Dano Roelvink

A Guide to Modeling Coastal Morphology

Wind Waves Currents Sediment Transport Morphological Processes Modelling Approaches Coastal Profile Models Coastline Models Coastal Area Models Case Studies Model System Development Modelling Procedure Example Applications Future Directions

River‐tide dynamics: Exploration of nonstationary and nonlinear tidal behavior in the Yangtze River estuary

River-tide dynamics remain poorly understood, in part because conventional harmonic analysis (HA) does not cope effectively with nonstationary signals. To explore nonstationary behavior of river tides and the modulation effects of river discharge, this work analyzes tidal signals in the Yangtze River estuary using both HA in a nonstationary mode and continuous wavelet transforms (CWT). The Yangtze is an excellent natural laboratory to analyze river tides because of its high and variable flow, its length, and the fact that there are do dams or reflecting barriers within the tidal part of the system. Analysis of tidal frequencies by CWT and analysis of subtidal water level and tidal ranges reveal a broad range of subtidal variations over fortnightly, monthly, semiannual, and annual frequencies driven by subtidal variations in friction and by variable river discharges. We employ HA in a nonstationary mode (NSHA) by segregating data within defined flow ranges into separate analyses. NSHA quantifies the decay of the principal tides and the modulation of M4 tide with increasing river discharges. M4 amplitudes decrease far upriver (landward portion of the estuary) and conversely increase close to the ocean as river discharge increases. The fortnightly frequencies reach an amplitude maximum upriver of that for over tide frequencies, due to the longer wavelength of the fortnightly constituents. These methods and findings should be applicable to large tidal rivers globally and have broad implications regarding management of navigation channels and ecosystems in tidal rivers.

Title: MICORE: DUNE EROSION AND OVERWASH MODEL VALIDATION WITH DATA FROM NINE EUROPEAN FIELD SITES

In this paper we present the first results of beach profile hindcasting with XBeach using recently measured coastal data acquired under storm conditions at eight European sites, including a comparison to model results obtained with off-the- shelf models. The results show consistently that the XBeach has skill in predicting the coastal profile, albeit that in most cases the erosion around the mean water line is overpredicted and the depositions at the lower beach face are overpredicted. The causes for this model effect are under active investigation but not resolved yet. Likely candidates are the modeling of onshore (asymmetry) transports which reduces the offshore transports due to undertow (currents) or the modeling of sediment motion in the swash zone.

Modeling centuries of estuarine morphodynamics in the Western Scheldt estuary

We hindcast a 110 year period (1860-1970) of morphodynamic behavior of the Western Scheldt estuary by means of a 2-D, high-resolution, process-based model and compare results to a historically unique bathymetric data set. Initially, the model skill decreases for a few decades. Against common perception, the model skill increases after that to become excellent after 110 years. We attribute this to the self-organization of the morphological system which is reproduced correctly by the numerical model. On time scales exceeding decades, the interaction between the major tidal forcing and the confinement of the estuary overrules other uncertainties. Both measured and modeled bathymetries reflect a trend of decreasing energy dissipation, less morphodynamic activity, and thus a more stable morphology over time, albeit that the estuarine adaptation time is long (approximately centuries). Process-based models applied in confined environments and under constant forcing conditions may perform well especially on long (greater than decades) time scales.

Video-Based Detection of Shorelines at Complex Meso-Macro Tidal Beaches

Abstract Almar, R.; Ranasinghe, R.; Sénéchal, N.; Bonneton, P.; Roelvink, D.; Bryan, K.R.; Marieu, V., and Parisot, J-P., 2012. Video-based detection of shorelines at complex meso–macro tidal beaches. Remote video imagery is widely used to acquire measurements of intertidal topography by means of shoreline detection, but, up to now, problems of accuracy were still encountered in the challenging case of energetic waves in nonuniform, meso–macro tidal environments. Unique, simultaneous, video-based and global positioning system (GPS)–based measurements of shoreline were undertaken at Truc Vert (France), a beach with such characteristics. An innovative video method, referred to herein as the Minimum Shoreline Variability (MSV) method, was developed to cope with highly variable spatiotemporal shoreline properties. The comparison of video-based and GPS-derived shoreline data sets showed that using images averaged over short periods (30 s), rather than the traditionally used 10-min averaged images, significantly improved the accuracy of shoreline determination. A local video-derived, swash-based shoreline correction was also developed to correct for the MSV error, which was found to be linearly correlated to local swash length. By combining shorter time-averaged images and video derived local swash correction factors, the horizontal root mean square error associated with MSV shorelines was reduced to 1.2 m, which is equivalent to errors reported at more uniform, microtidal, and less-energetic beaches.

