Exploiting remote imagery in an embayed sandy beach for the validation of a runup model framework

International audienceStorm surge and wave runup are key determinants of the potential for beach overwashing during storm events. However, the prediction and quantification of wave runup on embayed beaches is strongly influenced by particular characteristics (e.g., irregular morphology, low tides, absence of swell, etc.) which differ from those on open beaches, and have rarely been investigated in literature. In the present paper, a model framework aimed at predicting wave-induced runup on an embayed sandy beach is validated by means of measurements derived from a video-monitoring station, recently installed in South Italy, during two storm events in 2016. The numerical approach employs MeteOcean forecasted waves within SWAN and SWASH models (in both 2-d and 1-d mode). The combination of multibeam and d-RTK surveys with Unmanned Aerial Vehicle (UAV) imagery provides high resolution depth grid (m 0.015), particularly required in shallow waters, where wave hydrodynamics is highly influenced by the bottom. The results show and discuss the agreement between video measurements and 2-d predictions of runup. A sensitivity analysis of the Manningfls roughness factor is needed in 1-d simulations. The accuracy of the empirical formulas in predicting wave runup in an embayed beach is also investigated , showing mainly an overestimation of the observations

new coastal video monitoring system achievement and development

A new coastal video monitoring system for Apulia region, southeast of Italy, is under development. It is composed of visible and thermal streams and featured to be included in a wider meteo-oceanographic monitoring network. The system is designed for use on sandy beaches. The regional Basin Authority (AdBP) through previous field campaigns has identified for long-term monitoring purposes two hotspots, prone to erosion and flooding: Torre Canne (Fasano, BR) and Torre Lapillo (Porto Cesareo, LE), facing the Adriatic and Ionian seas, respectively. This paper presents the actual architecture and some initial findings of its implementation aiming at a complete automatic analysis of morphological features and hydrodynamic studies, mainly focused in swash zone

WALOWA (wave loads on walls) : large-scale experiments in the delta flume

Overtopping wave loads on vertical structures on top of a dike have been investigated in several small scale experiments in the past. A large-scale validation for a mild foreshore situation is still missing. Hence the WALOWA experimental campaign was carried out to address this topic. In the present paper the objectives of the WALOWA project are outlined in detail, the model and measurement set-up described and the test program presented. Furthermore, preliminary results featuring a single 1000 irregular waves test of the test program are highlighted. This includes the study of the mild and sandy foreshore evolution by comparing profiles before and after the test execution. The profile measurements are obtained with a mechanical profiler. The wave parameters offshore and at the dike toe are numerically simulated using a SWASH model. The numerical results are validated against the measurements. Finally, the force and pressure time series of the waves impacting against the wall are processed and filtered. The load cell measurements and the time series of integrated pressures are compared to each other and for each impact event the maximum force is derived.Hydraulic Structures and Flood RiskEnvironmental Fluid Mechanic

