Here we develop a computationally efficient method that evolves cross-shore profiles of sand beaches with or without cliffs along natural and urban coastal environments and across expansive geographic areas at decadal to centennial time-scales driven by 21st century climate change projections. The model requires projected sea level rise rates, extrema of nearshore wave conditions, bluff recession and shoreline change rates, and cross-shore profiles representing present-day conditions. The model is applied to the ~470-km long coast of the Southern California Bight, USA, using recently available projected nearshore waves and bluff recession and shoreline change rates. The results indicate that eroded cliff material, from unarmored cliffs, contribute 11% to 26% to the total sediment budget. Historical beach nourishment rates will need to increase by more than 30% for a 0.25 m sea level rise (~2044) and by at least 75% by the year 2100 for a 1 m sea level rise, if evolution of the shoreline is to keep pace with rising sea levels.
This study presents some recent developments in coastal morphological modeling focusing on flexible meshes, flexible coupling between models operating at different time scales, and a recently developed morphodynamic model for the intertidal and dry beach. This integrated modeling approach is applied to the Sand Engine mega nourishment in The Netherlands to illustrate the added-values of this integrated approach. A seamlessly coupled modeling system for Delft3D and AeoLiS has been developed and applied to compute the first years of evolution of the Sand Engine, both for the subaqueous and subaerial areas. The subaqueous bed level changes have been computed with the new Flexible Mesh version of Delft3D, resulting in comparable accuracy levels as to the standard Delft3D version. The integrated morphodynamic prediction of both subaqueous and subaerial reveals a qualitative behavior which is very similar to observations. Model results confirm that after the first year after construction the sand supply for aeolian transports is predominantly from the intertidal area. The AeoLiS model results indicate a significant intertidal erosion volume of about 230,000 m3 over the five year period, which is a not to be neglected volume, especially in multiyear or decadal predictions. Interestingly, the model results show that the spit, developed by the wave-related processes, is also subject to aeolian transports acting on the emerged spit during lower tides. The seamlessly coupled models are now able to combine the dry beach behaviour with subaqueous morphodynamic evolution, which is important in medium-term to decadal morphodynamic predictions but also relevant for designing such sandy solutions incorporating lakes, lagoons, and relief.
This paper reports on an experimental campaign focu sed on the hydromorphodynamics induced by sea waves propagating over a sloping sandy bed. The slope tri gge s a flow asymmetry that reflects on the bedform characteristics. Hydrodynamic characteristics have been acquired by means of a Vectrino Profiler at two measurement sec ions, one located over the sloping bed, the other at the plan e bottom. The velocity measurements showed that wav es re strongly skewed, with flat, wide troughs and sharp crests. A s a consequence, phase averaged velocity profiles e xhibit onshore velocities greater than offshore ones. An offshoredi cted steady current occurs, which near the bed is strongly influenced by the ripple-generated lee wake vortice s.
Changes in the inlet morphology of choked coastal lagoons often restrict the water exchange with the sea, making them vulnerable to pollution release and eutrophication processes. In this study, the role of tides for the water exchange was investigated in a choked lagoon for critical scenarios during the low river flow season. Both an integrated and spatially distributed approach were employed using Lagrangian tracking of particles. The spring tides were responsible for the most pronounced water exchange and the spatially distributed approach identified different zones with similar water exchange characteristics. The tide action was primarily limited to the southern portion of the lagoon presenting a zone with rapid exchange times, whereas the other parts involved lower exchange and consequently long renewal times making it a more vulnerable area to different types of pollution releases. (Less)
In the present time of sea-level rise and climate change a global shift has occurred toward sandy coastal protection measures and Building with Nature. These type of protection measures impose extra uncertainty on the instantaneous state of the coastal system over time for which present deterministic forecasting techniques are not capable of providing necessary information on uncertainties and hence could display a false sense of accuracy and skill. At present in long term morphological modeling a full systemic approach for uncertainty assessment has not yet been applied. This paper investigates the use of a Bayesian Network as a tool for uncertainty assessment in decadal scale morphological modeling for the evolution of a mega nourishment at the Dutch North-Holland coast, the Hondsbossche Dunes (HBD). The Bayesian Network is trained with an existing set of model data and field data of one year bed development. The Bayesian Network successfully transfers the bandwidth in input variables, model uncertainty and calibration uncertainty to an uncertainty bandwidth around the output parameter of choice.
