Mean Water Level Research Articles

Numerical simulation on the influence of artificial island on reef hydrodynamics

The construction of artificial island can greatly change the reef hydrodynamics, leading to increased water level and wave height as well as changes in flow field distribution. These alterations can affect sediment transport on the reef, and increase the risk of overtopping and structure instability. A numerical model based on the Reynolds Averaged Navier-Stokes (RANS) equations and k-ε turbulence closure model was developed to investigate the influence of artificial island on reef hydrodynamics. The numerical model was validated against the experimental results of wave height, mean water level, and wave breaking morphology. Detailed flow field, wave height, and wave set-up in front of artificial island were further analyzed based on the validated model. After building the reef-top structure, the wave breaking and offshore currents at reef edge were amplified. The flow stratification and increase of the wave set-up were also found on the reef flat. Furthermore, we found the relationship between maximum flow velocities on the reef flat and incoming wave conditions could be characterized by two non-dimensional parameters: |u¯|max/g(η¯+hr), (η¯+hr)/Hi.

Momentum, energy and vorticity balances in deep-water surface gravity waves

The particle trajectories in irrotational, incompressible and inviscid deep-water surface gravity waves are open, leading to a net drift in the direction of wave propagation commonly referred to as the Stokes drift, which is responsible for catalysing surface wave-induced mixing in the ocean and transporting marine debris. A balance between phase-averaged momentum density, kinetic energy density and vorticity for irrotational, monochromatic and spatially periodic two-dimensional water waves is derived by working directly within the Lagrangian reference frame, which tracks particle trajectories as a function of their labels and time. This balance should be expected as all three of these quantities are conserved following particles in this system. Vorticity in particular is always conserved along particles in two-dimensional inviscid flow, and as such even in its absence it is the value of the vorticity that fundamentally sets the drift, which in the Lagrangian frame is identified as the phase-averaged momentum density of the system. A relationship between the drift and the geometric mean water level of particles is found at the surface, which highlights connections between the geometry and dynamics. Finally, an example of an initially quiescent fluid driven by a wavelike pressure disturbance is considered, showing how the net momentum and energy from the surface pressure disturbance transfer to the wave field, and recognizing the source of the mean Lagrangian drift as the net momentum required to generate an irrotational surface wave by any conservative force.

Unsteady flow in a compound channel during flooding: Insights from a combined two-dimensional and three-dimensional numerical simulation

Natural channels often take the form of compound channels during flooding events. The unsteadiness of floods is crucial for flood management and river-floodplain ecosystems. However, the impact of unsteady flow on three-dimensional hydrodynamic processes within compound channels remains poorly understood. This study employed a validated two-dimensional/three-dimensional numerical model to examine how unsteady features influence key hydrodynamic indicators of compound channels, such as velocity distribution, turbulence characteristics, and discharge partitioning between the main channel and floodplain. A definitive positive correlation was found between water level fluctuation amplitudes and integrated discharge during unsteady events, as well as between mean water levels and mean discharges. The proportion of discharge conveyed by the main channel relative to the total was directly linked to the water depth ratio. The exchange at the main channel–floodplain interface exhibits a dual-layer flow pattern: surface waters spill into the floodplain, while deeper waters are siphoned into the main channel. Unsteady conditions principally modulate the intensity of these exchanges, altering hydraulic interactions and water transfer between areas. Pronounced wall shear stresses were detected at the main channel–floodplain interface due to swift channel flows and momentum exchange at the floodplain margin, with maxima adjacent to the floodplain edge correlating with peak flow velocities. Under deep conditions, cross-sectional flow across the compound channel's mixing layer was essentially two-dimensional. These findings elucidate the complex hydrodynamics inherent to unsteady overbank flows while holding implications for enhancing floodplain management and advancing the understanding of unsteady fluvial hydraulics.

