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Abstract
Tidal motions of the water table height inside a sloping beach are
investigated via field measurements and theoretical considerations. Only
the movements forced by the tide are considered, so a beach with
negligible wave activity was chosen for the field measurements. The data
show that even in the absence of precipitation the time averaged inland
water table stands considerably above the mean sea level. Also the water
table at a fixed point inside the beach is far from sinusoidal even
though its variation is forced by an essentially sinusoidal tide. This
latter effect is due to the boundary condition along the sloping beach
face which acts as a highly nonlinear filter. The observed behavior of
the water table is explained in terms of perturbation extensions to the
classical "deep aquifer solution." One extension deals with the
nonlinearity in the interior, the other with the boundary condition at
the sloping beach face.
... This inland constant flux was determined from a numerical simulation of the homogeneous coarse sand aquifer, with the inland boundary set to a constant head of 0.565 m, and in the absence of tides. The use of a constant-flux boundary meant that changes to the hydraulic conditions at the ocean boundary, such as the occurrence of tide-induced watertable overheight (i.e., the super-elevation of head conditions at the shoreline arising from tidal effects; see Nielsen, 1990), did not change the time-averaged freshwater component of the total SGD. ...
... Tidal parameters and the inland constant flux used in the laboratory-scale model were based on experimental conditions. To guide the laboratory-and field-scale models, we introduced a dimensionless scale λ, which is defined as the ratio of the aquifer depth to the tidal propagation distance Mo et al., 2021;Nielsen, 1990). ...
... The introduction of tides led to higher time-averaged heads in both aquifers. The difference between non-tidal and time-averaged tidal heads is referred to as watertable overheight, which is the super-elevation of heads near the coast arising from tidal effects (Nielsen, 1990). Tide-induced increases in heads were greater in the unconfined aquifer than in the semi-confined aquifer, as assessed by a comparison of hydraulic heads between Cases F-S and F-T. ...
Coastal aquifers are commonly layered, and thus, a clear understanding of groundwater flow and salt transport in layered coastal aquifers is important for managing fresh groundwater. However, the influence of leakage between adjacent aquifers on flow and transport processes remains largely unknown where the influence of tides is considered. This study used laboratory experiments and numerical simulation to examine the processes of flow and transport within a tidal aquifer‐aquitard system (i.e., an unconfined aquifer underlain by a semi‐confined aquifer, with an intervening thin aquitard). The laboratory‐scale observations of the current study are the first observations of offshore fresh groundwater within a semi‐confined coastal aquifer. The numerical and laboratory results are in close agreement, revealing that upward leakage from the semi‐confined aquifer into the saltwater wedge of the overlying unconfined aquifer caused buoyant instabilities to form. The development of freshwater fingers created complex saltwater‐freshwater mixing, leading to mixed saltwater influx‐efflux patterns across the sloping aquifer‐ocean interface. Compared with non‐tidal conditions, tidal forces reduced the net upward leakage from the semi‐confined aquifer to the overlying unconfined aquifer. This increased the horizontal flow toward the sea, which in turn reduced the extent of the saltwater wedge in the semi‐confined aquifer. The higher rates of both fresh and saline submarine groundwater discharge (SGD), caused by tides, led to lower groundwater ages in the semi‐confined aquifer. These findings have important implications for unveiling the complex characteristics of seawater intrusion, SGD and geochemical hotspots within layered coastal aquifers.
... Ocean tides can elevate the mean groundwater table at the coast well above mean sea level. This is referred to as tidal overheight (Jiao and Post, 2019;Li and Jiao, 2003;Nielsen, 1990;Turner et al., 1997). In regional-scale coastal groundwater flow models, tidal overheight is often either neglected (e.g., Babu et al., 2020;Comte et al., 2014;Green and MacQuarrie, 2014;Masterson and Garabedian, 2007;Oude Essink et al., 2010;Panteleit et al., 2018;Seibert et al., 2023;Sulzbacher et al., 2012) or it is implemented by assuming a time-invariant offset in hydraulic head from the mean sea level (e.g., Grünenbaum et al., 2020b;Holt et al., 2019;Masterson et al., 2013;Röper et al., 2013;Vandenbohede and Lebbe, 2006;Werner and Gallagher, 2006). ...
... The need to consider tidal overheight in a coastal aquifer model is often a matter of scale; i.e., of the interaction between spatial scale, time scale of the tidal forcing, aquifer properties, and other external forces acting on the groundwater system (e.g., recharge) (Pauw et al., 2014). The regional-scale impacts of tidal forcing on hydraulic heads and saltwater intrusion depend on tidal characteristics (e.g., tidal amplitude and spring-neap variations), hydraulic sediment properties, as well as coastal morphology (Jiao and Post, 2019;Nielsen, 1990), whereby tidal fluctuations in an aquifer are increasingly dampened with distance from the coast (Erskine, 1991;Ferris, 1952;Merritt, 2004). Sediments act as a low-pass filter dampening higher frequency signals (e.g., semi-diurnal tidal constituents) more readily than lower frequency ones (e.g., diurnal constituents) (Bye and Narayan, 2009;Rotzoll et al., 2008;Trefry and Bekele, 2004). ...
... Tidal overheight is essentially caused by transmissivity changes in an unconfined aquifer over a tidal cycle (Li and Jiao, 2003;Nielsen, 1990;Turner et al., 1997). The presence of a sloping beach can increase tidal overheight due to the asymmetric hydraulic gradients that develop during rising and falling tide (Ataie-Ashtiani et al., 2001;Nielsen, 1990;Turner et al., 1997). ...
Ocean tides cause water table overheight near the seaward boundary of coastal aquifers which can have a large impact on groundwater flows and saltwater intrusion. Despite this, regional-scale coastal groundwater flow models often neglect the effects of ocean tides or alternatively assume that the influences of ocean tides and sea level are constant over time. These simplifications are often required because simulation of the phase-resolved tidal signal at a coastal boundary is computationally demanding. Our objective was to derive a phase-averaged tidal boundary condition for application along gently sloping coastlines that enables simulation of real, complex tidal signals combined with sea-level changes due to meteorological effects. The boundary condition extends an existing analytical solution of tidal overheight by including an empirical correction function for conditions where the assumptions of the existing solution are violated. The correction function was developed by conducting a parametric study on an idealized two-dimensional cross-sectional coastal aquifer model and considering the effects of parameters including horizontal hydraulic conductivity, vertical anisotropy, specific yield, aquifer thickness, and beach slope. The performance of the new phase-averaged tidal boundary condition was assessed using a three-dimensional groundwater flow model of the island of Spiekeroog, Northwest Germany, which comprises complex coastal morphology with non-planar beach slopes. Simulations applying the new boundary condition were compared to simulations that adopted a phase-resolved tidal boundary condition and to observed groundwater levels. Accounting for time-varying changes in the tidal signal and sea level was found to be critical to simulate observed data and to adequately reproduce the transient tidal overheight simulated by the phase-resolved model. The performance of the new phase-averaged boundary condition in the regional-scale model varied depending on how the coastal morphology was represented in the boundary condition with local morphological features increasingly important for lower intertidal vertical infiltration capacity (low isotropic hydraulic conductivity or high vertical anisotropy) or specific yield. In conclusion, the new boundary condition presented overcomes current limitations in simulating transient tidal overheight in regional-scale groundwater models.