The coastal zone of Mekong delta is suffering under intense pressures from climate change as well as human intervention. Currently, the coastline evolution of Mekong delta is a complex combination of impacts due to (1) relative sea level rise i.e. the sum of eustatic sea level rise, natural and human induced subsidence (2) sediment transport rate changes at some sections due to change of wave condition by climate change (3) change of sediment sources from the Mekong estuaries by dam construction and sand mining and (4) mangrove degradation. A coastline monitoring is the basis to understand and manage coast. This study utilizes integrated techniques of remote sensing, geographic information system and statistics to monitor coastline change over the period of 1973 to 2015 from Landsat images of Multispectral (MSS), Thematic Mapper (TM), Enhanced Thematic Mapper Plus (ETM+), Operational Land Imager (OLI) at coastal area of Mekong delta. An advanced toolbox is developed for the work of atmospheric and radiometric correction of Landsat images as well as influence of tidal range is taken into account to obtain mean water level. Tasseled Cap and Normalized Difference Water Index (NDWI) algorithm is applied to separate land-water interface for extracting shorelines. Besides, a digital shoreline assessment system (DSAS) tool is used to analyze shoreline rate by statistic parameters as Shoreline Change Envelope (SCE), End Point Rate (EPR) and Linear Regression (LRR). Furthermore, uncertainty assessment for this methodology based on topographic surveying and Google Earth images. Moreover this research explored relationships between the accretion and erosion of land and the sediment load of the Mekong River. The results revealed a general pattern of accretion and erosion. The eastern coast, which is fragmented by 9 estuaries, was significant accretion and erosion, especially annual erosion rate of around 40 meter at Bo De estuary is noted. Meanwhile the western coast is rather stable, particularly annual accretion rate of up to 90-95 meter at Datmui commune of Camau province. This study indicated there is relative difference of coastline change rate among periods of 1973-1990, 1990-2005 and 2005- present. And the study illustrates the rate of shoreline change is significantly associated with sediment discharge on Mekong River through statistic approach, especially the phase of sediment flow decrease by dam and sand mining on Mekong River in recent 15 years. The results of methodology and maps from this research may be useful in planning and management of this exposed coastline.
In this paper we present a new numerical modelling approach for coastal and marine applications where a porous media conceptual model was combined with a free surface volume-of-fluid (VOF) model and an immersed boundary method (IBM). The immersed boundary model covers the method of describing a solid object in a simple computational mesh without resolving the object with a conventional body-fitted mesh. This model enables a detailed resolution of some parts of a stone cover layer for erosion protection with the IBM model while other parts are handled with the conceptual porosity model. In this paper, the model is applied to investigate two practical cases in terms of a cover layer of stones on a flat bed under oscillatory flow at different packing densities, and a rock toe structure at a breakwater.
A model to simulate long-term beach-dune evolution due to interacting longshore and cross-shore sediment transport processes is developed and tested. The work builds on a cross-shore model (CSM) previously developed at Lund University and includes changes to the equations describing aeolian transport and morphological evolution. The modifications are mainly based on existing conceptual geomorphological models which are translated into a numerical model, dealing with aeolian transport rates, transport limiting factors, and dune evolution under positive, negative, or stable beach sediment budgets. CSM is tested against a seven year data set of morphological evolution and sedimentgrain-size samples from Angelholm Beach, Sweden. Results show a satisfactory fit between the simulated and observed evolution, although not all processes could be validated due to the limited temporal extent of the data set. (Less)
Impact of spatial variability in the nearshore bathymetry on net sediment transport rates has been investigated for a selection of observed beach states at the Dutch coast. The beach states comprise a longshore bar trough and two transverse bar rip situations, which were present at the large scale Sand Motor nourishment at the Holland coast. These observed bathymetric features were then applied multiple times next to each other along a longer stretch of coast to obtain a repeating pattern of the considered beach state. The wave transformation towards the shore, alongshore wave - driven and water - level setup driven currents and sediment transport were computed with the Delft3D model, which has been applied successfully for many other studies at the Sand Motor. It was found that net sediment transport is considerably influenced for the most pronounced transverse bar rip configuration, which was most prominent for conditions with small wave angles (i.e. less than 10 ° from shore - normal). Furthermore, a decrease in transport rates is found for conditions from larger angles of wave incidence (i.e. 30 to 45° from shore - normal). Impacts of the bathymetries of longshore bar trough and the less pronounced transverse bar rip system on net sediment transport rates were much smaller. The actual cause for the enhancement (or decrease) of the net transport for the transverse bar rip configuration is expected to be related to 1) the oblique orientation of the rip - channel for the considered configuration as well as to 2) a more diffusive pattern of the wave breaking as a result of the refraction on the spatially variable bathymetry.