Skill assessment of a total water level and coastal change forecast during the landfall of a hurricane

The Total Water Level and Coastal Change Forecast (TWL&CC Forecast) provides coastal communities with 6-day notice of potential elevated water levels and coastal change (i.e., dune erosion, overwash, or inundation) on sandy beaches that threatens safety, infrastructure, or resources. This continuously operating model provides hourly information for select regions along U.S. Gulf of Mexico and Atlantic Ocean coastlines. The objective of this work is to assess the skill of forecasts during a period of elevated water levels along the coasts of North Carolina (NC) and South Carolina, USA caused by Hurricane Isaias in August 2020, using a combination of observations and model hindcasts. Water levels and waves were observed throughout the storm at three locations near Wrightsville Beach, NC, which provided information to assess forecast skill; a wave buoy offshore, a tide gage at a local pier, and a pressure sensor deployed at the pier. In addition to observations, the non-hydrostatic phase-resolving model SWASH (Simulating WAves till SHore) was forced with hourly wave energy spectra derived from a coupled Delft3D-SWAN simulation during the peak of Isaias, to complement observations by computing nearshore wave height and wave-induced setup and runup at the shoreline. During the storm peak, SWASH-simulated water levels at the sensor position were comparable to those at the maximum landward extent (bias = −0.05 m; gain = 0.26; r2 = 0.99), suggesting that observations at the USGS sensor location were a useful proxy for total water level (TWL; sum of tide, surge and wave runup) at the shoreline that are predicted by the TWL&CC Forecast. The TWL forecast at Wrightsville Beach was consistent with observations from the USGS sensor (bias = −0.38 m and −0.74 m, scatter index = 0.22 and 0.28 for the two forecast model grids considered, respectively; weighted regression considering model uncertainty explained 95 percent of variability in observed TWL). Observed TWL was within the confidence interval of the TWL&CC Forecast for the 5 h at the storm peak. Forecast mean water levels (MWL; sum of tide, surge and wave setup) and tide gage observations were also consistent (bias = 0.07 m and 0.02 m for the forecast model grids; scatter index = 0.46; r2 = 0.80). Forecast MWL at the storm peak was within 0.06 m of the observed MWL from the tide gage for both sites. In the region where Isaias made landfall, eight additional pressure sensors were compared to the peak TWL forecast (bias = 0.14 m; scatter index = 0.18). Forecast TWL explained 90 percent of observed variability in TWL when considering uncertainty of the forecast with a weighted regression. The results demonstrate that wave-driven water levels contributed a significant portion of the forecast TWL during Isaias (52 percent during the three peak hours of the storm), and that TWL were represented using the forecast model. Mean absolute error of the coastal change forecast and observed overwash is 0.4 and 0.14 for the two forecast model grids considered. The skill demonstrated by this computationally efficient method indicates that the forecasting system can provide fast and reliable predictions of TWL across hundreds of km of coastline at sub-km resolution, days to hours in advance of when storms threaten coastal regions.

Multi-step-ahead prediction of water levels using machine learning: A comparative analysis in the Vietnamese Mekong Delta

This study evaluates the efficacy of five machine learning algorithms Support Vector Regression (SVR), Decision Tree (DT), Random Forest (RF), Light Gradient Boosting Machine Regressor (LGBM), and Linear Regression (LR) in predicting water levels in the Vietnamese Mekong Delta's tidal river system, a complex nonlinear hydrological phenomenon. Using daily maximum, minimum, and mean water level data from the Cao Lanh gauging station on the Tien River (2000-2020), models were developed to forecast water levels one, three, five, and seven days in advance. Performance was assessed using Nash-Sutcliffe Efficiency, coefficient of determination, Root Mean Square Error, and Mean Absolute Error. Results indicate that all models performed well, with SVR consistently outperforming others, followed by RF, DT, and LGBM. The study demonstrates the viability of machine learning in water level prediction using solely historical water level data, potentially enhancing flood warning systems, water resource management, and agricultural planning. These findings contribute to the growing knowledge of machine learning applications in hydrology and can inform sustainable water resource management strategies in delta regions.

Discussion on “comparative study on volume conservation among various SPH models for flows of different levels of violence, coastal engineering, volume 191, August 2024, 104 521” by Wang et al

The paper by Wang et al. (2024) targets an interesting topic in the context of particle methods, namely, volume conservation. This important feature is studied by focusing on the time variation of mean water level under three different scenarios corresponding to hydrostatics, standing wave and dam break, and through consideration of a few variants of the Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) fluid model. In this discussion, the discussers briefly discuss some issues that could have been further considered in the paper by Wang et al. (2024) to ensure rigorousness and comprehensiveness of the conducted comparative analysis.

