Integrated Modelling of Beach Water Table Fluctuations

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Trend correlations for coastal eutrophication and its main local and whole-sea drivers – Application to the Baltic Sea

Coastal eutrophication is a major environmental issue worldwide. In the Baltic Sea, eutrophication affects both the coastal waters and the open sea. Various policy frameworks aim to hinder its progress but eutrophicationrelevant water quality variables, such as chlorophyll-a concentrations, still exhibit opposite temporal trends in various Baltic Sea marine and coastal waters. In this study, we investigate the temporal-trend linkages of measured water quality variables and their various anthropogenic, climatic and hydrospheric drivers over the period 1990-2020 with focus on the Swedish coastal waters and related marine basins in the Baltic Sea. We find that it is necessary to distinguish more and less isolated coastal waters, based on their water exchanges with the open sea, to capture different coastal eutrophication dynamics. In less isolated coastal waters, eutrophication is primarily related to nitrogen concentrations, while it is more related to phosphorus concentrations in more isolated coastal waters. In the open sea, trends in eutrophication conditions correlate best with trends in climatic and hydrospheric drivers, like wind speed and water salinity, respectively. In the coastal waters, driver signals are more mixed, with considerable influences from anthropogenic land-based nutrient loads and sea ice cover duration. Summer chlorophyll-a concentration in the open sea stands out as a main change driver of summer chlorophyll-a concentration in less isolated coastal waters. Overall, coastal waters are a melting pot of driver influences over various scales, from local land-based drivers to large-scale total catchment and open sea conditions. The latter in turn depend on long-term integration of pathway-dependent influences from the various coastal parts of the Baltic Sea and their land-based nutrient load drivers, combined with overarching climate conditions and internal feedback loops. As such, our results challenge any unidirectional local source-to-sea paradigm and emphasize a need for concerted local land-catchment and whole-sea measures for robust coastal eutrophication management. (c) 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).Peer reviewe

Modelling coastal ground- and surface-water interactions using an integrated approach

Beach water table fluctuations have an impact on the transport of beach sediments and the exchange of solute and mass between coastal aquifer and nearby water bodies. Details are given of the refinement of a dynamically integrated ground- and surface-water model, and its application to study ground- and surface-water interactions in coastal regions. The depth-integrated shallow-water equations are used to represent the surface-water flow, and the extended Darcy's equation is used to represent the groundwater flow, with a hydrostatic pressure distribution being assumed to apply for both these two types of flows. At the intertidal region, the model has two layers, with the surface-water layer being located on the top of the groundwater layer. The governing equations for these two types of flows are discretized in a similar manner and they are combined to give one set of linear algebraic equations that can be solved efficiently. The model is used to predict water level distributions across sloping beaches, where the water table in the aquifer may or may not decouple from the free water surface. Five cases are used to test the model for simulating beach water table fluctuations induced by tides, with the model predictions being compared with existing analytical solutions and laboratory and field data published in the literature. The numerical model results show that the integrated model is capable of simulating the combined ground- and surface-water flows in coastal areas. Detailed analysis is undertaken to investigate the capability of the model

Simulating moving boundary using a linked groundwater and surface water flow model

The moving boundary problem is a very important phenomenon in shallow water flows over wetlands and floodplains, particularly with regard to flooding. In considering the natural linkage between surface and groundwater flows for practical studies a new integrated surface and groundwater flow model has been developed for predicting shallow water flows, with particular emphasis being given to its capability in dealing with moving boundary problem. In linking a surface and groundwater model, a hydrostatic pressure distribution is assumed to enable the water surface gradient to be used as the driving force for the groundwater flow component of the model. The linked surface and groundwater flow model is capable of treating the moving boundary problem naturally and implicitly, without any specific algorithms being used to deal with the flooding and drying processes explicitly. Four typical numerical tests have been undertaken to study the performance of the new model with particular attention being focused on checking the accuracy, robustness and mass balance of the model. The numerical results show that: (a) the model is capable of treating moving boundary problems effectively without causing any substantial problem for mass conservation, and (b) the model is capable of giving reliable and accurate results

Random Walk Method for Modeling Water Exchange: An Application to Coastal Zone Environmental Management

This study introduces a modeling approach for simulation of water exchange based on the random walk particle tracking method. The water exchange is characterized by a water exchange matrix. The novel features of this model lie in its flexibility of operation and time saving in simulation. The feasibility of this model is demonstrated in simulation case studies conducted in the Tianjin coastal area. The model can be applied to track water exchange between water bodies in estuaries and coastal ocean for simulation and characterization of solute transport, and predict water body self-purification capacity. This model can serve as a useful tool for forecast and assessment of potential water quality issues in coastal environment

Nutrient Fluxes Across Sediment-Water Interface in Bohai Bay Coastal Zone, China

Sediment cores and overlying water samples were collected at four sites in Tianjin Coastal Zone, Bohai Bay, to investigate nutrient (N, P and Si) exchanges across the sediment-water interface. The exchange fluxes of each nutrient species were estimated based on the porewater profiles and laboratory incubation experiments. The results showed significant differences between the two methods, which implied that molecular diffusion alone was not the dominant process controlling nutrient exchanges at these sites. The impacts of redox conditions and bioturbation on the nutrient fluxes were confirmed by the laboratory incubation experiments. The results from this study showed that the nutrient fluxes measured directly from the incubation experiment were more reliable than that predicted from the porewater profiles. The possible impacts causing variations in the nutrient fluxes include sewage discharge and land reclamation

Storm Surges in the Bohai Sea: The Role of Waves and Tides

A storm surge is a complex phenomenon in which waves, tide and current interact. Even though wind is the predominant force driving the surge, waves and tidal phase are also important factors that influence the mass and momentum transport during the surge. Devastating storm surges often occur in the Bohai Sea, a semi-enclosed shallow sea in North China, due to extreme storms. However, the effects of waves on storm surges in the Bohai Sea have not been quantified and the mechanisms responsible for the higher surges that affect part of the Bohai Sea have not been thoroughly studied. In this study, we set up a storm surge model, considering coupled effects of tides and waves on the surges. Validation against measured data shows that the coupled model is capable of simulating storm surges in the Bohai Sea. The simulation results indicate that the longshore currents, which are induced by the large gradient of radiation stress due to wave deformation, are one of the main contributors to the higher surges occurring in some coastal regions. The gently varying bathymetry is another factor contributing to these surges. With such bathymetry, the wave force direction is nearly uniform, and pushes a large amount of water in that direction. Under these conditions, the water accumulates in some parts of the coast, leading to higher surges in nearby coastal regions such as the south coast of the Bohai Bay and the west and south coasts of the Laizhou Bay. Results analysis also shows that the tidal phase at which the surge occurs influences the wave–current interactions, and these interactions are more evident in shallow waters. Neglecting these interactions can lead to inaccurate predictions of the storm surges due to overestimation or underestimation of wave-induced set-up