Uncertainties in Sandy Shorelines Evolution under the Bruun Rule Assumption

Abstract

International audienceIn 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 low-and high-energy coasts. Hence, we incorporate uncertainties regarding the impacts of longshore processes, sea-level rise, storms, aeolian, and other cross-shore 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 defenses, 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, if greenhouse gas concentration in the atmosphere are not drastically reduced (scenarios RCP 4.5, 6.0, and 8.5)

Controls of local geology and cross-shore/longshore processes on embayed beach shoreline variability

“…With both model accuracy and computational power increasing, it is now feasible to simulate multidecadal shoreline change in complex coastal settings. At these timescales sea level rise contribution becomes significant (Le Cozannet et al, 2016) although largely uncertain (Le Cozannet et al, 2019). New developments are being carried out in LX-Shore to account for the effect of sea level change on shoreline variability (Robinet et al, in revision).…”

“…On longer timescales, other processes such as changes in sediment supply (Thom, 1983) and sea level rise (e.g. Bruun, 1962;Le Cozannet et al, 2016) become important, as well as longshore processes. However, the typical timescales associated with these processes mostly differ on embayed beaches, where inherent geology (rocky headlands, submerged outcrops and rocky platforms extending from the headlands) greatly complicates shoreline response, with longshore processes also impacting shoreline response on short timescales (Ojeda and Guillen, 2008).…”

Impact of model free parameters and sea‐level rise uncertainties on 20‐years shoreline hindcast: the case of Truc Vert beach (SW France)

“…Most studies addressing uncertainties of shoreline behaviour at decadal timescales rely on data‐driven approaches, based on extrapolated observations or empirical models calibrated with measured data (Allenbach et al, 2015; Casas‐Prat et al, 2016; Le Cozannet et al, 2016, 2019; Toimil et al, 2017; Ranasinghe, 2020). Reduced‐complexity models (RCMs) have been proposed as a way forward to increase the reliability of long‐term shoreline modelling (Yates et al, 2009; Davidson et al, 2013; Castelle et al, 2014; Splinter et al, 2014; Vitousek et al, 2017b; Robinet et al, 2018; Hallin et al, 2019).…”

Beach adaptation to intraseasonal sea level changes

“…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.…”