Narrow Results

Tidal inlets are an ecologically sensitive and significant constituent of the coastal environment, where an opening along the shoreline permits the free exchange of fresh and seawater. Predominant longshore currents along the shore result in the formation of sandbars, spits and shoals. Frequent occurrence of such sediment depositions acts as barriers, preventing the ingress of tidal flow. Conventionally training walls are constructed at the inlet to prevent sandbar and spit formations. The volume of water exchanged at the inlet mouth and ebb tidal currents primarily govern the inlet dimensions and the rate of littoral transport, respectively. The case study of a trained micro-tidal inlet at Karaikal (10${}^{\circ }54^{\prime }$52${}^{\prime \prime }$N; 79${}^{\circ }51^{\prime }09^{\prime \prime }$E), India is discussed in this paper. A numerical model using the finite volume method is applied to estimate the siltation rate and distribution within the study domain, driven by tide-induced currents and riverine discharge. A revetment structure is proposed to combat siltation within the inlet to facilitate smooth navigation. The study highlights the notable changes in the presence and absence of the proposed revetment.

A field investigation for deformation of a river-mouth sand bar in the Tenjin River, which is located at Tottori prefecture in Japan, was carried out. The topographic surveys using RTK-GPS, UAV and RC boat were conducted from July, 2015 to March, 2017. Moreover, 4 cameras were set at the both sides of the Tenjin River embankment in order to monitor the deformation of the river-mouth bar. From these measurements, the characteristics of the deformation of the sand bar tip in the opening of the river mouth were analyzed and the validity of an alternative survey technique using RTK-GPS, UAV and RC boat was confirmed.

Field, laboratory and modelling studies generally agree that the rate at which bedforms develop in response to changing conditions depends upon the pre-existing state of the bed but there is currently no consensus about the nature of this dependence. This study uses a simple abstracted model to investigate the way a field of ripples responds to a step change in forcing conditions. The results show that the influence of pre-existing bedforms is significant but subtle. Large pre-existing bedforms retard the growth of new bedforms but defects in the old bedform pattern increase the rate of development even above the growth rate from a flat bed. Wavelet analysis correctly identifies the sites of new ripple growth at defects. We suggest that defects trigger new bedform growth because they are regions of relatively flat bed, with upstream perturbations. Prediction of non-equilibrium bedforms will require information on defect densities.

The performance of most operational nearshore models is less limited by the quality of model physic or numerics than by the availability of input data for initial and boundary conditions. For example, circulation and sediment transport are very sensitive to the details of bathymetry, a data field that is notoriously hard to measure well and changes rapidly. Remote sensing approaches offer an increasingly attractive solution. While active sensors like radar and LIDAR have some advantages and will be discussed, this paper will focus more on optical remote sensing through the Argus program. Starting with a discussion of the required capabilities of an ideal sampling system, the paper will discuss the technical capabilities and future directions of optical remote sensing of the nearshore. Sampling gaps will be identified and possible solutions discussed.

The shoreline along sandy beaches is located at a unique position on the earth's surface where marine and terrestrial processes converge. The swash zone distinguishes the landward-most reach of wave action. Field observations from this shallow and highly energetic region reveal that individual waves regularly deposit or remove hundreds of kilograms of sand per meter width of beach. Such high rates of sand movement represent several centimeters of bed-level change and far exceed the underlying pace of beach evolution. Relatively large morphological changes caused by single swashes might suggest that very rapid beach erosion or accretion is a common occurrence. The contrasting reality shown by these new observations is that beaches generally exhibit a state of dynamic equilibrium.

A state-of-art process-based model is applied to simulate the hydrodynamics in the Yangtze Estuary in China, as a basic step to acquaint ourselves with the morphological development and the effect of human interference in the estuary. A major improvement with respect to previous models for the Yangtze Estuary is that the present model covers the entire tidal region of the Yangtze River. Two curvilinear grids are used to adapt to the complex geomorphologic setting and the large spatial magnitude of the study area by applying domain decomposition techniques. The calibration of the model against extensive hydrodynamics data shows a good representation of observed hydrodynamics. With the present calibrated and validated model, the morphological evolution, as well as the effects of discharge regime change and sediment load reduction on the Yangtze Estuary can be carried on.

In this study, we investigated the sediment transport due to the 2004 Indian Ocean tsunami along the natural coast at Hambantota, Sri-lanka. Bathymetry and topography surveys before and after the tsunami were conducted and results showed significant erosion by the tsunami, especially around the places where shoreline discontinuations were observed. We conducted a numerical simulation of tsunami propagation as well as the bathymetry change induced by the tsunami. Furthermore, we also estimated the bathymetry change due to the usual sea waves at the coast. Our numerical results suggested that the tsunami has strong bottom shear stress at specific landform such as a river mouth and a cape. We also found that the sediment erosion and accretion due to the tsunami had been mitigated by usual waves after the tsunami.

