Wave-driven Sediment Transport Research Articles

Representative residual transport pathways in a mixed-energy open tidal system

The North Frisian Wadden Sea (NFWS), an open tidal basin within the UNESCO World Heritage Wadden Sea, is characterized by its barrier systems, tidal inlets, intertidal flats, and estuaries. Unlike its East Frisian and Dutch counterparts, the NFWS is underexplored in terms of residual sediment and flow transport pathways—knowledge crucial for coastal conservation and nature-based protection. These pathways are a product of complex interplay between tides, winds and waves that together shape the morphology of the NFWS. This study investigates these interactions using a high-resolution process-based model to perform a representative period simulation derived through a novel unfiltered-reduction technique. Our results reveal an anticlockwise residual flow circulation in the back-barrier region, which was not discernible in the residual sediment pathways. Waves primarily dictated sediment transport over intertidal flats, showing high variability in transport direction under energetic conditions. The coastline orientation and fetch size favored southward wavedriven sediment transport, opposing the northward residual flow transport driven by tidal propagation. A dynamic residual divergence pattern in the nearshore region of the barrier islands is also revealed for both sediment and flow, which moves alongshore during energetic events. The discussion compares these patterns and their implications with earlier local measurements, conceptual pathways, and different systems globally to provide a comprehensive overview of the transport dynamics in the NFWS.

ADVANCES IN UNSTRUCTURED WAVEWATCH III AND APPLICATIONS TO NEARSHORE WAVES

The spectral wave generation and propagation model WAVEWATCH III (WW3) (WW3DG 2019) is undergoing rapid community development to extend capability and nearshore modeling applicability. An option for unstructured grids and implicit solution provides WW3 with the flexibility and efficiency to resolve complex shorelines and high-gradient wave zones to drive nearshore circulation, wave setup, and wave-driven sediment transport with multi-scale spatial coverage over approximately three orders of magnitude. A hybrid approach to parallelization uses spectral partitioning for advection in geographical space and domain decomposition for spectral advection and the source term integration. The advection part of wave action equation is integrated fully implicitly, and a new convergent action limiter derived from Komen et al. (1994) and Hersbach and Janssen (1999) is applied. The model is compatible with community-based coupling infrastructure to facilitate two-way coupling with circulation models for simulating hurricane storm surge and waves. Abdolali et al. (2020) evaluated WW3 enhancements on a large-scale numerical domain employing the new parallelization algorithm and implicit solver for unstructured grids and compared to the existing parallelization algorithm, domain decomposition, and robust explicit numerical solver of WW3 on both structured and unstructured grids. These new capabilities in the wave model push the limitations of the model, including minimum resolution (10s of meters), maximum number of model grid points (~2 million), and computational scalability, as well as better accuracy in contrast to the explicit scheme. This paper describes model enhancements and validation.

Wave-driven sediment transport potential on a tropical coast: Implications for the northeastern Australian sediment budget

Coastal dynamics are governed by the interface of complex, multidimensional processes that result from the interaction between offshore wave climate and local geomorphology. Nearshore waves are usually the primary driver of sediment transport in the coastal zone. However, nearshore wave processes in sub-tropical to tropical regions are often poorly understood, since deep-water waves are often substantially transformed, via interaction with coral reefs, as they propagate towards the nearshore zone. Here, we produce a suite of nearshore wave conditions using a discrete set of representative deep-water wave parameters to determine the potential for large-scale, wave-driven longshore sediment transport along the sub-tropical to coral reef mediated tropical northeastern coast of Australia (15-28oS). Representative conditions at deep-water source points, determined using a Maximum Dissimilarity Algorithm (MDA), were used as boundary conditions for a coastal wave model. Potential longshore sediment transport is largely dependent on offshore wave direction and the presence (or lack thereof) of the Great Barrier Reef (GBR). There is a decrease in mean, wave-driven transport potential from >500,000 m3/year in southern Queensland to <50,000 m3/year north of Fraser Island and along the section of the coast regulated by wave transformation through the GBR. Seasonal changes in the deep-water wave regime control patterns of sediment transport along the entire coast, even in the low wave energy regions. We also show that there are complex sediment transport pathways on this tropical coast with large variability in both direction (up-coast vs. down-coast transport) and magnitude in the nearshore. Our results provide insights into the potential volumes, pathways, and complexity of sediment transport on tropical and sub-tropical coasts and a methodological approach that can be applied globally to efficiently construct wave-driven sediment flux estimates on tropical coastlines where coral reefs influence nearshore wave processes.

