Morphodynamic Instability Research Articles

A New Morphodynamic Instability Associated With Cross‐Shore Transport in the Nearshore

AbstractThe existing theory for shore‐transverse rhythmic sand bars relies on morphodynamic instabilities involving the wave‐driven longshore current and rip currents. Intriguingly, transverse finger bars are common on coasts with sediment excess, something not related to those currents. Here we show that, if the actual beach profile is above the equilibrium profile, cross‐shore transport can induce an instability triggered by onshore transport together with wave refraction by the emerging bars. We use a numerical model that filters out the dynamics associated to longshore and rip currents but includes a simplified version of cross‐shore transport and is able to reproduce the formation of shore‐transverse bars. The alongshore spacing scales with the wavelength of the incident waves and the cross‐shore extent is approximately equal to the distance from shore to the depth of closure. The modeled bars compare qualitatively well with observations at El Trabucador back‐barrier beach (Ebro delta, Western Mediterranean Sea).

Resilience of River Deltas in the Anthropocene

At a global scale, delta morphologies are subject to rapid change as a result of direct and indirect effects of human activity. This jeopardizes the ecosystem services of deltas, including protection against flood hazards, facilitation of navigation, and biodiversity. Direct manifestations of delta morphological instability include river bank failure, which may lead to avulsion, persistent channel incision or aggregation, and a change of the sedimentary regime to hyperturbid conditions. Notwithstanding the in‐depth knowledge developed over the past decades about those topics, existing understanding is fragmented, and the predictive capacity of morphodynamic models is limited. The advancement of potential resilience analysis tools may proceed from improved models, continuous observations, and the application of novel analysis techniques. Progress will benefit from synergy between approaches. Empirical and numerical models are built using field observations, and, in turn, model simulations can inform observationists about where to measure. Information theory offers a systematic approach to test the realism of alternative model concepts. Once the key mechanism responsible for a morphodynamic instability phenomenon is understood, concepts from dynamic system theory can be employed to develop early warning indicators. In the development of reliable tools to design resilient deltas, one of the first challenges is to close the sediment balance at multiple scales, such that morphodynamic model predictions match with fully independent measurements. Such a high ambition level is rarely adopted and is urgently needed to address the ongoing global changes causing sea level rise and reduced sediment input by reservoir building.

Interaction between hydrokinetic turbine wakes and sediment dynamics: array performance and geomorphic effects under different siting strategies and sediment transport conditions

In-stream hydrokinetic energy conversion devices can be deployed in large scale rivers to produce energy with minimal infrastructure costs. They are however shown to actively interact with the channel bathymetry and sediment transport generating a scour and deposition pattern similar to bridge pier. Symmetric, streamwise, aligned turbine installations have shown to introduce only local effects, yet complex configurations may trigger non-local morphodynamic instabilities. Experimental investigations, based on continuous spatio-temporal measurements of bed topography, explore a number of inflow conditions and siting strategies for axial-flow hydrokinetic turbine models. Results show that asymmetric turbine installations in a portion of the channel cross section may introduce weak non-local deformation of the mean bed topography and alter bedform migration velocities. Geomorphic effects become stronger with increasing shear stress and with rotors deployed up to half of the channel width, resulting in mean flow distortion within the channel cross section and inducing an alternating scour-deposition pattern resembling the signature of steady, forced fluvial bars. Non-local effects can be mitigated, narrowing the turbine array, or amplified, distributing turbines in a vane-like installation, leading to different estimates of energy production averaged at the power plant scale. A discussion on the key quantities governing geomorphic effects and the potential benefits of asymmetric turbine deployments is provided as a preliminary guideline towards the expansion of Marine Hydrokinetic energy in rivers.

