Magnus Larson

Prediction of cross-shore sediment transport at different spatial and temporal scales

Abstract Coastal sediment transport and associated morphological change can be examined at many spatial and temporal scales. These scales range from instantaneous movement of single grains on a time scale of the local turbulence to seasonal and long-term movement of large sand bodies such as longshore bars and tidal deltas. Researchers have begun to recognize that a first-principles (microscale) approach to calculating long-term and large-scale (megascale) coastal change may not be possible or even adequate. Therefore, new approaches are being sought to advance quantitative understanding and prediction of large-scale coastal behavior. The present paper opens with a discussion of compatible spatial and temporal scales for calculating sediment transport and morphology change. Motivation is given for the need to calculate morphologic change at scales greater than the typical one of microscale. Example calculations are made at different scales, showing the assumptions and approximations of each. The main conclusion is that calculations at different scales can be related and reconciled if limitations in prediction of initial and boundary conditions and in the fluid forcing are recognized.

BEACH PROFILE CHANGE: MORPHOLOGY, TRANSPORT RATE, AND NUMERICAL SIMULATION

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.

Measurements of the Velocity Field Downstream of an Impeller

The velocity field downstream of a model impeller operating in water was measured using a two-component laser doppler velocimeter. The investigation focused on the spatial development of the mean velocity in the axial, radial, and circumferential direction, although simultaneous measurements were performed of the velocity unsteadiness from which turbulence characteristics were inferred. The measurements extended up to 12 impeller diameters downstream of the blades displaying the properties of the generated swirling jet both in the zone of flow establishment and the zone of established flow. The division between these zones was made based on similarity of the mean axial velocity profile. Integral properties of the flow such as volume and momentum flux were computed from the measured velocity profiles. The transverse spreading of the impeller jet and its development towards self-similarity were examined and compared with non-swirling jets and swirling jets generated by other means.

Coastal Evolution Modeling at Multiple Scales in Regional Sediment Management Applications

Abstract : A numerical model called GenCade is introduced that simulates shoreline change relative to regional morphologic constraints upon which these processes take place. The evolution of multiple interacting coastal projects and morphologic features and pathways, such as those associated with inlets and adjacent beaches can also be simulated. GenCade calculates longshore sediment transport rates induced by waves and tidal currents, shoreline change, tidal inlet shoal and bar volume evolution, natural bypassing, and the fate of coastal restoration and stabilization projects. It is intended for project- and regional-scale applications, engineering decision support, and long-term morphology response to physical and anthropogenic forcing. Capabilities of the model are illustrated by an application to the south shore of Long Island, NY. The Long Island application has multiple coastal structures and features that are maintained to varying degrees of frequency. Cumulative response of the beaches from a variety of coastal projects leads to complexity in regional coastal management. GenCade is presented as a tool to unify management of local projects at regional scales.

A New Approach to Represent Tidal Currents and Bathymetric Features in the One-Line Model Concept

The GENESIS model has been widely used for predictions of long-term shoreline evolution along wave-dominated open-coast beaches. However, to be applied in the vicinity of coastal inlets new capabilities needed to be implemented. This paper describes two new capabilities recently implemented in the model. The first is the extension of the model to calculate the longshore sediment transport rate by means of a generalized version of the Bagnold formula (1963) which makes it possible to describe longshore transport and shoreline change originating from multiple sources of a longshore current, such as wind and tide. The second is provision for inclusion of a pre-specified offshore contour to allow dominant local or regional bathymetric features to be represented in the wave transformation. Illustrative examples indicate promising results for these major enhancements of the model. INTRODUCTION Significant offset between upand down-drift beaches are typically found adjacent to inlet jetties where a dominant direction of longshore sediment transport exists. Down-drift beaches are often suffering from chronic erosion and in need of remedial measures. Several remediation measures have been proposed and implemented for reducing erosion and increasing the longevity of material placed in such hot spots. These include lengthening the down-drift jetty, placement of an external spur 1) Department of Water Resources Engineering, Lund University, Box 118, Lund, Sweden S-221 00. Email: Hans.H~.nson@,tvd.lth.se. 2) U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Rd., Vicksburg, MS 39180.

Performance of Longshore Sediment Transport Formulas Evaluated with Field Data

The skill of six well-known formulas developed for calculating the longshore sediment transport rate was evaluated in this study. Formulas proposed by Bijker (1967, 1971), Engelund-Hansen (1967), Ackers-White (1973), Bailard-Inman (1981), Van Rijn (1984), and Watanabe (1992) were investigated because they are commonly employed to calculate the time-averaged net sediment transport rate in the surf zone. Detailed, high-quality data on hydrodynamics and sediment transport from Duck, NC, collected during the DUCK85, SUPERDUCK, and SANDYDUCK field experiments were utilized to assess the predictive capability of these six formulas. Overall, the Van Rijn formula was found to yield the most reliable predictions over the range of swell and storm conditions that the field data covered. The Engelund-Hansen formula worked reasonably well, although with large scatter for the storm cases, whereas the Bailard-Inman formula systematically overestimated the swell cases and underestimated the storm cases. The formulas by Watanabe and Ackers-White produced satisfactory results for most cases, although the former overestimated the transport rates for swell cases and the latter yielded considerable scatter for storm cases. Finally, the Bijker formula systematically overestimated the transport rates for all cases. 1 Department of Water Resources Engineering, Lund University, Box 118, S-22100, Lund, SWEDEN. 2 U.S. Army Engineer Research and Development Center, Field Research Facility, 1261 Duck Road, Duck, NC, 27949-4471, U.S.A. 3 U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road, Vicksburg, MS, 39180-6199, U.S.A.

PREDICTION OF BEACH PROFILE CHANGE AT MESOSCALE UNDER RANDOM WAVES

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

Sheet flow and suspension under wave groups in a large wave flume (SISTEX99)

Detailed measurements of sediment concentrations in the sheet flow layer and in the suspension layer for flat bed conditions under prototype wave groups are presented. These measurements are part of experiments on near-bed sand transport processes, carried out in the Large Wave Flume in Hannover, Germany (SISTEX99). The results show that concentrations in the sheet flow layer are highly coherent with the instantaneous near-bed velocity. In the suspension layer, however, concentrations respond much slower to changes in near-bed velocity: at some distance from the bed concentrations increase and decrease on the time scale of the wave group, with a time delay relative to the peak wave within the wave group.