Uncertainties in the Projected Patterns of Wave-Driven Longshore Sediment Transport Along a Non-straight Coastline

This study quantifies the uncertainties in the projected changes in potential longshore sediment transport (LST) rates along a non-straight coastline. Four main sources of uncertainty, including the choice of emission scenarios, Global Circulation Model-driven offshore wave datasets (GCM-Ws), LST models, and their non-linear interactions were addressed through two ensemble modelling frameworks. The first ensemble consisted of the offshore wave forcing conditions without any bias correction (i.e., wave parameters extracted from eight datasets of GCM-Ws for baseline period 1979–2005, and future period 2081–2100 under two emission scenarios), a hybrid wave transformation method, and eight LST models (i.e., four bulk formulae, four process-based models). The differentiating factor of the second ensemble was the application of bias correction to the GCM-Ws, using a hindcast dataset as the reference. All ensemble members were weighted according to their performance to reproduce the reference LST patterns for the baseline period. Additionally, the total uncertainty of the LST projections was decomposed into the main sources and their interactions using the ANOVA method. Finally, the robustness of the LST projections was checked. Comparison of the projected changes in LST rates obtained from two ensembles indicated that the bias correction could relatively reduce the ranges of the uncertainty in the LST projections. On the annual scale, the contribution of emission scenarios, GCM-Ws, LST models and non-linear interactions to the total uncertainty was about 10–20, 35–50, 5–15, and 30–35%, respectively. Overall, the weighted means of the ensembles reported a decrease in net annual mean LST rates (less than 10% under RCP 4.5, a 10–20% under RCP 8.5). However, no robust projected changes in LST rates on annual and seasonal scales were found, questioning any ultimate decision being made using the means of the projected changes.

Numerical simulations of wave climate in the Baltic Sea: a review

Efforts towards the numerical simulation of the Baltic Sea wave properties, started in the 1950s, have reached maturity by the implementation of contemporary third generation spectral wave models, such as WAM and SWAN. The purpose of this paper is to give an overview of the relevant efforts since the beginning of numerical wave simulations. The Sverdrup-Munk-Bretschneider (SMB) type models are still valuable tools for rapid estimates of some properties of wave climate in single locations. The spatial resolution of spectral wave models for the entire sea has increased from about 20 km to 1 km, and to 100–200 m in specific areas. The number of directional bins has increased from 10–15 to 24–36 and the number of spectral frequency bins from about 15 to 35–42. The models replicate all main features of the wave climate of the Baltic Sea, such as an overall mild but intermittent wave climate, the predominance of short windseas and the scarcity of long swell, east-west asymmetry, the strong impact of seasonal ice, and the specific properties of wave growth in some areas. The wave climate changes involve variations in regional wave intensity, core properties of wave-driven sediment transport and wave set-up. Reconstruction of wave properties in the nearshore, archipelago areas, and in narrow subbasins remains a challenge. These areas require finer spatial resolution and possibly advancement of wave physics to account for changes in the spectral composition of wave fields and specific features of wave growth in narrow basins. Progress in these fields is a pillar for a number of applications, from the quantification of sediment transport to proper input into management issues of the coastal zone.

