Thresholds for estuarine compound flooding using a combined hydrodynamic-statistical modelling approach

Estuarine compound flooding can happen when extreme sea level and river discharges occur concurrently, or in close succession, inundating low-lying coastal regions. Such events are hard to predict and amplify the hazard. Recent UK storms, including Storm Desmond (2015) and Ciara (2020), have highlighted the vulnerability of mountainous Atlantic-facing catchments to the impacts of compound flooding including risk to life and short- and long-term socio-economic damages. To improve prediction and early warning of compound flooding, combined sea and river thresholds need to be established. In this study, observational data and numerical modelling were used to reconstruct the historic flood record of an estuary particularly vulnerable to compound flooding (Conwy, North Wales). The record was used to develop a method for identifying combined sea level and river discharge thresholds for flooding using idealised simulations and joint-probability analyses. The results show how flooding extent responds to increasing total water level and river discharge, with notable amplification in flood extent due to the compounding drivers in some circumstances, and sensitivity (∼7%) due to a 3h time lag between the drivers. The influence of storm surge magnitude (as a component of total water level) on the flooding extent was only important for scenarios with minor flooding. There was variability as to when and where compound flooding occurred; it was most likely under moderate sea and river conditions (e.g. 60th-70th and 30th-50th percentiles) and only in the middle-estuary zone. For such cases, joint-probability analysis is important for establishing compound flood risk behaviour. Elsewhere in the estuary, either the sea state (lower estuary) or river flow (upper estuary) dominated the hazard, and single-value probability analysis is sufficient. These methods can be applied to estuaries worldwide to identify site-specific thresholds for flooding to support emergency response and long-term coastal management plans

Impact of dams and climate change on suspended sediment flux to the Mekong delta

The livelihoods of millions of people living in the world's deltas are deeply interconnected with the sediment dynamics of these deltas. In particular a sustainable supply of fluvial sediments from upstream is critical for ensuring the fertility of delta soils and for promoting sediment deposition that can offset rising sea levels. Yet, in many large river catchments this supply of sediment is being threatened by the planned construction of large dams. In this study, we apply the INCA hydrological and sediment model to the Mekong River catchment in South East Asia. The aim is to assess the impact of several large dams (both existing and planned) on the suspended sediment fluxes of the river. We force the INCA model with a climate model to assess the interplay of changing climate and sediment trapping caused by dam construction. The results show that historical sediment flux declines are mostly caused by dams built in PR China and that sediment trapping will increase in the future due to the construction of new dams in PDR Lao and Cambodia. If all dams that are currently planned for the next two decades are built, they will induce a decline of suspended sediment flux of 50% (47–53% 90% confidence interval (90%CI)) compared to current levels (99 Mt/year at the delta apex), with potentially damaging consequences for local livelihoods and ecosystems

Characterising the structure and fluvial drag of emergent vegetation

Plants function as large-scale, flexible obstacles that exert additional drag on surface water flows, affecting local scale turbulence and the structure of the boundary layer. Hence, vegetation plays a significant role in controlling flood water and modulating geomorphic change. This makes it an important, but often under-considered, component when undertaking flood or erosion control actions or designing river restorations strategies. Vegetative drag varies depending on flow conditions and the associated vegetation structure and temporary reconfiguration of the plant. Whilst several approaches have been developed to describe this relationship, they have been limited due to the difficulty of accurately and precisely characterising the vegetation itself, especially during flow. In practice, vegetative drag is commonly expressed through bulk parameters that are typically derived from lookup tables. Terrestrial Laser Scanning (TLS) has the ability to capture the surface of in situ objects as 3D point clouds, at high resolution (mm), precision and accuracy, even when submerged in water. In this study, the potential for characterising the 3D structure of vegetation from high resolution TLS data is explored. Novel methods capable of converting unstructured TLS 3D point clouds to structured 2D and 3D grid arrays are developed enabling the accurate representation of plant structure. These methods are adapted and combined with physical modelling experiments to investigate a series of structurally variable plants at a range of flow scenarios, including the collection of precise hydraulic measurements and capturing of plant deformation during flow. Models capable of predicting vegetation’s fluvial drag from the combination of bulk porous plant structure and extent are developed. Small scale flow characteristics in the vicinity of plant elements, including adjustments of the velocity profile, the extent and intensity of the wake region, the development of secondary flow and adjustments of the turbulent kinetic energy, are quantified and associated with the explicitly characterised plant structure. The results show a promising potential for transferring the methods to field studies. They highlight the potential of employing vegetation in natural flood management applications and can help to inform decisions regarding the choice of plant types to be used for the reforestation of floodplains as well as the optimal plant spacing for a satisfactory control of flood water during floods

