A Holistic Modelling Approach to Simulate Catchments-Estuary-Coastal System Behaviour at Macro Time Scales

Probabilistic assessment of the long-term evolution of inlet-interrupted coasts has always been a challenge, thus continuing to remain a significant knowledge gap. Therefore, this research study was undertaken to gain new insights into the climate change (CC) driven evolution of inlet-interrupted coasts at macro time scales in a probabilistic manner, while giving due consideration to terrestrial and oceanic processes. Here, this was achieved through the development and application of a reduced-complexity (RC) model at a number of locations around the world. Application of this RC model to four sites in USA, UK, Sri Lanka and Australia shows that the long-term evolution of coastlines would vary markedly. These variations are linked to the catchment size, basin surface area and arid/non-arid nature of CC projection. Broader application of the RC model to 41 inlet systems worldwide shows that 93% of the inlet-interrupted coasts will erode under changing climate. The projections also show that the long-term evolution of inlet-interrupted coastlines may not always be governed by the CC-induced sea-level rise, as commonly believed, but may at some locations be governed by the terrestrial processes. Uncertainties in the model projections emphasize the need for probabilistic approaches to investigate the long-term evolution of inlet-interrupted coasts

Impact of ebb-delta dynamics on shoreline evolution along inlet-interrupted coasts

Shorelines adjacent to tidal inlets are highly dynamic landforms affected by oceanic (e.g., sea-level rise) and terrestrial (e.g., fluvial sediment supply) processes. Climate change is thus expected to have substantial physical impacts on these inlet-interrupted coasts. Numerical simulation of such impacts requires a holistic approach where at least the major governing processes that affect the local sediment budget are considered. The Generalized-Scale-aggregated Model for Inlet-interrupted Coasts (i.e., G-SMIC) is such a model that is capable of holistically simulating the evolution of inlet-interrupted coasts over multi-decadal to century time periods. However, in its present form, G-SMIC does not consider the effects of ebb-delta dynamics in its computations. Here, we improve the model to include ebb-delta dynamics and pilot the improved model (G-SMIC+) at two selected case study sites in Vietnam (Thu Bon estuary) and Wales, United Kingdom (Mawddach estuary). Model hindcasts of G-SMIC+ at both case study locations show reasonable agreement with available records of shoreline variations. The evolution of the two inlet-estuary systems was assessed over the 21st century under four of the IPCC’s sixth assessment report climate scenarios (viz., SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5). Results show that both systems switch between sediment exporting and sediment importing systems over the study period (2031 - 2100). Moreover, while the inclusion of ebb-delta dynamics may decrease the erosion volumes of the up-drift shoreline by up to 37% and 46% at Thu Bon and Mawddach estuaries, respectively (by 2100, relative to 2030), the down-drift coast is only affected in a noticeable way at the Mawddach estuary, where the accretion volume is projected to reduce by ~50%. As a result, the ebb-delta effect decreases the up-drift shoreline retreat by up to 37% and 48% at Thu Bon and Mawddach estuaries, respectively, while it reduces shoreline progradation of the down-drift coast of Mawddach estuary by up to ~50%. These results highlight the importance of including ebb-delta dynamics in modelling efforts to assess the climate change responses of inlet-interrupted coasts worldwide

Comparison of process-based and lumped parameter models for projecting future changes in fluvial sediment supply to the coast

Fluvial sediment supply (FSS) is one of the primary sources of sediment received by coasts. Any significant change in sediment supply to the coast will disturb its equilibrium state. Therefore, a robust assessment of future changes in FSS is required to understand the coastal system’s status under plausible climatic variations and human activities. Here, we investigate two modelling approaches to estimate the FSS at two spatially heterogeneous river basins: the Irrawaddy River Basin (IRB), Myanmar and the Kalu River Basin (KRB), Sri Lanka. We compare the FSS obtained from a process-based model (i.e., Soil Water Assessment Tool: SWAT) and an empirical model (i.e., the BQART model) for mid- (2046–2065) and end-century (2081–2100) periods under climate change and human activities (viz, planned reservoirs considered here). Our results show that SWAT simulations project a higher sediment load than BQART in the IRB and vice versa in KRB (for both future periods considered). SWAT projects higher percentage changes for both future periods (relative to baseline) compared to BQART projections in both basins with climate change alone (i.e., no reservoirs) and vice versa when planned reservoirs are considered. The difference between the two model projections (from SWAT and BQART) is higher in KRB, and it may imply that empirical BQART model projections are more in line with semi-distributed SWAT projections at the larger Irrawaddy River Basin than in the smaller Kalu River Basin