A new framework for quantifying alongshore variability of swash motion using fully convolutional networks

Waves running up and down the beach (‘swash’) at the landward edge of the ocean can cause changes to the beach topology, can erode dunes, and can result in inland flooding. Despite the importance of swash, field observations are difficult to obtain in the thin, bubbly, and potentially sediment laden fluid layers. Here, swash excursions along an Atlantic Ocean beach are estimated with a new framework, V-BeachNet, that uses a fully convolutional network to distinguish between sand and the moving edge of the wave in rapid sequences of images. V-BeachNet is trained with 16 randomly selected and manually segmented images of the swash zone, and is used to estimate swash excursions along 200 m of the shoreline by automatically segmenting four 1-h sequences of images that span a range of incident wave conditions. Data from a scanning lidar system are used to validate the swash estimates along a cross-shore transect within the camera field of view. V-BeachNet estimates of swash spectra, significant wave heights, and wave-driven setup (increases in the mean water level) agree with those estimated from the lidar data.

Simulating sea level extremes from synthetic low-pressure systems

Abstract. In this article we present a method for numerical simulations of extreme sea levels using synthetic low-pressure systems as atmospheric forcing. Our simulations can be considered to be estimates of the high sea levels that may be reached when a low-pressure system of high intensity and optimal track passes the studied region. We test the method using sites located along the Baltic Sea coast and simulate synthetic cyclones with various tracks. To model the effects of the cyclone properties on sea level, we simulate internal Baltic Sea water level variations with a numerical two-dimensional hydrodynamic model, forced by an ensemble of time-dependent wind and air-pressure fields from synthetic cyclones. The storm surges caused by the synthetic cyclones come on top of the mean water level of the Baltic Sea, for which we used a fixed upper estimate of 100 cm. We find high extremes in the northern Bothnian Bay and in the eastern Gulf of Finland, where the sea level extreme due to the synthetic cyclone reaches up to 3.5 m. In the event that the mean water level of the Baltic Sea has a maximal value (1 m) during the cyclone, the highest sea levels of 4.5 m could thus be reached. We find our method to be suitable for use in further studies of sea level extremes.

Wave setup estimation at regional scale: Empirical and modeling-based multi-approach analysis in the Mediterranean Sea

Wave setup is a physical process that induces a temporal increase of the mean water level due to wave dissipation by bottom friction and breaking in the surf zone, extending over tens to hundreds of meters in the cross-shore direction. Wave setup contribution to coastal sea level solely induced by wind and atmospheric effects can increase by more than 100% under extreme events and conditions favoring its formation. It is therefore crucial to consider this phenomenon when assessing sea-level-related coastal hazards. Previous studies estimated the wave setup effect by means of numerical modeling and empirical formulations at regional and global scale. Such analyses require either high computational capacity to implement high-resolution numerical models over large domains, and/or accurate information on coastal morphological features from global or regional databases. Although the Mediterranean Sea is a fetch-limited environment, waves generated from extra-tropical cyclones are powerful enough for wave setup to develop, and subsequently for a potential significant wave setup contribution to extreme coastal sea level. Through the use of both numerical and empirical methods, we investigate the uncertainty associated to wave setup representation on the frequency and magnitude of coastal extreme sea levels occurring on sandy beaches in the Mediterranean Sea. Wave setup values are compared at beach scale between process-based modeling and empirical approaches, showing highly variable results. We also quantify the impact of wave setup on return levels of coastal sea level extremes using reconstructed sea levels. We employ various methods to calculate the wave setup component. Results show high spatial dispersion, with clear differences between the numerical and empirical approaches, especially in regions prone to the development of energetic waves. The total inter-method dispersion of 100-year return levels is often higher than 30 cm for average values of 62.4 cm. We emphasize the important limitations related to wave setup modeling (i.e., its underestimation) at large scale, and call for caution when applying empirical formulations (generally developed from local studies) at regional to global scale, which can lead to unrealistic wave setup values.

