Background

Basin-infilling (i.e., the sediment volume needed by an estuary to satisfy the sea-level-rise-driven increase in its accommodation space) is a key process that needs to be considered when assessing the long-term evolution of inlet-adjacent coastlines. Due to the differences in the time scales involved, there is a time lag between hydrodynamic forcing (viz., sea-level rise) and morphodynamic response (viz., basin infilling) at tidal inlet systems. Our work reveals that the time lag between sea-level rise and basin infilling directly depends on the estuarine surface area. We derive indicative scale-dependent factors (M) accounting for the time lag between sea-level rise and basin infilling.

Why Is It Important to Consider the Time Lag between Sea-Level Rise and Basin Infilling at Tidal Inlets?

Throughout the past, inlet-adjacent coastlines have supported livelihoods and provided habitat for a vast majority of the population. The projected changes in the climatic conditions over the twenty-first century and beyond will likely result in substantial modifications of these densely populated coastlines. Such climate-change-driven changes will inevitably manifest into substantial socio-economic impacts. Therefore, it is essential to understand how these inlet-adjacent coastlines may evolve over century-scale periods to inform management strategies geared to minimise potential socio-economic damages¹. Such long-term assessments consider the sediment volume exchanged between tidal-inlet systems and their adjacent coastlines^2–4. In numerical model simulations, the sediment volume due to basin infilling, more often than not, is found to be the dominating driver of coastline change⁵. To date, one single lag factor value (M = 0.50) has been used to represent the time lag between sea-level rise and basin infilling volume for all estuaries, which may lead to inaccuracies in numerical model projections of inlet-adjacent shorelines. The consequences of such errors may be enormous, with the potential to significantly affect the socio-economic well-being of societies, especially if shoreline retreats are underpredicted.

What Did We Discover?

The lag factor (M) depends on the estuary's surface area.

The lag factor (M) between sea-level rise and basin infilling depends on the estuary's surface area. Based on a quality-controlled estuary dataset, here, we clustered the inlet-estuary systems into three classes by considering the surface area (A_B). For the first time, using analytical considerations, we derive indicative optimal lag factors (i.e., M values) for the above-identified three estuary classes.

The use of M values other than an optimal lag factor may result in significant errors in shoreline change projections over the twenty-first century.

We tested the consequences of the use of sub-optimal lag factors on the 21^st-century projections of inlet-adjacent shoreline changes by applying a probabilistic, reduced-complexity model (viz., G-SMIC)⁶, under four IPCC AR6 climate scenarios to each of the above-identified three tidal-inlet system types over the mid-century (2056 - 2065) and end-century (2091 – 2100) periods.

References

1. Toimil, A., Losada, I. J., Nicholls, R. J., Dalrymple, R. A. & Stive, M. J. F. Addressing the challenges of climate change risks and adaptation in coastal areas: A review. Coastal Engineering 156, 103611 (2020).
2. Ranasinghe, R., Duong, T. M., Uhlenbrook, S., Roelvink, D. & Stive, M. Climate-change impact assessment for inlet-interrupted coastlines. Nat Clim Chang 3, 83–87 (2013).
3. Ranasinghe, R. On the need for a new generation of coastal change models for the 21st century. Sci Rep 10, 2010 (2020).
4. Bamunawala, J. et al. A Holistic Modeling Approach to Project the Evolution of Inlet-Interrupted Coastlines Over the 21st Century. Front Mar Sci 7, 542 (2020).
5. Bamunawala, J. et al. Twenty-first-century projections of shoreline change along inlet-interrupted coastlines. Sci Rep 11, 14038 (2021).
6. Bamunawala, J. et al. Probabilistic Application of an Integrated Catchment-Estuary-Coastal System Model to Assess the Evolution of Inlet-Interrupted Coasts Over the 21st Century. Front Mar Sci 7, 1104 (2020).