... Fluctuations in ocean water levels owing to tides and storms cause fluctuations in the hydraulic head at the surface aquifer-ocean interface, which generate waves that propagate inland (Anderson and Lauer, 2008;Cartwright et al., 2004;Cartwright and Gibbes, 2011;Erskine, 1991;Ferris, 1951;Jacob, 1950;Nielsen, 1990;Raubenheimer et al., 1999;Rotzoll and El-Kadi, 2008;Trglavcnik et al., 2018). The hydraulic gradients resulting from the groundwater waves influence mixing between ocean water and freshwater within the aquifer, affect the fate and transport of contaminants, and modulate the rate of groundwater discharge to the ocean (Boufadel et al., 2007;Moore, 2010;Robinson et al., 2006Robinson et al., , 2014, and also may affect sediment transport and evolution of the beach profile (Bakhtyar et al., 2013;Chardón-Maldonado et al., 2016;Sous et al., 2013;Turner and Masselink 1998;Xin et al 2010). ...
... Forecasts of coastal flooding resulting from ocean-driven groundwater increases owing to sea level rise and storm intensification, which are important for coastal management, often are based on these linear theory solutions (Vitousek et al., 2017;Woodruff et al., 2013). However, amplitude decay rates estimated from observations can be 2 or 3 times larger than expected from linear theory in comparison with phase lag rates, resulting in uncertainty in the estimates of aquifer parameters (Erskine, 1991;Fakir and Razack, 2003;Nielsen, 1990;Rotzoll and El-Kadi, 2008;Smith and Hick, 2001;Trefry and Bekele, 2004). The ability of higher-order theories to characterize natural aquifers is unknown. ...
... Non-shallow aquifers have been encountered in studies around the world, including Australia (Robinson et al., 2007;Trefry and Bekele 2004), Japan (Uchiyama et al., 2000), Germany (Röper et al., 2013), UK (Erskine, 1991), and the USA (Schultz and Ruppel, 2002). Despite several studies indicating the importance of aquifer depth (Parlange, 1984;Nielsen et al., 1997;Robinson et al., 2007), many studies of coastal water table fluctuations do not report all of the geologic information (depth, hydraulic conductivity, and specific yield) necessary to evaluate higher-order solutions and neglect the potential effects of aquifer depth (Carr and Van der Kamp 1969;Fadili et al., 2016;Fakir and Razack, 2003;Jha et al., 2008;Nielsen 1990;Robinson et al., 2006;Sous et al., 2016, and many more). Additionally, a wide range of ocean wave periods is needed to identify aquifer depth effects on the propagation of hydraulic head fluctuations, but few field studies have had long enough durations to examine hydraulic head fluctuations driven by both tides and storm surges. ...
... Previously, substantial efforts were devoted to predicting watertable fluctuations in coastal unconfined aquifers (e.g., Parlange et al. 1984, Nielsen 1990, Barry et al., 1996, Raubenheimer et al. 1999, Li et al. 2000a, b, Robinson et al. 2006, Heiss and Michael 2014, Shoushtari et al. 2016. Most of these studies investigated watertable fluctuations based on the one-dimensional Boussinesq equation since it is computationally efficient and reveals explicit relations between parameters that affect watertable fluctuations. ...
... Most of these studies investigated watertable fluctuations based on the one-dimensional Boussinesq equation since it is computationally efficient and reveals explicit relations between parameters that affect watertable fluctuations. Initially, the Boussinesq equation was used to describe groundwater flow in the saturated zone (e.g., Parlange et al. 1984, Nielsen 1990, Bear 2012. With time, more laboratory and field data were presented, and it was found that the predictions of the classical Boussinesq equation deviate from measurements. ...
... Most of abovementioned work regarding watertable fluctuations assumes a vertical sea boundary, although sloping beaches are common (Vos et al., 2020). Nielsen (1990) developed an analytical approximation to predict watertable fluctuations in coastal unconfined aquifers with a sloping sea boundary using a perturbation method. However, this analytical solution only matches the time-varying sea boundary condition approximately. ...
... Philip [3] modelled the groundwater response to tidal forcing for a vertical beach and found that the water table is elevated above mean sea level even in the absence of recharge. Nielsen [2] further investigated the inland overheight by considering the effects of a sloping beach via a perturbation solution. His result, given to second order, demonstrated that a sloping beach enhanced the inland overheight as well as the asymmetry in the water table fluctuations and their shift from the tidal signature. ...
... The nonlinear nature of the system of equations (9)-(13) with (16) is such that an exact solution is not possible. Teo et al. [4] considered hydraulic conductivities in the range 50-1000 m/day, with aquifer depths of 5-10 m and tidal amplitudes of 1-2 m, while Nielsen [2] considered tidal amplitudes and aquifer depths on the order of 0.5 m, with hydraulic conductivities on the order of a few mm/day. Given a soil porosity in the range 0.2-0.4 and considering semi-diurnal and diurnal tides, these values correspond to parameter values in the range 0.1 to 0.6. ...
... During the rising tide, water flows landward at a faster rate than it flows seaward during the ebb tide, which is balanced by an elevation in the water table; thus a greater tidal amplitude and hydraulic conductivity, which arises implicitly through β 0 , see (15), enhances the extent of the overheight, while a deeper aquifer mitigates this effect. A sloping beach enhances the overheight as water can infiltrate into the aquifer vertically during the rising tide, but is only able to seep from the aquifer horizontally during the ebb [2]. ...
