Effects of intratidal and tidal range variability on circulation and salinity structure in the Cape Fear River Estuary, North Carolina

Tidal influences on circulation and the salinity structure are investigated in the largely unstudied Cape Fear River Estuary (CFRE), North Carolina, a partially mixed estuary along the southeast coast of the United States. During two different tidal conditions (high versus low tidal range) and when river flow was low, salinity and velocity data were collected over a semidiurnal tidal cycle in a 2.8 km long transect along the estuary axis. Water level data were recorded nearby. Mechanisms that influence salt transport characteristics are diagnosed from an analysis of the field data. Specifically, we investigated the relationship between tidal range and salinity through comparison of along-channel circulation characteristics, computed salt fluxes, and coefficients of vertical eddy diffusivity (Kz) based on a parameterization and on salt budget analysis. Findings indicate up-estuary tidally driven salt fluxes resulting from oscillatory salt transport are dominant near the pycnocline, while mean advective transport dominates near the bottom during both tidal range periods. Earlier research related to salt transport in estuaries with significant gravitational circulation suggests that up-estuary salt transport increases during low tidal ranges as a result of increased gravitational circulation. In the CFRE, in contrast, net (tidally averaged) near-bottom along-channel velocities are greater during higher tidal range conditions than during lower tidal range conditions. Findings indicate stronger tidal forcing and associated mixing contribute to greater near-bottom salinity gradients and, consequently, increased baroclinic circulation. Lower near-bottom salinities during the higher tidal range period are a result of a combination of increased vertical turbulent salt fluxes near the pycnocline and increased bottom-generated mixing

Wind-induced Sediment Resuspension and its Impact on Algal Growth for Lake Balaton

This report includes two articles which were published concerning IIASA's joint study with the Hungarian Academy of Sciences on eutrophication management of Lake Balaton. The first one, "Dynamic Behavior of Suspended Sediment Concentrations in a Shallow Lake Perturbed by Episodic Wind Events," by Luettich, Jr. et al, deals with the understanding and modeling of physical processes influencing resuspension, while the second one, "Influence of Sediment Resuspension on the Light Conditions and Algal Growth in Lake Balaton," by Somlyody and Koncsos, builds on its achievements and estimates the impact on light conditions and algae biomass. Due to an increased internal phosphorus load and the appearance of nitrogen-fixing blue-green algae, nowadays nutrients are not a limiting factor for Lake Balaton (in spite of the significant load reduction realized since 1983). The short-term dynamics of algae biomass are primarily determined by light, as described in the second article

Sea level anomalies exacerbate beach erosion

Sea level anomalies are intra-seasonal increases in water level forced by meteorological and oceanographic processes unrelated to storms. The effects of sea level anomalies on beach morphology are unknown but important to constrain because these events have been recognized over large stretches of continental margins. Here, we present beach erosion measurements along Onslow Beach, a barrier island on the U.S. East Coast, in response to a year with frequent sea level anomalies and no major storms. The anomalies enabled extensive erosion, which was similar and in most places greater than the erosion that occurred during a year with a hurricane. These results highlight the importance of sea level anomalies in facilitating coastal erosion and advocate for their inclusion in beach-erosion models and management plans. Sea level anomalies amplify the erosive effects of accelerated sea level rise and changes in storminess associated with global climate change

Effects of Model Resolution and Coverage on Storm-Driven Coastal Flooding Predictions

Predictions of storm surge and flooding require models with higher resolution of coastal regions, to describe fine-scale bathymetric and topographic variations, natural and artificial channels, flow features, and barriers. However, models for real-time forecasting often use a lower resolution to improve efficiency. There is a need to understand how resolution of inland regions can translate to predictive accuracy, but previous studies have not considered differences between models that both represent conveyance into floodplains and are intended to be used in real time. In this study, the effects of model resolution and coverage are explored using comparisons between forecast-ready and production-grade models that both represent floodplains along the US southeast coast, but with typical resolutions in coastal regions of 400 and 50 m, respectively. For two storms that impacted the US southeast coast, it is shown that, although the overall error statistics are similar between simulations on the two meshes, the production-grade model allowed a greater conveyance into inland regions, which improved the tide and surge signals in small channels and increased the inundation volumes between 40% and 60%. Its extended coverage also removed water level errors of 20-40 cm associated with boundary effects in smaller regional models

CaMEL and ADCIRC storm surge models-A comparative study

The Computation and Modeling Engineering Laboratory (CaMEL), an implicit solver-based storm surge model, has been extended for use on high performance computing platforms. An MPI (Message Passing Interface) based parallel version of CaMEL has been developed from the previously existing serial version. CaMEL uses hybrid finite element and finite volume techniques to solve shallow water conservation equations in either a Cartesian or a spherical coordinate system and includes hurricane-induced wind stress and pressure, bottom friction, the Coriolis effect, and tidal forcing. Both semi-implicit and fully-implicit time stepping formulations are available. Once the parallel implementation is properly validated, CaMEL is evaluated against ADCIRC, an established storm surge model, using a hindcast of storm surge due to Hurricane Katrina. Observed high water marks are used to verify that both models have comparable accuracy. The effects of time step on the stability and accuracy of the models are investigated and indicate that the semi- and fully-implicit solvers in CaMEL allow the use of larger timesteps than ADCIRC's explicit and semi-implicit solvers. However, ADCIRC outperforms CaMEL in parallel scalability and execution wall clock times. Wall times of CaMEL improve significantly when the largest stable time step sizes are used in respective models, although ADCIRC still is faster