Validation of swash model for run-up prediction on a natural embayed beach

Attualmente, gli eventi estremi di mareggiata rientrano tra i processi responsabili dell'alterazione dello stato dei litorali e dell'inondazione delle aree costiere, recentemente aggravati dall'innalzamento del livello medio del mare indotto, sia a scala globale che locale, dai cambiamenti climatici in atto. Tali fenomeni inducono effetti potenzialmente dannosi sugli ecosistemi e sulla salute dell'uomo, con ripercussioni sia sul sistema sociale che sulle attività economiche. La valutazione del rischio inondazione rappresenta, quindi, un aspetto cruciale al fine di garantire una efficace gestione integrata del territorio costiero. Si impone, dunque, l'utilizzo di strumenti dedicati per una corretta determinazione del rischio costiero, allo scopo di sviluppare efficienti politiche di mitigazione, protezione e prevenzione, basate su modelli predittivi affidabili, sistemi di allerta rapida e mappe di vulnerabilità, utili al fine di prevenire condizioni spesso irreversibili. In tal senso risulta prioritario definire un programma di monitoraggio ben strutturato e continuo dei litorali, da implementare all'interno di una strategia di gestione costiera nel lungo periodo. In questa prospettiva, i sistemi video rappresentano una soluzione in via di diffusione e di largo utilizzo in tutto il mondo nelle strategie di monitoraggio costiero, poiché permettono un'elevata frequenza di campionamento temporale e spaziale dei processi costieri, con bassi sforzi logistici e costi complessivi di installazione e manutenzione. Nel presente lavoro viene brevemente descritta l'implementazione e la configurazione di un nuovo sistema di video monitoraggio installato in Puglia, orientato ad alcune caratteristiche fondamentali quali la rapida installazione, robustezza, basso costo, efficienza delle fasi di acquisizione e della catena di elaborazione dei dati, attraverso l'installazione di due stazioni. Il progetto del sistema si pone l'obiettivo principale di ottenere uno strumento in grado di operare in maniera quasi esclusivamente automatica. Il sistema consente l'elaborazione delle immagini acquisite, la geo-rettifica, l'estrazione della linea di riva e l'archiviazione dei risultati in tempo reale su un portale web di dominio pubblico (http://91.121.30.84/). Il sistema è principalmente finalizzato al monitoraggio dei processi di morfodinamica della zona di riva, ma è stato anche ben predisposto per la misura dell'idrodinamica sottocosta. A tal fine, quindi, è stato implementato un tool di gestione composto da una serie di routine supportate da un applicativo web, finalizzate all'elaborazione di immagini (e.g. estrazione linea di riva e geo-rettifica), all'analisi ed alla condivisione dei dati sullo stato attuale della spiaggia e della sua evoluzione, quasi in tempo reale. Presso la baia di Torre Lapillo (Porto Cesareo, Lecce) il sistema è stato utilizzato per la raccolta di dati di run-up indotto dal moto ondoso sulla spiaggia, utilizzando immagini time-stack. I dati raccolti hanno offerto la possibilità di analizzare la sensitività di un modello ad elevata risoluzione per simulare l'interazione di un campo d'onde irregolari con la spiaggia fino alla risalita stessa delle onde, comunemente utilizzato per la risoluzione delle NLSWE. L'approccio numerico si basa sull'accoppiamento del modello spettrale SWAN (Booij et alii, 1996) ed il modello non-idrostatico SWASH (Zijlema et alii, 2011), utilizzando come forzanti al contorno i risultati in forecasting del modello MeteOcean, sviluppato dal Dipartimento DICCA dell'Università di Genova (Mentaschi et alii, 2015), al fine di modellare con elevata precisione le onde a differenti scale spaziali, fino alla dissipazione energetica nella swash zone. Le stime di run-up confrontate con le osservazioni da video-analisi dimostrano di essere in grado di rappresentare fedelmente la zona di swash, avendo ottenuto bias nel calcolo del R 2% pari a circa 5 cm ed un errore quadratico medio di circa 8 cm, entrambi in linea con l'accuratezza complessiva della restituzione topografica in uso. Al fine di risolvere la topo-batimetria del sito, l'area di studio è stata oggetto di diverse campagne di campo. E' stato dimostrato come la particolare conformazione topografica tipica degli ambienti costieri, sia adatta ad essere risolta mediante immagini acquisite da drone (UAV). Utilizzando l'algoritmo Structure from Motion (SfM) è stato infatti possibile ricostruire il Modello Digitale di Superficie (DSM), ottenendo una risoluzione a terra pari a 1.5 cm. Viene di seguito descritta brevemente l'elaborazione dei dati e la validazione della metodologia utilizzata, per la quale ci si è avvalsi, inoltre, di dati derivanti da rilievi effettuati mediante metodi tradizionali, quali d-GPS e Multibeam. In definitiva, la metodologia proposta può essere utilizzata al fine di creare un tool predittivo per stimare il rischio da inondazione in diverse aree sensibili della costa, come quella pugliese già notoriamente esposta a rischi di erosione avanzata

Video Depth Inversion at a Microtidal Site Exposed to Prevailing Low-energy Short-period Waves and Episodic Severe Storms