... Watertable fluctuations induced by oceanic oscillations are an important signal for quantifying these processes, and their prediction is a longstanding topic (e.g., Philip, 1973;Smiles & Stokes, 1976;Parlange et al., 1984). Such fluctuations have been investigated by field measurements (e.g., Nielsen et al., 1990;Raubenheimer et al., 1999;Robinson et al., 2006;Heiss & Michael, 2014;Trglavcnik et al., 2018), laboratory experiments (e.g., Cartwright et al., 2004;Robinson & Li, 2004;Shoushtari et al., 2016, numerical simulations (e.g., Li et al., 1997;Cartwright et al., 2006;Shoushtari et al., 2015;Brakenhoff et al., 2019) and analytical solutions (e.g., Parlange & Brutsaert, 1987;Barry et al., 1996;Nielsen et al., 1997;Li et al., 2000a,b;Teo et al., 2003;Song et al., 2007;Kong et al., 2013Kong et al., , 2015. Among these, analytical solutions based on the 1D Boussinesq equation describe watertable fluctuations and give results that are easily computed, present explicit relations between parameters that impact watertable fluctuations, and can be used as benchmarks for numerical simulations. ...
... The Dupuit-Forchheimer-based Boussinesq equation describes groundwater flow in the saturated zone (Bear, 2012). It is amenable to analytical investigations, and so is used to reveal mechanisms that influence watertable wave propagation (e.g., Nielsen, 1990;Li et al., 2000a;Teo et al., 2003;Jeng et al., 2005a). To examine the effects of vertical flow on watertable fluctuations, Liu and Wen (1997) and Nielsen et al. (1997) proposed a Boussinesq equation based on different approximation methods. ...
... Here, an extensive set of existing 2D experimental results is used to examine the modified governing Eq. (18) (3) Eq. (A3) only with the dynamic effective porosity. Following previous studies (e.g., Nielsen, 1990;Barry et al., 1996;Li et al., 2000a;Cartwright et al., 2004;Kong et al., 2015), the dispersion relation linking the amplitude decay rate (k r D) with phase lag increase rate (k i D) is adopted to characterize the propagation of watertable waves. A total of 122 sand flume experiments, covering a wide range of Γ values from 4 to 415, were reported by Shoushtari et al. (2016) and are used here to examine the validity of these predictions. ...
Watertable fluctuations are a characteristic feature of coastal unconfined aquifers. They interact with the vadose zone creating a dynamic effective porosity, for which a new (empirical) expression is proposed based on a dimensionless parameter related to the fluctuation frequency. After comparing with both experimental data and numerical simulations, the new expression is implemented into a modified Boussinesq equation, allowing for examination of the effects of the dynamic effective porosity on watertable fluctuations. The dispersion relation arising from the modified Boussinesq equation predicts laboratory experimental data accurately, highlighting the importance of the dynamic effective porosity in modeling watertable fluctuations in coastal unconfined aquifers. This in turn confirms the applicability of the real-valued expression of the dynamic effective porosity. An outcome is that the phase lag between the total moisture (above the watertable) and watertable height measured in laboratory experiments using vertical soil columns (1D systems) can be ignored when predicting watertable fluctuations in coastal unconfined aquifers (2D systems). The dynamic effective porosity is always smaller than the soil porosity, and so by comparison there is a reduction in vertical water exchange between the saturated and vadose zones. Consequently, watertable waves can propagate further landward without standing wave behaviors.
... There is some confusion in beach groundwater literature concerning the terms dipwell/stilling well/standpipe piezometer and piezometer; these have been usedsomewhat indiscriminately and interchangeably (e.g. Dominick et al., 1971;Nicholls, 1985;Nielsen, 1990a;Hegge & Masselink, 1991;Aseervatham et al., 1993;Turner, 1993&;Kang & Nielsen, 1994). A well open to the atmosphere and perforated throughout its length measures the elevation of the water table. ...
... The amplitude of tidally-forced groundwater fluctuation attenuates to shoreward of the beachface (Emery & Foster, 1948;Erickson, 1970). Such landward dampening can be identified within 10 m (Nielsen, 1990a;Turner, 1993a), but can extend up to 30 m . The inland position, where oscillations of the water table due to tidal forcing are negligible, is used as the landward boundary in some analytical models for tidally-forced groundwater oscillations (e.g. ...
... The inland position, where oscillations of the water table due to tidal forcing are negligible, is used as the landward boundary in some analytical models for tidally-forced groundwater oscillations (e.g. Dominick et al., 1971;Nielsen, 1990a). ...
p>A field investigation of a macro-tidal, ridge and runnel beach at Morfa Dyffryn, North Wales, is described, which examines the distinctive characteristics of a mixed (sand and shingle) beach.
The moments of the velocity field and wave reflection at Morfa Dyffryn are compared with those from a similarly macro-tidal, sandy beach at Neiuwpoort-aan-Zee, Belgium. Strong, wind-enhanced, mean longshore currents persist into shallow water. Results of the velocity moment analysis indicate that the mean flows dominate the sediment transport patterns, particularly in shallow water, early in the flood and late in the ebb phases of the tide. The mean longshore current can be responsible for mobilising (in addition to transporting) sediment, when in excess of 0.3 m s<sup>-1</sup>. Mean fluxes dominate the measured suspended sediment transport patterns, in common with other macro-tidal beaches. Suspended sediment fluxes are generally at their highest in shallow water, although extremely high fluxes were observed just after High Water. These patterns are attributed to increased re-suspension due to wind/resolved mean current interaction. Therefore, high transport rates can occur, even in the absence of storm conditions. The relict sand/shingle ridge has little influence on reflection of wind wave energy. However, reflection of swell waves is enhanced, once the swash zone inundates the sand/shingle berm, due to its steeper gradient.
Fluctuations in the beach water table due to tidal inundation are compared to groundwater behaviour on a sandy beach at Canford Cliffs, Dorset. The GRIST I model (which was derived for sandy beaches) predicts well both the location and translation of a seepage face across the sand/shingle profile. The sand fraction of the sediment determines the response of the water table. Laboratory hydraulic conductivity tests undertaken on sand/shingle mixtures indicate that the shingle content of a beach should be in excess of 80% to 90%, before increased dissipation of energy through percolation can occur.
The main significance of the sand/shingle mixture, in relation to the morphodynamic response of the ridge is through its ability to maintain a steeper slope than would be supported by a sandy beach. The change in gradient across the mixed beach profile has more influence on the hydrodynamics, than does the sediment composition.</p
... Thus, the extent of nitrate contamination increased as the flux of fresh groundwater discharge decreased. Fresh groundwater discharge under tidal conditions was lesser than that under static conditions, with a decrease of 16.90% ((0.284-0.236)/0.284) between case 1 and case 2. Tidal oscillations gave rise to the overheight phenomenon of the groundwater level on the seaward side (Ataie-Ashtiani et al., 1999;Nielsen, 1990). Groundwater level overheight decreased the hydraulic gradient of the aquifer and subsequent fresh groundwater discharge (Robinson et al., 2007). ...
