Measurement of longshore current with a Doppler radar velocimeter at the research pier HORS

Various reliable data sets from sites (USA and Netherlands) have been selected to analyse the longshore transport process and to establish the relationship between wave height, wave incidence angle and longshore transport, yielding a relatively simple transport formula. The data set was too small to detect any influence of particle size, wave period and profile shape. These aspects were studied by using the results of a detailed process-based model, which were parameterized and implemented in the simplified formula. The main overall result is a general expression for longshore transport of sand and gravel, including the effects of profile slope and tidal velocity. INTRODUCTION The computation of a reliable estimate of longshore sand transport remains of considerable practical importance in coastal engineering applications such as the derivation of sediment budgets for coastal areas with and without structures (breakwaters, groynes) and long-term beach stability with and without beach nourishments or coarse-grained beach protections. Most research on longshore transport has concentrated on sand sized sediment, but research on longshore transport along gravel and shingle beaches has also been performed to deal with the erosion problems along these types of beaches, which are quite common along midand high-latitude (formerly glaciated) parts of the world. The most widely used formula for longshore transport (LST) is commonly known as the CERC equation (Shore Protection Manual, US Army Corps of Engineers, 1984). This method is based on the principle that the longshore transport rate (LST, incl. bed load and suspended load) is proportional to longshore wave power P per unit length of beach; LST=K P, with K=calibration coeffcient. The CERC formula has been calibrated using field data from sand beaches. The effects of particle diameter and bed slope have been studied systematically by Kamphuis (1991), resulting in a more refined equation for longshore sediment transport. This latter equation was found to give the best agreement between computed and measured transport rates (Schoonees and Theron, 1996). Both equations (CERC and KAMPHUIS) have been used in the present study. Furthermore, a detailed process-based model (CROSMOR2000) has been used in this study to compute the longshore sand transport distribution along the cross-shore bed profile. First, this model 1 Senior engineer, Delft Hydraulics, PO Box 177, 2600 MH Delft, Netherlands; leo.vanrijn@wldelft.nl.

Sediment transport model of the southeastern Mediterranean coast

Changes in deep-water wave climate drive coastal morphologic change according to unique shoaling transformation patterns of waves over local shelf bathymetry. The Southern California Bight has a particularly complex shelf configuration, of tectonic origin, which poses a challenge to predictions of wave driven, morphologic coastal change. Northward shifts in cyclonic activity in the central Pacific Ocean, which may arise due to global climate change, will significantly alter the heights, periods, and directions of waves approaching the California coasts. In this paper, we present the results of a series of numerical experiments that explore the sensitivity of longshore sediment transport patterns to changes in deep water wave direction, for several wave height and period scenarios. We outline a numerical modeling procedure, which links a spectral wave transformation model (SWAN) with a calculation of gradients in potential longshore sediment transport rate (CGEM), to project magnitudes of potential coastal erosion and accretion, under proscribed deep water wave conditions. The sediment transport model employs two significant assumptions: (1) quantity of sediment movement is calculated for the transport-limited case, as opposed to supply-limited case, and (2) nearshore wave conditions used to evaluate transport are calculated at the 5-meter isobath, as opposed to the wave break point. To illustrate the sensitivity of the sedimentary system to changes in deep-water wave direction, we apply this modeling procedure to two sites that represent two different coastal exposures and bathymetric configurations. The Santa Barbara site, oriented with a roughly west-to-east trending coastline, provides an example where the behavior of the coastal erosional/accretional character is exacerbated by deep-water wave climate intensification. Where sheltered, an increase in wave height enhances accretion, and where exposed, increases in wave height and period enhance erosion. In contrast, all simulations run for the Torrey Pines site, oriented with a north-to-south trending coastline, resulted in erosion, the magnitude of which was strongly influenced by wave height and less so by wave period. At both sites, the absolute value of coastal accretion or erosion strongly increases with a shift from northwesterly to westerly waves. These results provide some examples of the potential outcomes, which may result from increases in cyclonic activity, El Nino frequency, or other changes in ocean storminess that may accompany global climate change.

A Three-Dimensional Numerical Model of Surface Waves in the Surf Zone and Longshore Current Generation over a Plane Beach

Longshore sediment transport and foreshore change in the swash zone of an estuarine beach

Storm-generated morphological change and longshore sand transport in the intertidal zone of a multi-barred macrotidal beach

Effect of wave climate change on longshore sediment transport in Southwestern Black Sea

Data were acquired continuously during the 19‐day DELILAH nearshore experiment with a specific objective of examining variability of the longshore current at tidal frequencies. It is hypothesized that breaking wave heights inside the surf zone are strong functions of the depth which are modulated by the tidal variations, and since radiation stress is a function of the wave height, longshore currents are forced at the tidal frequency inside the surf zone. The measured longshore current variations at tidal frequency are the same order of magnitude as the mean longshore current variations for moderate wave height conditions, indicating that the tide is a dominant mechanism associated with longshore current variability. Simulations of the magnitude and phase of the longshore current variability with tide elevation using the model by Thornton and Guza (1986) are used to explain observations. The measured tidal elevation and longshore current are in phase in the inner surf zone and out of phase in the outer surf zone as predicted by the model, verifying the hypothesis.

