Rodrigo Pedreros

Application of grain size trend analysis for the determination of sediment transport pathways in intertidal areas

Abstract In this paper, we apply the Gao and Collins procedure of grain size trend analysis for the first time in an intertidal sedimentary system. Our purpose is to evaluate a reliable method to determine residual transport directions in intertidal environments, where sediment movement is not well understood. Residual transport patterns can be determined by grain size trend analysis if the following criteria are applied: (1) a single sediment transport unit must be precisely defined and investigated; (2) the thickness of sampling has to be chosen in order to investigate the most recent sedimentary event; and (3) a regular mesh grid is sampled to avoid bias in the calculation of grain size trends. The Gao and Collins method was undertaken simultaneously with a topographic survey and a tracer experiment, during consecutive meteorological conditions of fair weather and a storm event. The topographic changes observed between these two situations and the directions of sediment movement deduced from tracer tracking are coherent with the residual transport pattern calculated from the grain size trend analysis. The method was tested on a ridge and runnel system which corresponds to the intertidal part of a macrotidal beach, called La Salie beach, located along the southwest French coast, in a high wave energy environment. This study permits us to recognize two different residual transport patterns for this intertidal ridge and runnel system; one for fair weather and a second for storm conditions. During fair weather, specific sites are subject to sand accretion, while during storm conditions these sand stocks are subsequently spread out over the whole area, and remain partly on the beach.

Present Wave Climate in the Bay of Biscay: Spatiotemporal Variability and Trends from 1958 to 2001

AbstractClimate change impacts on wave conditions can increase the risk of offshore and coastal hazards. The present paper investigates wave climate multidecadal trends and interannual variability in the Bay of Biscay during the past decades (1958–2001). Wave fields are computed with a wave modeling system based on the WAVEWATCH III code and forced by 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) wind fields. It provides both an extended spatiotemporal domain and a refined spatial resolution over the Bay of Biscay. The validation of the wave model is based on 11 buoys, allowing for the use of computed wave fields in the analysis of mean and extreme wave height trends and variability. Wave height, period, and direction are examined for a large array of wave conditions (by seasons, high percentiles of wave heights, different periods). Several trends for recent periods are identified, notably an increase of summer significant wave height, a southerly shift of autumn extreme wa...

Teleconnection Pattern Influence on Sea-Wave Climate in the Bay of Biscay

Abstract The potential modification of hydrodynamic factors, such as waves, is a source of concern for many coastal communities because of its potential effect on shoreline evolution. In the northern Atlantic, swell is created by storm winds that cross the Atlantic following west–east tracks. These tracks are shifted more southward or northward depending on the season and on recurring large-scale atmospheric pressure anomalies, also called teleconnection patterns. This study investigates the trends of sea-wave patterns in the Bay of Biscay and relates their interannual variability to teleconnection patterns. Sea-wave parameter time series from the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) show a satisfying correlation with an in situ buoy of Meteo-France during the period they overlap. Using a k-means algorithm, data from this 44-yr-long time series were clustered into a few sea-state modes, each of them corresponding to an observable sea state associated with a...

Evaluating uncertainties of future marine flooding occurrence as sea-level rises

As sea-level rises, the frequency of coastal marine flooding events is changing. For accurate assessments, several other factors must be considered as well, such as the variability of sea-level rise and storm surge patterns. Here, a global sensitivity analysis is used to provide quantitative insight into the relative importance of contributing uncertainties over the coming decades. The method is applied on an urban low-lying coastal site located in the north-western Mediterranean, where the yearly probability of damaging flooding could grow drastically after 2050 if sea-level rise follows IPCC projections. Storm surge propagation processes, then sea-level variability, and, later, global sea-level rise scenarios become successively important source of uncertainties over the 21st century. This defines research priorities that depend on the target period of interest. On the long term, scenarios RCP 6.0 and 8.0 challenge local capacities of adaptation for the considered site.

