GM6.7

Dikes are the conventional means of flood defence along rivers and estuaries. However, dikes gradually lead to the superelevation of waterbodies, and the common method of enforcing dikes is unsustainable as this is expensive, contends with limited space for urbanisation and may aggravate ecological deterioration. Therefore, future flood management requires new, sustainable strategies that not only minimise flood risk, but also steer land-level rise and improve ecology. An example is controlled floodbasins, where a part of land is temporarily opened to the tide to capture sediment and rise well above mean sea-level. This study explores how the sequence of opening controlled floodbasins affects sediment capture and large-scale estuary dynamics through 2D modelling in Delft3D. To this end, different floodbasin configurations and delays of opening floodbasins were tested along the Western Scheldt Estuary (NL). Findings show land-level rise in all configurations. However, opening more floodbasins results in a lag of muddy sediment capture in floodbasins opened later in a sequence, most likely due to a deficit of fines. Opening of the more landward located floodbasins generally leads to a stronger reduction in tidal range if opened alone or at the start of an opening sequence compared to more seaward located floodbasins. Also, the floodbasins seem to result in stronger erosion and deposition patterns in the estuary seaward of the floodbasin inlets, but it is still unclear whether and how this trend influences the channel migration rate. The results imply that a well-chosen location and timing of opening floodbasins, which may vary for different estuaries, can have a positive impact on reducing flood risk.

How to cite: Weisscher, S., Baar, A., and Kleinhans, M.: Controlled floodbasins: driving land-level rise along estuaries, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11058, https://doi.org/10.5194/egusphere-egu22-11058, 2022.

Artificial beach nourishment has become a common practice in coastal engineering. On sandy beaches, initiatives are emerging concerning pebble nourishment. Indeed, by increasing the granulometry, coastal managers hope to reduce shoreline recession more significantly than what would have been achieved by artificial sand nourishment.

In this presentation we estimate the effectiveness and impact of artificial pebble nourishment on two microtidal beaches along the French Mediterranean coastline: (1) exposed to wave in Camargue (annual Hsig = 3.5m)  and (2) subjected to low wave (annual Hisg < 2.0m) along the shoreline of the Etang de Berre. In both cases the beach has been recharged in order to widen it. On the beach exposed to waves, the nourishment did not significantly slow the retreat of the shoreline, whereas it succeeded in stabilizing it on the less exposed beach. In the Camargue, pebble deposits reworked by waves take the form of overwash. In both cases, the pebbles are subject to the dominant longshore littoral drift which exports the pebbles outside the nourished zone. We do not observe any significant morphological evolution of the surf zone, although we would expect an increase in slope in relation to the morphology that has become rather reflective in the swash zone. The pebbles remain on the foreshore and beach without moving offshore.

How to cite: sabatier, F.: Artificial pebbles nourhisment on microtidal sandy beaches, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12335, https://doi.org/10.5194/egusphere-egu22-12335, 2022.

To study on a regional basis, the relation between fluvial sediment delivery and coastal erosion, the historical record of coastline migration of Sicily was analyzed with respect to the estimated sediment delivery to the coast obtained from the spatially distributed sediment delivery WaTEM/SEDEM model. The latter was directly acquired from the ESDAC database as a 25 m pixel layers, being based on the combination between the RUSLE model and a transport capacity routing algorithm.

At the same time, the coastline-evolution (accretion/retreatment) data for 1960/1994 and 1994/2012 intervals were processed. This dataset, provided by ISPRA (Italian Institute for Environmental Protection and Research), is made by vectorial polygons, corresponding to erosion or accretion areas obtained by the intersection between two coastlines. The dataset contains polygons related to the 1960-1994 and 1994-2012 periods.

Once a common baseline was extracted from 2019 satellite images, 22 Physiographic Units (PU) were identified. The PU was defined based on geomorphologic criteria and by assuming a null net sediment budget (null sediment transport between two PU neighboring). Each coastal PU was connected to its contributing fluvial basins, also assigning the expected sediment delivery at the coastline.

