A Holistic Modeling Approach to Project the Evolution of Inlet-Interrupted Coastlines Over the 21st Century

Abstract

Approximately one-quarter of the World's sandy beaches, most of which are interrupted by tidal inlets, are eroding. Understanding the long-term (50-100 year) evolution of inletinterrupted coasts in a changing climate is, therefore of great importance for coastal zone planners and managers. This study, therefore, focuses on the development and piloting of an innovative model that can simulate the climate-change driven evolution of inlet-interrupted coasts at 50-100 year time scales, while taking into account the contributions from catchment-estuary-coastal systems in a holistic manner. In this new model, the evolution of inlet-interrupted coasts is determined by: (1) computing the variation of total sediment volume exchange between the inlet-estuary system and its adjacent coast, and (2) distributing the computed sediment volume along the inlet-interrupted coast as a spatially and temporally varying quantity. The exchange volume, as computed here, consists of three major components: variation in fluvial sediment supply, basin (or estuarine) infilling due to the sea-level rise-induced increase in accommodation space, and estuarine sediment volume change due to variations in river discharge. To pilot the model, it is here applied to three different catchmentestuary-coastal systems: the Alsea estuary (Oregon, United States), Dyfi estuary (Wales, United Kigdom), and Kalutara inlet (Sri Lanka). Results indicate that all three systems will experience sediment deficits by 2100 (i.e., sediment importing estuaries). However, processes and system characteristics governing the total sediment exchange volume, and thus coastline change, vary markedly among the systems due to differences in geomorphic settings and projected climatic conditions. These results underline the importance of accounting for the different governing processes when assessing the future evolution of inlet-interrupted coastlines.

Probabilistic Application of an Integrated Catchment-Estuary-Coastal System Model to Assess the Evolution of Inlet-Interrupted Coasts Over the 21st Century

“…The reduced complexity model used here is described in detail by Bamunawala et al (2020), and therefore, only a summary is presented below. In this model (G-SMIC), which is based on the SMIC model originally presented by Ranasinghe et al (2013), the long-term evolution of inlet-interrupted coasts is represented by combining two major components: (1) coastline change due to the variation in total sediment volume exchanged ( V T ) between the estuary and the adjacent inlet-interrupted coast, (2) sea-level rise-driven landward movement of the coastline (i.e., the Bruun effect).…”

“…where V T is the cumulative change in the total sediment volume exchange between the estuary and its adjacent coast, V BI is the sediment demand of the basin due to sea-level rise-driven change in basin volume (i.e., basin infilling volume), V BV is the change in basin volume due to variation in river discharge, and V FS is the change in fluvial sediment supply due to combined effects of climate change and anthropogenic activities, with all volumes in m 3 . A brief description of the three sediment volume components of equation [1]is given below [for detailed derivations, please see Ranasinghe et al (2013) and Bamunawala et al (2020)].…”

“…Additionally, inlet-interrupted coasts will undergo further coastal recession due to sea-level rise driven basin infilling as well (Stive et al, 1990(Stive et al, , 1998Stive, 2004;Ranasinghe et al, 2013). Along with these influences, future changes in temperature, precipitation and anthropogenic activities at catchment scale will alter the fluvial sediment supply received by the coasts (Syvitski and Milliman, 2007;Overeem and Syvitski, 2009;Syvitski et al, 2009;Ranasinghe et al, 2019), which in turn would affect sedimentation patterns, including beach behavior on inletinterrupted coasts (Bamunawala et al, 2018a(Bamunawala et al, , 2020.…”

“…Even if such a multi-scale highly detailed modeling technique were developed, the computational demand and the simulation time per each model realization would likely to make it impractical to be used in a probabilistic framework to estimate the coastline changes along inlet-interrupted coasts (Ranasinghe, 2016. These drawbacks can be overcome via the use of reducedcomplexity models, which have proven to be very useful in obtaining insights into long-term coastal zone evolution at regional scales at low computational cost (Ranasinghe, 2016van Maanen et al, 2016;Bamunawala et al, 2020). Due to their computational efficiency (compared to highly detailed models), reduced complexity models can be easily applied within a probabilistic framework to quantify the uncertainties in future changes along inlet-interrupted coastlines.…”

“…Here, a novel probabilistic modeling framework is presented to quantify the input uncertainties associated with projections derived from the reduced complexity model developed and demonstrated (albeit in deterministic mode) by Bamunawala et al (2020). To enable a direct comparison of the two modeling approaches (i.e., deterministic vs. probabilistic), the probabilistic approach presented here is applied to the same coastal systems used by Bamunawala et al (2020).…”