Longshore Sediment Movements from Tracers and Models, Praia de Faro, South Portugal

Abstract This work aims to improve the knowledge of sediment circulation in the coastal zone, focusing on the quantification of sediment movement due to longshore drift. The studied field site is the Praia de Faro beach on the Peninsula do Ancão in Algarve (Portugal), which is located on the updrift coast of the Barra Nova tidal inlet. The main objective of the study was to quantify the sediment transport rate that will enter in the inlet system. Measurements were undertaken during low hydrodynamic conditions prevailing in this area and permitted to estimate a longshore transport rate of 2.6–4.2 m/s, consistent with other transport estimations in Algarve. Four theoretical models for longshore transport rate calculation were confronted to tracer results to choose a extrapolation tool. Kamphuis (1991) formula appeared to be the more appropriate in this area. It was used to extrapolate the results in order to be compared to the observed morphological evolution. The calculated short term sediment flux (10500 m3/month) corresponds to the accretion on the updrift swash platform of the Barra Nova tidal inlet and is thus considered to be the contribution of the adjacent coast to this system. Moreover, the estimated longshore transport rate permit us to predict a spit progradation of 60–70 m/year. This result is consistent with the observed migration rate of the Barra Nova inlet during the last 25 years. This study shows that, with a good knowledge of the environmental context, sediment fluxes obtained by sand tracers are accurate enough to estimate the long term longshore transport and long term morphological response of a stretch of shoreline.

Very low frequency motions of a rip current system: observations and modeling

Biscarrosse beach, located on the French Aquitanian Coast, is a high-energy meso-macrotidal double-barred beach. The outer and the inner bars exhibit most of the time crescentic patterns and transverse bar and rip morphology, respectively. Breaking waves over these three-dimensional features induce strong rip currents that are responsible of several drowning accidents each year. To improve our knowledge on this kind of complex environment, an intensive 5-day field experiment was carried out in June 2007 at Biscarrosse Beach. The present study is focused on Very Low Frequency motions (VLF) of a rip current system over a well-developed bar-rip morphology. Using both a drifter experiment and virtual drifter modeling, the study aims at analyzing the rip current pulsations and the drifter retention in the surf zone. The main results show the oscillating behavior of the rip currents, in particular within the rip neck where the VLF pulsations are intense (reaching 1m/s on time scales of 10 to 30 minutes). In addition, most of the drifters are retained within the surf zone (about 80%), with the other 20% exiting the surf zone. These results are reproduced by our numerical model, which shows that shear instabilities of the rip current can be the cause of such retention/expulsion proportions. In addition, here we present the spatial variability of the VLF motion over the entire rip current system.

Uncertainties in sandy shorelines evolution under the Bruun rule assumption

In the current practice of sandy shoreline change assessments, the local sedimentary budget is evaluated using the sediment balance equation, that is, by summing the contributions of longshore and cross-shore processes. The contribution of future sea-level-rise induced by climate change is usually obtained using the Bruun rule, which assumes that the shoreline retreat is equal to the change of sea-level divided by the slope of the upper shoreface. However, it remains unsure that this approach is appropriate to account for the impacts of future sea-level rise. This is due to the lack of relevant observations to validate the Bruun rule under the expected sea-level rise rates. To address this issue, this article estimates the coastal settings and period of time under which the use of the Bruun rule could be (in)validated, in the case of wave-exposed gently-sloping sandy beaches. Using the sedimentary budgets of Stive (2004) and probabilistic sea-level rise scenarios based on IPCC, we provide shoreline change projections that account for all uncertain hydrosedimentary processes affecting idealized coasts (impacts of sea-level rise, storms and other cross-shore and longshore processes). We evaluate the relative importance of each source of uncertainties in the sediment balance equation using a global sensitivity analysis. For scenario RCP 6.0 and 8.5 and in the absence of coastal defences, the model predicts a perceivable shift toward generalized beach erosion by the middle of the 21st century. In contrast, the model predictions are unlikely to differ from the current situation in case of scenario RCP 2.6. Finally, the contribution of sea-level rise and climate change scenarios to sandy shoreline change projections uncertainties increases with time during the 21st century. Our results have three primary implications for coastal settings similar to those provided described in Stive (2004) : first, the validation of the Bruun rule will not necessarily be possible under scenario RCP 2.6. Second, even if the Bruun rule is assumed valid, the uncertainties around average values are large. Finally, despite these uncertainties, the Bruun rule predicts rapid shoreline retreat of sandy coasts during the second-half of the 21st century without strong reductions of greenhouse gas emissions.