ADVANCES IN THE UNSTRUCTURED WAVEWATCH III AND APPLICATION TO HURRICANE DORIAN

The spectral wave generation and propagation model WAVEWATCH III (WW3) is undergoing rapid development to extend capability and applicability. An option for unstructured grids and implicit solution provides WW3 with the flexibility and efficiency to resolve complex shorelines and high-gradient wave zones to drive nearshore circulation, wave setup, and wave-driven sediment transport with multi-scale spatial coverage over approximately three orders of magnitude. The model is compatible with community-based coupling infrastructure to facilitate two-way coupling with circulation models for simulating hurricane storm surge and waves. Unstructured WW3 is applied for 2019 Hurricane Dorian and validated with National Data Buoy Center buoys and nearshore gauges at the US Army Corps of Engineers Field Research Facility.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/kz9G46xUD0k

Armoring and exposure effects on the wave-driven sediment transport

Coastal sediment transport of graded spherical particles with the material properties of sand was simulated and compared to transport of the uniform-sized spherical particles using a two-phase Eulerian-Lagrangian model. The fluid phase solver was based on SedFoam developed in OpenFOAM and the open-source discrete element method solver LIGGGHTS was used for the particle phase. We validated the model for sheet flow of well-sorted medium sand (d50 = 0.28 mm) and mixed sand with bimodal size distribution in velocity-skewed oscillatory flows as well as well-sorted coarse sand (d50 = 0.51 mm) in velocity-skewed and acceleration-skewed oscillatory flows. Simulation results of graded particles showed the formation of inverse grading (upward coarsening) in sediment bed under oscillatory flows, suggesting that the effects of armoring and exposure were important in the resulting transport rate. Examining different particle size distributions under onshore velocity-skewed flows, it was found that the largest increase of the net onshore sediment transport rate due to size gradation corresponded to the moderately sorted particle size distribution (d90/d10 = 3.41), where the coarse fraction (d > d50) had the maximum contribution to the transport. By analyzing intra-wave sediment transport quantities, the response of size gradation to the flow skewness and asymmetry and velocity intensity was investigated. Model results revealed that the armoring effect (reduction of sediment flux due to inverse grading) was dominant when flow velocity magnitude was lower (wave trough) or the fluid acceleration was higher. On the other hand, when flow velocity magnitude was larger (wave crest), the armoring effect was reduced or the exposure effect (enhancement of sediment flux due to inverse grading) may become more pronounced. Overall, we found that onshore net sediment transport was enhanced up to 30% due to particle size gradation under onshore velocity-skewed oscillatory flows. Conversely, the size gradation reduced the net onshore transport rate up to 35% under onshore acceleration-skewed oscillatory flows. Model results also suggested that the thickness of the active layer (surface layer affected by vertical sorting) in sheet flows can be quantified by the peak erosion depth. The simulation results presented here provide insights into the role of sediment size gradation (armoring and exposure effects) in wave-driven onshore transport which is important for predicting morphological evolution.

Mapping the Shoreface of Coastal Sediment Compartments to Improve Shoreline Change Forecasts in New South Wales, Australia

The potential response of shoreface depositional environments to sea level rise over the present century and beyond remains poorly understood. The shoreface is shaped by wave action across a sedimentary seabed and may aggrade or deflate depending on the balance between time-averaged wave energy and the availability and character of sediment, within the context of the inherited geological control. For embayed and accommodation-dominated coastal settings, where shoreline change is particularly sensitive to cross-shore sediment transport, whether the shoreface is a source or sink for coastal sediment during rising sea level may be a crucial determinant of future shoreline change. While simple equilibrium-based models (e.g. the Bruun Rule) are widely used in coastal risk planning practice to predict shoreline change due to sea level rise, the relevance of fundamental model assumptions to the shoreface depositional setting is often overlooked due to limited knowledge about the geomorphology of the nearshore seabed. We present high-resolution mapping of the shoreface-inner shelf in southeastern Australia from airborne lidar and vessel-based multibeam echosounder surveys, which reveals a more complex seabed than was previously known. The mapping data are used to interpret the extent, depositional character and morphodynamic state of the shoreface, by comparing the observed geomorphology to theoretical predictions from wave-driven sediment transport theory. The benefits of high-resolution seabed mapping for improving shoreline change predictions in practice are explored by comparing idealised shoreline change modelling based on our understanding of shoreface geomorphology and morphodynamics before and after the mapping exercise.

