Maitane Olabarrieta

The role of morphology and wave‐current interaction at tidal inlets: An idealized modeling analysis

The outflowing currents from tidal inlets are influenced both by the morphology of the ebb-tide shoal and interaction with incident surface gravity waves. Likewise, the propagation and breaking of incident waves are affected by the morphology and the strength and structure of the outflowing current. The 3-D Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system is applied to numerically analyze the interaction between currents, waves, and bathymetry in idealized inlet configurations. The bathymetry is found to be a dominant controlling variable. In the absence of an ebb shoal and with weak wave forcing, a narrow outflow jet extends seaward with little lateral spreading. The presence of an ebb-tide shoal produces significant pressure gradients in the region of the outflow, resulting in enhanced lateral spreading of the jet. Incident waves cause lateral spreading and limit the seaward extent of the jet, due both to conversion of wave momentum flux and enhanced bottom friction. The interaction between the vorticity of the outflow jet and the wave stokes drift is also an important driving force for the lateral spreading of the plume. For weak outflows, the outflow jet is actually enhanced by strong waves when there is a channel across the bar, due to the “return current” effect. For both strong and weak outflows, waves increase the alongshore transport in both directions from the inlet due to the wave-induced setup over the ebb shoal. Wave breaking is more influenced by the topography of the ebb shoal than by wave-current interaction, although strong outflows show intensified breaking at the head of the main channel.

Morphodynamics of river‐influenced back‐barrier tidal basins: The role of landscape and hydrodynamic settings

We investigate the morphodynamics of river-influenced barrier basins numerically, with a particular emphasis on the effects of landscape and hydrodynamic settings. The simulated morphologies are qualitatively comparable to natural systems (e.g., tidal inlets along the East Coast of the USA). Model results suggest that the basin morphology is governed by the relative importance of tidal and fluvial forcing which is reflected, to the first-order approximation, in the ratio (rQ) between the mean tidal and river discharge. In agreement with empirical knowledge, the model indicates that riverine influence can be neglected when rQ is larger than 20. On the other hand, the river may dominate when rQ is smaller than 5. Pronounced differences in morphodynamic evolution are observed for different landscape settings (i.e., initial basin bathymetries and river inflow locations), indicating their fundamental importance in governing the evolution of barrier basins. Model results also show that the addition of a river tends to compensate the flood dominance in the tidal basin. Overall, the river flow has limited influence on the volumetric change of tidal flats, while it plays a more important role in determining the depth of the tidal channels and the size of the ebb delta. The riverine sediment source appears to be more important in shaping the basin morphology when the fluvial forcing is stronger. Finally, we show that the presence of a large river in a tidal inlet system influences the performance of the widely adopted relation between tidal prism and inlet cross-sectional area.

A comparative study of physical and numerical modeling of tidal network ontogeny

We investigate the initiation and long-term evolution of tidal networks by comparing controlled laboratory experiments and their associated scaling laws with outputs from a numerical model. We conducted numerical experiments at both the experimental laboratory scale (ELS) and natural estuary scale (NES) and compared these simulations with experimental data and field observations. Sensitivity tests show that initial bathymetry, frictional parametrization, sediment transport, and bed slope terms play an important role in determining the morphodynamic evolution and the final landscape. Consistent with experimental observations, the morphodynamic feedbacks between flow, sediment transport, and bathymetry gradually lead the system to a less dynamic state, finally reaching a stable network configuration. In both the ELS and NES simulations, the initially planar lagoon with large intertidal areas is subject to erosion, indicating ebb-dominance. Based on quantitative analyses of the ELS and the NES simulations (e.g., geometric characteristics and relationship between modified tidal prism and cross-sectional area), we conclude that numerical simulations are consistent with laboratory experiments and show that both type of models provide a realistic, albeit simplified, representation of natural systems. The combination of laboratory and numerical experiments also allowed us to explore the possibility of reaching a long-term morphodynamic equilibrium. Both the physical and numerical models approach a dynamic equilibrium characterized by negligible gradients in sediment fluxes. The equilibrium configuration appears to be consistent with traditional relationships linking tidal prism and cross-sectional area of the inlet. Finally, this contribution highlights the significance of complementary research between experimental and numerical modeling in investigating long-term morphodynamics of tidal networks.