A new instability mechanism related to high-angle waves

Waves with a large incidence angle in deep water can drive a morphodynamic instability on a sandy coast whereby shoreline sand waves, cuspate forelands, and spits can emerge. This instability is related to bathymetric perturbations extending offshore in the shoaling zone. Here, we explore a different mechanism where the large incidence angle is supposed to occur at breaking and the bathymetric perturbations occur only in the surf zone. For wave incidence angles at breaking above ≈ 45∘, the one-line approximation of coastal dynamics predicts an unstable shoreline. This instability (EHAWI) is scale-free and the growth rate increases without bound for decreasing wavelength. Here we use a 2DH morphodynamic model resolving surf zone instabilities to investigate whether EHAWI could approximate a real instability in nature with a characteristic length scale. Assuming very idealized conditions on the bathymetric profile and sediment transport, we find a 2DH instability mode consisting of shore-oblique up-current bars coupled to a meandering of the longshore current. This mode grows for high-angle waves, above about 30∘ (offshore) and the maximum growth rate occurs for the angle maximizing the angle at breaking, about 70∘ (offshore). The dominant wavelength is of the order of the surf zone width. Interestingly, for long sand waves, the growth rate never becomes negative and it matches very well the anti-diffusive behavior of EHAWI. This distinguishes the present instability mode from other modes found in previous studies for other bathymetric and sediment transport conditions. Thus, we conclude that EHAWI approximates a real morphodynamic instability only for quite particular conditions. In such case, a characteristic length scale of the instability emerges thanks to surf zone processes that damp short wavelengths.

How Observed Kilometric Sandy Shoreline Undulations depend on Wave Climate

Idier, D.; Falques, A.; Garcin, M., and Rohmer, J., 2018. How observed kilometric sandy shoreline undulations depend on wave climate. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 191–195. Coconut Creek (Florida), ISSN 0749-0208.Coastlines sometimes exhibit km-scale undulations, also called shoreline sandwaves. This work focuses on self-organized undulations and investigates how frequent they are, on which parameters they depend and whether existing theories of morphodynamic instabilities can explain them. We perform a wavelet analysis on shoreline data from areas appearing free of human or geological constraints and representing totally 1268 km around the world. It is found that 61% of cases exhibit undulations with wavelengths in the range 1–10 km. The wave parameter that correlates the best with their existence is the incidence angle with respect to shore normal and, to a lesser extent, the closure depth. The shoreline undulation probability increases with the angle, from 50% for low angles to almost 100% for large angles, suggesting that the High-Angle Wave Instability mechanism (HAWI) would be their primary cause, but that Low-Angle Wave Instability (LAWI) may also play a role. These results are consistent with recent theoretical investigations.

Multitemporal Analysis of Coastal Erosion Based on Multisource Satellite Images, Ponta Negra Beach, Natal City, Northeastern Brazil

This paper describes the use of moderate and high-resolution remote sensing images integrated with a Geographic Information System to evaluate the sedimentary balance and morphological changes seen on the sandy beach of Ponta Negra in northeastern Brazil. High-energy forces such as wind, tides, waves, and currents cause widespread erosion and morphodynamic instability along the shoreline. This investigation uses diverse images from multitemporal and multiresolution analyses and a Post-Processing Kinematic Global Positioning System (PPK GPS) survey as the primary resources to study the short and long-term effects of shoreline changes and the processes controlling the persistent horizontal and vertical erosion on the beach. We used moderate-resolution remote sensing images from 1973 to 2012 and high-resolution images from 2003 to 2011. Between 1973 and 2012 a total area of 121,494.52 m2 was affected by erosion and 37,266.58 m2 was affected by accretion. Shoreline progradation rates of 4.2 m/year and recession rates of -3.7 m/year were estimated for the period between 1973 and 1986 and mean erosion rates of 1.5 m/year were estimated from 1986 to 2012. Digital Elevation Model (DEM) comparison resulting from different PPK GPS surveys showed a negative sediment balance of approximately -3,261.94 m3 in a plot of 12,500 m2 between May and September 2012. The coast has also been facing a drastically increased human influence in recent decades. In summary, this article aims to contribute information to support the mitigation of coastal erosion and to protect seaside infrastructure.

