and a wetter period between 500 and 900 y BP. This implies that during wetter periods the suspended-sediment supply from the Amazon (and the Orinoco) was (is) higher.

Properties of short-crested waves in water of finite depth

AbstractShort-crested waves are defined as propagating surface gravity waves which are doubly-periodic in the horizontal plane. Linearly, the short-crested wave system we consider occurs when two progressive wavetrains of equal amplitude and frequency are propagating at an angle to each other.Solutions are calculated via a computer-generated perturbation expansion in wave steepness. Harmonic resonance affects the solutions but Padé approximants can be used to estimate wave properties such as maximum wave steepness, frequency, kinetic energy and potential energy.The force exerted by waves being reflected by a seawall is also calculated. Our results for the maximum depth-integrated onshore wave force in the standing wave limit are compared with experiment. The maximum force exerted on a seawail occurs for a steep wave in shallow water incident at an oblique angle. Results are given for this maximum force.

EFFECTS OF THE GULF STREAM ON NEARSHORE WAVE CLIMATE

Large scale ocean currents, such as the Gulf Stream, Kuroshlo, Peru Current, Agulhaus Current, etc., strongly modify the surrounding wave characteristics. As the Gulf Stream moves along the Continental Shelf of the southeast coast of the United States, the local ocean environment is divided into three wave climatic regimes. They are the offshore, the Gulf Stream, and the nearshore regimes. The nearshore zone is bounded by the land to the west and the Gulf Stream to the east. The distance between land and the Gulf Stream varies from 10 to 60 miles. Most of the waves in this regime are generated offshore and cross the Gulf Stream. The correlation of local wind and waves in the nearshore regime is poor except in the presence of a persistent onshore storm. A semi-empirical approach has been developed to compute the nearshore wave climate. The hindcast/forecast directional waves from the Spectral Ocean Wave Model (SOWM) of the Navy Fleet Numerical Oceanography Center have been used as the source of the initial offshore wave conditions. After crossing the Gulf Stream, which is assumed to be a uniform current with a velocity of 2 m/s, the waves are either refracted to the nearshore regime or reflected to the offshore regime following ray theory. The onshore waves in the nearshore zone are confined to the sector from 30 to 150 degrees. The computed results are then compared with measured data with good agreement. In summary, the Gulf Stream acts as a barrier to damp long waves and to regroup short waves. The refraction of long waves can be predicted by using ray theory. Further field experiments are needed to quantify the variation of the Gulf Stream and to investigate the interaction with approaching long waves and local wind generated short waves.

Development of Rigid Inflatable Boats for U.S. Navy Shipboard Use

The Rigid Inflatable Boat (RIB) was originally developed as a gasoline outboard powered craft for surf zone rescue use by the Royal National Lifeboat Institution in the United Kingdom. The objectives were to provide a craft with extremely good stability characteristics to operate in steep onshore breaking waves without capsizing, and to improve the safety of operations alongside other vessels. Open ocean rescue and boarding applications were of interest to the U.S. Coast Guard because of the inherent stability and alongside safety of the RIB. Subsequent feasibility studies by the U.S. Navy indicated that the RIB would be superior in performance to the present 26-ft motor whaleboat, and could also reduce topside weight. The Navy opted for diesel inboard power for the RIB's designated to be carried aboard combatant ships. A diesel-powered RIB was procured for concept evaluation by the U.S. Navy, and has undergone a series of trials and tests to establish smooth-and rough-water performance characteristics. The data acquired confirmed the theoretical performance predicted during the feasibility studies. This first RIB was deployed on a U.S. Navy DDG-993 Class ship utilizing an existing single-point davit. During that deployment, the RIB was launched and retrieved successfully at ship's speeds up to 12 knots. Early indications are that this development program will result in a new ship's boat and an accompanying davit system which will provide the Fleet with a safe, high-performance craft which will greatly enhance operational capability and safety, and substantially reduce topside weight. The enthusiasm of test and Fleet personnel who have operated the RIB attests to its superior performance and to a high degree of probability for success of the program.