Assessing storm impact on a French coastal dune system using morphodynamic modeling

ABSTRACT Muller, H.; van Rooijen, A.; Idier, D.; Pedreros, R., and Rohmer, J., 2017. Assessing storm impact on a French coastal dune system using morphodynamic modeling. Natural coastal dune systems provide protection against flooding from the ocean for many coastal communities around the world. An in-depth knowledge of their erosion mechanisms under oceanic stress can help to identify potential weak spots along the coastline and eventually prevent damage to the hinterland. The objective of this study was to assess the effects of hydro-morphodynamic phenomena on the coastal dune morphology during storm events and to evaluate their level of protection. This paper focuses on the physical processes that cause erosion and breaching of sandy dune systems, which are typically a function of the coastal morphology in combination with the hydraulic loading. An XBeach model was set up for the coast of Les Boucholeurs (France), which suffered from severe erosion, dune breaching (at six locations), and inundation of the hinterland during storm Xynthia in February 2010. The model was run for the full storm period, and it accurately reproduced the reported dune breach locations and dimensions with limited calibration. The model results were further utilized to study the temporal and spatial breaching dynamics and to understand the effect of the prestorm morphology on dune breach development. The results show that the dune behavior during the storm was strongly related to the submersion height of the dune crest and anticorrelated with its prestorm geometry (i.e. dune height and width). The governing parameters for the poststorm dune state are, in terms of alongshore-averaged erosion and in order of relative contribution: Submersion height over the dune crest, prestorm dune width, and prestorm dune crest height. Finally, additional simulations showed the limited effects of infragravity waves and the initial bathymetry used in the simulations.

Casting light on forcing and breaching scenarios that lead to marine inundation: Combining numerical simulations with a random-forest classification approach

Abstract Identifying the offshore forcing and breaching conditions that lead to marine inundation is of high importance for risk management. This task cannot be conducted by using a numerical hydrodynamic model due to its high computation time cost (of several minutes or even hours). In the present study, we show how the random forest (RF) classification technique can approximate the numerical model to explore these critical conditions. We focus on the Boucholeurs site, which is located on the French Atlantic coast and exposed to overflow processes. An iterative strategy is developed for selecting the numerical simulations (a total of 200) to train the RF model. The sensitivity to the input parameters is studied using permutation-based importance measures and extended versions of the partial dependence plots. The results highlight the key interplay among the high-tide level, the surge peak and the phase difference, and the complex role of the breaching location.

Influences relatives des agents hydrodynamiques sur la mobilité sédimentaire des pertuis charentais: Cas des sédiments non cohésifs

ABSTRACT This paper investigates the relative influence of waves, wind-induced current and tidal current on the sediment mobility of a macrotidal environment belonging to the inner shelf seaward of the pertuis charentais (France). Past studies, based on hydrodynamic measurements already showed the dominant role of swells in the initiation of the motion of sand in an area located between the Ré and Oléron islands. Even if the method has some limits, using wave (SWAN) and 2DH hydrodynamic (MARS) modelling, we extend spatially the knowledge of the relative contribution of waves (without consideration of wave-induced currents), tides and wind-induced current in the non-cohesive sediment mobility.