Submarine channel initiation, filling and maintenance from sea‐floor geomorphology and morphodynamic modelling of cyclic steps

AbstractAdvances in acoustic imaging of submarine canyons and channels have provided accurate renderings of sea‐floor geomorphology. Still, a fundamental understanding of channel inception, evolution, sediment transport and the nature of the currents traversing these channels remains elusive. Herein, Autonomous Underwater Vehicle technology developed by the Monterey Bay Aquarium Research Institute provides high‐resolution perspectives of the geomorphology and shallow stratigraphy of the San Mateo canyon‐channel system, which is located on a tectonically active slope offshore of southern California. The channel comprises a series of crescent‐shaped bedforms in its thalweg. Numerical modelling is combined with interpretations of sea‐floor and shallow subsurface stratigraphic imagery to demonstrate that these bedforms are likely to be cyclic steps. Submarine cyclic steps compose a morphodynamic feature characterized by a cyclic series of long‐wave, upstream‐migrating bedforms. The bedforms are cyclic steps if each bedform in the series is bounded by a hydraulic jump in an overriding turbidity current, which is Froude‐supercritical over the lee side of the bedform and Froude‐subcritical over the stoss side. Numerical modelling and seismic‐reflection imagery support an interpretation of weakly asymmetrical to near‐symmetrical aggradation of predominantly fine‐grained net‐depositional cyclic steps. The dominant mode of San Mateo channel maintenance during the Holocene is interpreted to be thalweg reworking into aggrading cyclic steps by dilute turbidity currents. Numerical modelling also suggests that an incipient, proto‐San Mateo channel comprises a series of relatively coarse‐grained net‐erosional cyclic steps, which nucleated out of sea‐floor perturbations across the tectonically active lower slope. Thus, the interaction between turbidity‐current processes and sea‐floor perturbations appears to be fundamentally important to channel initiation, particularly in high‐gradient systems. Offshore of southern California, and in analogous deep‐water basins, channel inception, filling and maintenance are hypothesized to be strongly linked to the development of morphodynamic instability manifested as cyclic steps.

Instability and finite-amplitude self-organization of large-scale coastline shapes

Recent research addresses the formation of patterns on sandy coastlines on alongshore scales that are large compared with the cross-shore extent of active sediment transport. A simple morphodynamic instability arises from the feedback between wave-driven alongshore sediment flux and coastline shape. Coastline segments with different orientations experience different alongshore sediment fluxes, so that curvatures in coastline shape drive gradients in sediment flux, which can augment the shoreline curvatures. In a simple numerical model, this instability, and subsequent finite-amplitude inter-actions between pattern elements, lead to a wide range of different rhythmic shapes and behaviours--ranging from symmetric cuspate capes and bays to alongshore migrating 'flying spits'--depending on the characteristics of the input wave forcing. The scale of the pattern coarsens in some cases because of the merger of migrating coastline features, and in other cases because of non-local screening interactions between coastline protrusions, which affect the waves reaching other parts of the coastline. Features growing on opposite sides of an enclosed water body mutually affect the waves reaching each other in ways that lead to the segmentation of elongated water bodies. Initial tests of model predictions and comparison with observations suggest that modes of pattern formation in the model are relevant in nature.

Modeling shoreline sand waves on the coasts of Namibia and Angola

The southwestern (SW) coast of Africa (Namibia and Angola) features long sandy beaches and a wave climate dominated by energetic swells from the Southsouthwest (SSW), therefore approaching the coast with a very high obliquity. Satellite images reveal that along that coast there are many shoreline sand waves with wavelengths ranging from 2 to 8 km. A more detailed study, including a Fourier analysis of the shoreline position, yields the wavelengths (among this range) with the highest spectral density concentration. Also, it becomes apparent that at least some of the sand waves are dynamically active rather than being controlled by the geological setting. A morphodynamic model is used to test the hypothesis that these sand waves could emerge as free morphodynamic instabilities of the coastline due to the obliquity in wave incidence. It is found that the period of the incident water waves, Tp, is crucial to establish the tendency to stability or instability, instability increasing for decreasing period, whilst there is some discrepancy in the observed periods. Model results for Tp = 7–8 s clearly show the tendency for the coast to develop free sand waves at about 4 km wavelength within a few years, which migrate to the north at rates of 0.2–0.6 km yr−1. For larger Tp or steeper profiles, the coast is stable but sand waves originated by other mechanisms can propagate downdrift with little decay.

