Quantifying uncertainties of sandy shoreline change projections as sea level rises

Abstract

Sandy shorelines are constantly evolving, threatening frequently human assets such as buildings or transport infrastructure. In these environments, sea-level rise will exacerbate coastal erosion to an amount which remains uncertain. Sandy shoreline change projections inherit the uncertainties of future mean sea-level changes, of vertical ground motions, and of other natural and anthropogenic processes affecting shoreline change variability and trends. Furthermore, the erosive impact of sea-level rise itself can be quantified using two fundamentally different models. Here, we show that this latter source of uncertainty, which has been little quantified so far, can account for 20 to 40% of the variance of shoreline projections by 2100 and beyond. This is demonstrated for four contrasting sandy beaches that are relatively unaffected by human interventions in southwestern France, where a variance-based global sensitivity analysis of shoreline projection uncertainties can be performed owing to previous observations of beach profile and shoreline changes. This means that sustained coastal observations and efforts to develop sea-level rise impact models are needed to understand and eventually reduce uncertainties of shoreline change projections, in order to ultimately support coastal land-use planning and adaptation.

Beach adaptation to intraseasonal sea level changes

“…Multi-scale coastal evolution, due to sea level variations, remains poorly known. As noted earlier, only the sea-level rise is well known to lead to a recession of the shoreline at decadal to centenary scales (Passeri et al 2014, Le Cozannet et al 2019. At shorter time scales, from hours to years, sea level variations also influence the beach variability.…”

“…Global seal-level rise is well known to lead to a recession of the shoreline (Bruun 1954, 1962, 1983, 1988, Rosati et al 2013, Shand et al 2013, Dean and Houston 2016, Le Cozannet et al 2016, Atkinson et al 2018. It is an important contributor to erosion hotspots at decadal to centenary scales (Zhang et al 2004, Nicholls and Cazenave 2010, Passeri et al 2014, Le Cozannet et al 2019. Conventionally, it is thought that at shorter time scales, from hours to years, waves, tides, sedimentary processes, and anthropogenic factors drive beach changes that surpass sea level impact (Stive 2004, Ranasinghe 2016, Anthony et al 2019.…”

A holistic modelling approach to simulate catchment-estuary-coastal system behaviour at macro-time scales

“…This could be achieved via the global sensitivity analysis approach presented by Sobol′ (2001). One application of this method to coastline projection is presented by Le Cozannet et al (2019). However, due to time limitations, such an analysis could not be completed in the present study, and this is recommended as a Future research direction in Section 5.3.…”

“…In this regard, a vast majority of the world's coastlines fall in the 'open coast' category, in which the coastlines are exposed to the direct impacts from the oceans. These open coasts contain estuaries, cliffed and sandy coasts and gravel beaches, in which open sandy coasts comprise about one-third of world's coastlines (Le Cozannet et al, 2019;Luijendijk et al, 2018;Ranasinghe, 2016), and are extensively subjected to human utilization (Davenport and Davenport, 2006). Sandy coasts are considered to be one of the most complex coastal systems with respect to its geomorphological behaviour because they are continually varying due to the influences of both natural and anthropogenic drivers (Anthony et al, 2015;Besset et al, 2019;Ranasinghe, 2016;Stive, 2004;Toimil et al, 2017).…”

“…There are significant uncertainties in future climate change and anthropogenic activities driven impacts that would affect the shoreline changes along sandy coasts (Le Cozannet et al, 2019;Ranasinghe, 2016). As a result, (in addition to the uncertainties associated with the modelling technique(s) adopted, (i.e., forcing uncertainties)), projections of future changes along inlet-interrupted coastlines will inherit the variabilities related to the climate change impacts and anthropogenic activities (i.e., input uncertainties) considered.…”