To perform the analysis, cross profiles along the coastline were generated and intersected with the polygons, calculating a response value, in terms of retreatment or accretion, to each of the cross-profile centroids. Finally, for each PU, the cumulated variations were computed.

PUs with significant cumulative variations (more than 2 km) in at least one of the two epochs were identified and three different patterns were detected: accretion/retreatment, retreatment/accretion, and retreatment/retreatment. The response observed for the different PUs was then analyzed considering estimated sediment delivery, recognizing coherent (large sediment delivery = accretion) and incoherent (large sediment delivery = retreatment) behaviors, which have been interpreted as controlled by the history of soil/coastal erosion management practices.

In particular, in spite of a very high expected sediment delivery, more than three-quarters of the Tyrrhenian coast resulted as affected by a marked retreat in 60-94 (same tens of meters) and a moderate accretion in 94-12, as the result of extensive coastal works which have been realized to mitigate coastal erosion. 

How to cite: Azzara, G., Manno, G., Martinello, C., Lo Re, C., Mercurio, C., Basile, M., Ciraolo, G., and Rotigliano, E.: A regional approach for exploring the relation between sediment transport and coastal erosion in Sicily., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5295, https://doi.org/10.5194/egusphere-egu22-5295, 2022.

The coasts of Greenland mostly consist of hard rock with relatively small rates of erosion. However, isostatic uplift following the retreat of Holocene ice sheets is creating soft sediment bluffs consisting of deltas or beach ridges. Hitherto, very little has been known about the rates and processes of soft sediment bluff erosion along the coast of Greenland. Here, we investigate a bluff section at the south coast of Disko Island in western Greenland. The height of the bluff ranges from one to 30 m along a three km long section of the coast. The bluff consists of a heterogeneous matrix of hard rock outcrops with pockets of coarse clastic sediments in between, overlain by parallel beach ridges with discontinuous permafrost. Sea ice along the coast limits wave activity between December and May.

A series of oblique aerial images from July 2019 and July 2021 was obtained to create Structure from Motion Multi-View Stereo (SfM MVS) point clouds. Changes were detected by multi-scale model-to-model cloud comparison (M3C2). Climate data from the nearby town of Qeqertarsuaq were used to identify precipitation events to estimate erosion events at the bluff. This data were utilized in conjunction with satellite derived bathymetry and wave data to estimate wave run-up and erosion at the coastline for a series of hydrodynamic conditions.

We find the absence of soil on top of the uplifted beach ridges strongly influences runoff patterns. Without the water retention capability of the soil, water directly infiltrates into the soft sediments, or runs off the surface of the hard rock areas. This leads to an accumulation of water in the soft sediment pockets and gullies, making them especially vulnerable to erosion. The sedimentary bluff is eroding by two coupled processes: (i) Precipitation-driven surface runoff downslope the bluff face and (ii) wave-driven erosion at the bluff base of sediment that is delivered by the surface runoff. Typical erosion rates are up to half a meter per year. Longer ice-free periods with extended wave action should further increase coastal bluff erosion rates in Greenland in the future.

How to cite: Luetzenburg, G., Townsend, D., Svennevig, K., Bendixen, M., Bjørk, A. A., Eidam, E. F., and Kroon, A.: Arctic Coastal Bluff Erosion on Disko Island, Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3470, https://doi.org/10.5194/egusphere-egu22-3470, 2022.

The Baltic Sea is a semi-enclosed sea that connects with the Global ocean through the Danish straits. The Lithuanian coast of the Baltic Sea is a generic type of almost straight, relatively high-energy, actively developing coasts that (1) contain a large amount of finer, mobile sediment, (2) are open to predominating wind and wave directions, and (3) are exposed to waves from a wide range of directions.