Morphodynamic modeling of the basal boundary of ice cover on brackish lakes

AbstractThe analysis considers a cold‐region brackish lake that develops an ice cover on its surface in winter. As the ice cover thickens due to freezing, black ice grows on the bottom side. The freezing process excludes the salt from the ice, resulting in an increase in dissolved salt concentration in the lake water just below the bottom of the ice. The density profile in this type of lake is then governed by two opposing factors: the water temperature distribution generally produces a stable stratification, whereas the excess salinity produced by exclusion tends to increase the density of the upper layers of the lake, resulting in an unstable contribution to stratification. Competition between these two factors can lead to unstable density gradients, so an initially motionless lake with a slowly thickening plane ice cover develops a convective flow field. This flow field can then feed back into the evolution of the morphology of the ice cover itself, resulting in a morphodynamic interaction between water and ice. This paper aims at investigating this morphodynamic instability. A mathematical model for the stability of a lake covered with an initially plane ice cover growing in time is proposed here. The results reveal threshold values of the main dimensionless parameters, expressed in the form of Rayleigh numbers, and in particular define a ratio between buoyancy effects and diffusivity that governs neutral stability conditions. When the system is unstable, i.e., for Rayleigh numbers above ~103 depending on the values of the input parameters, the analysis predicts the growth of convective flow circulation cells responsible for the morphodynamic evolution of the ice‐water interface.

WHAT DETERMINES THE WAVELENGTH OF SELF-ORGANIZED SHORELINE SAND WAVES?

Shoreline undulations extending into the bathymetric contours with a length scale larger than that of the rhythmic surf zone bars are referred to as shoreline sand waves. Many observed undulations along sandy coasts display a wavelength in the order 1-7 km. Several models that are based on the hypothesis that sand waves emerge from a morphodynamic instability in case of very oblique wave incidence predict this range of wavelengths. Here we investigate the physical reasons for the wavelength selection and the main parametric trends of the wavelength in case of sand waves arising from such instability. It is shown that the existence of a minimum wavelength depends on an interplay between three factors affecting littoral drift: (A) the angle of wave fronts relative to local shoreline, which tends to cause maximum transport at the downdrift flank of the sand wave, (B) the refractive energy spreading which tends to cause maximum transport at the updrift flank and (C) wave focusing (de-focusing) by the capes (bays), which tends to cause maximum transport at the crest or slightly downdrift of it. Processes A and C cause decay of the sand waves while process B causes their growth. For low incidence angles, B is very weak so that a rectilinear shoreline is stable. For large angles and long sand waves, B is dominant and causes the growth of sand waves. For large angles and short sand waves C is dominant and the sand waves decay. Thus, wavelength selection depends on process C, which essentially depends on shoreline curvature. The growth rate of very long sand waves is weak because the alongshore gradients in sediment transport decrease with the wavelength. This is why there is an optimum or dominant wavelength. It is found that sand wave wavelength scales with λ0/β where λ0 is the water wave wavelength in deep water and β is the mean bed slope from shore to the wave base.

Experimental and numerical findings on the long‐term evolution of migrating alternate bars in alluvial channels

Migrating alternate bars form in alluvial channels as a result of morphodynamic instability. Extensive literature can be found on their origin and short‐term development, but their long‐term evolution has been poorly studied so far. In particular, it is not clear whether migrating bars eventually reach a (dynamic) equilibrium, as in previous studies bars were observed to elongate with time. We studied the long‐term evolution of alternate bars by performing two independent long‐duration laboratory experiments and some numerical tests with a physics‐based depth‐averaged model. In a straight flume with constant water flow and sediment recirculation, migrating bars followed a cyclic variation. They became gradually longer and higher for a while, then quickly much shorter and lower. In one case, all migrating bars simultaneously vanished almost completely only to reform soon after. At the same time, steady bars, two to three times as long, progressively developed from upstream, gradually suppressing the migrating bars. We also observed simultaneous vanishing of migrating bars in an annular flume experiment, this time at intervals of 6–8 d. Numerical simulations of long alluvial channels with constant flow rate and fixed banks show periodic vanishing of a few migrating bars at a time, occurring at regular spacing. Under constant flow rates, migrating bars appear as a transition phenomenon of alluvial channels having a cyclic character. These observations, however, might hold only for certain morphodynamics conditions, which should be further investigated.

