Robert M. Sorensen

DEVELOPMENT OF SHIP WAVE DESIGN INFORMATION

This report will update the coastal zone practitioner on the National Flood Insurance Program (NFIP) as it affects the implementation of manmade changes along the coastline. It is our intent to place in proper perspective this fast-changing and often difficult to interpret national program. Readers will achieve an overall understanding of the NFIP on the coast, and will be in a position to apply the programs requirements in their efforts. We will begin with a history of the application of the NFIP to the coastal zone. The history of the problems encountered will lead into current regulations, methodologies, and the changes the Federal Emergency Management Agency plans for the future.The spatial variability of the nearshore wave field is examined in terms of the coherence functions found between five closely spaced wave gages moored off the North Carolina coast in 17 meters depth. Coherence was found to rapidly decrease as the separation distance increased, particularly in the along-crest direction. This effect is expressed as nondimensional coherence contours which can be used to provide an estimate of the wave coherence expected between two spatial positions.Prediction of depositional patterns in estuaries is one of the primary concerns to coastal engineers planning major hydraulic works. For a well-mixed estuary where suspended load is the dominant transport mode, we propose to use the divergence of the distribution of the net suspended load to predict the depositional patterns. The method is applied to Hangzhou Bay, and the results agree well qualitatively with measured results while quantitatively they are also of the right order of magnitude.

Finite-Amplitude Waves

The small-amplitude wave theory was formulated as a solution to the Laplace equation with the required surface (two) and bottom (one) boundary conditions [Eqs. (2.1), (2.3), (2.4), and (2.6)]. But the two surface boundary conditions had to be linearized and then applied at the still water level rather than at the water surface. This requires that H/d and H/L be small compared to unity. Consequently, the small-amplitude wave theory can be applied over the complete range of relative water depths (d/L), but it is limited to waves of relatively small amplitude relative to the water depth (for shallow water waves) and wave length (for deep water waves).

Two-Dimensional Wave Equations and Wave Characteristics

A practicing coastal engineer must have a basic and relatively easy to use theory that defines the important characteristics of two-dimensional waves. This theory is required in order to analyze changes in the characteristics of a wave as it propagates from the deep sea to the shore. Also, this theory will be used as a building block to describe more complex sea wave spectra. Such a theory—the small amplitude wave theory—is presented in this chapter along with related material needed to adequately describe the characteristics and behavior of twodimensional waves.

Wind-Generated Waves

The most apparent and usually the most important waves in the spectrum of waves at sea (see Figure 5.2) are those generated by the wind. Wind-generated waves are much more complex than the simple monochromatic waves considered to this point. We must briefly look into how these waves are generated by the wind and some of the important characteristics that result. It is important to have a means to quantify wind-generated waves for use in various engineering analyses. It is also important to be able to predict these waves for a given wind condition both wave hindcasts for historic wind conditions and wave forecasts for predicted impending wind conditions. Finally, we also need to look at procedures for extreme wave analysis, i.e., to predict those extreme wind-generated wave conditions that will be used as the limit for engineering design.

Coastal Zone Processes

The zone of interest in this chapter is that segment of the coast located between the offshore point where shoaling waves begin to move sediment and the onshore limit of active marine processes. The latter is usually delineated by a dune field or cliff line, unless a line of structures is constructed along the coast. Most of the world’s coastlines consist of sandy beaches. In some locations the beach is covered partially or completely with coarser stone known as shingle. Many shorelines consist of long beaches occasionally interrupted be a river, tidal inlet, or rocky headland. In other locations there are short pocket beaches between large headlands that limit the interchange of sediment between adjacent beaches.

Wave Refraction, Diffraction, and Reflection

Consider the design of a protective breakwater for a small marina that is located on the open coast. A typical design concern would be to predict wave conditions at interior points in the marina (where vessels will be moored) for a given deep water design wave height, period, and direction. There must be an analysis of the change in wave height owing to the change in relative depth from deep water to the marina interior (as discussed in Chapters 2 and 3). We must also evaluate the effects of refraction on wave height and crest orientation as the wave propagates over the nearshore bottom contours from deep water to the vicinity of the marina. Then the effects of diffraction and possibly further refraction as the wave propagates into the lee of the marina breakwater must be evaluated. The combined effects of shoaling, refraction, and diffraction will yield the resulting wave height and direction of propagation pattern within the marina. If any of the interior borders of the marina (e.g., vertical bulkheads and quay walls) have a high reflection coefficient, reflected waves may also be active at the points of interest.

Coastal Water Level Fluctuations

This chapter is concerned with coastal water level fluctuations caused by waves having longer periods than those generated directly by the wind, and other nonwave fluctuations in coastal water levels. In particular, these may be classified as: 1. Astronomical tide—periodic fluctuations caused by the interaction of gravitational and centrifugal forces primarily between the earth, sun, and moon 2. Tsunamis—surface waves generated by underwater disturbances primarily of seismic origin 3. Basin oscillations—resonant response of water bodies to long period wave and nonwave excitations 4. Storm surge—setup and setdown of coastal water levels caused by meteorological forces 5. Climatological/geological effects—long-term changes in relative sea level owing to atmospheric warming coupled with coastal uplift or subsidence.

Field and Laboratory Investigations

The coastal zone—where the land, sea, and air meet—is one of the most complex areas for conducting civil engineering analysis and design. Owing to the nature of the coastal zone, most coastal engineering analysis and design procedures have a partial or complete empirical basis. This empirical basis must be developed and continually improved through extensive field, and in some cases, laboratory investigations.

