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The book describes an integrated theory that links estuary shape to tidal hydraulics, tidal mixing and salt intrusion.The shape of an alluvial estuary is characterised by exponentially varying width and the absence of bottom slope.

Saltwater intrusion (SWI) has a negative environmental impact on groundwater quality in coastal areas. Therefore, effective management strategies are required to preserve fresh groundwater resources. Historically, vertical barriers have been exclusively considered in both numerical studies and practical applications. The novelty of this study consists in investigating the SWI mitigation effectiveness of inclined physical subsurface barriers (PSBs), and specifically cutoff walls (CWs) and subsurface dams (SDs). An initial benchmark analysis of the Henry problem was performed. Following verification, the proposed model was applied to a real case study - the Biscayne aquifer (Southeastern Florida, USA). The model simulations run for different scenarios considering the vertical placement of the PSB, an inclined placement of the PSB according to different slopes (1/4, 1/2 and 1/1, at sea- and landside) and the combination of the best scenario. The results showed that CWs are more effective in limiting SWI in comparison with SDs. The most positive impact in both cases was achieved for a slope of 1/4, indicating that a moderate vertical inclination of the PSB better preserve coastal groundwater resources. The model presented in this work can be a valuable tool for policy makers in predicting the coastal aquifer response. However, a comprehensive cost–benefit analysis is required to further account for the feasibility and the economic costs related to the construction of inclined PSBs.

Saltwater intrusion (SWI) is a type of pollution that adversely affects the quality of groundwater in coastal aquifers. The Nile Delta aquifer (NDA) in Egypt contains a large amount of freshwater. Increasing abstraction from the aquifer and sea level rise have led to an increase in SWI, which has reached up to 100 km inland. Therefore, practical measures are required to prevent further SWI. This study aims to identify an optimal well system to manage the intrusion of saline water in NDA using a number of management systems, including pumping of brackish water, aquifer recharge, and abstraction of the freshwater. SEAWAT code is used to simulate SWI in the aquifer considering different scenarios of pumping and sea level rise. Four scenarios are used to control SWI, including: decreasing pumping from the aquifer, increasing recharge using treated waste water, increasing abstraction of brackish water for desalination, and a combination of these systems. The results showed that increasing recharge could lead to greater retardation of SWI (19.5%) than decreasing pumping (6.2%) and abstraction of brackish water (5.9%). However, a combined well system of pumping, recharge and abstraction is shown to be a more effective tool to control SWI in coastal aquifers, with retardation percentage of 21.3%.

Among the well-known approaches for controlling seawater intrusion during extensive freshwater abstraction from coastal aquifers is the construction of subsurface dams. In the current research, the SEAWAT code is being implemented to examine the impact of groundwater extraction on the effectiveness of a damaged subsurface dam for controlling saltwater intrusion. Simulations were performed numerically to check impact of the subsurface dam height, dam location, well height, well location, abstraction rate, fracture aperture, fracture location, seawater density and fracture dimension on the effectiveness of subsurface dam as a countermeasure to prevent saltwater intrusion in coastal aquifers. Increasing the abstraction rate from 1 × 10−6 to 5 × 10−6 m3/s caused the seawater to advance more into the freshwater, and the loss of effectiveness increased. The minimum and maximum value of loss of subsurface dam effectiveness was recorded to be 34.6% to 93%, respectively, for the abstraction rates from the well equal 1 × 10−6 and 5 × 10−6 m3/s, consequentially. When the dimensionless value of well height location Lw/Ld is increased from 1.0 to 2.0, the effectiveness of the subsurface dam is reduced by around 20%. The findings demonstrate that the well location, well depth, abstraction rate, location of the dam, fracture aperture, and density of saltwater all affect the effectiveness impairment of the fractured subsurface dam for controlling saltwater intrusion. Decision makers could use findings of this research to better manage groundwater resources in coastal aquifers.

With continued sea level rise and over-exploitation, saline water extends farther inland, causing changes in soil salinity and water quality and leading to permanent land salinization and ecosystem damage. Saltwater intrusion (SWI), causing numerous ecosystem problems and disasters, brings risk to urban ecosystems in coastal cities. Ecological risk, in the Greater Bay Area in China, should be assessed based on the effect of SWI status on ecosystem health. In this study, we built a new ecological risk-assessment model based on the geographic information system (GIS) technique and spatial data. At the conceptual level, four main stressors were identified based on literature reading and fieldwork. Four stress factors (SFs) were thoroughly investigated, namely, SF1: the intensity gradient immersed in saltwater; SF2: the mountain phreatic water supply; SF3: the salinity tolerance of urban greenbelt vegetation; and SF4: the supply capacity of irrigation water to suppress saline water. After a comprehensive evaluation using GIS and the analytic hierarchy process (AHP), we mapped and assessed the ecological risk level of the urban greenbelt for the SWI. Our results showed that the area of urban green space affected by the SWI was approximately 49.31 km2, almost 12.05%. Ecological risk was sorted into five ranks: (1) very low risk 47.53%, (2) low risk 26.29%, (3) medium risk 22.92%, (4) high risk 2.45%, and (5) very high risk (0.8%). The ecological infrastructure of sponges should include freshwater conservation in coastal cities, and more attention should be paid to fresh groundwater discharge from coastal ecosystems in Shenzhen.