... This demonstrated the nonlinear effect of tidal amplitude on the extent of nitrate contamination. Nielsen (1990) has proposed a prediction of the tide-averaged rise in water table elevation (i.e., groundwater level overheight) as follows: ...
... The growth of the extent of nitrate contamination was more evident with the increase in tidal amplitudes. This was mainly due to the nonlinear relationship between the tide-averaged rise in groundwater level and the tidal amplitude (Ataie-Ashtiani et al., 1999;Nielsen, 1990). In addition, the effect of subsurface dam height on nitrate pollution was also nonlinear. ...
The construction of subsurface dams for controlling seawater intrusion triggers the accumulation of nitrate upstream of a dam. This is raising the concerns about nitrate contamination in those regions of coastal aquifers that are supposed to be used as a fresh groundwater source behind a subsurface dam. Research on this subject has been mostly restricted to the use of a simplified sea boundary (e.g., static and no slope), ignoring sea level fluctuations driven by tides. In this study, the combined effect of tides and subsurface dams on nitrate pollution in upstream groundwater was examined through laboratory experiments and numerical simulations. The results revealed that the difference in the extent of nitrate contamination under various conditions (i.e., static, tidal, static with a dam, and tidal with a dam) was related to the temporal pollution behavior. In the early stage, nitrate contamination in upstream groundwater was essentially identical for different scenarios. Both tides and subsurface dams were found to increase nitrate contamination in upstream aquifers. The extent of nitrate contamination increased with higher tidal amplitudes, whereas the increment was more evident for a large tidal amplitude. The effects of tides and subsurface dams on nitrate contamination were also regulated by the locations and infiltration rates of the pollution source. Interestingly, under the joint action of tides and subsurface dams, the increment in the extent of nitrate pollution was greater than the sum of their individual effects. The increased pollutions caused by subsurface dams and tides were quantified as 9.47% and 37.22%, respectively, whereas the increased value caused by their joint action was measured as 51.10%. These findings suggest that tidal activity should not be overlooked when assessing nitrate contamination in upstream groundwater.
... In 1990, Nielsen proposed that when the ratio of the tidal amplitude to the tidal-affected distance of the aquifer is much smaller than the sine of the coast angle (as shown in Fig. 6), that is, when A L cot ≪ 1 , the coast can be described as vertical coast (Nielsen 1990). ...
... Referring to the solution method proposed by Nielsen (Nielsen 1990), the groundwater level fluctuation equation in coastal phreatic aquifer is In Eq. (6), S represents groundwater level (L), d represents specific yield (), K represents hydraulic conductivity (L/T), M represents aquifer thickness (L), and x represents the distance from land to coastline (L). ...
Taking advantage of the periodic fluctuation of groundwater level caused by tides in coastal areas, the hydrogeological parameters of aquifers can be solved analytically. This method greatly simplifies the acquisition of hydrogeological parameters in coastal areas. However, it is not appropriate to generalize the sea level to a simple wave function for the complex types of tides, such as irregular diurnal tide and irregular semi-diurnal tide. Dongshan Bay, Fujian Province, China is a bay characterized by irregular semi-diurnal tide. In this study, a period of sea level monitoring and groundwater level monitoring in coastal phreatic aquifer were carried out in the irregular semi-diurnal tide area. Fourier series method is used to analyze the time series of sea level. The fluctuation equation of sea level in the irregular semi-diurnal tide area is represented by the superposition of multiple harmonics. The fluctuation equation of groundwater in the phreatic aquifer in the irregular semi-diurnal area is derived using the fluctuation equation of sea level. Finally, the measured data of groundwater level is used as a reference basis. The goal is to minimize the error between the measured groundwater level and the calculated groundwater level. The least square method is used to invert and identify the hydrogeological parameters of the aquifer. This analytical solution provides a useful mathematical tool for obtaining hydrogeological parameters in aquifers in areas similar to irregular semi-diurnal tides and irregular diurnal tides.
... [1,2,3,4,5,6,7]. The beach groundwater circulation is mainly controlled by the combination of the cross-barrier gradients due to tides or large-scale fluctuations of the mean water level and the wave forcing at the shoreline, including swell and long infragravity waves [8,9,10,11,12]. The swash zone is the ultimate part of the beach exposed to ocean forcing, alternatively covered and uncovered by wave run-up (uprush) and run-down (backwash). It moves across the beachface following the fluctuations of 10 the mean water level at the shoreline due to tides, atmospheric pressure variations, wind or wave setup. ...
... Owing to the difficulty to measure experimentally the groundwater parameters, and in particular to monitor their spatio-temporal dynamics over a relevant area, a series of numerical approaches have been 25 proposed during the last few decades. First models based on Darcy's theory and purely horizontal groundwater dynamics (Dupuit-Forchheimer assumption) [8,31] allowed to describe the tidal dynamics. Similar assumptions of purely horizontal flows were made in the coupled approach using Boussinesq model for wave propagation and Darcy-Forchheimer within the porous medium [22,32]. ...
This study introduces a model based on Richards’ equation to describe variably-saturated beach groundwater flow. The surface wave propagation is computed by the phase-resolving non-hydrostatic SWASH code. The SWASH data are used to make a suitable dynamic boundary condition at the beach face to force Richards’ equation. The latter is solved by a weighted discontinuous Galerkin method together with adaptive mesh refinement. The model is validated by comparison with a laboratory experiment of a transient water table recharge problem. Then, the BARDEX II prototype-scale experiment is considered to assess the model abilities for beach groundwater dynamics. The barrier beach is studied for three cases with different lagoon levels. Steady-state results with no-wave conditions show excellent agreement. Transient waves simulations are evaluated in terms of pressure heads, saturations, water table position and groundwater velocities for time-averaged, swash-resolving and spectral analysis. Results bring interesting insights about beach groundwater modelling by comparison with the experimental data as well as a Darcy’s equation-based model. A first investigation is carried out to assess the groundwater effect on the bed sediment dynamics through the modification of sediment relative weight.
... Second, the rise and fall of the tide against a sloping shoreline propagates a tidally produced elevation in the hydraulic gradient into the coastal aquifer. This is commonly recognized as tidal pumping (Nielsen 1990). Tidal variation at the shoreline will propagate landward on the water table roughly at a speed dependent on the ratio of K and T (Metcalf 1995). ...
... The action of the tide against a sloping beach face elevates the mean hydraulic gradient landward of the shoreline and drives a component of the net seaward flow of groundwater landward of the shoreline due to the tidally induced over height. To a first approximation, the magnitude if this impact is calculated as (Nielsen 1990) ...