VALIDATION OF UNSTRUCTURED WAVEWATCH III FOR NEARSHORE WAVES

Unstructured wave model grids in the nearshore coastal region provide flexibility and efficiency to resolve complex shorelines and high-gradient wave zones to drive nearshore circulation, wave setup and wave-driven sediment transport. Recent improvements to the unstructured version of WAVEWATCH III (WW3) (WW3DG 2016) to support nearshore application include an implicit solution scheme and domain decomposition for multi-scale spatial coverage over approximately three orders of magnitude. The hybrid approach to parallelization involves spectral partitioning for advection in geographical space and domain decomposition for spectral advection and the source term integration. The advection part of wave action equation is integrated fully implicitly, and a new convergent action limiter derived from Komen et al. (1994) and Hersbach and Janssen (1999) is applied. New Block-Jacobi and Block-Gauss-Seidel solvers are applied with improved convergence. The purpose of this paper is to evaluate the upgraded unstructured WW3 for nearshore application.

MODELING SHEET FLOW UNDER BREAKING WAVES ON A SURF ZONE SANDBAR

Wave-driven sediment transport is one of the main drivers of beach morphodynamics. However, the creation of a comprehensive numerical model remains to be a challenging task due to complex mechanisms associated with unsteadiness and free-surface effects. Particularly for highly non-linear and skewed-asymmetric breaking waves, the boundary layer approximation (i.e., assuming horizontal pressure gradient is equal to local free-stream acceleration) is questionable. Moreover, wave-breaking-induced turbulence may approach the bed and further enhance sediment transport. Thus, a numerical model that can resolve the entire water column from the bottom boundary layer to the free-surface can be a powerful tool to understand wave-driven sediment transport.

Spectral wave-driven sediment transport across a fringing reef

A laboratory experiment was conducted to investigate the dynamics of cross-shore sediment transport across a fringing coral reef. The aim was to quantify how a highly bimodal spectrum of high-frequency (sea-swell) and low-frequency (infragravity and seiching) waves that is typically present on coral reef flats, influences the various sediment transport mechanisms. The experiments were conducted in a 55m wave flume, using a 1:15 scale fringing reef model that had a 1:5 forereef slope, a 14m long reef flat, and a 1:12 sloping beach. The initial 7m of reef flat had a fixed bed, whereas the back 7m of the reef and the beach had a moveable sandy bed. Four seven-hour irregular wave cases were conducted both with and without bottom roughness elements (schematically representing bottom friction by coral roughness), as well as for both low and high still water levels. We observed that the wave energy on the reef flat was partitioned between two primary frequency bands (high and low), and the proportion of energy within each band varied substantially across the reef flat, with the low-frequency waves becoming increasingly important near the shore. The offshore transport of suspended sediment by the Eulerian mean flow was the dominant transport mechanism near the reef crest, but a wide region of onshore transport prevailed on the reef flat where low-frequency waves were very important to the overall transport. Ripples developed over the movable bed and their properties were consistent with the local high-frequency wave orbital excursion lengths despite substantial low-frequency wave motions also present on the reef flat. This study demonstrated that while a proportion of the sediment was transported by bedload and mean flow, the greatest contributions to cross-shore transport were due to the skewness and asymmetry of the high and low-frequency waves.

An abrupt change in winds that may radically affect the coasts and deep sections of the Baltic Sea

We explore long-term variations in the properties of waves in the Baltic Sea and geostrophic air-flow over this water body. Records of visual wave observations from 1946 to 2012 at 8 observation sites at the eastern coast of the Baltic Sea reveal a multitude of changes. A substantial decrease in the wave heights took place until about 1970, and considerable decadal variations occurred since then. The coherence between annual average wave heights at different locations was lost at the end of the 1980s. A rotation of the usual wave approach direction by almost 90° was identified at one location. We then couple this data with a reconstruction of wave fields and wave-driven sediment transport for 1970 to 2007. The reconstruction is based on adjusted geostrophic winds from the Swedish Meteorological and Hydrological Institute. The simulated net potential sediment transport along the eastern coast of the sea reveals a major change at the end of the 1980s. This change is associated with an abrupt turn of the geostrophic air-flow over the southern Baltic Sea by ~40° since 1987. This change may serve as an alternative explanation for a radical decrease in the frequency of major inflows of saltier, oxygen-rich water into the Baltic Sea since the mid-1980s.