An experimental approach to characterising vegetation roughness in forested floodplains using dilution tracing and terrestrial laser scanning

The vegetation on forested floodplains plays an essential role in floodplain flow routing, flow dynamics and therefore the transfer of sediments and associated nutrients and contaminants. The flow resistance introduced by the presence of vegetation assemblages leads to changes in flow velocity and depth that, in turn, modulate sediment transport. Attempts to quantify these processes remain elusive due to difficulties in accurately estimating the altered flow parameters around and through vegetation. Here we employ complementary techniques of characterising the bulk flow properties and vegetation structure. Physical modelling of a prototype scale (2 x 45 m) shallow earthen channel containing small trees, herbaceous plants and leaf-litter was undertaken to explore the bulk flow characteristics. Vegetation structure and fluid retention was measured in controlled discharge experiments for: (1) natural cover of herbs and trees; (2) trees only and; (3) earthen channel only. Dilution-curve data were analysed within the Aggregated Dead Zone (ADZ) framework to yield bulk flow parameters including dispersion, fluid retention and flow resistance parameters. Vegetation structure is recorded using TLS, with data processed to represent vegetation structure as a 3D volume porous media. The main response of flow to vegetation removal was an increase in bulk velocity, with depth and wetted width decreasing slightly. Retentiveness was more prominent during low flow and all three experimental conditions tended to converge on a constant low value for high discharges. Reach mean travel times and the advective time delays decreased very slightly from experiments (1) to (2). The ADZ analysis and TLS data shows that in these two initial experiments, the trees provided the majority of the resistance in contrast to the aggregate effect of herbaceous plants and litter. Removing the trees further decreased travel times such that the ADZ residence time was more than halved moving from condition (1) to (3). The overall bulk flow effect of tree cover on retention is here expressed by the dispersive fraction parameter, which reduced from 0.4 to 0.2 when vegetation was removed

Development and validation of flood inundation models for estuaries

Estuaries, at the interface between catchment and coast, are vulnerable to flooding from the combination of riverine and marine inputs. High river flows generated from intense precipitation can occur synchronously with high tides and storm surges, amplifying flood hazard. In the United Kingdom 20 million people are estimated to live near estuaries, with estuarine flooding regarded as the costliest impact to these areas and second highest hazard to civil emergency. On-going global warming increases sea-levels and modifies hydroclimate variability, thus affecting river fluxes, tidal maxima and the intensify of storm surges. There is therefore a need for improved methods and tools to understand compound flooding events, their impacts and how they may change into the future. In the present paper we developed a validated flood inundation model for the Conwy estuary in North Wales, one of the flashiest catchments in Britain where flooding makes headline news at least once every year. The Caesar-Lisflood 2D hydrodynamic flow model was combined with a range of publicly available datasets to represent channel bathymetry, land elevation, location and heights of flood defences and the hydraulic roughness across the model domain. The model was forced with recorded time-series (15-minute resolution) of tidal oscillations and river discharge data and validated by comparing simulated water levels against observations from existing water level gauges within the estuarine channel. Flood predictions were validated against observed flood extents extracted from SAR imagery using the Google Earth Engine. Calibrated, ortho-corrected (GRD) C-band interferometric Synthetic Aperture Radar (SAR) images captured by the Sentinel-1 constellation of satellites using a dual-band cross-polarization (VH) was used. SAR images were filtered to remove speckle noise and Otsu’s method of thresholding was adopted to automatically extract inundated areas from each available image. Comparison of model-based simulated flood extents against their SAR-derived equivalents was used as a means to validate the flood inundation model