Subterranean estuaries (STEs), like open-water estuaries are zones of mixing between seawater and freshwater with a characteristic structure. Despite the diverse manifestations of STEs, the mixing processes have elements in common with open-water estuaries, which can serve as a basis for their classification. A typology for STEs might provide a road map for further distilling a working definition of STEs. By analogy with open-water estuaries, a typology for STEs might include characteristic physical drivers and processes, morphology, and biologically relevant parameters. I suggest that such a typology be based on salinity structure to include at a minimum the 1) coastal slope, 2) tidal range, 3) hydraulic conductivity, and 4) recharge. Even a partially applicable definition permits classification, encourages comparisons and can provide a framework for management.
... Coastal aquifers are characterized by the influence of tides that generate periodic changes in the groundwater table [1,2]. The study of tidal influences has allowed the authors to determine hydraulic properties [3,4] and identify tens of tidal constituents with different fluctuation frequencies and amplitudes [5]. ...
... Semidiurnal and diurnal frequencies, as well as fortnightly components, are the most important constituents in groundwater head time series measured in unconsolidated aquifers [6,7]. The amplitude of the fluctuation decreases exponentially with the distance from the shoreline [8,9], while the time-lag between the sea level and the water table increases [1,10]. ...
The temperature distribution of shallow sectors of coastal aquifers are highly influenced by the atmospheric temperature and recharge. However, geothermal heat or vertical fluxes due to the presence of the saline wedge have more influence at deeper locations. In this study, using numerical models that account for variable density, periodic oscillations of temperature have been detected, and their origin has been attributed to the influence exerted by recharge and tides. The combined analysis of field data and numerical models showed that the alternation of dry and wet periods modifies heat distribution in deep zones (>100 m) of the aquifer. Oscillations with diurnal and semidiurnal frequencies have been detected for groundwater temperature, but they show differences in terms of amplitudes and delay with electrical conductivity (EC). The main driver of the temperature oscillations is the forward and backward displacement of the freshwater–saltwater interface, and the associated thermal plume generated by the upward flow from the aquifer basement. These oscillations are amplified at the interfaces between layers with different hydraulic conductivity, where thermal contours are affected by refraction.
... A stability diagram to predict the stability of the USP was proposed by Greskowiak, (2014) based on two dimensionless parameters; π 1 from the stability analysis of Oostrom et al. (1992) and ∊ introduced by Nielsen (1990). By definition, π 1 is the ratio of gravitational force to circumferential lateral groundwater flow, whereas the perturbation parameter ∊ is the ratio of the horizontal tidal propagation distance to ...
Iron‐(hydro)oxide formation in subterranean estuaries is visualised in a sand tank experiment. The so‐called ‘iron curtain’ forms at the interfaces where anoxic, iron (II)‐rich groundwater mixes with circulating oxic seawater.
... First, atoll aquifers have high hydraulic conductivities ranging from 10 −1 to 10 −4 m s −1 (Ketabchi et al., 2014;Oberdorfer et al., 1990;Werner et al., 2017). Because of the high hydraulic conductivity, a change in sea level during a storm surge can propagate inland rapidly with a weak attenuation in amplitude (e.g., Li et al., 1997;Li et al., 2000;Nielsen, 1990;Nielsen et al., 1997). Second, typical atolls have narrow widths ranging from 400 to 1,000 m (Bailey et al., 2013;Chui & Terry, 2013;Underwood et al., 1992), and the entire subsurface of an atoll is susceptible to changes in sea level during storm surges. ...
... A different study found that tides on Jeju Island, Korea, affect the coastal groundwater system up to 3 km inland (Kim et al., 2006). According to studies (Carr and van der Kamp, 1969;Nielsen, 1990), the oscillation in tide movement modulates the groundwater head in coastal zones, which results in periodic variations in the groundwater table. The freshwaterseawater mixing zone is directly impacted by such changes in groundwater head (Bear, 1972;Strack, 1976;Wang and Tsay, 2001). ...
... A stability diagram to predict the stability of the USP was proposed by Greskowiak, (2014) based on two dimensionless parameters; π 1 from the stability analysis of Oostrom et al. (1992) and ∊ introduced by Nielsen (1990). By definition, π 1 is the ratio of gravitational force to circumferential lateral groundwater flow, whereas the perturbation parameter ∊ is the ratio of the horizontal tidal propagation distance to ...
... Coastal aquifer dynamics and the width of the transition zone between salt-and freshwater are strongly influenced by inland and tidal forcing (Heiss & Michael, 2014;LeRoux et al., 2021;Pool et al., 2014;Robinson et al., 2007;Robinson & Li, 2004). Analytical solutions have been developed to quantify how tidal signals propagate into coastal aquifers and influence groundwater dynamics (Bakker, 2019;Inouchi et al., 1990;Li & Jiao, 2001;Nielsen, 1990). Such solutions have been applied in an inverse manner to infer aquifer properties from observed groundwater tidal signals that are damped and lagged in comparison to surface water tides (Zhang et al., 2020). ...
Climate change will increase sea levels, driving saltwater into coastal aquifers and impacting coastal communities and land use viability. Coastal aquifers are also impacted by tides that control groundwater‐ocean interactions and maintain an “upper saline plume” (USP) of brackish groundwater. Coastal dikes are designed to limit the surface impacts of high‐amplitude tides, but, due to ongoing sea‐level rise (SLR), low‐lying dikelands and underlying aquifers are becoming increasingly vulnerable to flooding from high tides and storm surges. This study combines field observations with numerical modeling to investigate ocean‐aquifer mixing and future saltwater intrusion dynamics in a mega‐tidal (tidal range >8 m) dikeland along the Bay of Fundy in Atlantic Canada. Field data revealed strong connectivity between the ocean and coastal aquifer, as evidenced by pronounced tidal oscillations in deeper groundwater heads and an order of magnitude intra‐tidal change in subsurface electrical resistivity. Numerical model results indicate that SLR and surges will force the migration of the USP landward, amplifying salinization of freshwater resources. Simulated storm surges can overtop the dike, contaminating agricultural soils. The presence of dikes decreased salinization under low surge scenarios, but increased salinization under larger overtopping scenarios due to landward ponding of seawater behind the dike. Mega‐tidal conditions maintain a large USP and impact aquifer freshening rates. Results highlight the vulnerability of terrestrial soil landscapes and freshwater resources to climate change and suggest that the subsurface impacts of dike management decisions should be considered in addition to protection measures associated with surface saltwater intrusion processes.
