The thin blue line: A review of shoreline dynamics across time scales and environments

A major thread of theoretical research on the response of shorelines to changing boundary conditions has adapted the moving-boundary approach from heat transfer and solidification/melting. On sufficiently short time scales, shorelines respond to changes in relative sea level in a simple, geometrically predictable way. On longer time scales, their behaviour becomes far more complex and interesting, because the surface over which the shoreline moves is itself continually modified by morphodynamics that depend sensitively on shoreline location. This makes the shoreline the archetype of moving-boundary problems in morphodynamics, and subject to potentially counterintuitive behaviours over time scales on which the sediment surface modifies itself as relative sea level changes. We review existing moving-boundary theories and propose two significant extensions to allow inclusion of first-order effects of waves and tides. First, we show how wave effects can be included via planform diffusion linked to river-mouth location, which results in shoreline smoothing during delta-lobe growth and localized transgression after channel abandonment. Tides produce a low-gradient region in which the sea and land overlap; we show how this can be treated in a moving-boundary framework by replacing the shoreline with a ‘mushy region' so that the handoff from land to water occurs over a zone rather than a line. We also propose that the moving-boundary approach can be readily generalized to other dynamic moving boundaries, such as those separating different regimes of river transport. The shoreline thus serves as a prototype for modelling dynamic facies boundaries along the whole source–sink system. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

A baroclinic coastal trapped wave event in the Gulf of Naples (Tyrrhenian Sea)

In this study, a baroclinic coastal trapped wave, with period ~?4.5 days and cross-shore scale ~?3 km, is identified in the outputs of a very-high-resolution ocean circulation model of the Campania coastal system (central Tyrrhenian Sea; including the Gulfs of Salerno, Naples, and Gaeta). The outputs are from a simulation spanning June 2003, a month in which the surface winds were always weak, except for a strong northeasterly wind event that lasted 1 day (20 June). This event is found to generate a strong upwelling along the Campania coasts, whose relaxation—virtually free, because of the weakness of the winds—produces coastal current fluctuations that propagate to the north, spanning the three gulfs. The dynamics (analyzed with a focus on the Gulf of Naples) is found to share important features with baroclinic Kelvin waves in a two-layer model, such as the sharp cross-shore decrease of the perturbation amplitude and the vertical reversal of the long-shore current velocities. The simulated phase speed, cross-shore extension, and wavelength of the perturbation are also close to those obtained using a two-layer approximation of the dynamics. Moreover, the propagation described by the models is shown to be compatible with current measurements that were made in June 2003 at the southern entrance of the Gulf of Naples. Experimental implications related to the specific oceanographic problem are finally discussed, and an experimental strategy—inspired by our modeling approach—aimed at identifying the phenomenon is proposed.     

Validation and application of an ensemble Kalman filter in the Selat Pauh of Singapore

The effectiveness of an ensemble Kalman filter (EnKF) is assessed in the Selat Pauh of Singapore using observing system simulation experiment. Perfect model experiments are first considered. The perfect model experiments examine the EnKF in reducing the initial perturbations with no further errors than those in the initial conditions. Current velocity at 15 observational sites from the true ocean is assimilated every hour into the false ocean. While EnKF reduces the initial velocity error during the first few hours, it fails after one tidal cycle (approximately 12 h) due to the rapid convergence of the ensemble members. Successively, errors are introduced in the surface wind forcing. A random perturbation ε is applied independently to each ensemble member to maintain the ensemble spread. The assimilation results showed that the success of EnKF depends critically on the presence of ε, yet it is not sensitive to the magnitude of ε, at least in the range of weak to moderate perturbations. Although all experiments were made with EnKF only, the results could be applicable in general to all other ensemble-based data assimilation methods.     

Comparing landslide inventory maps

Landslide inventory maps are effective and easily understandable products for both experts, such as geomorphologists, and for non experts, including decision-makers, planners, and civil defense managers. Landslide inventories are essential to understand the evolution of landscapes, and to ascertain landslide susceptibility and hazard. Despite landslide maps being compiled every year in the word at different scales, limited efforts are made to critically compare landslide maps prepared using different techniques or by different investigators. Based on the experience gained in 20 years of landslide mapping in Italy, and on the limited literature on landslide inventory assessment, we propose a general framework for the quantitative comparison of landslide inventory maps. To test the proposed framework we exploit three inventory maps. The first map is a reconnaissance landslide inventory prepared for the Umbria region, in central Italy. The second map is a detailed geomorphological landslide map, also prepared for the Umbria region. The third map is a multi-temporal landslide inventory compiled for the Collazzone area, in central Umbria. Results of the experiment allow for establishing how well the individual inventories describe the location, type and abundance of landslides, to what extent the landslide maps can be used to determine the frequency-area statistics of the slope failures, and the significance of the inventory maps as predictors of landslide susceptibility. We further use the results obtained in the Collazzone area to estimate the quality and completeness of the two regional landslide inventory maps, and to outline general advantages and limitations of the techniques used to complete the inventories.     

Linking the Recurrence Time of Earthquakes to Source Parameters: A Dream or a Real Possibility?