Modeling of submarine cyclic steps: Controls on their formation, migration, and architecture

Submarine cyclic steps are a newly recognized manifestation of fundamental morphodynamic instability of Froude-supercritical flow over an erodible bed. There is a growing recognition of the global presence and importance of cyclic steps. An attempt was made here to: (1) outline submarine cyclic steps in the context of the sediment waves of various origins; (2) elucidate the physics and key parameters governing their formation, migration, and architecture; and (3) summarize selected numerical experiments on net-depositional and net-erosional cyclic steps in a useful form. The paper also addresses frequent terminology confusion between net-depositional cyclic steps and sediment waves in general.

Conséquences de l’instabilité morphodynamique liée à l’exploitation du soufre dans le massif volcanique du Calimani (Roumanie)

En Roumanie, la seuleexploitation du soufre en carrière a été ouverte en 1973 dans le secteur du Negoiul Romanesc situé dans la partie centrale du massif du Calimani (Carpates Orientales roumaines). Mais, à cause de la faible rentabilité, cette activité a été arrêtée définitivement en 1997. Actuellement, les versants du cône transformés en carrière et terrils sont très instables, caractérisés par une morphodynamique active (coulées de débris, glissements de terrain, etc.). Utilisant des orthophotoplans, des cartes topographiques 1/25000 et 1/5000 et les informations provenant des observations de terrain, des sources bibliographiques diverses ou des témoignages, nous avons réalisé une carte actuelle de l’occupation du sol de cette région. Sur cette carte, on observe l’extension spatiale des zones de carrière, des terrils ou de la plateforme industrielle réalisée au détriment des forêts. En aval des terrils, les glissements de terrain et les coulées de débris ont affecté les forêts. Des mesures topographiques ont été effectuées afin de déterminer la vitesse de remplissage des sédiments transportés par les coulées de débris et déposés derrière les barrages construits pour stopper leur progression. Les valeurs obtenues indiquent des vitesses de remplissage de 1430 m3 par an dans le cas du barrage Dumitrelul, respectivement 3250 m3 par an pour le barrage Pinul. Des travaux de réhabilitation des terrils, de la carrière et de la plateforme industrielle ont commencé en juillet 2008, 11 ans après l’arrêt définitif de l’activité minière.

Modelling the morphodynamic impact of offshore sandpit geometries

We investigate the hydrodynamic effects and morphodynamic impact of large-scale offshore sand extraction, for a variety of pit designs. We use a process-based idealized model for flow, sediment transport and bed evolution in a tide-dominated environment. Legislation and other practical considerations motivate our assumption that sandpits are both wide (horizontal dimensions of the order of kilometres) and shallow (the ratio of pit depth to water depth being small). This results in a semi-analytical tool that, unlike previous studies, enables a quick and extensive study into the effects of varying the physical characteristics as well as the pit design parameters. These parameters include pit length, width, and orientation with respect to the tide. The model results show that sandpits experience flow contraction, which is enhanced by friction–topography interactions and also affected by Coriolis effects. As a result, sandpits trigger the morphodynamic instability associated with the formation of large-scale bed features known as tidal sandbanks. It implies a gradual deepening and deformation of the pit itself, as well as the appearance of adjacent humps. The time scale of this behaviour is of the order of decades to centuries. A sensitivity analysis is then carried out to determine the effects of pit geometry on the model results. The morphodynamic response is found to be strongest for sandpits elongated in the preferred direction of sandbank formation (giving the largest area of morphodynamic influence), and weakest for pits perpendicular to this direction. The migration of the pit is shown not to depend on the pit geometry, but rather on the ambient flow conditions.

On the diffusivity in coastline dynamics

The 1D diffusion equation for the dynamics of the coastline is revisited. It is found that the classical evaluation of the diffusivity coefficient over‐predicts it by a factor ranging from 1.25 up to infinity since the diffusivity may become zero while the classical prediction is always positive. The over‐prediction depends on wave steepness and wave incidence angle. It is larger for swell than for sea waves and it increases with increasing angle. For moderate angles it can easily be about a factor 10. Negative diffusivity occurs in case of large angles, consistent with the large angle morphodynamic instability.