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Longshore sand transport: a comparison between field observations and predictions of numerical models and implications for coastal erosion studies
Abstract and Figures
Field measurements of waves, currents and longshore transport were carried out at Culatra Island in southern Portugal in a semi-diurnal meso-tidal regime. Observed wave conditions were uniform (H rms 0.24-0.26 m) during one and a half tidal cycles while mean longshore currents reached a peak (0.28 mjs) half way throughout the experiment, due to the presence of a moderate longshore wind. Sand tracers measured a peak in the longshore transport rate (1 .38 x 10. 2 m 3 js) in agreement with the increase in current speed. Simulations of the field conditions using a computer model (LlTPACK) do not predict successfully the measured transport rates. Calculations using sim pie empirical formulas provided a better approximation, despite being one order of magnitude smaller than the field measurements. If the model was used employing theoretical or extrapolated physical parameters it would have been impossible to determine the errors in the predictions. It is praposed that computer models of sand transport should produce probabili ~tic results with a clear indication of the expected levei of errar. Field monitoring of beach morphodynamics ~till remains the best and most reliable method of pracess assessment.
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... Three consecutive readings were taken [6]. Number of waves and Angle of approach of waves have been measured by visual observation and by using magnet respectively [7]. Quantitative sampling methods were used for the sampling process. ...
... Despite the use of LITPACK in a "chain" of wave and hydrodynamic models considering the main nearshore processes affecting coastal deposition/erosion, discrepancies may be attributed on the assumption that these conditions, produced for year 2008, are repeated for the entire 31-year period . Other investigators as Lakhan and Trenhaile [1989] and Giavola et al. [1996] also question the ability of this deterministic model to produce efficient predictions. ...
In this paper, the combined use of satellite images and morphodynamic models is used to assess the coastline changes of an erosion-prone deltaic zone in Northern Greece (Nestos River delta). Satellite images over the last thirty-one years were used to estimate the erosion rates at the eastern, central and western parts of the delta. The numerical model LITPACK, coupled with wave and hydrodynamic nearshore models, was calibrated based on these results, and was further utilized to predict the shoreline evolution with and without “hard” protection works. Results show that the model underestimated erosion rates over all studied sub-sections, but showed fair agreement to remote sensing analysis when focusing on the zones of major erosion. “Hard” coastal protection structures were tested to assess their mitigation efficiency in terms of longshore and cross-shore sediment transport reduction. LITPACK considered that the scenario of seven rubble-mound breakwaters with low seaward and lee-side slopes, positioned between 9 and 19 m depicted the best results, even reversing coastline retreat process.
... Certainly the fact that both methods give predictions within comparable orders of magnitude is already satisfactory for sediment transport estimations. On the other hand, we are within the same error band given by predictions calculated for the presented physical conditions using either bulk transport equations (Ciavola et al. 1997a) or deterministic numerical modelling (Ciavola et al. 1996). ...
The effective management of the Coastal Zone requires an appreciation of the complex nature of the processes at work. Decisions relating to its management require the modelling of these processes which in turn calls upon a large number of disparate data sets. The most effective way to relate these data to each other, to facilitate their subsequent analysis, and to allow clear visualisation, is by geographical position. This thesis describes work aimed at improving the efficiency and applicability of coastal process models in shoreline management decisions by integrating spatial data and coastal processes in a GIS environment. A Geographic Information System has been integrated with a coastal wave model. The resulting system is shown to simplify the modelling procedures, to facilitate the generation of input data and incorporate functionality for the visualisation and analysis of model results. Increased automation makes coastal process modelling less complicated and more accessible to non- specialists. The integrated system developed has extended the capability of the wave model to generate wave data. Wave height, period, and direction can be generated for discrete points at any density along an inshore contour. These results are visualised and analysed in the GIS. The system has been further developed to demonstrate its decision support capabilities. It is adapted for a typical dredging application. New tools allow the bathymetry to be 'excavated' simulating sediment removal; and subsequent modelling quantifies its impact on the inshore wave conditions, which are then reported back to the user. The system and tools are assessed using two test areas. The benefits of using the integrated system are evaluated by comparison with the experience of using wave models in isolation when investigating coastal management issues. Finally, directions and suggestions for further work are discussed.
Forcing functions and sediment response were measured during two comprehensive surf zone experiments. The experiments included simultaneous measurements of waves and currents, and the movement of sediment as bed and suspended load. The longshore transport of suspended load was found to be about 10 to 20% of the tracer-measured load. Results from tracer measurements of the longshore transport of bed load indicate that previous measurements may have misestimated the effective "tracer layer thickness," and a more rigorous method is proposed.
Simultaneous field measurements of wave and current parameters in the surf zone and the resulting longshore transport of sand have been made on two beaches under a variety of conditions. The direction and flux of wave energy was measured from an array of digital wave sensors placed in and near the surf zone. Quantitative measurements of the longshore sand transport rate were obtained from the time history of the center of gravity of sand tracer. The measurements have been used to test two models for the prediction of the longshore transport rate of sand. The first model gives the immersed weight longshore transport rate of sand, Il, as proportional to the longshore component of wave energy flux (power), Il = K(ECn)b sin αb cos αb, where E is the energy density, Cn is the wave group velocity, and αb is the breaker angle. The second model assumes that the waves provide the power to move and support the sand and that the superimposed longshore current 〈νl〉 provides a longshore component that results in the longshore transport of sand according to the relationship Il = K′ (ECn)b cos αb〈νl〉/um, where um is the magnitude of the maximum horizontal component of orbital velocity near the bottom under the breaking wave, assumed to be proportional to the rate of energy dissipation by friction on the beach bed. The measurements show that both models successfully predict the sand transport rate, with values of the dimensionless coefficients K = 0.77 and K′ = 0.28. The coherence of the models implies that they are interrelated, their common solution giving the relation as 〈νl〉 = K″ um sin αb, where K″ is a dimensionless constant equal to 2.7. This relation can be obtained directly by equating the longshore current and the longshore component of the momentum flux (radiation stress) of the breaking waves. Thus, the coherence of the models appears to be based on the generation of the longshore currents by the longshore radiation stress. The models will not be equivalent if 〈νl〉 owes its origin to some other generating mechanism such as tides or winds.