Numerical study of wave-induced flow and wave set-up on a platform reef with steep slope

A 2D numerical model was developed in this study to investigate the wave-induced flow and wave set-up on a platform reef with steep frontal slope. The model solves the Reynolds Averaged Navier-Stokes (RANS) equations, with the classical k−ε turbulence closure model. The inflow boundary was modified by adding fluid replenishment to balance the water level in the numerical flume to ensure long-term simulation in the platform reef system. The numerical model was first validated by systematically comparing the wave breaking, flow structure, and mean water level against laboratory measurements. The validated model was employed to further analyze the detailed flow field and reveal the influence of the reef morphology on the wave-induced flow and wave set-up. The complex distribution pattern of wave-induced flow and wave set-up on the reef flat were found to be related to the Froude number. The results also reveal that reef slope and reef roughness will not only change the wave breaking morphology but also affect the distribution pattern of wave-induced flow and wave set-up.

Wave Basin Experiments Of Wave-Driven Hydrodynamics Over Submerged Coastal Structures And Artificial Reefs

Submerged coastal structures, such as submerged breakwaters and artificial reefs, modify incident wave fields and alter a wide range of nearshore hydrodynamic processes. Submerged breakwaters are usually designed for a singular function of wave attenuation; whereas nature-based artificial reef structures aim to both attenuate wave energy while enhancing ecosystem services, including the creation of habitat for marine organisms and the promotion of biodiversity. However, to date the application of artificial reefs has remained more limited due to the poorer understanding of how reefs influence key nearshore processes that determine their effectiveness. Existing research on submerged breakwaters has mostly focused on quantifying the wave transmission, which is ratio between the incident wave energy onshore and offshore of the reefs (e.g., van Gent 2023). The wave transmission coefficient has conventionally served as a primary design criterion related to the effectiveness of reefs in offering coastal protection. However, a meta-analysis of the shoreline response to constructed submerged breakwaters found that in the majority of cases erosion occurred in their lee (Ranasinghe and Turner., 2006), which demonstrates our limited knowledge on how reefs modify coastal processes. Wave-reef interactions can lead to the generation of mean (wave-averaged) currents and water levels (setup). Numerical modelling studies have found that two and four-cell mean circulation patterns can develop in response to changes to the wave field caused by submerged structures (Ranasinghe et al., 2006, da Silva et al., 2022). A two-cell circulation is characterized by diverging currents behind the reefs and at the shoreline, which could lead to an erosive shoreline. In contrast, a four-cell circulation is characterized by diverging currents in the immediate lee but converging currents at the shoreline, which would result in beach accretion. While these modelling studies advanced the understanding of how reefs influence shoreline hydrodynamics, the absence of comprehensive experimental observations replicating submerged coastal structures has hindered their rigorous validation. Here we present the findings of an extensive set of 3D wave basin experiments that were designed to investigate the detailed wave-driven hydrodynamics around submerged coastal structures subject to a range of wave conditions, water levels and reef layouts.

Wave and Roller Transformation Over Barred Bathymetry

AbstractThe cross‐shore transformation of breaking‐wave roller momentum and energy on observed barred surfzone bathymetry is investigated with a two‐phase Reynolds Averaged Navier Stokes model driven with measured incident waves. Modeled wave spectra, wave heights, and wave‐driven increases in the mean water level (setup) agree well with field observations along transects extending from 5‐m water depth to the shoreline. Consistent with prior results the roller forcing contributes 50%–60% to the setup, whereas the advective terms contribute ∼20%, with the contribution of bottom stress largest (up to 20%) for shallow sandbar crest depths. The model simulations suggest that an energy‐flux balance between wave dissipation, roller energy, and roller dissipation is accurate. However, as little as 70% of the modeled wave energy ultimately dissipated by breaking was first transferred from the wave to the roller. Furthermore, of the energy transferred to the roller, 15%–25% is dissipated by turbulence in the water column below the roller, with the majority of energy dissipated in the aerated region or near the roller‐surface interface. The contributions of turbulence to the momentum balance are sensitive to the parameterized turbulent anisotropy, which observations suggest increases with increasing turbulence intensity. Here, modeled turbulent kinetic energy dissipation decreases with increasing depth of the sandbar crest, possibly reflecting a change from plunging (on the steeper offshore slope of the bar) to spilling breakers (over the flatter bar crest and trough). Thus, using a variable roller front slope in the roller‐wave energy flux balance may account for these variations in breaker type.