Morphodynamic Response to Wave Group Forcing

A numerical model is used to compute the time-dependent behaviour of the nearshore bathymetry under wave group forcing using the depth-averaged shallow water equations in combination with an advection-diffusion equation for the sediment concentration. After verification with prototype flume measurements, a morphodynamic computation for a pocket beach has been carried out for two different conditions. The first case consideres wave groups made up by a bichromatic wave resulting in an alongshore rhythmic double-barred beach. The second case considers wave groups made up by a directionally spread short wave field resulting in a single barred beach intersected by irregularly spaced rip-channels.

A SIMPLE METHOD TO PREDICT LONG-TERM MORPHOLOGICAL CHANGES

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

Exploring the impacts of multiple tidal constituents and varying river flow on long‐term, large‐scale estuarine morphodynamics by means of a 1‐D model

Tidal asymmetry is an important mechanism generating tidal residual sediment transport (TRST) in tidal environments. So far, it is known that a number of tidal interactions (e.g., M2-M4 and M2-O1-K1) can induce tidal asymmetry and associated TRST; however, their variability and morphodynamic impacts are insufficiently explored. Inspired by the river and tidal forcing conditions in the Yangtze River Estuary, we explore the morphodynamic development of a 560 km long estuary under the boundary forcing conditions of varyingly combined tidal constituents and river discharges using a schematized 1-D morphodynamic model for long-term (millennial) simulations. We then employ an analytical scheme which integrates sediment transport as a function of flow velocities to decompose the contribution of different tidal interactions on TRST and to explain how the river and tidal interactions control TRST and associated morphodynamics. Model results display varying equilibrium bed profiles. Analytical results suggest that (1) a series of tidal interactions creates multiple tidal asymmetries and associated TRST, (2) river flow modulates tidal asymmetry nonlinearly in space, and (3) more tidal constituents at the sea boundary persistently enhance the seaward TRST through river-tide interactions. It is the combined effects of multiple tidal asymmetries and river-tide interactions that determine the net TRST and consequent morphodynamic development. It thus suggests that tidal harmonics of significant amplitudes need to be considered properly as boundary conditions for long-term, large-scale morphodynamic modeling.

WAVE CELERITY FROM VIDEO IMAGING: A NEW METHOD

Video systems are emerging tools to monitor high frequency nearshore morphodynamics. Bathymetry can be inverted from surface wave celerity. We present here a new and robust method to estimate this celerity. The method is a time and space cross-correlation based on time-stack images. The celerity estimation is local, within a distance smaller than typical encountered wavelength and uses a time-domain correlation that integrates all wave effects within the duration of the time-stack. A validation is given from available in-situ data of the Pre-ECORS experiment. Global RMS error is lower than 0.2 m/s for the available period of comparison. Thus, the method seems to be well adapted to complex morphologies. Sources of error on the celerity estimation in the nearshore area are also investigated.

Evolution of the Bengal Delta and Its Prevailing Processes

ABSTRACT Akter, J.; Sarker, M.H.; Popescu, I., and Roelvink, D., 2016. Evolution of the Bengal Delta and its prevailing processes. Bangladesh, occupying low-lying floodplains and tidal plains, has one of the largest and the most disaster-prone populous deltas in the world. The Bengal Delta is a tide-dominated delta, where tides play the key role in the sediment dispersal process and in shaping the delta. There are many studies and reports on river-dominated deltas, but research is sparse on tide-dominated deltas. The Ganges and Brahmaputra Rivers, which combined form one of the three largest riverine sources of water and sediment for the worlds oceans, have developed the Bengal Delta to its present form with an aerial extent of 104 km2. About 1012 m3 of water with 109 tonnes of sediment per year make this system morphologically active. In the last five decades, the Bengal Delta has prograded at a rate of 17 km2/y, whereas most large deltas elsewhere in the world suffered from sediment starvation. Delta progradation always makes the river system unstable, and rapid changes cause the delta to become dynamic. Sea level rise induced by unequivocal climate change and subsidence would make the delta more vulnerable in the coming decades. Although some literature is available on the millennium-scale development process of the Bengal Delta, sound knowledge on the decade- to century-scale processes of the delta development for facing the threats of climate change and deltaic subsidence is limited. In addition, there are significant differences in opinions and widely varying findings in the literature to the response of the delta to different natural and human interventions. Against this backdrop, relevant available literature on Bengal Delta and deltas elsewhere in the world, is reviewed and evaluated to provide direction for future research that would help to form a way out of the present situation and a way into sustainable planning for this delta.