Sand mining far outpaces natural supply in a large alluvial river

The world's large rivers are facing reduced sediment loads due to anthropogenic activities such as hydropower development and sediment extraction. Globally, estimates of sand extraction from large river systems are lacking, in part due to the pervasive and distributed nature of extraction processes. For the Mekong River, the widely assumed estimate of basin-wide sand extraction is 50 Mt per year. This figure is based on 2013 estimates and is likely to be outdated. Here, we demonstrate the ability of high-resolution satellite imagery to map, monitor, and estimate volumes of sand extraction on the Lower Mekong River in Cambodia. We use monthly composite images from PlanetScope imagery (5m resolution) to estimate sand extraction volumes over the period 2016-2020 through tracking sand barges. We show that rates of extraction have increased on a yearly basis from 24 Mt (17 to 32 Mt) in 2016 to 59 Mt (41 to 75 Mt) in 2020 at a rate of ~ 8 Mt yr-1 (6 to 10 Mt yr-1), where values in parentheses relate to lower and upper error bounds, respectively. Our revised estimates for 2020 (59 Mt) are nearly 2 times greater than previous best estimates for sand extraction for Cambodia (32 Mt) and greater than current best estimates for the entire Mekong Basin (50 Mt). We show that over the 5-year period, only 2 months have seen positive (supply exceeds extraction) sand budgets under mean scenarios (5 months under the scenarios with the greatest natural sand supply). We demonstrate that this net negative sand budget is driving major reach-wide bed incision with a median rate of -0.26ma-1 over the period 2013 to 2019. The use of satellite imagery to monitor sand mining activities provides a low-cost means to generate up-to-date, robust estimates of sand extraction in the world's large rivers that are needed to underpin sustainable management plans of the global sand commons

Water quality modelling of the Mekong River basin: climate change and socioeconomics drive flow and nutrient flux changes to the Mekong Delta

The Mekong delta is recognised as one of the world's most vulnerable mega-deltas, being subject to a range of environmental pressures including sea level rise, increasing population, and changes in flows and nutrients from its upland catchment. With changing climate and socioeconomics there is a need to assess how the Mekong catchment will be affected in terms of the delivery of water and nutrients into the delta system. Here we apply the Integrated Catchment model (INCA) to the whole Mekong River Basin to simulate flow and water quality, including nitrate, ammonia, total phosphorus and soluble reactive phosphorus. The impacts of climate change on all these variables have been assessed across 24 river reaches ranging from the Himalayas down to the delta in Vietnam. We used the UK Met Office PRECIS regionally coupled climate model to downscale precipitation and temperature to the Mekong catchment. This was accomplished using the Global Circulation Model GFDL-CM to provide the boundary conditions under two carbon control strategies, namely representative concentration pathways (RCP) 4.5 and a RCP 8.5 scenario. The RCP 4.5 scenario represents the carbon strategy required to meet the Paris Accord, which aims to limit peak global temperatures to below a 2 °C rise whilst seeking to pursue options that limit temperature rise to 1.5 °C. The RCP 8.5 scenario is associated with a larger 3–4 °C rise. In addition, we also constructed a range of socio-economic scenarios to investigate the potential impacts of changing population, atmospheric pollution, economic growth and land use change up to the 2050s. Results of INCA simulations indicate increases in mean flows of up to 24%, with flood flows in the monsoon period increasing by up to 27%, but with increasing periods of drought up to 2050. A shift in the timing of the monsoon is also simulated, with a 4 week advance in the onset of monsoon flows on average. Decreases in nitrogen and phosphorus concentrations occur primarily due to flow dilution, but fluxes of these nutrients also increase by 5%, which reflects the changing flow, land use change and population changes