Impacts of vegetation and tidal conditions on porewater and salt transport in coastal wetlands: Numerical simulations and field evidence

Coastal wetlands are crucial ecosystems with high fragility to environmental alterations. Despite early efforts, interactions of water and salt with vegetation in coastal wetlands have rarely been investigated especially with increasing anthropogenic disturbances. Here, field data and numerical simulations were utilized to understand the influences of vegetation, evapotranspiration (ETa), tide, and inundation events on porewater and salt transport in highly regulated tidal riparian wetlands. At the site, the presence of Phragmites australis significantly enhanced soil hydraulic conductivity (KS) and facilitated hydraulic connections between groundwater and surface soil, resulting in the rapid response of soil salinity to groundwater level fluctuations. Simulation results further demonstrated the combined effects of tides and ETa on the salinity distributions, revealing the formation of a high salinity zone in the middle region due to the inflow of water from the tides and ETa-driven upward movements of water and salt; however, as salinity levels increased within the high salinity zone, the growth of Phragmites australis was constrained, leading to reduced transpiration (Ta). Tidal parameters such as amplitudes and mean water levels could affect salt accumulation processes in soils and further the high-salinity zone. Finally, inundation events from extreme upstream discharge had a flushing and diluting effect on accumulated salts, alleviating salinity stress and promoting vegetation Ta and growth; however, further increases in ETa led to salt re-accumulation, potentially reinstating the salinity distribution pattern after inundation. These findings enhance our understanding of ecohydrological processes in coastal wetlands and have important implications for wetland management and restoration.

Comparative study on volume conservation among various SPH models for flows of different levels of violence

The weakly-compressible smoothed particle hydrodynamics (WCSPH) models offer distinct advantages in simulating free-surface flow. Within the rapidly evolving WCSPH field, Classical WCSPH model, based on the artificial viscous term, and its derived models, which additionally introduce numerical diffusive terms into the continuity equations - including the δ-SPH, δ-SPHC, δ-SPHC+, δ-LES-SPH, and δ-SPHF models have gained prominence recently. The numerical diffusive term is found to alter the particle distributions in the free-surface region, leading to a change in the mean water level that reflects the volume conservation, which might result in inaccuracies in some simulations. This study evaluates the volume conservations of the aforementioned WCSPH models for flows at quasi-hydrostatic states evolved from varying levels of flow violence under prolonged simulations, through a series of numerical tests involving hydrostatic, standing-wave, and dam-break cases. For the hydrostatic and standing-wave cases, the performance of different types of WCSPH models remains consistent overall. The Classical WCSPH and δ-LES-SPH models exhibit minimal changes in volume, with almost no decrease in the mean water level occurring. The δ-SPH, δ-SPHC, δ-SPHC + models demonstrate decreases in the mean water level converging to around 0.2 times the SPH particle diameter (Δx), indicating favorable volume conservation. Conversely, the δ-SPHF model consistently exhibits a decrease in the mean water level exceeding 1 Δx at t = 2000s in a linear fashion, resulting in a noticeable reduction in volume. Under the dam-break case, most of the mean water levels simulated by different models experience a small increase, which is close to the decrease observed under the hydrostatic and standing-wave cases with corresponding models. Only the δ-SPHF model continues to show a continuous decreasing tendency. In summary, considering volume conservation, the δ-LES-SPH model demonstrates the best performance (excluding the Classical WCSPH model as it cannot simulate violent flow), followed by the δ-SPHC, δ-SPHC+, δ-SPH and δ-SPHF models.

Numerical investigation of infragravity wave hydrodynamics at fringing reef with a permeable layer