A nearshore wave and current operational forecasting system.

An operational forecasting system for nearshore waves and wave-induced currents is presented. The forecasting system (FS) has been built to provide real time information about nearshore conditions for beach safety purposes. The system has been built in a modular way with four different autonomous submodels providing, twice a day, a 36-hour wave and current forecast, with a temporal resolution of 1 hour. Making use of a mild slope parabolic model, the system propagates hourly deep water wave spectra to the shore. The resulting radiation stresses are introduced in a depth-integrated Navier-Stokes model to derive the resulting current fields. The system has been implemented in a beach located in the northeastern part of Mallorca Island (western Mediterranean), characterized by its high touristic pressure during summer season. The FS has been running for 3 years and is a valuable tool for local authorities for beach safety management.

Observed and modeled drifters at a tidal inlet

Material transport and dispersion near the mouth of a tidal inlet (New River Inlet, NC) are investigated using GPS-tracked drifters and numerical models. For ebb tide releases, velocities are largest (>1 m s−1) in two approximately 30 m wide channels that bisect the 1–3 m deep ebb shoal. In the channels, drifter and subsurface current meter velocities are similar, consistent with strong vertical mixing and 2-D hydrodynamics. Drifters were preferentially entrained in the channelized jets where drifter cluster lateral spreading rates μin were small ( μin≈0.5 m2 s−1). At the seaward edge of the ebb shoal, jet velocities decrease linearly with distance (to ≤0.2 m s−1, about 1 km from shore), and cluster spreading rates are larger with μout≈3 m2 s−1. Although the models COAWST and NearCom generally reproduce the observed trajectory directions, certain observed drifter properties are poorly modeled. For example, modeled mean drifter velocities are smaller than observed, and upon exiting the inlet, observed drifters turn north more than modeled drifters. The model simulations do reproduce qualitatively the spreading rates observed in the inner inlet, the flow deceleration, and the increase in μout observed in the outer inlet. However, model spreading rates increase only to μout<1 m2 s−1. Smaller modeled than observed μout may result from using unstratified models. Noncoincident (in space) observations show evidence of a buoyant plume ( Δρ=1 kg m−3) in the outer inlet, likely affecting drifter lateral spreading. Generally, drifter-based model performance is good within the inlet channels where tidal currents are strongest, whereas model-data differences are significant farther offshore.

The BIG’95 Submarine Landslide–Generated Tsunami: A Numerical Simulation

This article presents a reasonable present-day, sea-level highstand numerical simulation and scenario for a potential tsunami generated by a landslide with the characteristics of the BIG’95 debris flow, which occurred on the Ebro margin in the western Mediterranean Sea in prehistoric times (11,500 cal yr BP). The submarine landslide deposit covers an area of 2200 km2 of the slope and base of slope (200–1800-m water depth), involving a volume of 26 km3. A leapfrog finite difference model, COMCOT (Cornell multigrid coupled tsunami model), is used to simulate the propagation of the debris-flow-generated tsunami and its associated impact on the nearby Balearic Islands and Iberian coastlines. As a requisite of the model, reconstruction of the bathymetry before the landslide occurrence and seafloor variation during landsliding have been developed based on the conceptual and numerical model of Lastras et al. (2005). We have also taken into account all available multibeam bathymetry of the area and high-resolution seismic profiles of the debris flow deposit. The results of the numerical simulation are displayed using plots of snapshots at consecutive times, marigrams of synthetic stations, maximum amplitude plots, and spectral analyses. The obtained outputs show that the nearest shoreline, the Iberian coast, would not be the first one hit by the tsunami. The eastward, outgoing wave would arrive at Eivissa Island 18 min after the triggering of the slide and at Mallorca Island 9 min later, whereas the westward-spreading wave would hit the Iberian Peninsula 54 min after the slide was triggered. This noticeable delay in the arrival times at the peninsula is produced by the asymmetric bathymetry of the Catalano-Balearic Sea and the shoaling effect due to the presence of the wide Ebro continental shelf, which in addition significantly amplifies the tsunami wave (>9 m). The wave amplitudes attain 8 m in Eivissa, and waves up to 3 m high would arrive to Palma Bay. Resonance effects produced in the narrow Santa Ponça Bay in Mallorca Island could produce waves up to 9 m high. A similar event occurring today would have catastrophic consequences, especially in summer when human use of these tourist coasts increases significantly.