Assessing Climate Change Impacts on the Stability of Small Tidal Inlets: Part 2- Data Rich Environments

Climate change (CC) is likely to affect the thousands of bar-built or barrier estuaries (here referred to as Small tidal inlets - STIs) around the world. Any such CC impacts on the stability of STIs, which governs the dynamics of STIs as well as that of the inlet-adjacent coastline, can result in significant socio-economic consequences due to the heavy human utilisation of these systems and their surrounds. This article demonstrates the application of a process based snap-shot modelling approach, using the coastal morphodynamic model Delft3D, to 3 case study sites representing the 3 main STI types; Permanently open, locationally stable inlets (Type 1), Permanently open, alongshore migrating inlets (Type 2) and Seasonally/Intermittently open, locationally stable inlets (Type 3). The 3 case study sites (Negombo lagoon - Type 1, Kalutara lagoon - Type 2, and Maha Oya river - Type 3) are all located along the southwest coast of Sri Lanka. After successful hydrodynamic and morphodynamic model validation at the 3 case study sites, CC impact assessment are undertaken for a high end greenhouse gas emission scenario. Future CC modified wave and riverflow conditions are derived from a regional scale application of spectral wave models (WaveWatch III and SWAN) and catchment scale applications of a hydrologic model (CLSM) respectively, both of which are forced with IPCC Global Climate Model output dynamically downscaled to approximately 50 km resolution over the study area with the stretched grid Conformal Cubic Atmospheric Model CCAM. Results show that while all 3 case study STIs will experience significant CC driven variations in their level of stability, none of them will change Type by the year 2100. Specifically, the level of stability of the Type 1 inlet will decrease from 'Good' to 'Fair to poor' by 2100, while the level of (locational) stability of the Type 2 inlet will also decrease with a doubling of the annual migration distance. Conversely, the stability of the Type 3 inlet will increase, with the time till inlet closure increasing by approximately 75%. The main contributor to the overall CC effect on the stability of all 3 STIs is CC driven variations in wave conditions and resulting changes in longshore sediment transport, not Sea level rise as commonly believed

Twenty-first-century projections of shoreline change along inlet-interrupted coastlines

Sandy coastlines adjacent to tidal inlets are highly dynamic and widespread landforms, where large changes are expected due to climatic and anthropogenic influences. To adequately assess these important changes, both oceanic (e.g., sea-level rise) and terrestrial (e.g., fluvial sediment supply) processes that govern the local sediment budget must be considered. Here, we present novel projections of shoreline change adjacent to 41 tidal inlets around the world, using a probabilistic, reduced complexity, system-based model that considers catchment-estuary-coastal systems in a holistic way. Under the RCP 8.5 scenario, retreat dominates (90% of cases) over the twenty-first century, with projections exceeding 100 m of retreat in two-thirds of cases. However, the remaining systems are projected to accrete under the same scenario, reflecting fluvial influence. This diverse range of response compared to earlier methods implies that erosion hazards at inlet-interrupted coasts have been inadequately characterised to date. The methods used here need to be applied widely to support evidence-based coastal adaptation

Significance of Fluvial Sediment Supply in Coastline Modelling at Tidal Inlets

The sediment budget associated with future coastline change in the vicinity of tidal inlets consists of four components; sea level rise-driven landward movement of the coastline (i.e., the Bruun effect), basin infilling effect due to sea level rise-induced increase in accommodation space, basin volume change due to variation in river discharge, and coastline change caused by change in fluvial sediment supply. These four components are affected by climate change and/or anthropogenic impacts. Despite this understanding, holistic modelling techniques that account for all the aforementioned processes under both climate change and anthropogenic influences are lacking. This manuscript presents the applications of a newly-developed reduced complexity modelling approach that accounts for both climate change and anthropogenically-driven impacts on future coastline changes. Modelled results corresponding to the year 2100 indicate considerable coastline recessions at Wilson Inlet (152 m) and the Swan River system (168 m) in Australia and Tu Hien Inlet (305 m) and Thuan An Inlet (148 m) in Vietnam. These results demonstrate that coastline models should incorporate both climate change and anthropogenic impacts to quantify future changes in fluvial sediment supply to coasts to achieve better estimates of total coastline changes at tidal inlets. Omission of these impacts is one of the major drawbacks in all the existing coastline models that simulate future coastline changes at tidal inlets. A comparison of these modelled future coastline changes with the predictions made by a relevant existing modelling technique (Scale Aggregated Model for Inlet-interrupted Coasts (SMIC)) indicates that the latter method overestimates total coastline recessions at the Swan River system, and the Tu Hien and Thuan An Inlets by 7%, 10%, and 30%, respectively, underlining the significance of integrating both climate change and anthropogenic impacts to assess future coastline changes at tidal inlets