In tropical and subtropical coastal regions, coral reefs are abundant and play a vital role in maintaining ecosystem balance. Additionally, they effectively dissipate a significant amount of wave energy that propagates from the open sea towards the coastline, providing coastal areas with protection against wave impacts. Consequently, numerous scholars have conducted extensive research to investigate the hydrodynamic characteristics of wave propagation and transformation over coral reef topography. However, previous studies have often simplified the bottom boundary of coral reefs as impermeable layers, neglecting the fact that the coral reef consists of a permeable canopy structure in the actual marine environment. To fill the knowledge gap of previous research, this study is based on the Non-Hydrostatic Wave Model Solver (NHWAVE) to numerically simulate the propagation process of random waves over permeable coral fringing reefs. The study takes a comprehensive approach by considering the influences of various factors on the hydrodynamic characteristics of random waves over the fringing reef, including incident wave height, reef-flat water depth, peak wave period, permeable layer thickness, permeable layer porosity, and median diameter of the permeable layer. This paper focuses primarily on analyzing the variations in sea-swell wave height (HSS), infragravity wave height (HIG), and mean water level (MWL) along the fringing reef, while conducting a comparative analysis between fringing reefs with a permeable layer and impermeable fringing reefs. The findings reveal that the presence of a permeable layer reduces the shallow water deformation of waves on the fore-reef slope and mitigates the wave-breaking phenomenon near the reef edge, thereby significantly reducing the sea-swell wave height (HSS), infragravity wave height (HIG), and wave setup on the reef-flat. Furthermore, the permeable layer's existence also leads to a decrease in the maximum wave run-up height on the back-reef slope. The research findings of this study can further enhance our understanding of the hydrodynamic characteristics of infragravity waves over fringing reefs, which is significant for studying the impact of random waves on coastal areas and understanding the protective mechanisms of coral reefs in coastal regions.

A continuous non-ergodic theory for the wave set-up

Inhomogeneities in the wave field due to wave groups, currents, and shoaling among other ocean processes can affect the mean water level. In this work, the classical and unsolved problem of continuously computing the set-down and the following set-up induced by wave breaking on a shoal of constant finite slope is tackled. This is possible by using available theoretical knowledge on how to approximate the distribution of wave random phases in finite depth. Then, the non-homogeneous spectral analysis of the wave field allows the computation of the ensemble average by means of the phase distribution and the inversion of the integral of the second moment for the special case of a shoaling process with uniform phase distribution. In doing so, I am able to obtain a direct effect of the slope magnitude on the phases distribution. Therefore, an analytical and slope-dependent mean water level with continuity over the entire range of water depth is provided.

A moderately chocked estuary: Influence of a constriction on the water level variations of the Wouri estuary (Cameroon)

We study the water-level (WL) evolution in the Douala basin (DB), a sub-basin of the Wouri estuary, separated from the ocean by a natural constriction and influenced by tides and river inflow. Our objective is to assess whether floods in Douala city could result from water overflow from the Douala basin due to Wouri river floods. We first evaluate the constriction’s damping effect by analyzing tide amplitude variations from the ocean to the basin. A simplified model for basin WL variation is developed, incorporating a dissipation parameter (Г) that is evaluated using weak tidal amplitude modulation from the ocean to the basin over the dry season of 2023. Results in the DB reveal nondimensional choking-parameter (P) values of 12–19, indicating moderate dissipation. The basin's relatively small area results in a nondimensional river discharge parameter (S) of 0–12, indicating high river discharge regimes. Subsequently, the model is used to evaluate the mean and maximum WL, as well as tide amplitude variations of the DB, as functions of river flow and ocean tides. The mean WL increases, but the maximum tide amplitude decreases as river input increases. The maximum WL in the basin always exceeds the maximum amplitude of ocean tides but is only significantly higher when river input is above 1000 m3/s. In current conditions, where maximum observed river input is around 1800 m3/s, it is unlikely that flooding will extend beyond the basin borders into the city of Douala. For a moderate increase (20%) in future maximum river fluxes, it is also unlikely that the DB will overflow into Douala city. Only a drastic increase in mean ocean level and river fluxes (which are possible scenarios associated with climate change) could potentially lead to a significant rise in basin WL, resulting in severe flooding.

Change trends of mean ice thickness, water level, and surface salinity in the Laptev Sea in winter-spring period under further climate warming conditions

he purpose of the paper is to identify the most likely trends in interannual changes of the ice cover thickness on sections of the Northern Sea Route in the Laptev Sea in the winter-spring months under conditions of further climate warming. A hypothesis that in the modern period such changes can be significantly impacted by melting of submarine permafrost layers located inside the sea shelf, — a process caused by their heat exchange with bottom sea waters, is put forward. To verify the hypothesis, trends in interannual changes in ice thickness are compared with trends in variations in sea level and surface salinity. As factual material, data from the global reanalysis GLORYS12v1 is used. The research methodology is based on the standard methods of mathematical statistics. A correlation between the trends of the above processes has been found. It allows us to confirm the hypothesis put forward. The results obtained indicate that, under the conditions of ongoing climate warming, there remains a high probability that the average thickness of the ice cover will not decrease, and the ice situation in the Laptev Sea in the winter-spring months will not improve significantly. The latter confirms the need and feasibility of further development of the Russian icebreaker fleet