By using a single-degree-of-freedom spring-slider analog fault model, we generate a synthetic catalog of nearly 500 different seismic sequences. We explore the parameter space by assuming different values of constitutive parameters and tectonic environment. We also consider three different versions of the rate-dependent and state-dependent friction laws [the Dieterich-Ruina (DR), the Ruina-Dieterich (RD) and the Chester-Higgs (CH) models], and different approximations of the behavior of the friction at high sliding speeds, as well as the radiation damping effects. Our results indicate that for all the considered models, the recurrence time (T _cycle) exhibits an inverse proportionality on the loading rate; a linear, positive dependence on the effective normal stress; and a linear, negative dependence on the characteristic distance controlling the state variable evolution. These results confirm and generalize previous studies. Remarkably, we found here that the coefficients of proportionality strongly depend on the adopted friction model, on the high speed behavior and on the reference set of parameters. Notably, we also found that the positive proportionality between T _cycle and the difference b – a, confirmed for DR and RD laws, does not hold in general for the CH law. Overall, we conclude that even in the simplest (and idealized) case of characteristic earthquakes considered here, in which the limiting cycle is reached by the system, and even in the framework of a very simplified fault model, the possibility to a priori predict, through an universal analytical relation, the inter-event time of an impending earthquake still remains only a dream. On the other hand, a numerical prediction of T _cycle would require the exact knowledge of the rheological model (and its parameters at all times over the entire life of the fault) and the actual state of the fault, which indeed are often unknown.     

Earthquakes from 1820 to 1936 in Grahamstown and surroundings (Eastern Cape Province,South Africa)

The seismicity of Grahamstown, in the Eastern Cape Province of South Africa, for the years between 1820 and 1936 is investigated with recourse to contemporaneous documentary sources, leading to the development of a seismic history incorporating consideration of the broader geo-political context. Individual studies of five regional events that were felt in Grahamstown during that period, and of one that was not, are presented. Each study includes the development of a full set of intensity data points, which are used to determine reappraised epicentral locations and magnitudes, some of which differ significantly from previously listed parameters. The results thus obtained highlight the value of seeking out additional contemporary sources from different language groups when revisiting the source parameters of earthquakes for which no or only very limited instrumental information is available.     

40Ar–39Ar dating of volcanogenic products from the AND-2A core (ANDRILL Southern McMurdo Sound Project, Antarctica): correlations with the Erebus Volcanic Province and implications for the age model of the core

The AND-2A drillcore (Antarctic Drilling Program—ANDRILL) was successfully completed in late 2007 on the Antarctic continental margin (Southern McMurdo Sound, Ross Sea) with the aim of tracking ice proximal to shallow marine environmental fluctuations and to document the 20-Ma evolution of the Erebus Volcanic Province. Lava clasts and tephra layers from the AND-2A drillcore were investigated from a petrographic and stratigraphic point of view and analyzed by the ⁴⁰Ar–³⁹Ar laser technique in order to constrain the age model of the core and to gain information on the style and nature of sediment deposition in the Victoria Land Basin since Early Miocene. Ten out of 17 samples yielded statistically robust ⁴⁰Ar–³⁹Ar ages, indicating that the AND-2A drillcore recovered ≤230 m of Middle Miocene (∼128–358 m below sea floor, ∼11.5–16.0 Ma) and >780 m of Early Miocene (∼358–1093 m below sea floor, ∼16.0–20.1 Ma). Results also highlight a nearly continuous stratigraphic record from at least 358 m below sea floor down hole, characterized by a mean sedimentation rate of ∼19 cm/ka, possible oscillations of no more than a few hundreds of ka and a break within ∼17.5–18.1 Ma. Comparison with available data from volcanic deposits on land, suggests that volcanic rocks within the AND-2A core were supplied from the south, possibly with source areas closer to the drill site for the upper core levels, and from 358 m below sea floor down hole, with the “proto-Mount Morning” as the main source.     

Changes in biomass structure and trophic status of the plankton communities in a highly dynamic ecosystem (Gulf of Venice,Northern Adriatic Sea)

The changes in the plankton biomass structure in relation to nutrient inputs were studied in the Gulf of Venice (Northern Adriatic Sea), an area characterized by a very marked trophic state variability. The investigation was carried out at two stations, in March, May and July 2005 and 2006, considering the whole water column. The size structure (from picoplankton to mesozooplankton) of both autotrophs and heterotrophs was analysed. Signals of diluted waters and nutrient inputs were more marked in 2005 than in 2006. In 2005, the total plankton biomass was almost double (87 ± 37 μg·C·l^?1) that in 2006 (44 ± 26 μg·C·l^?1). The variations were determined mainly by phytoplankton, with a 70% decrease, and a shift from a community dominated by microphytoplankton (49 ± 12%) in 2005 to one dominated by bacteria (43 ± 11%) in 2006 was observed. The relationship between the heterotrophic (H) and autotrophic (A) biomass indicated a rapid decline of the H/A ratio with increasing phytoplankton biomass. This study, although temporally limited, is consistent with the results reported for other marine environments and it seems to confirm the importance of nutrient inputs in structuring the biomass of plankton community.