THE BEHAVIOUR OF PROTOTYPE BOULDER REVETMENT WALLS

This paper reports the results of on-site observations of coastal revetment structures under extreme storm conditions on the Gold Coast of Australia. The Gold Coast is located at approximately Lat. 27° S on the East Coast of Australia facing the Tasman sea behind a narrow continental shelf and exposed to a relatively high energy wave climate as depicted in Fig 1. Tropical cyclones generate the highest storm activity on the Gold Coast with Ho values commonly exceeding 10m with the resultant onshore wave i.e. either the second or third wave reformed breaks within the range of 2.5 to 3.5m. Storm wave periods are usually between 8 to 18 seconds. The ocean beach on the Gold Coast, some 30km long has been receding since the early forties and this has resulted in the construction of nearly 20km of revetment walls to "protect" the rear beach. Whilst some walls in particularly erosion-prone areas were constructed in the 1920 decade, most have been constructed since 1967 which represented a particularly high cyclone prone year. Since the latter period the walls have been exposed to three further periods of high cyclone energy attack in 1972, 1974 and 1976. Nearly all revetment walls demonstrated at least some settlement and damage but over the three storm periods at least 0.8km of wall was completely destroyed. Most wall failures were monitored on site and whilst the construction of the walls varied in quality the observational results might well be classified as full scale prototype performance tests.

Experimental control of beach face dynamics by watertable pumping

The dynamic sediment budget on the intertidal face of sandy surf beaches is influenced by interaction of swash/backwash flows with the beach watertable. Watertable height variations are coupled to tides and pass into the beach as a slow wave of diminishing amplitude and increasing lag. High-frequency pulses from the onshore wave train also propagate into the beach. The height at which the watertable outcrops on the beach face is affected by antecedent tide and wave history and influences balance of sand deposition and entrainment during swash and backwash. Experimental lowering of the watertable by pumping an array of wells induces sand deposition. Energy expended in pumping is less than 0.3% of the onshore flux in the wave train.

Sedimentary Processes and Their Roles in Formation of Future Source and Reservoir Rocks: ABSTRACT

In order to understand more fully the principles of transportation in shallow parts of all marine systems, a series of studies by diving geologists at the University of Southern California are aimed at providing new data on the mechanisms of sediment movement from the surf zone to depths of approximately 100 ft. It is evident from the first results of this program that much of the theoretical information on wave transport based on wave-tank observations must be modified. Measurements have been made of rates of sand movement using dyed sand (Vernon, 1965), magnitude of wave-generated surges over the bottom in shallow depths (Vernon et al., 1966), changes in energy in the surf zone (Ingle, 1966; Schiffman, 1965), regional changes in beach characteristics (Gorsline, 1966), movement and quantity of suspended sediment over the shelf (Rodolfo, 1964; Wildharber, 1966), and movement of fine sediment in canyons (Gorsline, in progress). These various measurements show that the wave transport of sediments is active to depths of 60-80 ft off California during an average year and that the flow of sand along the coast probably is matched in magnitude by the flow of fine suspended material. It is also demonstrated that sand moves around headlands below surf depth an is then moved back into the surf system by onshore wave action. Much of the sand entering submarine canyon heads probably is moved in below surf depth by this same ripple-transport mechanism. All of these systems are strongly controlled by bottom or coastal physiography. In addition to the commonly considered physiographic barriers to sedimentation, End_Page 2175------------------------------ numerous contemporary examples of water barriers also exist that have effects at all scales on the distribution and character of marine sediments. Because these are also the precursors of most source and reservoir rocks, an understanding of their effects is of basic importance to petroleum geologists. On a relatively small scale, circulation patterns in Florida Bay, at the southern tip of the Florida peninsula, are slow tidally controlled gyres which create a flow that probably prohibits sediment transport into the central portions of the individual lakes of this broad shallow embayment. Thus the sediment accumulation occurs around the periphery of the individual segments and these lines of sedimentation in turn appear to coincide with the locus of points of small or zero tidal amplitude. Current transport of these materials also takes place and thus they ultimately come to rest in the deep water of the Florida Straits. In large coastal bays on the Pacific coast, water circulation also plays a strong part in the distribution of sediment types. Sebastian Viscaino Bay is an open, broad, north-facing embayment that also forms the southern extremity of the continental borderland off California and Baja California. Within this huge embayment the California Current turns back on itself and forms a large gyre. The patterns of texture, bioclastics, and organic content are strongly controlled by this circulation pattern and, in fact, parallel the contours of flow. Work by K. S. Rodolfo at U.S.C. shows that the shift in the monsoon and the period of strong river flow combine in the Andaman Sea to restrict Irrawaddy sedimentation to the confines of the sea even though no physiographic barrier is present to hinder flow to the adjacent Bay of Bengal. Thus, the sedimentation in the two areas is from two different sources producing lenses of sedimentation of geosynclinal scale side by side from different sources. The development of the entire Andaman margin is effectively controlled by these circumstances. Off the southern Atlantic coast of the United States, the Gulf Stream forms an effective boundary to the detrital terrigenous sediments of the upper shelf and the bioclastic sediments of the outer shelf and Blake Plateau. The combination of broad shelf and strong regional current also influences the form of the coast and apparently also prevents the active formation of submarine canyons. End_of_Article - Last_Page 2176------------