The role of hydrodynamics in explaining variability in fish populations

A review of the physical processes present in coastal regions and their effect on pelagic stages of flatfish populations is presented. While quantitative understanding of processes affecting cross-shelf transport and exchange continues to be a fundamental problem shared by physical oceanographers and fisheries scientists studying the early life history of flatfish, advances in hydrodynamic and coupled physical-biological models have made it possible to begin to examine population-level implications of environmental processes. There is now a need to rank these processes in terms of their impact on recruit strength. Existing paradigms provide testable frameworks for explaining the role of physical variability in the observed population patterns, abundance and variability. Identifying explicit links between physical variability and recruitment could result in new approaches to fisheries management strategies

Low frequency water level correction in storm surge models using data assimilation

Research performed to-date on data assimilation (DA) in storm surge modeling has found it to have limited value for predicting rapid surge responses (e.g., those accompanying tropical cyclones). In this paper, we submit that a well-resolved, barotropic hydrodynamic model is typically able to capture the surge event itself, leaving slower processes that determine the large scale, background water level as primary sources of water level error. These “unresolved drivers” reflect physical processes not included in the model's governing equations or forcing terms, such as far field atmospheric forcing, baroclinic processes, major ocean currents, steric variations, or precipitation. We have developed a novel, efficient, optimal interpolation-based DA scheme, using observations from coastal water level gages, that dynamically corrects for the presence of unresolved drivers. The methodology is applied for Hurricane Matthew (2016) and results demonstrate it is highly effective at removing water level residuals, roughly halving overall surge errors for that storm. The method is computationally efficient, well-suited for either hindcast or forecast applications and extensible to more advanced techniques and datasets

Storm-driven erosion and inundation of barrier islands from dune-to region-scales

Barrier islands are susceptible to erosion, overwash, and breaching during intense storms. However, these processes are not represented typically in large-domain models for storm surge and coastal inundation. In this study, we explore the requirements for bridging the gap between dune-scale morphodynamic and region-scale flooding models. A high-resolution XBeach model is developed to represent the morphodynamics during Hurricane Isabel (2003) in the North Carolina (NC) Outer Banks. The model domain is extended to more than 30km of Hatteras Island and is thus larger than in previous studies. The predicted dune erosion is in good agreement with post-storm observed topography, and an ‘‘excellent’’ Skill Score of 0.59 is obtained on this large domain. Sensitivity studies show the morphodynamic model accuracy is decreased as the mesh spacing is coarsened in the cross-shore direction, but the results are less sensitive to the alongshore resolution. A new metric to assess model skill, Water Overpassing Area (WOA), is introduced to account for the available flow pathway over the dune crest. Together, these findings allow for upscaled parameterizations of erosion in larger-domain models. The updated topography, obtained from XBeach prediction, is applied in a region-scale flooding model, thus allowing for enhanced flooding predictions in communities along the Outer Banks. It is found that, even using a fixed topography in region-scale model, the flooding predictions are improved significantly when post-storm topography from XBeach is implemented. These findings can be generalized to similar barrier island systems, which are common along the U.S. Gulf and Atlantic coasts

Transient Response of the Gulf Stream to Multiple Hurricanes in 2017

Autonomous underwater glider observations collected during and after 2017 Hurricanes Irma, Jose, and Maria show two types of transient response within the Gulf Stream. First, anomalously fresh water observed near the surface and within the core of the Gulf Stream offshore of the Carolinas likely resulted from Irma's rainfall being entrained into the Loop Current-Gulf Stream system. Second, Gulf Stream volume transport was reduced by as much as 40% for about 2 weeks following Jose and Maria. The transport reduction had both barotropic and depth-dependent characteristics. Correlations between transport through the Florida Straits and reanalysis winds suggest that both local winds in the Florida Straits and winds over the Gulf Stream farther downstream may have contributed to the transport reduction. To clarify the underlying dynamics, additional analyses using numerical models that capture the Gulf Stream's transient response to multiple tropical cyclones passing nearby in a short period are needed

Spatial differences in wind-driven sediment resuspension in a shallow, coastal estuary

Two locations approximately 11 km apart along the axis of the New River Estuary near Jacksonville, NC USA were continuously monitored for eight years. Included in the observations are vertical profiles of turbidity, temperature, salinity, chl-a, dissolved oxygen, pH and water velocity as well as local wind velocity. Differences between the two sites result from a number of factors, including bathymetry, wind strength, direction and fetch, estuarine morphology, tidal currents and sediment properties. The site near the head of the estuary, Morgan Bay, is deeper, experiences generally weaker winds and has less fetch in most directions. Stones Bay, the down-estuary site, is shallower, experiences stronger winds and has longer fetch, particularly in the prevailing wind directions. Current speeds also differ along the estuary with the down-estuary Stones Bay site being more tidal. The observations were used together with a simple wave model to analyze the estuarine turbidity response to different forcing mechanisms. Results suggest that sediments are resuspended primarily by wind-wave generated bottom stress at both locations. While turbidity is generally higher in Stones Bay than in Morgan Bay, turbidity as a function of the local wave-induced bottom stress (including forcing from all directions) is similar at both locations at low stress but diverges at higher stresses. At higher bottom stresses, turbidity in Stones Bay responds primarily to winds from the NE, S and NW while turbidity in Morgan Bay responds primarily to winds from the NW and S. Accounting for sediment resuspension within an approximate spatial advection scale around each of the observation sites, yields a similar turbidity vs bottom stress response curve for the three primary directions in Stones Bay and the S direction in Morgan Bay but a greater turbidity response for winds from the NW in Morgan Bay. In the latter case, waves are crossing the section of the New River Estuary just downstream of the confluence with the New River and are presumably encountering sediments that are more easily resuspended. Average sediment export is down-river with more sediment leaving Stones Bay than Morgan Bay