Eight fluorescent sand tracer experiments were performed in energetic surf zones on natural beaches and on beaches near structures to measure the short-term longshore sand transport rate. Tracer of up to four distinct colors was injected on a line crossing the surf zone to investigate the on-offshore distributions of the longshore sand adveetion velocity and transport rate. The tracer advection velocity, v , and the depth of mixing into the bed, b, were determined from large numbers of cores taken in situ throughout the sampling area. The sand advection velocity and mixing depth were not constant across the surf zone, but usually exhibited a maximum either toward the shoreline or toward the breaker line, or in both regions. The local breaking wave height, Hb, and horizontal current velocity in the surf zone (yielding an average longshore current velocity V ¯ ) were also measured. The data were interpreted with simple dimensional arguments to give the following results: b2 = 0.027 Hb, va = 0.014 V ¯ , and the volumetric transport rate Q = 0.024 H b 2 V ¯ . Agreement was also found between the measured total longshore sand transport rate and a predictive expression due to Bagnold involving the breaking wave power and average longshore current velocity. Although the results appear reasonable and consistent, a problem remains concerning the apparent decrease in tracer advection speed alongshore recorded in most experiments at the longer sampling times.
The Generalized Model for Simulating Shoreline Change (GENESIS)(Hanson and Kraus, 1989) is designed to simulate the long-term shoreline changes at coastal engineering sites resulting from spatial and temporal differences in longshore sediment transport. GENESIS is used by the coastal engineering and planning communities for predicting the behaviour of shorelines in response to coastal engineering and/or beach replenishment activities that may alter longshore transport. GENESIS is also used by some modelers to develop regional scale sediment budgets. In most cases, the assumptions used in the model fail to be met or are so oversimplified that the model's effectiveness as a predictive tool is limited at best. -from Authors
This chapter provides an overview of systems concepts and the systems approach. The coastal system is a complex, decomposable, and large-scale system that cannot be fully comprehended with time-limited studies. As complex systems involve interrelationships among variables and parameters, the best way to gain insights into their structure, organization, and functioning is the use of models. The chapter discusses two types of models employed in the study of natural systems: physical models and mathematical models. Physical models are used (1) to seek a qualitative insight into a phenomenon, (2) to obtain measurements for verifying (or disprove) a theoretical result, and (3) to obtain measurements for phenomena that are so complex that they have not been accessible to theoretical approaches. However, mathematical models are most widely used in systems studies because of their generality, versatility, and flexibility.
Dispersions of solid spherical grains of diameter D = 0\cdot 13 cm were sheared in Newtonian fluids of varying viscosity (water and a glycerine-water-alcohol mixture) in the annular space between two concentric drums. The density sigma of the grains was balanced against the density rho of the fluid, giving a condition of no differential forces due to radial acceleration. The volume concentration C of the grains was varied between 62 and 13%. A substantial radial dispersive pressure was found to be exerted between the grains. This was measured as an increase of static pressure in the inner stationary drum which had a deformable periphery. The torque on the inner drum was also measured. The dispersive pressure P was found to be proportional to a shear stress T attributable to the presence of the grains. The linear grain concentration lambda is defined as the ratio grain diameter/mean free dispersion distance and is related to C by lambda =1/(C0/C)1/3-1, where C0 is the maximum possible static volume concentration. Both the stresses T and P, as dimensionless groups Tsigma D2/lambda eta 2 and Psigma D2/lambda eta 2, were found to bear single-valued empirical relations to a dimensionless shear strain group lambda 1/2sigma D2(dU/dy)/eta for all the values of lambda < 12 (C = 57% approx.) where dU/dy is the rate of shearing of the grains over one another, and eta the fluid viscosity. This relation gives T propto \ sigma (lambda D)2 (dU/dy)2 and T propto \ lambda 3/2eta dU/dy, according as dU/dy is large or small, i.e. according to whether grain inertia or fluid viscosity dominate. An alternative semi-empirical relation T = (1 + lambda ) (1 + 1/2lambda ) eta dU/dy was found for the viscous case, when T is the whole shear stress. The ratio T/P was constant at 0\cdot 3 approx. in the inertia region, and at 0\cdot 75 approx. in the viscous region. The results are applied to a few hitherto unexplained natural phenomena.
The use of mathematical modeling to predict the behavior of beaches does not work. Some of the major assumptions behind the models used by coastal engineers in the United States are wrong or are unverified. In addition, the models are deterministic and fail to account for the uncertainty of storms or for the chaotic nature of the nearshore environment. The Dutch and Australians do a better job of predicting beach behavior with an observational approach, predicting how a particular beach might behave in the future by studying its past behavior. -Author
Experiments which were made to determine the depth of disturbance of beach sand by waves are discussed. The method of the experiments, which were made with dyed sand, is described. A significant correlation was found between the depth of disturbance of the sand and the wave height, and the results have been analyzed statistically. The depth of sand disturbance was in general very small, but it was found to increase with increasing wave height and energy. The disturbance was found to be greatest in the relatively shallow water at and inside the break-point of the waves. It was found that the depth of disturbance was greater as the sand became coarser. The significance of the results of the experiment is discussed briefly.