... First, atoll aquifers have high hydraulic conductivities ranging from 10 −1 to 10 −4 m s −1 (Ketabchi et al., 2014;Oberdorfer et al., 1990;Werner et al., 2017). Because of the high hydraulic conductivity, a change in sea level during a storm surge can propagate inland rapidly with a weak attenuation in amplitude (e.g., Li et al., 1997;Li et al., 2000;Nielsen, 1990;Nielsen et al., 1997). Second, typical atolls have narrow widths ranging from 400 to 1,000 m (Bailey et al., 2013;Chui & Terry, 2013;Underwood et al., 1992), and the entire subsurface of an atoll is susceptible to changes in sea level during storm surges. ...
Storm surges associated with tropical cyclones endanger atolls through groundwater flooding, wherein groundwater is discharged from the land surface at elevated sea levels. Atolls are characterised by a ‘dual‐aquifer’ configuration, where recent Holocene sediments unconformably overlie highly permeable Pleistocene limestone creating an interface called a ‘Thurber discontinuity.’ This study aimed to quantitatively analyse how the dual‐aquifer configuration of atolls controls the temporal dynamics of groundwater flooding caused by storm surges. To this end, we ran surface‐subsurface coupled synthetic numerical simulations using the HydroGeoSphere code and compared 12 scenarios with different Thurber discontinuity elevations and hydraulic conductivities of the Pleistocene aquifer ( K P ). The results showed that the shallower the Thurber discontinuity and the higher the K P , the higher the maximum water depth in the freshwater swamp on the atoll during the storm surge and the longer the flooding duration. Despite the effects of the different dual‐aquifer configurations, the initial water table elevation and salinity distribution were almost identical in all simulation cases. These findings suggest that accurate information on the dual‐aquifer configuration is necessary to evaluate the potential risk of groundwater flooding in atolls accompanying storm surges. Furthermore, the results indicate that groundwater flooding caused by storm surges substantially contributes to cyclone‐driven flooding on atolls, and hence, it should not be neglected in flood predictions to avoid underestimation.
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... One of the most important aspects of this modeling study is to explicitly and fully consider the effects of tidal forcing on surface water-groundwater interactions. While some existing studies (e.g., Nielsen, 1990;Robinson, Gibbes, et al., 2007;Y. Zhang et al., 2020) focus on the groundwater flow dynamics near the beaches or nearshore areas in response to the ocean tides, this study is believed to be among the very few that explicitly investigate how the ocean tides impact the regional groundwater flow system as the tidal forcing propagates far away from the coast through the river channel network (Hoitink & Jay, 2016). ...
River deltas typically have high population density and support a wide range of intensive and prosperous socioeconomic activities. The hydrological processes in these regions are complex, primarily due to the interactions among the river, aquifer, and sea. However, a systematic and quantitative elaboration of the river-aquifer-sea interactions is still lacking. Here we developed an integrated hydrological flow model for the Pearl River Delta (PRD), which contains the world's largest urban area in both size and population, to gain a deeper understanding of the complexities in the river-aquifer-sea interactions. The model performance was validated and cross-checked via observations at gauging stations and independent remote-sensing products (e.g., soil moisture, ET and total water storage anomalies). Based on the 10-year simulation results (2004-2013), the major findings of this study are as follows: 1) accurate representation of the tidal effect is important not only for simulating short-term flow dynamics but also for capturing the characteristics of long-term hydrological fluxes and states; 2) the flow-model-computed average groundwater discharge rate per unit length of the coastline for the PRD is 3.01 m 3 /d/m, which is comparable with those derived from water budget approaches but 1-2 orders of magnitude lower than the total submarine groundwater discharge (SGD) estimated by using isotope tracer-based methods; 3) the temporal variation of SGD is controlled by tidal forcing on an hourly time scale, but by terrestrial hydrological processes on monthly and annual time scales; and 4) an integrated hydrological flow model can be used to identify distinct and large subsurface zones sensitive to tidal fluctuations, quantifying the pivotal role of ocean tides in shaping the coastal groundwater system. This study represents a first step in using an integrated hydrological model to explore river-aquifer-sea interactions and their effects on the regional groundwater system simultaneously driven by meteorological and tidal forcings.
... The lower frequencies of wind pumping, compared to frequencies of waves or diurnal tides, drives water table fluctuations and associated mixing further inland because the signal propagation is directly linked to its period (e.g. Nielsen 1990). During high water associated with W wind, oxygen-rich lagoon water recharges the coastal supra-permafrost aquifer, interacting with the coastal sediments and organic matter. ...
Groundwater discharge transports dissolved constituents to the ocean, affecting coastal carbon budgets and water quality. However, the magnitude and mechanisms of groundwater exchange along rapidly transitioning Arctic coastlines are largely unknown due to limited observations. Here, using first-of-its-kind coastal Arctic groundwater timeseries data, we evaluate the magnitude and drivers of groundwater discharge to Alaska’s Beaufort Sea coast. Darcy flux calculations reveal temporally variable groundwater fluxes, ranging from −6.5 cm d ⁻¹ (recharge) to 14.1 cm d ⁻¹ (discharge), with fluctuations in groundwater discharge or aquifer recharge over diurnal and multiday timescales during the open-water season. The average flux during the monitoring period of 4.9 cm d ⁻¹ is in line with previous estimates, but the maximum discharge exceeds previous estimates by over an order-of-magnitude. While the diurnal fluctuations are small due to the microtidal conditions, multiday variability is large and drives sustained periods of aquifer recharge and groundwater discharge. Results show that wind-driven lagoon water level changes are the dominant mechanism of fluctuations in land–sea hydraulic head gradients and, in turn, groundwater discharge. Given the microtidal conditions, low topographic relief, and limited rainfall along the Beaufort Sea coast, we identify wind as an important forcing mechanism of coastal groundwater discharge and aquifer recharge with implications for nearshore biogeochemistry. This study provides insights into groundwater flux dynamics along this coastline over time and highlights an oft overlooked discharge and circulation mechanism with implications towards refining solute export estimates to coastal Arctic waters.
... Groundwater flow under sandy beaches can be considered as flow in shallow aquifers, with only horizontal groundwater flow so that the pressure distribution is hydrostatic (Baird et al., 1998;Brakenhoff et al., 2019;Nielsen, 1990;Raubenheimer et al., 1999). The cross-shore flow dominates temporal variations of groundwater levels. ...
... Central Guizhou lay within South China Craton was influenced by the far-fields effects of LPIA and extraordinary isotopic features of seawater (indicated by carbonates at that time; Liu et al., 2015;Qie et al., 2011) and ancient exposure surface (Wang et al., 2013b). GWTs fluctuated driven by those variations of sea level in central Guizhou due to its paleogeographic environment (a coastal karst plain), which is also seen in Quaternary and modern records that show that sea-level fluctuations influence groundwater hydrology (Ataie-Ashtiani et al., 2001;Ian, 1993Ian, , 1995Li et al., 1997Li et al., , 2000Nielsen et al., 1990;Nicholls and Cazenave, 2010;Philip, 1973;Rotzoll and Fletcher, 2012). In addition, eustatic fluctuations may be responsible for changes in the amount of seawater present in the karst indicated by the B/Ga ratio (Fig. 8), as central Guizhou was located on a coastal karst plain (Fig. 1B). ...