Most commonly applied models for reference concentrations or sediment pickup rates do not account for wave breaking effects on sand resuspension. Consequently, they tend to underpredict the suspended sand load in the surf zone. Here, we present a new method to account for wave breaking turbulence effects in reference concentration models or pickup functions. An adapted reference concentration model is validated using recent laboratory measurements of near-bed turbulent kinetic energy and suspended sand concentration, yielding good agreement (r2 = 0.60).
Tilt Current Meters (TCM’s) are relatively simple and inexpensive instruments for measuring currents in rivers and in the sea. Their low cost and easy deployment means t h t a relatively large number of TCM’s can be deploy ed compared to more conventional current meters such as Acousti c Doppler Velocimeters (ADV’s) or Acoustic Doppler Current Profiler (ADCP’s). Although, the accuracy of the ind ivi ual measurements may not be as good as conventi onal current meters, the possibility of deploying many instrumen ts is a great advantage when studying spatial varia tions in flows. This is especially the case when data is later used for comparison with numerical models whose results are also associated with considerable uncertainty. Previous st dies have mainly considered steady current or ti dal flows in which velocities were relatively low and the import ance of waves limited. The presence of waves adds a number of important challenges to the measurements as the hyd rod namic forcing changes and the oscillations of t he TCM cannot necessarily be averaged out as for a steady current . This study addresses some of these challenges by anal zing the performance of a TCM in the surf zone where wave orb ital motion is dominant.
Observations in a mangrove in the Whangapoua Harbour, New Zealand, have shown that deposition rates are greatest in the fringing zone between the tidal flats and the mangrove forest, where the vegetation is dominated by a cover of pneumatophores (i.e. pencil roots). Current speeds and suspended sediment concentrations dropped substantially across this zone. Near-bed turbulence within the fringe was substantially lower where the pneumatophore canopy was denser, facilitating the enhanced deposition in this zone. However, the near-bed conditions were not the primary control on the instantaneous sediment concentrations at this site. The total deposition across the different zones was the combined result of the reduced near-bed turbulence inside the vegetation and the larger-scale dynamics over the spatially variable vegetation cover, along with other confounding factors such as changing sediment inputs.
This paper presents initial results from an on-goin study on the influence from wave nonlinearity on the wave height distribution in deepand depth-limited nonlinear w ave conditions. A fully nonlinear VOF model, IH-2VO F, is applied to model the propagation of irregular waves on a slopi ng sea bed from deep to shallow water, including th e effects of wave breaking. Different wave nonlinearities are evaluat ed in the model and the effects of the wave nonline arity, described by the so-called Ursell-number, on the wave height distrib u ions along the sloping sea bed are evaluated. The widely used Battjes & Groenendijk (2000) shallow water wave height dist ribution is concluded in the present study to signi f cantly underpredict the low-exceedance wave heights in case of very non linear waves. A modification of the Battjess & Groe nendijk (2000) distribution is suggested in order to include the e ff cts of wave nonlinearity.