AN ENGINEERING STUDY OF OCEAN CITY'S BEACHES, NEW JERSEY, USA

The Labrador Sea Extreme Waves Experiment (LEWEX), is an international basic research programme concerned with full-scale measurements, analysis, modelling and simulation in test basins of 3-dimensional seas. The research is carried out in order to assess the significance of 3-dimensional sea states in engineering applications. The first phase of the programme full scale wave measurements in the North Atlantic Ocean was performed at a site and time that had a high probability of encountering severe sea states. The present publication shows examples of measured bi-modal directional sea spectra obtained with the WAVESCAN buoy and directional sea spectra measured with an airborne Synthetic Aperture Radar (SAR). Directional spectra of gravity waves are obtained with the SAR both in open waters and below an ice cover. Further work is needed in order to verify SAR-measurements with in-situ observations. In-situ measured directional spectra are also compared with hindcast spectra from the 3G-WAM model. Hindcast significant wave heights were found to be lower than the in-situ measurements.The littoral drift model developed at DHI and ISVA, see Deigaard et al. (1986b) has been extended to include the effects of the irregularity of the waves, of a coastal current and a wind acting on the surf zone. Further, a mathematical model to simulate the near-shore current pattern along a barred coast with rip channels has been developed. The influence on the littoral drift of the irregularity of waves, wind, coastal current, and rip channels is discussed. It is concluded that irregularity of waves and presence of rip channels must be considered while coastal current and wind action are of minor importance.At present, the Prodhoe Bay oil field in Alaska contributes a substantial amount of the domestic oil production of the United States. Oil is also expected to be present on the continental shelf of Alaska, and it is estimated that approximatedly 28 percent of the total U. S. reserve is located beneath the shallow ice covered seas of the Alaskan continental shelf. To expolre and to exploit these oil rich resources, engineers are confronted with hostile oceanographic conditions such as high tides, waves, strong currents and sea ice. The same area is also rich in fishery resources. Being one of the most productive fishing grounds in the nothern Pacific, the potential ecological impact due to an oil spill is of a major concern. This paper describes the methologies used for the development of a modeling system for the oil risk analysis. The system is designed with generality in mind so it can be used for other coastal areas. The development of three dimensional models used in the modeling system described here have been published in the earlier International Coastal Engineering Conferences (Liu and Leendertse, 1982, 1984, 1986) and a report published recently by RAND (Liu and Leendertse, 1987). In the oil-spill risk analysis, these three dimensional hydrodynamic models are coupled to a two-dimensional stochastic weather model and an oil weathering model.The two projects (LUBIATOWO 79 and LUBIATOWO 86) were aimed at study pore pressure behaviour in natural sand bed in the coastal zone of the Baltic sea under the action of storm waves. During both projects, the wave induced pore pressures at the various levels in the sea bed were measured. The collected data were used to verify the applicability of the various theoretical approaches. In the conclusion, the range of the application of the particular analytical method is given.The rapid recession of the shingle bank of Hurst Beach (up to 3.5m/yr) makes it an excellent natural laboratory for the study of the factors which influence the stability of shingle beaches. Studies have included: the significance of long period, high energy, swell waves the classification and quantification of overwash processes run-up and seepage characteristics the effect of settlement of the underlying strata and the implications for practices in shingle nourishment. The studies have revealed the distinctive character of shingle beaches as compared with the more fully researched sand beaches. More detailed research on shingle beaches is justified particularly in relation to (i) the run-up characteristics including its interaction with swash cusps and (ii) the influence of the subsidiary sand fraction on the beach characteristics.Environmental assessment, engineering studies and designs were completed for a new 26.5 m3/s seawater intake system in the Persian Gulf. The original intake facility consisted of a curved, 60m breakwater with one end attached to the shoreline, a settling basin immediately adjacent to the shoreline and dredged to a maximum depth of approximately 5m, and a pumphouse structure located on shore such that the seaward wall formed one side of the settling basin. The facility located on an island in the Gulf, which served multiple seawater uses, had experienced both structural and operational problems, the latter consisting principally of excessive ingestion of sediment and seaweed. These factors plus the requirement for additional demands for seawater beyond plant capacity caused the owner to initiate a study of alternative intake systems, produce a design for the most effective solution and construct the new intake system.A two-dimensional wave prediction model suitable for use on personal computers is described. The model requires the twodimensional time-dependent wind field as input. Output consists of wave height, wave period, and wave direction estimates at all grid points on a computational grid representing an enclosed or semi-closed basin. Model predictions compare favorably with observations from a wave research tower in Lake Erie. A formula is provided to estimate how long a model simulation would take on a personal computer given the surface area of the computational domain, the grid size, and the computer clock speed.Cullera Bay is a neritic ecosystem placed on the Spanish Mediterranean Littoral largely influenced by the Jucar River, that brings about lower salinities than surrounding waters, and broad variations of its values. An extensive research, with 9 samplings throughout the year, was carried out, measuring both physical and chemical parameters, and the planktonic communities. The trophic status of the ecosystem, the spatial and temporal variations of the nutrients and the planktonic communities were studied, evaluating the influence of the river loads and the littoral dynamics. Some essential basis to allow a suitable emplacement of waste waters disposals along the Valencian littoral are set up in order to minimize the gradual eutrophication of this coast.In the last two years a whole of studies was realized in order to determine precise solutions to the regeneration of Villajoyosas beach, in the Spanish mediterranean coast. Investigations were carried out to the surrounding coastal areas based in field investigations and laboratory analyses of the beaches materials.