NearCoM-TVD — A quasi-3D nearshore circulation and sediment transport model

The newly developed nearshore circulation model, SHORECIRC, using a hybrid finite-difference finite-volume TVD-type scheme, is coupled with the wave model SWAN in the Nearshore Community Model (NearCoM) system. The new modeling system is named NearCoM-TVD and the purpose of this study is to report the capability and limitation of NearCoM-TVD for several coastal applications. For tidal inlet applications, the model is verified with the semi-analytical solution of Keulegan (1967) for an idealized inlet-bay system. To further evaluate the model performance in predicting nearshore circulation under intense wave–current interaction over complex bathymetry, modeled circulation patterns are validated with measured data during RCEX field experiment (MacMahan et al., 2010). For sediment transport applications, two sediment transport models are applied to predict three sandbar migration events at Duck, NC, during August to October 1994 (Gallagher et al., 1998). The model of Kobayashi et al. (2008) incorporates wave-induced onshore sediment transport rate as a function of the standard deviation of wave-induced horizontal velocities. The modeled beach profile evolution for two offshore events and one onshore event agrees well with the measured data. The second model investigated here combines two published sediment transport models, namely, the total load model driven by currents under the effect of wave stirring (Soulsby, 1997) and the wave-driven sediment transport model due to wave asymmetry/skewness (van Rijn et al., 2011). The model study with limited field data suggests that the parameterization of wave stirring is appropriate during energetic wave conditions. However, during low energy wave conditions, the effect of wave stirring needs to be re-calibrated.

Modelling wave-driven sediment transport in a changing climate: a case study for northern Adriatic Sea (Italy)

In this paper, we investigate the impact of climate change on coastal sediment transport in a deltaic system in the northern Adriatic Sea, with reference to the period 2070–2099 in the IPCC A1B emission scenario. Wind fields obtained by means of the high-resolution regional climate model Consortium for Small-scale Modelling–Climate Limited-area Modelling were employed for computing wave climate at basin scale by means of the spectral wave model Simulating Waves Nearshore. This was used as a constraint for a nearshore hydromorphodynamic model (MIKE LITPACK), which was applied to a test site on the Po River Delta, located in northern Italy. Relevant sediment transport processes have been studied at storm and decadal time scales in order to capture climate change effects on single events and as an overall trend. The transport rates in the A1B climate change scenario were then compared with the corresponding results of a control analysis (period 1965–1994) representing the actual climate. Although predicted wave climate in the investigated scenario displays an overall decrease in sea severity offshore in the northern Adriatic Sea, the effects of these modifications are modulated during the onshore propagation, with different impacts on sediment transport depending on the considered process and time scale. The strategy presented in this work can find fruitful applications in the long-term modelling of coastal and transitional environments, in which morphology is strongly influenced by sediment transport in the nearshore zone, suggesting a methodological approach for coastal planning and management.

Review of wave-driven sediment resuspension and transport in estuaries

Waves are fundamentally important to the physical and biological functioning of estuaries. Understanding and predicting contaminant transport, development of sedimentary structures, geomorphological response to changes in external forcings such as rising sea level, and response of estuarine ecosystems to contaminant stressors require understanding of the relative roles of wave- and current-driven sediment transport. We review wave-driven sediment resuspension and transport in estuaries, including generation of bed shear stress by waves, initiation of sediment motion by waves, and the ways waves modulate, add to, and interact with sediment transport driven by currents. A key characteristic of the wave-induced force on the seabed is extreme spatial and temporal variations; simple analytical models are revealing of the way such patterns develop. Statistical methods have been widely applied to predict wave resuspension of intertidal-flat bed sediments, and physically based predictors of resuspension developed from open-coast studies appear to also apply to short-period estuarine waves. There is ample experimental evidence to conclude that over the long term, waves erode and tidal currents accrete intertidal flats. Waves indirectly add to the formation of fluid mud by adding to the estuarine pool of fine sediment, and waves may fluidize subtidal seabeds, changing bed erodibility. Models have been used to explore the dynamic balance between sediment transport by waves and by currents and have revealed the key control of waves on estuarine morphology. Estuarine intertidal flats are excellent natural laboratories that offer opportunities for working on a number of fundamental problems in sediment transport.