A Holistic Modelling Approach to Simulate Catchments-Estuary-Coastal System Behaviour at Macro Time Scales

Probabilistic assessment of the long-term evolution of inlet-interrupted coasts has always been a challenge, thus continuing to remain a significant knowledge gap. Therefore, this research study was undertaken to gain new insights into the climate change (CC) driven evolution of inlet-interrupted coasts at macro time scales in a probabilistic manner, while giving due consideration to terrestrial and oceanic processes. Here, this was achieved through the development and application of a reduced-complexity (RC) model at a number of locations around the world. Application of this RC model to four sites in USA, UK, Sri Lanka and Australia shows that the long-term evolution of coastlines would vary markedly. These variations are linked to the catchment size, basin surface area and arid/non-arid nature of CC projection. Broader application of the RC model to 41 inlet systems worldwide shows that 93% of the inlet-interrupted coasts will erode under changing climate. The projections also show that the long-term evolution of inlet-interrupted coastlines may not always be governed by the CC-induced sea-level rise, as commonly believed, but may at some locations be governed by the terrestrial processes. Uncertainties in the model projections emphasize the need for probabilistic approaches to investigate the long-term evolution of inlet-interrupted coasts

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

Probabilistic assessment of the long-term evolution of inlet-interrupted coasts has always been a challenge, thus continuing to remain a significant knowledge gap. Therefore, this research study was undertaken to gain new insights into the climate change (CC) driven evolution of inlet-interrupted coasts at macro time scales in a probabilistic manner, while giving due consideration to terrestrial and oceanic processes. Here, this was achieved through the development and application of a reduced-complexity (RC) model at a number of locations around the world. Application of this RC model to four sites in USA, UK, Sri Lanka and Australia shows that the long-term evolution of coastlines would vary markedly. These variations are linked to the catchment size, basin surface area and arid/non-arid nature of CC projection. Broader application of the RC model to 41 inlet systems worldwide shows that 93% of the inlet-interrupted coasts will erode under changing climate. The projections also show that the long-term evolution of inlet-interrupted coastlines may not always be governed by the CC-induced sea-level rise, as commonly believed, but may at some locations be governed by the terrestrial processes. Uncertainties in the model projections emphasize the need for probabilistic approaches to investigate the long-term evolution of inlet-interrupted coasts

Littoral Drift Impoundment at a Sandbar Breakwater: Two Case Studies along the Bight of Benin Coast (Gulf of Guinea, West Africa)

This study assessed the deposition of sediment and shoreline evolution at two newly constructed port facilities in the Bight of Benin, West Africa. Based on the Building with Nature approach, the concept of a sandbar breakwater was implemented at the study sites. The coastal system of the bight is characterized by a sand barrier-lagoon system and a uniform prevailing wave climate, making it a favorable location for this innovative port solution. The case studies were undertaken at the Port of Lomé, Togo, and the Lekki Deep Sea Port (Dangote Sea Port), Nigeria, using remotely sensed shoreline positions and the one-line coastline change model for different periods. After construction of the breakwater, we estimated that the updrift coastline at the two locations accreted in the range of 10–23 m/year and the rates of sediment deposition were estimated to be in the magnitude of 1.0–7.0 × 105 m3/year. The comparative study conducted also showed that these rates could further reach a magnitude of 106 m3/year at other sediment-accreting landforms within the bight. We found that these large magnitudes of longshore sediment transport generated from very oblique incident waves (10°–20°) and sediment input from rivers (in orders of 106 m3/year) have enabled the realization of expected morphodynamic changes on the updrift shoreline of the ports. From these results, downdrift morphological changes should not be underestimated due to potential imbalances induced in the sedimentary budget along the coastline. Future developmental plans within the bight should also continuously aim to adopt nature-based solutions to protect the ecosystem while mitigating unforeseen implications