Previous numerical modelling studies based on 2DH morphodynamical model show that oblique waves tend to inhibit the formation of rip channel systems, but the mechanisms were not investigated. Field observations do not always agree with this model result, thus, understanding the mechanisms seems essential. To this end, the global analysis technique, originally developed to describe the long term behavior of bars (saturation of the bar growth), is also applied here to the initial stage of the bar evolution (formation of the bars). As a result, rip channels grow slower for larger wave angle because of the weakening of the instability mechanism that only depends on the cross-shore current-rather than the increase of the damping due to the diffusive bedslope transport.

The shore behind the offshore fishing port has never been attached to the facility for 15 years after the construction. The main object is to investigate the littoral drift behavior around the port. As the result of wave and current observation, the differences in the time-averaged surface elevation were found at the right and left sides during typical storm. By using a volumetric analysis and a simple method to estimate the longshore sediment transport rate, 55 percents of the net longshore sediment transport is estimated to go through behind the port. In this study, a numerical simulation is applied and gives a good validation against the observed data and estimates the current field and suspended sediment transport during typical storm to understand the littoral drift behavior.

It has been suggested that vertical perforated tubes placed below the beach surface will increase the drainage of the beach, and hence increase the deposition of sand on the beach. The system is called the PEM-system, Pressure Equalization System, and the Danish company SIC (www.shore.dk) is doing the marketing. Although it for a coastal engineer seems obvious that such a device can't drain the beach (nearly no driving forces ), SIC has succeeded in installing the system in more than 75 locations around the world (according to the company). In Denmark a full scale experiment at the exposed west coast has been performed through 2005-08, and a similar Dutch test is going on right now at Egmond, Holland. In this paper, we model the flow in the beach taking into account the presence of (high-permeable) tubes and demonstrate that the drainage effect is negligible. Further, the morphodynamic behavior of the coast in relation to the Danish field test is described, and it is concluded that all morphological changes can be explained by natural causes.

The south part of the Gulf of Lions coast (Mediterranean Sea) and its double crescentic sandbar system has been observed monthly during a 3-year long period (2005-2008) and yearly from 2000. This paper proposes a conceptual model of the morphodynamic evolution of this microtidal bar system supported by field bathymetric surveys and hydrodynamics measurements. This model is based on the different evolutive sequences observed. It is then compared with other observations made in the literature.

Sandbars are often present in the nearshore zones of sandy coasts. As a result of the interaction with the incoming waves, sandbars change in shape and location over time. Trends in cross-shore sandbar behavior can often be observed on many timescales, ranging from a few months to several years. Models for sandbar behavior can be specified on different abstraction levels and corresponding scales. In the present work we investigate the ability of two models specified on different abstraction levels to predict long-term (several months) sandbar behavior, using a high-resolution 15 year-long profile dataset collected at the Hasaki Oceanographic Research Station in Japan. We find that it is very difficult to predict the rate of offshore-directed trends in sandbar behavior on the timescales of months, resulting in overall poor model performance. We discuss the implications of our findings for the application of models in the understanding of long-term sandbar behavior.

The development and evolution of crescentic patterns in double-barred systems is explored using a morphodynamic stability model. The description of the surf zone hydrodynamics is based on depth and wave averaged conditions while sediment transport is calculated using a total load formula. The linear stability analysis predicts that an initially rectilinear coast, subject to infinitesimal perturbations and under normal wave incidence, is unstable and can result in the development of crescentic shapes that can be coupled either in-phase (highs and lows of both sandbars are at the same alongshore position) or out-of-phase (highs and lows of one sandbar correspond to lows and highs of the other sandbar). Results of numerical simulations are sensitive to some of the parameterizations used in the description of hydrodynamics. Changes in the breaker index can have an effect not only on the spacing and growth rate of the pattern but also on the type of pattern that develops. An increase in the breaker index leads to a faster growth of the pattern but also to a smaller alongshore spacing. The role of parameterizations in lateral mixing and roughness length appear to be limited.