International audienceBouvier, C., Balouin Y., Castelle B. and Valentini N., 2020. Video depth inversion at a microtidal site exposed to prevailing low-energy short-period waves and episodic severe storms. Over the last decades, a wide range of depth inversion algorithms have been developed, which aim to infer local water depth from remotely-sensed wave parameters based on physical relations. Particularly, the depth inversion algorithm named cBathy (Holman et al., 2013) showed good results for a range of wave conditions and environments but the method was only applied to beaches exposed to moderate or highly energetic wave climate, with wave peak period often larger than 8 s. This paper applies the cBathy algorithm for the very first time on a Mediterranean Sea beach, Sète, southeast France, a semidiurnal microtidal environment exposed to a moderate-energy modal wave climate and episodic severe storms. The objective of the paper is to assess the ability of the cBathy algorithm to estimate the nearshore bathymetry for waves with large energy spread in both direction and frequency. After a comparison between the remotely sensed bathymetry and a ground truth bathymetric survey, we use a non-linear, depth-averaged morphodynamic model, 2DBeach (Dubarbier et al., 2017), on remotely sensed bathymetries to determine the sensitivity of nearshore waves and horizontal circulations to the estimated morphology. The depth inversion technique estimates the nearshore bathymetry with good accuracy (RMSE = 0.39 m) for a beach area extending respectively 1600 m and 600 m in the longshore and cross-shore directions, respectively, suggesting the technical feasibility of coupling remotely-sensed bathymetry to 2DH numerical model, even at short-waves exposed Mediterranean beaches

New algorithms for shoreline monitoring from coastal video systems

Video systems have become widely used all around the world in coastal monitoring strategies, allowing both high temporal and spatial sampling frequency, with low logistic and costs efforts. The present paper deals with a new tool for coastal images processing, aimed at the automatic shoreline detection and data analysis. The tool is composed by a shoreline detection routine implemented in a web-application, addressed at images processing (i.e. shoreline extraction and geo-rectification), data analysis and sharing results about beach actual state and shore evolution in quasi-real time. The Shoreline Detection Model (SDM) is based on a new algorithm, implementing image-processing procedures, which allows extracting the sea/land boundary from automatic segmented Timex images. The SDM calibration and validation has been performed on different coastal images derived from a video monitoring system installed at Alimini (Lecce, IT) in 2005, by comparing automatic shoreline contours with the manual detected ones. Moreover, in December 2015, new video monitoring systems were installed in South Italy (Porto Cesareo and Torre Canne, Apulia region), at sandy beaches affected by erosion phenomena. The application of the SDM on images recorded by the new systems has allowed testing the model feasibility at sites characterized by different morphological features and geographical exposition. The present describes in detail the SDM algorithm and the image processing procedures used. The results of the model calibration and validation performed at Alimini and the tests performed at Porto Cesareo on first images are reported

Laboratory Investigation on the Evolution of a Sandy Beach Nourishment Protected by a Mixed Soft–Hard System

A new experimental campaign on a 2D movable-bed physical model, reproducing a typical nourishment sandy beach profile, is being carried out in the wave flume of the Laboratory of Coastal Engineering at Politecnico di Bari (Bari, Italy). The main aim is to assess the short-term evolution of a sandy beach nourishment, relying on a mixed solution built on the deployment of a Beach Drainage System (BDS) and a rubble-mound detached submerged breakwater. This paper aims at illustrating the experimental findings. Tests presented herein deal with both unprotected and protected configurations, focusing on the hydrodynamic and morphodynamic processes under erosive conditions. Results show that, with respect to the unprotected conditions, BDS reduces the shoreline retreat and the beach steepen within swash and surf zone as well. Moreover, a reduction of net sediment transport rate is observed. When BDS is coupled with the submerged sill, a reversal of the prevalent direction of the net sediment transport seaward occurs offshore the sheltered region. Less considerable positive effects on shoreline retreat are induced by the submerged structure, whereas the mean beach slope remains quite stable. Secondary effects of drain on the submerged sill performance are also highlighted. BDS reduces wave-induced setup on beach, by mitigating the mean water level raising, typically experienced by such structures