Climate controls on longshore sediment transport

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.

Establishing uniform longshore currents in a large-scale sediment transport facility

Morphodynamic variability of surf zones and beaches: A synthesis

The erosional and accretional profile changes of an intertidal mudflat are examined using available field data and the cross-shore numerical model CSHORE that is extended to allow for a mixture of sand and mud. The semidiurnal migration of the still water shoreline and surf zone is resolved numerically to predict the net cross-shore and longshore sediment transport rates influenced by the small cross-shore (undertow) and longshore currents induced by breaking waves of about 0.2 m height. Alongshore sediment loss or gain is included by approximating the alongshore sediment transport gradient using an equivalent alongshore length. The calibrated CSHORE reproduces the measured erosional (accretional) profile change of about 0.1 m (0.1 m) over a cross-shore distance of 950 m during the erosional (accretional) interval of 206 (195) days. The mudflat profile changes are equally affected by mud characteristics, the semidiurnal tide amplitude, and the wave height, period, and direction. In addition, the alongshore water level gradient and wind stress influence longshore current and sediment transport. This study shows the importance of sediment transport in the surf zone that may have been excluded in previous numerical modeling.

The present study is focused on the influence of tidal cycle on heavy minerals variation in the beaches at Kottilpaducoast of Kanyakumari district, Tamil Nadu. The study area is enriched with a variety of heavy minerals, mostcommonly with ilmenite, along with monazite, rutile, zircon, garnet etc. These placer deposits often vary indistribution due to the effect of tides as well as waves. 5 sampling stations were selected to represent the beach and daily monitoring was carried out during high and low tide levels. oreover, wave parameters such as wave height, wave period, littoral drift and wave direction are also taken into consideration. Further, the sediment samples were analyzed for size fraction and mineral composition. The beach morphology was assessed based on beach profile data. The results reveal that high percentage of heavy minerals is noticed at high tide and low percentage at low tide. Exceptions are noticed in few samples which may be due to varied hydrodynamic conditions prevails in the study area. In all samples more than 40% constitute opaques followed by sillimanite <25% and other constitute rest of the percentage. From the XRD analysis, the peak positions shown by diffractogram is ilmenite (FeTiO3) pyrope and pseudorutile (Fe2Ti3O9), sillimanite (AlFeO2.SiO5), zircon (ZrSiO4) and rutile (TiO2). X-ray fluorescence analytical results also reflect what was inferred from XRD and point counting data. The heavy mineral assemblage of the beach sediment indicates the possibility of mineral supply from alongshore and offshore sources.

PURPOSE: The calculation of longshore sediment transport (LST) rates is a key component of most coastal engineering studies. While the LST process is conceptually simple, in practice the development of reliable rates is made difficult by problems associated with collecting accurate field data, by limitations to model predictions, and by substantial variations of the rates in time and space. This Coastal and Hydraulic Engineering Technical Note (CHETN) summarizes the state of understanding of the influence of grain size on surf zone sediment transport and is a companion to Smith et al. (2004). This CHETN discusses details of bed-load and suspended load transport, and the classical bed-load regime is shown to encompass two distinct modes of transport. Four LST models with varying levels of complexity are discussed to show how they incorporate the physics of grain size variation and its effect on the transport rate. In addition, a relationship between the K coefficient in the CERC formula (Coastal Engineering Manual (2002), Section III-2-3-a) and grain size is presented. Finally, some inconsistencies between theory and data are discussed in the context of the interrelationship between grain size and beach slope. The final conclusion is repeated here. In general, an increase in the median grain size will decrease LST rates in the surf zone. If a simple exponential relationship between transport rate and grain size (D) is needed (Equation 7: LST rate ∝ D n ), the most appropriate value for the exponent n should be of the order of -1, as seen from Equation 23. However, this tech note argues that this is clearly a simplistic view of surf zone sediment dynamics. A more realistic (though still highly simplified) approach would be that, for fine grain sediments (D50 on the order of 0.15 to 0.30 mm), suspended load transport should dominate and n should be somewhere within the range of -0.5 to -3.0. For coarse sands (D50 around 1.0 mm), sheetflow bed-load transport should dominate (Figure 6), and the transport rate should be nearly independent of grain size (n = 0). For large gravel and shingle (D50 > 20 mm), the dominant transport should be in the IM (Initial Motion) bed-load regime, with n within the range of -0.5 to -2.0. To state this another way, the exponent n in Equation 7 is itself a function of grain size. BACKGROUND: Though most researchers recognize the median grain size as a parameter of first order importance in its effects on the magnitude of sediment transport, its variation is not as easily studied as the variation of many of the other primary parameters. In the field, it is easy to obtain data for a range of values for wave heights and periods, for instance, as these vary constantly in time. However, the grain size on a beach typically shows no (or insignificant) variations in time. Similar problems can occur in the laboratory. Changing the wave height or period in a laboratory study is usually as simple as reprogramming a wave paddle. However, usually, changing the grain size in a large model is infrequently done because of the large cost in both time and money.