A 40 year data set collected annually and a 3.4 data set with a daily resolution both collected at Noordwijk in The Netherlands are compared with results obtained with a process based profile model over a 3.3 year simulation period. The simulation period approximately encompasses a complete bar cycle of generation in the intertidal zone, migration through the surf zone and decay at the seaward extend of the active profile. Evaluation of the model is aimed at obtaining insight in the ability of the model to predict the characteristics of such cyclic bar behavior by comparing bar amplitude, migration rates and dominant time scales using analysis techniques such as CEOF. The observed maximum bar amplitudes, its cross-shore position and the distance between bars were accurately predicted by the model. Weekly, seasonal and annual migration rates were over-predicted quantitatively, but showed good qualitative agreement. The quantitative disagreement is probably the combination of model error and the fact that the considered periods in the data analysis and model simulation were different.

A morphodynamic model has been developed to gain more fundamental knowledge about the formation of transverse finger sand bars. The model describes the feedback between waves, rollers, depth-averaged currents and bed evolution, so that self-organized processes can develop. The wave and bathymetric conditions measured at Egmond site are firstly applied and the modeled longshore current and wave height are compared with field data of that beach. Subsequently, the wave and bathymetric conditions measured at Noordwijk site are used to compare model results with the up-current oriented bars observed there. Realistic positive feedback leading to formation of the observed bars only occurs if the resuspension of sediment due to bore turbulence is included in the model. The modeled wavelength, crest orientation and growth rate agree with data but the model overestimates the migration rates.

The utility of numerical models of beach morphodynamics is constrained by our ability to establish that the theoretical dynamics match reality. The inherent difficulty in collecting suitable validation data for spatial and temporal bathymetric models of beach evolution has resulted in relatively few studies which perform empirical validation of nearshore morphological models. The present study addresses the validity of morphological modelling of an exposed beach by qualitatively comparing the evolution of a numerically modelled beach state transition with data observed using remote imaging. The application of the numerical model was broadly validated, in that, when forced with parameterised wave conditions, the morphological development is consistent with that observed via optical sensing.

A Geological model of continental shelf sand ridges is combined with a numerical modeling system designed for shallow marine physical processes to assess the potential for recovering beach quality sand. The shelf sand ridge geological model includes a coarsening upward sequence beginning with silts, clay and silty fine sand grading upward into relatively coarse sands having minimal silt and clay fraction. The upper meter of the sequence includes a clean cross-bedded sand unit that is reworked by episodic storms and waves on the inner to mid-continental shelf. Results of a numerical model investigation of modern sand ridges on the inner continental shelf of northeast Florida are consistent with the elements of the geological model. Sand ridge crests at relatively shallow depth activated by passing storms are predicted to be subject to topographic changes of up to 1 m consisting of both deposition and erosion. It is concluded that numerical modeling combined with the appropriate geological modeling can be applied as a tool for identifying potential sand and gravel deposits on the inner continental shelf.

A series of mobile bed experiments on small-, medium and large-scales using identical wave conditions based on Froude scaling showed the erosion of the upper beach by swash motions and the generation of a submerged breaker bar for erosive test data. The accretive test data shows onshore movement of the breaker bar. A Delft3D two-dimensional-vertical (2DV) profile model was used with default model settings to model beach profile development under erosive and accretive conditions at the three different laboratory scales. It was shown from Brier Skill Scores for the model predictions that the model is capable of simulating wave-induced erosion along plane sloping beaches at laboratory scales under erosive conditions. Model computations for accretive wave conditions were in qualitative agreement with the observed onshore bar movement of the breaker bar.

The nearshore hydrodynamic and morphological model CSHORE has been under development for the past several years, utilizing a multitude of laboratory data sets. Although the model has a physically-based foundation, all practical morphological models, including CSHORE require empirical parameters. The model is compared with a large set of field data and two representative examples are provided in detail. The storm response data sets, one from the east coast of the US and one from the west coast, are similar in bulk wave statistic but differ in that the East Coast case has a significant storm surge. The model performs reasonably well without site-specific calibration, but some improvement is realized by increasing the effect for the dissipation due to breaking waves on the West Coast.

In this paper, results of sediment transport at Exe Estuary, Devon, UK, obtained from a process-based model under a number of wave and tide scenarios, are presented. This study uses a nested modelling system, which consists of an oceanic scale wave model WAM and a tide/surge model POLCOMS, to transform the meteorological information to nearshore wave and tide conditions for a fine resolution local coastal process-based model to carry out detailed predictions of nearshore hydrodynamics and morphodynamics at the study area. The work is focused on studying the impacts of yearly and 1 in 50 year return period storm events on morphology at the mouth of the Exe Estuary and the adjacent coasts. Comparisons of model results are made with the beach survey data carried out by the local authorities in March 2008, in addition to the model tests on both yearly calm and storm conditions in November 2006.