Simulated wave-driven sediment transport along the eastern coast of the Baltic Sea

Alongshore variations in sediment transport along the eastern Baltic Sea coast from the Sambian (Samland) Peninsula up to Pärnu Bay in the Gulf of Riga are analysed using long-term (1970–2007) simulations of the nearshore wave climate and the Coastal Engineering Research Centre (CERC) wave energy flux model applied to about 5.5km long beach sectors. The local rate of bulk transport is the largest along a short section of the Sambian Peninsula and along the north-western part of the Latvian coast. The net transport has an overall counter-clockwise nature but contains a number of local temporary reversals. The alongshore sediment flux has several divergence and convergence points. One of the divergence points at the Akmenrags Cape divides the sedimentary system of the eastern coast of the Baltic Proper into two almost completely separated compartments in the simulated wave climate. Cyclic relocation of a highly persistent convergence point over the entire Curonian Spit suggests that this landform is in almost perfect dynamical equilibrium in the simulated approximation of the contemporary wave climate.

Wave-driven sediment transport and beach-dune dynamics in a headland bay beach

This paper presents a comprehensive study of wave refraction, wave and wind driven sediment transport, foredune development and Holocene barrier evolution over a variety of time scales at Moçambique Beach, Santa Catarina State, Southern Brazil. The motivation for this study was to examine the relationships between estimated wave driven sediment transport, medium and long term aeolian transport, foredune development, and Holocene barrier volumes and morphology. An analysis of 2years of wave rider data for the region shows two well defined peaks of frequencies: one of swell waves from the south with a period of 12s, and another peak of sea waves from east-northeast with a period of 7s. Longshore sediment transport rates were calculated using high resolution hindcast wave data for the South Atlantic Ocean, which was calibrated by the measured buoy data. The dominant southerly waves drive a longshore gradient in wave energy from south to north, and nearshore/surf zone sediment transport increases from south to north. Beach grain size is fine in both the south and north, and medium in the center. The combination of grain size and wave energy variations creates a marked alongshore variation in surf zone beach types ranging from low energy dissipative in the south, moderate energy intermediate in the central area to moderate to high energy dissipative in the north. Estimates of aeolian sediment transport, volumetric changes in foredune deposition and total Holocene dunefield volumes were also calculated. Results indicate that the morphological behavior of the foredune profiles reflects the gradients of longshore sediment transport and the exposure to wind energy and aeolian sediment delivery, with a maximum of sediment volume and transport towards, and in the central part and northern end of the barrier. Holocene (0–7000years) barrier volumes vary along the embayment, mimicking the trends in both calculated wave and wind driven sediment transport. The Short and Hesp (1982) model was more suitable in predicting the long term (Holocene) barrier evolution than the size and volume of the foredune. The combination of wide beach, high energy dissipation, maximum sediment supply, and high exposure to strong onshore winds is found in the northern end of the embayment where Holocene subaerial barrier volumes are at a maximum. Overall, this study proves that foredune development is closely related to wind exposure, wave energy and gradients of longshore sediment transport, and that dunefield development reflects the increase downdrift in potential sediment supply from the southern to the northern end of the beach, and the change alongshore from low energy dissipative, through intermediate, to high energy dissipative beach-surf zone environments. This paper significantly extends previous models of beach and dune interactions in that it (i) actually determines wave driven sediment transport for various sections of a long embayment, (ii) relates those transport patterns to surf zone/beach morphodynamic types and variations in alongshore sediment patterns, and (iii) correlates foredune volumes and Holocene barrier volumes (which reflect proxy Holocene onshore transport rates) with wave driven sediment transport rates.