The Carboniferous karstic bauxite-bearing rock series within the Jiujialu Formation were widely distributed in central Guizhou, South China, with a prominent enrichment of ferrous clay materials at the lower part and fluctuations of Al contents at the upper part of those deposits. Yet, the evolution, distribution and formation mechanism of those bauxite ore-bearing rock series require further investigation. We used thickness data from ∼800 drillcores in central Guizhou to generate a high-precision paleo-geomorphological reconstruction of the study area, which reveals a series of karstic depressions from Southwest to the Northeast on a coastal karstic plain. Based on a typical drillcore of ZK47-12 located at a karstic depression, a relatively complete depositional records of bauxite deposits in the Jiujialu Formation have been yielded. The study bauxite deposit contains the lower Fe-rich layer and upper Al-rich layer. Within Al-rich deposits, bauxite ore comprises Al-bearing minerals including diaspore, boehmtie, with slight amounts of Fe-bearing and clay minerals, while bauxitic claystone yields higher contents of clay minerals. And Fe-rich layers comprise Fe-bearing minerals (mainly for chamosite). Geochemistry of the drillcore samples and mass changes reveal strong losses of most mobile elements, relative enrichment of Al within the bauxite ores and enrichment of some trace elements and REEs within claystones, caused by leaching process. Mineralogical and geochemical cycles within the upper Al-rich layer directly link with variations of the groundwater table (GWT) during the bauxitization in central Guizhou. The B content and B/Ga ratios indicate that the Jiujialu Formation in central Guizhou contains variations of paleosalinity, this evidence further supports that the fluctuations of GWT may cause by the Early Carboniferous sea-level changes. Our new data indicate that multiple factors control the formation of the Carboniferous bauxite deposit in central Guizhou. Paleo-geomorphological features of a series of karstic depressions upon the underlying carbonate rocks providing deposition space, tropical climate near the paleo-equator contributing to the weathering process, frequent fluctuation of groundwater table enhancing or releasing the intensity of leaching process, glacio-eustatic cycles promoting karstification and driving the fluctuations of GWT, and input of seawater influencing the intensity of leaching process during bauxitization.
... At location ML1, the tidal signal is reflected stronger in the deeper wells and more damped in the shallow ones. At a closer look, the heads observed in the shallow wells (6 m and to a smaller extent also at 12 m) near the MHWL at ML2, do not follow the typical tidal signal of sinusoidal shape but are rather influenced by the beach slope, revealing the so-called tidal overheight (Nielsen, 1990). Moreover, the head observed in ML2 at 6 m is even higher than the incoming tidal signal of the North Sea which can be related to the occurrence of high-energy waves near the MHWL (Waska et al., 2019). ...
Subterranean estuaries are connective zones between inland aquifers and the
open sea where terrestrial freshwater and circulating seawater mix and undergo
major biogeochemical changes. They are biogeochemical reactors that modify
groundwater chemistry prior to discharge into the sea. We propose that
subterranean estuaries of high-energy beaches are particularly dynamic
environments, where the effect of the dynamic boundary conditions
propagates tens of meters into the subsurface, leading to strong spatiotemporal
variability of geochemical conditions. We hypothesize that they form
a unique habitat with an adapted microbial community unlike other typically
more stable subsurface environments. So far, however, studies concerning
subterranean estuaries of high-energy beaches have been rare and therefore
their functioning, and their importance for coastal ecosystems, as well as for
carbon, nutrient and trace element cycling, is little understood. We are
addressing this knowledge gap within the interdisciplinary research project DynaDeep by studying the combined effect of surface (hydro- and
morphodynamics) on subsurface processes (groundwater flow and transport,
biogeochemical reactions, microbiology). A unique subterranean estuary
observatory was established on the northern beach of the island of Spiekeroog
facing the North Sea, serving as an exemplary high-energy research site and
model system. It consists of fixed and permanent infrastructure such as a pole
with measuring devices, multi-level groundwater wells and an electrode chain.
This forms the base for autonomous measurements, regular repeated sampling,
interdisciplinary field campaigns and experimental work, all of which are
integrated via mathematical modelling to understand and quantify the
functioning of the biogeochemical reactor. First results show that the
DynaDeep observatory is collecting the intended spatially and temporally
resolved morphological, sedimentological and biogeochemical data. Samples
and data are further processed ex-situ and combined with experiments and
modelling. Ultimately, DynaDeep aims at elucidating the global relevance of
these common but overlooked environments.
... At location ML1, the tidal signal is reflected stronger in the deeper wells and more damped in the shallow ones. At a closer look, the heads observed in the shallow wells (6 m and to a smaller extent also at 12 m) near the MHWL at ML2, do not follow the typical tidal signal of sinusoidal shape but are rather influenced by the beach slope, revealing the so-called tidal overheight (Nielsen, 1990). Moreover, the head observed in ML2 at 6 m is even higher than the incoming tidal signal of the North Sea which can be related to the occurrence of high-energy waves near the MHWL (Waska et al., 2019). ...
Subterranean estuaries are connective zones between inland aquifers and the open sea where terrestrial freshwater and circulating seawater mix and undergo major biogeochemical changes. They are biogeochemical reactors that modify groundwater chemistry prior to discharge into the sea. We propose that subterranean estuaries of high-energy beaches are particularly dynamic environments, where the effect of the dynamic boundary conditions propagates tens of meters into the subsurface, leading to strong spatio-temporal variability of geochemical conditions. We hypothesize that they form a unique habitat with an adapted microbial community unlike other typically more stable subsurface environments. So far, however, studies concerning subterranean estuaries of high-energy beaches have been rare and therefore their functioning, and their importance for coastal ecosystems, as well as for carbon, nutrient and trace element cycling, is little understood. We are addressing this knowledge gap within the interdisciplinary research project DynaDeep by studying the combined effect of surface (hydro- and morphodynamics) on subsurface processes (groundwater flow and transport, biogeochemical reactions, microbiology). A unique subterranean estuary observatory was established on the northern beach of the island of Spiekeroog facing the North Sea, serving as an exemplary high-energy research site and model system. It consists of fixed and permanent infrastructure such as a pole with measuring devices, multi-level groundwater wells and an electrode chain. This forms the base for autonomous measurements, regular repeated sampling, interdisciplinary field campaigns and experimental work, all of which are integrated via mathematical modelling to understand and quantify the functioning of the biogeochemical reactor. First results show that the DynaDeep observatory is collecting the intended spatially and temporally resolved morphological, sedimentological and biogeochemical data. Samples and data are further processed ex-situ and combined with experiments and modelling. Ultimately, DynaDeep aims at elucidating the global relevance of these common but overlooked environments.