A one-dimensional vertical (1DV) turbulence-closure flow model, coupled with sediment transport capabilities, is extended to incorporate graded sediment mixtures. The hydrodynamic model solves the horizontal component of the incompressible Reynolds-averaged Navier–Stokes (RANS) equations coupled with k–ω turbulence closure. The sediment transport description includes both bed and suspended load descriptions. So-called high-concentration effects (turbulence damping and hindered settling velocities) are likewise included. The sediment transport model treats the bed and suspended load individually for each grain fraction, including effects associated with increased exposure of larger particles within a mixture. The suspended sediment transport model also makes use of modified reference concentration approach, wherein reference concentrations computed individually for each fraction are translated to a common level, conveniently enabling use of a single computational grid for the simulation of suspended sediments. Parametric study shows that these two effects combine to help alleviate an otherwise systematic tendency towards over- (under-) predicted transport rates for fine (coarse) sand fractions. The sediment transport model is validated against sheet-flow experimental oscillatory tunnel measurements beneath velocity-skewed wave signals, and demonstrates similar accuracy (transport rates generally within a factor of two) for both graded and uniform sands. The model is likewise validated against an extensive data set involving sheet-flow transport beneath acceleration-skewed wave signals (limited to uniform sands). It is then utilized to study potential effects of gradation on the net transport beneath such flows. The simulations suggest that gradation effects can both increase, as well as decrease, the total transport rate, depending largely on the behavior of the fine sand fraction. The model is implemented within the Matlab environment, and is freely available upon request to the corresponding author.
Overwash hydrodynamics datasets are mixed in quality and scope, being hard to obtain due to fieldwork experimental difficulties. Aiming to overcome such limitations, this work presents accurate fieldwork data on overwash hydrodynamics, further exploring it to model overwash on a low-lying barrier island. Fieldwork was performed on Barreta Island (Portugal), in December 2013, during neap to spring-tides, when significant wave height reached 2.64 m. During approximately 4 hours, more than 120 shallow overwash events were measured with a video-camera (at 10 Hz), a pressure transducer (at 4 Hz) and a current-meter (at 4 Hz). This high-frequency fieldwork dataset includes runup, overwash number, depth and velocity. Fieldwork data along with information from literature were used to setup XBeach model in non-hydrostatic mode. The baseline model had variable skills over the duration of the overwash each 30 minutes. The baseline model was forced to simulate overwash with different nearshore morphology, grain-size and lagoon water level. An average decrease of about 30% overwash was obtained due to changes in the nearshore episode, performing better during the rising tide than during the falling tide. Model average number of events RMSE (root-mean-square-error) was 7 events morphology, mostly a small vertical growth of the submerged bar. The coarser and finer grain-sizes tests produced an 11% change in overwash, with less overwash on the coarser barrier. Changing lagoon water levels had a reduced effect on overwash hydraulics.
The role of seagrass in sediment dynamics is studied through observations and model simulations focused on the Zostera marina seagrass meadows in the Isles of Scilly UK. Observations are presented which indicate a clear selection for fine grained sediments inside seagrass meadows and coarser sediments surrounding them. A recent modification of the 1DV General Ocean Turbulence Model (GOTM) known as mGOTM has been performed and is presented including a validated methodology for simulating the suspension of a distribution of sediment sizes. Using hind cast wave data from the study site, mGOTM simulations are used to demonstrate how the interactions between seagrass and flow can lead to both a reduction of bed stress but an enhancement of turbulent kinetic energy throughout the flow and how this contributes to the grain size selection observed in this location.
Incident-band and infragravity wave transformation on five rocky shore platforms is simulated using a numerical model. The model is calibrated using measurements of bed roughness and hydrodynamics under mild to energetic wave conditions. Validation of the model show that the model is capable of reproducing wave heights across the platform with little scatter (scatter index 0.07 – 0.27) and bias (-0.05 – + 0.02 m). Model calibration shows a strong relation between incident-band wave-related bed friction parameter values and platform roughness, as well as between infragravity wave-related bed friction parameter values and roughness once small-scale topographic details are removed from the model bed elevation. Sensitivity studies show that on even relatively smooth platforms, bed friction may dissipate significant amounts of infragravity wave energy.
Large repetitive patterns on the sea bed are commonly observed in sandy areas. The formation of the bed forms have been studied extensively in literature using linear stability analyses, commonly conducted analytically and with simplifications in the governing equations. This work presents a shallow water equation model that is used to numerically simulate the morphodynamics of the water-bed system. The model includes separate formulations for bed load and suspended load, featuring bed load correction due to a sloping bed and modelled helical flow effects. Horizontal gradients are computed with spectral accuracy, which proves highly efficient for the analysis. Numerical linear stability analysis is used to identify the likely emergence of dominant finite sized bed forms, as a function of governing parameters. These are then used for interpretation of the results of a long time morphological simulation.