Dynamics of salt intrusion in the Mekong Delta: Results of field observations and integrated coastal-inland modelling

On the list of challenges facing the world largest deltas, increased saline water intrusion (SWI) in the surface water system and its role in jeopardizing freshwater supply are often ranked very high. Yet, detailed process-based studies of SWI at the whole delta scale are limited, and the trends are regularly associated with global sea level rise. Here, using field measurements and a sophisticated 3D model that integrates the riverine, rural, estuarine, and coastal dynamics within one numerical domain, we study SWI at the scale of the Mekong Delta in extensive detail. While many studies downscale the SWI problem to a topic within an estuary, we show that the physical processes on the continental shelf, such as monsoon-driven ocean surge, directly influence salinity dynamics within the delta. Typical values of 20-40gcm surge over the continental shelf contribute to up to 10gkm of further SWI. The delta's estuarine system is also more sensitive than many other systems to variations of river discharge. Furthermore, spring-neap variability plays a key role in SWI in the delta. The estuarine variability from a stratified to a mixed system between neap and spring tides develops 3D processes such as estuarine circulation and tidal straining that become the main upstream salt transport mechanisms. The 3D nature of salinity dynamics, and the role of upstream and downstream processes, suggests that compromising on dimension or extent of the numerical domain can limit the accuracy of predictions of SWI in the delta. The study also showcases the fact that riverbed incision in response to anthropogenic sediment starvation in the last 2 decades has increased stratification and activated or magnified 3D salt transport subprocesses that amplify upstream salt transport. With all the external forces on the delta, namely climate change and an altered hydrological regime by the upstream dams, due to deeper estuarine channels (driven by sand mining and upstream impoundments) compared to its near past, the delta itself has become far more vulnerable to even mild natural events. This exemplifies the fundamental importance of preserving the sediment budget and riverbed levels in protecting the world's deltas against SWI

Dynamics of salt intrusion in the Mekong Delta: Results of field observations and integrated coastal-inland modelling

On the list of challenges facing the world largest deltas, increased saline water intrusion (SWI) in the surface water system and its role in jeopardizing freshwater supply are often ranked very high. Yet, detailed process-based studies of SWI at the whole delta scale are limited, and the trends are regularly associated with global sea level rise. Here, using field measurements and a sophisticated 3D model that integrates the riverine, rural, estuarine, and coastal dynamics within one numerical domain, we study SWI at the scale of the Mekong Delta in extensive detail. While many studies downscale the SWI problem to a topic within an estuary, we show that the physical processes on the continental shelf, such as monsoon-driven ocean surge, directly influence salinity dynamics within the delta. Typical values of 20-40gcm surge over the continental shelf contribute to up to 10gkm of further SWI. The delta's estuarine system is also more sensitive than many other systems to variations of river discharge. Furthermore, spring-neap variability plays a key role in SWI in the delta. The estuarine variability from a stratified to a mixed system between neap and spring tides develops 3D processes such as estuarine circulation and tidal straining that become the main upstream salt transport mechanisms. The 3D nature of salinity dynamics, and the role of upstream and downstream processes, suggests that compromising on dimension or extent of the numerical domain can limit the accuracy of predictions of SWI in the delta. The study also showcases the fact that riverbed incision in response to anthropogenic sediment starvation in the last 2 decades has increased stratification and activated or magnified 3D salt transport subprocesses that amplify upstream salt transport. With all the external forces on the delta, namely climate change and an altered hydrological regime by the upstream dams, due to deeper estuarine channels (driven by sand mining and upstream impoundments) compared to its near past, the delta itself has become far more vulnerable to even mild natural events. This exemplifies the fundamental importance of preserving the sediment budget and riverbed levels in protecting the world's deltas against SWI