C3: A finite volume-finite difference hybrid model for tsunami propagation and runup

A numerical model that couples Finite Difference and Finite Volume schemes has been developed for tsunami propagation and runup study. An explicit leap-frog scheme and a first order upwind scheme has been considered in the Finite Difference module, while in the Finite Volume scheme a Godunov Type method based on the f-waves approach has been used. The Riemann solver included in the model corresponds to an approximate augmented solver for the Shallow Water Equations (SWE) in the presence of variable bottom surface. With this hybrid model some of the problems inherent to the Godunov type schemes are avoided in the offshore region, while in the coastal area the use of a conservative method ensures the correct computation of the runup and wave breaking. The model has been tested and validated using different problems with a known analytical solution and also with laboratory experiments, considering both non breaking and breaking waves. The results are very satisfactory, showing that the hybrid approach is a useful technique for practical usages.

Development of a GIS-based oil spill risk assessment system

The Prestige crisis proved the importance of developing scientific- and application- oriented activities which allow us to improve the oil spill preparedness and response systems having efficient tools to minimize the spill impact in case of an emergency. In this work a methodology has been developed in which oil spill risk is calculated assuming its dependency on the hazard, H, and vulnerability, V, components. To estimate the probability of an oil spill reaching a specific target area, H, an approach based on numerically generated data has been used. Regarding the other risk component, the oil spill vulnerability V, a new approach is presented which focuses on the integration in one single index of physical, biological and socio-economic aspects of the coast. To illustrate the presented methodology it has been applied to the Cantabrian coast, Northern coast of Spain (Bay of Biscay) where a user-friendly application which incorporates a Geographic Information System has been built. This application integrates the two components of the oil spill risk, H and V, to support spill response planning along this coast.

An Alert System for Beach Hazard Management in the Balearic Islands

A real-time beach hazard level associated with nearshore hydrodynamics is presented in this article. The suitability of the discussed alert system is illustrated via its application to fifteen beaches in the Balearic Islands (Western Mediterranean Sea) providing nearshore safety conditions for beach safety manager. The system provides daily forecasts of nearshore wave conditions using the deep water wave forecasts. The shallow water wave data (wave height, period, and direction) together with the morphology of the site (presence of bars, capes, beach type, etc.) are used to define a hazard level (low, medium, and high) associated with local conditions. The resulting hazard level is transmitted via SMS to lifeguards and local authorities for real-time beach management. The low computational cost of this system after the initial implementation and subsequent calibration results in a very suitable approach for beach management in order to mitigate risks related to local hydrodynamics.

Observations and modeling of a tidal inlet dye tracer plume

A 9 km long tracer plume was created by continuously releasing Rhodamine WT dye for 2.2 h during ebb tide within the southern edge of the main tidal channel at New River Inlet, NC on 7 May 2012, with highly obliquely incident waves and alongshore winds. Over 6 h from release, COAWST (coupled ROMS and SWAN, including wave, wind, and tidal forcing) modeled dye compares well with (aerial hyperspectral and in situ) observed dye concentration. Dye first was transported rapidly seaward along the main channel and partially advected across the ebb-tidal shoal until reaching the offshore edge of the shoal. Dye did not eject offshore in an ebb-tidal jet because the obliquely incident breaking waves retarded the inlet-mouth ebb-tidal flow and forced currents along the ebb shoal. The dye plume largely was confined to <4 m depth. Dye was then transported downcoast in the narrow (few 100 m wide) surfzone of the beach bordering the inlet at 0.3 m s−1 driven by wave breaking. Over 6 h, the dye plume is not significantly affected by buoyancy. Observed dye mass balances close indicating all released dye is accounted for. Modeled and observed dye behaviors are qualitatively similar. The model simulates well the evolution of the dye center of mass, lateral spreading, surface area, and maximum concentration, as well as regional (“inlet” and “ocean”) dye mass balances. This indicates that the model represents well the dynamics of the ebb-tidal dye plume. Details of the dye transport pathways across the ebb shoal are modeled poorly perhaps owing to low-resolution and smoothed model bathymetry. Wave forcing effects have a large impact on the dye transport.