Abstract Climate change and anthropogenic pressure stress groundwater resources and facilitate saltwater intrusion. Raigad, a coastal district on the west side of Maharashtra, is susceptible to the problem of saltwater intrusion. Over the region, varied growth rate with increasing as well as stagnant growth trend of mangrove has been observed. The present study attempts to analyze the scenario of saltwater intrusion vulnerability and its relation to mangrove growth during the past two decades. The GALDIT model has been employed for saltwater intrusion vulnerability assessment. It is an indicator-based model that depends on the hydrogeological parameters of a region. The GALDIT scores have been calculated for the years 2000 and 2019. It has been observed that GALDIT scores have increased from moderately vulnerable to highly vulnerable. The results reveal four distinct saline hotspots in the study area. Salinity has explicitly increased over Shivardhan, Murud, Alibag, and Uran. Representative mangrove patches show that spatial growth of mangrove species depends on the area's salinity, with Avicennia marina being the most salt tolerant and Excoecaria agallocha being the least salt tolerant. The present work will help in decision-making for plantation and conservation programs of mangrove forests.

Laboratory and numerical experiments were conducted to provide a quantitative steady-state analysis of the effect of incremental variations of water level on saltwater intrusion. The purpose was to seek mathematical correlations relating both the wedge toe length and the height along the coastline to the boundary head difference. The laboratory experiments were completed in a 2D sand tank where both freshwater and seawater levels were varied. The experiments were conducted for two bead sizes having different hydraulic conductivities. The numerical model SEAWAT was used to validate the results and then to perform sensitivity analysis. The experimental results show that at steady-state conditions, the logarithmic toe length could be expressed as a linear function of the boundary head difference. The linear relationship was recorded in both advancing and receding wedge phases. The linearity of the correlation was also well demonstrated with analytical solutions. Similar relationships were also derived in the scenarios where the sea level fluctuated while the freshwater boundary head was constant. The height of the saltwater wedge along the coastline was also found to be a linear function of the boundary head difference. The sensitivity analysis shows that the regression coefficients were sensitive to the hydraulic conductivity, the dispersivity, and the saltwater density, while the porosity and the rate of boundary head change induced negligible effects. The existence of a linear relationship between the logarithmic toe length and the boundary head difference was also well evidenced in a field-scale aquifer model for all the different hydrogeological aquifer conditions tested. This study is the first attempt in identifying the underlying correlation between the boundary water level variations and the main seawater intrusion (SWI) external metrics under controlled laboratory conditions, which is of great relevance from a water resources management perspective.

Saltwater intrusion typically develops in the along-channel direction but exceptions can be found in bifurcated estuaries. Based on the observational data, we found that the saltwater intrusion in the upper reaches of the North Channel (NC) of Changjiang Estuary is dominated by the lateral saltwater intrusion from a small northern outlet (denoted as NONC) of this channel. This phenomenon has severe effects on the freshwater usage in this region. To investigate the underlying mechanisms of this pattern of intrusion, numerical experiments were conducted using a well-validated model. A flux decomposing method was used to decompose the process of saltwater intrusion into several mechanisms. During the neap tide, the saltwater begins to intrude landward into the NONC through shear transport induced by estuarine circulation. During the transition period between the neap tide and the following spring tide, the saltwater that previously reached the NONC further intrudes into the NC via Lagrangian and tidal pumping transports, causing a significant salinity increase in the middle and upper reaches of the NC. During the spring tide and the subsequent middle tide, saltwater intrusion in the NONC retreats. The impacts of the topography of the NONC and the wind stress on this lateral saltwater intrusion were also evaluated in this study.

Saltwater intrusion is the leading edge of sea-level rise, preceding tidal inundation, but leaving its salty signature far inland. With climate change, saltwater is shifting landward into regions that previously have not experienced or adapted to salinity, leading to novel transitions in biogeochemistry, ecology, and human land uses. We explore these changes and their implications for climate adaptation in coastal ecosystems. Biogeochemical changes, including increases in ionic strength, sulfidation, and alkalinization, have cascading ecological consequences such as upland forest retreat, conversion of freshwater wetlands, nutrient mobilization, and declines in agricultural productivity. We explore the trade-offs among land management decisions in response to these changes and how public policy should shape socioecological transitions in the coastal zone. Understanding transitions resulting from saltwater intrusion—and how to manage them—is vital for promoting coastal resilience.