Beach and sea-cliff dynamics as a driver of long-term rocky coastline evolution and stability

We investigate rocky coastline evolution over millennial time scales using exploratory analytical and numerical models based on interactions between beaches and sea cliffs. In the models, wave-driven sea-cliff retreat is a nonlinear function of beach width, where cliff retreat is maximized by sediment abrasion and minimized by either a lack of beach sediment or too much sediment (which prevents waves from reaching the sea cliff). As sea cliffs retreat, beach sediment is produced and distributed alongshore by wave-driven sediment transport, and local beach widths determine future cliff retreat rates. Numerical experiments indicate that through such interactions, rocky coastlines can reach an equilibrium configuration where headlands and embayments remain stable through time, even in the absence of alongshore variations in sea-cliff lithology. Furthermore, the equilibrium coastline configuration, or the alongshore proportion of rocky headland to cliff-backed pocket beach, can be predicted analytically. Initial tests suggest that predictions match well qualitatively with actual landscapes.

Progress in coupling models of human and coastal landscape change

Humans are increasingly altering the Earth's surface, and affecting processes that shape and reshape landscapes. In many cases, humans are reacting to landscape-change processes that represent natural hazards. Thus, the landscape is reacting to humans who are reacting to the landscape. When the timescales for landscape change are comparable to those of human dynamics, human and ‘natural’ components of developed environments are dynamically coupled—necessitating coupling models of human and physical/biological processes to study either environmental change or human responses. Here we focus on a case study coupling models of coastal economics and physical coastline change. In this modeling, coastline change results from patterns of wave-driven sediment transport and sea-level rise, and shoreline stabilization decisions are based on the benefits of wide beaches (capitalized into property values) balanced against the costs of stabilization. This interdisciplinary modeling highlights points that may apply to other coupled human/natural systems. First, climate change, by accelerating the rates of landscape change, tends to strengthen the coupling with human dynamics. In our case study, both increasing sea-level-rise rates and changing storm patterns tend to increase shoreline change rates, which can induce more vigorous shoreline stabilization efforts. However, property values can fall dramatically as erosion rates and stabilization costs rise, which can also lead to the abandonment of expensive stabilization methods as shoreline change rates increase. Second, socio-economic change can also strengthen the human/landscape coupling. Changing costs of shoreline stabilization can alter stabilization decisions, which in turn alters patterns of coastline change. The coupled modeling illuminates the long-range effects of localized shoreline stabilization efforts; communities arrayed along a coastline are unwittingly affecting each other's erosion rates, and therefore each other's economies. Our coupled modeling experiments show that spatial distributions of property values and erosion rates can jointly affect economic outcomes, resource allocation between communities, and patterns of shoreline change. These findings raise questions about coastal management strategies, and efficient and equitable allocation of scarce resources among coastal communities.

Simulation of fine sediment transport in oscillatory boundary layer

Sediment transport and resulting morphodynamics take place at vastly different spatial and temporal scales. Bottom boundary layer dynamics is one of the most critical small-scale processes controlling sediment transport. However, it is very challenging as it involves highly nonlinear turbulent flow phenomenon that occurs at a scale as small as Kolmogorov scales. The unsteady nature of the flow forcing in the coastal environment, such as waves, is another important factor that further complicates the process. This paper aims to take a step to better understand wave-driven turbulent boundary layer and corresponding transport process. The problem of wave-driven sediment transport is simplified into fully developed oscillatory channel flow. Particles are assumed to be fine and dilute and hence they can be approximately considered as passive except gravitational settling with its settling velocity estimated from Stokes' law. Highly accurate pseudo-spectral flow solver has been employed and Direct Numerical Simulation (DNS) is carried out to resolve all scales of flow turbulence without sub-grid closure. Sediment concentration is then updated via mass conservation. Clear fluid simulation for intermittently turbulent condition (i.e., Stokes Reynolds number Re Δ = 1000) is validated with earlier DNS results reported by Spalart and Baldwin [1989. Direct simulation of a turbulent oscillating boundary layer. Turbulent Shear Flows 6. Springer]. Fine sediment with dilute concentration is then added in the numerical simulation to study sediment suspension events under oscillatory flow. Suspension events are evaluated both qualitatively, by observing vortex structures, and quantitatively by the statistics of vertical fluxes. For fine particles and dilute concentration considered here, it is observed that particle phase is well-mixed in the boundary layer. Suspension is highly dependent on turbulent vortical structures at different wave phases.