The combination of angular wind distribution and coastal geometry is such that the longshore sediment transport caused by waves is, on average, to the north through the entire Curonian Spit and the mainland coast of Lithuania. This predominant sediment flow means that sediment availability or transportation changes in these areas significantly impact the sediment budget north of Klaipeda. While sediment flows along the spit predominantly occur under natural conditions, further sediment transport to Lithuania's mainland coast is obstructed by jetties and breakwaters of Klaipeda Port, out-flowing currents from Klaipeda Strait, dredging of the port entrance channel, and other factors.

Knowledge of the cross-shore distribution of longshore sediment transport in the surf zone is necessary to design and plan groins, jetties, weirs, and pipeline landfalls.

Accurate estimation of the longshore sediment transport distribution helps understand spit development, migration of sediments, natural or artificial, and the development of other coastal morphologic features.

How to cite: Sakurova, I., Kondrat, V., Baltranaite, E., and Kelpsaite-Rimkiene, L.: Estimation of longshore sediment transport: the case of Lithuania, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2552, https://doi.org/10.5194/egusphere-egu22-2552, 2022.

Coastal areas in Thailand are largely under threat from flooding as a result of a range of factors including coastal erosion, coastal land subsidence, sea level rise and climate change. The erosion of coastal areas in Thailand has been a huge concern to national and local authorities as 17% of the Thai population (approximately 11 million people) live on the coastal plains and a large part of the Thai economy revolves around the exploitation of coastal resources via tourism, fishing or farming.

A recent study estimated a mangrove forest loss of just over 45% between 1961 and 1996 (Sampantamit et al., 2020), while measurements of the shoreline evolution in 2011 by the Thai Department of Marine and Coastal Resources (DMCR) revealed high rates of erosion (>5m per year) along long stretches of sandy beaches of Southern Thailand. As a response to those emergencies, Thailand has invested a lot of money in protecting and restoring their coastal mangroves since the mid-90s, and more recently financed large realignment or replenishment plans for their beaches.

This study assessed the long-term state of coastal evolution of both the Krabi and Nakhon Si Thammarat coastlines (560 km of coastline approximately) in southern Thailand. This was undertaken using the toolkit Coastsat to digitise a time series of shoreline positions from freely available satellite images between 1990 and 2019. . Based on these digitised shorelines and the use of the software DSAS, it was possible to identify shoreline change, which varied between -66 to +16.4 m/y in the mangroves of Nakhon Si Thammarat and -22.2 to +10.6 m/y on its sandy beaches. Shoreline change rates along the Krabi coast varied -34.5 to +21.7 m/year in the mangroves and -4.1 to +4 m/y on sandy beaches.

The analysis of the spatial and temporal variations of the shoreline position during the survey period reveals, in some places along the Nakhon Si Thammarat coastline, how efficient coastal defence work has been. This work also revealed the synchronicity between large and sudden coastal erosional movements and the occurrence of typhoons or tropical storms coming from the Sea of China, highlighting the importance of extreme weather events on sediment remobilisation on the Eastern coast of Southern Thailand.

This work also supports the use of freely available semi-automated toolkits such as CoastSat to deliver crucial and reliable time series shoreline data over extensive areas. The relevance of those newly developed tools is emphasised by the current COVID 19 travel bans and restrictions, which limit travel abroad for coastal managers and researchers to. The ability to collect, visualise and analyse remotely large datasets of environmental data has been essential over the last two years.

How to cite: Curoy, J., Ward, R., Barlow, J., Moses, C., and Nakhapakorn, K.: Coastal evolution in Southern Thailand between 1990 and 2018: an application of the CoastSat toolkit., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6215, https://doi.org/10.5194/egusphere-egu22-6215, 2022.

How to cite: Pastier, A.-M., Huppert, K., and Malatesta, L.: A global dataset of Holocene reef morphometrics to improve numerical modelling of coral reef development, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12157, https://doi.org/10.5194/egusphere-egu22-12157, 2022.