Relative Sea Level Rise in the Winyah Bay-Waccamaw River Tidal System Over the Last Thirteen Years

Prediction of sea level rise (SLR) in response to climate change has been the focus of worldwide research, most focusing on the impact by human development. The research has been limited to estuaries and tidal rivers near harbors dealing with the hydrodynamics of reversing tidal flows. This article focuses on the Waccamaw River National Wildlife Refuge in coastal South Carolina where freshwater unidirectional flow is common. We examined the record of water levels in the Waccamaw and Pee Dee Rivers over the period 2007–2019 and the length of record of the United States Geographical Survey (USGS) gauge at Pawleys Island on the Waccamaw River. The Atlantic Ocean, off the southeastern coast of the US, has experienced accelerated SLR since 2000. National Oceanic and Atmosphere Administration (NOAA) tide gauges from Fort Pulaski on Cockspur Island in Georgia to Beaufort, North Carolina, show significant increase in long-term SLR since then with an average since 2007 of approximately 10 mm y-1. Since the study period was less than the 18.6-year cycle of lunar precession, tidal ranges were expanding for much of the study period resulting in the rate of rise of Mean Higher High Water (MHHW; the average of the highest tide levels during each day) being greater than the rate of increase of Mean Lower Low Water (MLLW; the average of the lowest tide levels during each day) in all ocean stations. We examined water levels at NOAA and USGS gauges from Oyster Creek, in North Inlet to Conway on the Waccamaw River and Near Bucksport on the Pee Dee River. We found mean water levels increased more rapidly with distance from the ocean with an apparent SLR > 40 mm y-1 at Conway on the Waccamaw and Bucksport on the Pee Dee. In contrast to the ocean NOAA gauges, the estuary/river gauges showed more rapid increase of daily minimum water level (an approximation of MLLW) than daily maximum water level (an approximation of MHHW) with an extreme of apparent rise of minimum water levels of 58 mm y-1 at Bucksport on the Pee Dee. Nearly 50% of the increase in apparent SLR was due to an increase in the annual average freshwater flow of the Pee Dee and Waccamaw Rivers. Over the past 13 years the Waccamaw National Wildlife Refuge has experienced an apparent SLR that was more than double that observed at the edge of the ocean. The rise has been greater in the height of daily low water than in the height of daily high water. The increase was driven by both tidal hydrodynamics and an increase in the rate of flow in the Pee Dee and Waccamaw Rivers. These findings have important implications for land managers, policymakers, and homeowners in the region as people in the middle to upper estuaries need to plan for rates of relative SLR rise much greater than the frequently discussed rates in the ocean.

Integration of life cycle and habitat conditions in modeling fish biomass in the floodplain of the Lower Mekong Basin

Migratory fishes utilize several distinct habitats during their life cycles, and thus the conservation of such species requires proper habitat management in both spatial and seasonal dimensions. However, their life cycle in the freshwater environment has not been well investigated, in particular in the tropical region. Therefore, our study aimed to elucidate the importance of the integration of life cycle and habitat conditions in modeling the biomass of migratory fish species. Focusing on the hydrological years 2000–2015, we collected the biomass of 74 migratory fish species and their seasonal habitat conditions in the floodplain of the Lower Mekong Basin, including Tonle Sap Lake. Then, generalized linear, generalized additive, and random forest models were applied to the collected data with and without life cycle information. As a result, the models and their performance indicate that the integration of the life cycle and seasonal habitat conditions in modeling the biomass is significantly important for migratory fishes. The results further showed that changing ecohydrological indices in the feeding and refuge seasons negatively affected the fish biomass. Those indices were hydrologic factors (the maximum water levels, the mean water levels, the start date of the flood, and the end date of the flood) and climate factors (water temperatures). Interestingly, the models indicated that the total area of inundated forests negatively influences the biomass of planktivorous fishes while it positively affects the biomass of piscivores. These results clearly showed the importance of a natural flow regime to ensure the fish life cycle and biomass production in the Lower Mekong Basin. The presented approach can be applied to model the biomass of migratory fishes in spatially connected and changing environments.