Seasonal Distribution of Magnetite and Ilmenite in Beach Sand of Malaga Cove, California: ABSTRACT

In 1961, 1962, and 1963, surveys of part of the black beach at Malaga Cove, California, included profiles for measuring seasonal variations in beach erosion and accretion, and sampling for grain-size, mineral composition, and magnetite percentage. Profiles show maximum beach erosion during August and maximum accretion during January, contrary to the cycle of summer accretion and winter erosion for most beaches. Mean grain-sizes, determined from analyses of samples extracted on October 19, 1963, are largest for all depth intervals within a narrow strip midway between the cliff and the swash. Particle size gradually decreases both oceanward and shoreward and increases northward. The average percentage of magnetite decreases with an increase in grain-size. Magnetite, ilmenite, epidote, zircon, and quartz are the most abundant of 27 minerals identified petrographically in the very fine size. For all samplings, the magnetite content in the beach sand increases with depth and shoreward; maximum concentrations are at the slope break near the cliff. During beach buildup, high-energy waves temporarily erode the foreshore, carry the lighter, fine materials offshore, and concentrate the magnetite and other heavy minerals as laminations and layers at depth in the beach. The magnetite concentration high on the beach reflects the superiority of onshore wave energy over other types of wave energy. The magnetite and ilmenite in the sand at Malaga Cove are believed to have been transported from the western San Gabriel Mountains to Santa Monica Bay when the Los Angeles River channel followed the present course of Ballona Creek. The minerals were carried southward along the coast by predominant littoral movement in this direction and concentrated by wave action on the beach at Malaga Cove. End_of_Article - Last_Page 648------------

(RESPONSES OF NYMPHON GRACILE (PYCNOGONIDA) TO PRESSURE CYCLES OF TIDAL FREQUENCY.

ABSTRACT The responses of Nymphon gracile to changes in hydrostatic pressure are described. (M, the unit of pressure used here, denotes 1 metre sea water.) A sudden increase in pressure tended to inhibit swimming, while a reduction promoted it, a response opposite in sense to that normally associated with depthregulation. Nymphon showed no adaptive response to cyclical pressure changes similar to those produced by onshore waves. Pressure changes of similar range (1M) but with a 2 sec. period usually induced swimming in previously sluggish individuals. Pressure cycles of 8M range and periods ranging from 14 to 160 min. evoked a depth-regulatory type of response in several individuals, but the majority responded in the opposite way. Nymphon responded to cyclical pressure changes of approximately tidal range (8M) and frequency, by swimming most actively during the late ebb or low water of the artificial cycles. The data of Fage (1932) support the hypothesis that Nymphon swims most often when stimulated by decreasing pressure as the tide ebbs.

Notes on Shallow-Water Sand Structures

Three widely contrasted types of sand structures are discussed in this paper-strand-line ripples, ridge and trough structures, and sand waves. The first is allied to but distinct from ordinary oscillation ripple marks. The third is analogous to ordinary, asymmetrical ripple marks but has crests rising above the troughs a foot or more instead of an inch or two. The ridge and trough structures are approximately symmetrical like oscillation ripple marks but are spaced at intervals of many feet instead of one or two or a very few inches. They are considered to be the product of the joint action of on-shore waves and undertow.