... Regarding the influence of topography on groundwater dynamics, more previous studies focused on the change of straight beachface slope (Nielsen 1990;Raubenheimer et al. 1999;Li et al. 2008;Evans and Wilson 2016). As a typical feature of beach morphology, few studies have been conducted on the impact of a beach berm (a horizontal parallel ridge formed along the beach due to transport of beach material by the wave swash). ...
Beach recovery describes the processes by which there is a natural restoration of beach material and coastal morphology following storm events, and these processes are common across the globe. However, the effects of beach recovery on salinity distribution and solute transport in unconfined coastal aquifers are poorly understood. This study examined the changes in salinity distribution in tidally influenced aquifers in response to beach recovery, based on numerical modeling. The extent and location of the upper saline plume and saltwater wedge were found to vary with the beach recovery. The variations in salinity distribution directly changed the particle travel times in the aquifers. Compared with the erosion profile after the storm (storm profile), an increase of up to 743% of the particle travel time in the intertidal zone was observed when the beach recovered to a berm (silting) profile. The berm profile increased the residence time and peak concentration of the land-sourced solute plume in the beach aquifer compared with the storm profile. The berm profile also enhanced the aquifer–ocean mass exchange, resulting in increased intertidal saltwater infiltration and submarine groundwater discharge. On the other hand, the storm profile can generate much higher solute efflux than the berm profile. The storm profile is more favorable in diluting the land-sourced conservative solute and shortening its residence time in an aquifer.
... Although coastal aquifers have been intensively studied on a global scale [6,[8][9][10], less attention has been given to them from the microbiological point of view, despite their fundamental role in the fertilization process of the ocean [11][12][13][14]. ...
Coastal aquifers have been extensively studied from the hydrodynamic and geochemical points of view, but there is still a significant gap in the knowledge of their microbial diversity. The bacterial communities of four coastal aquifers at different depths and salinities were studied in order to infer the anthropogenic and physico-chemical influences on groundwater microbiota. At the physico-chemical level, samples from different aquifers, but with similar salinities, are more similar than those taken within the same aquifer. The microbial community at the phylum level shows the dominance of Proteobacteria, Firmicutes, and Actinobacteria. Samples from the same aquifer, although having very different salinities, are more similar than samples with similar physico-chemical characteristics. Therefore, the taxa present in these media are resilient to environmental variations. The aquifer preserving the most pristine conditions harbors the lowest values of biodiversity, compared to those affected by anthropic activities. The incorporation of pollutants into the aquifer favors the development of a so-called “rare biosphere”, consisting of a high number of taxa which represent a low percentage (<1%) of the total microbial community. The analysis of microbial biodiversity in a coastal aquifer could be used as an indicator of the degree of anthropic alteration.
... The influence of fluctuating sea levels diminishes with distance from the shoreline, with tidal variations attenuating more rapidly than intra-(e.g., seasonal) and inter-annual (e.g., extreme events, such as floods and droughts) variation (Ferris, 1952;Li et al., 2004;Nielsen, 1990). Precipitation recharges groundwater by vertical percolation through the overlying unsaturated zone or by direct recharge from surface-water bodies that fill during storm events. ...
The sustainability of limited freshwater resources in coastal settings requires an understanding of the processes that affect them. This is especially relevant for freshwater lenses of oceanic islands. Yet, these processes are often obscured by
dynamic oceanic water levels that change over a range of time scales. We use regression deconvolution to estimate an Oceanic Response Function (ORF) that accounts for how sea-level fluctuations affect measured groundwater levels, thus providing a clearer understanding of recharge and withdrawal processes. The method is demonstrated using sea-level and groundwater-level measurements on the island of Norderney in the North Sea (Northwest Germany). We expect that the method is suitable for any coastal groundwater system where it is important to understand processes that affect freshwater lenses or other coastal freshwater resources.
... During the PoM2020, discharge at 20 m (rSGD) showed a decreasing trend from ~1000 cm/day during the first hours of sampling and gradually decreased to ~200 cm/day at the last hours of sampling. This is due to the recirculated seawater flushing out of the USP zone of the STE during the falling tide (Gardner, 2005;Nielsen, 1990). ...
Submarine Groundwater Discharge (SGD) is the flow of fresh groundwater and recirculated seawater to the sea. SGD plays a crucial role in the transport of solute-rich terrestrial groundwater as well as recirculated seawater. We conducted seasonal investigations for two years on a tropical, high-rainfall occurring coastline in southwestern India to quantify the SGD from the nearshore. A combination of subsurface seepage meters and porewater samplers was used for an entire tidal cycle to estimate the seepage rates and to understand the processes controlling the discharge. The estimated seepage rates from this region are one of the highest reported in the world. This could be due to the large span of the highly porous coastal aquifer and the high annual rainfall (over 450 cm) in this region. The seepage rates are 754 cm/day, 572 cm/day and 296 cm/day for the pre-monsoon (February 2020), post-monsoon (December 2020) and pre-monsoon (February 2021) seasons, respectively and showed high spatial and temporal variability. The end-member concentrations of groundwater and seawater were used to delineate the fresh and recirculated SGD from the total seepage. The recirculated SGD (rSGD) dominates during all the seasons taking up to 99 % of the total SGD during the pre-monsoon seasons (February) and 70 % during the post-monsoon (December) season. The fresh SGD (fSGD) is low (∼1 %) during the pre-monsoon season and increases up to 30 % during the post-monsoon. A considerable amount (0.1–10 % of the total SGD) of salinity enrichment was observed at the upper saline plume. We suspect this could be due to the evaporation of recirculated seawater due to ambient weather conditions. The fSGD in the study area is mainly controlled by the high inland hydraulic head, and the rSGD is regulated by the tides. The subsurface seepage meters used in this study eliminate the instabilities in seepage measurements and can be replicated in less-studied tropical coastal zones to quantify the volume of SGD entering the world oceans. The inferences reported can benefit the public and decision-makers in managing coastal groundwater resources and are interesting to the researchers working on delineating the hydrological and geochemical processes in the nearshore.
... The above process can be reverted in some areas due to irregular terrestrial flow caused by changes in hydrostatic pressure. In certain circumstances, the tidal or wave setup affects how coastal aquifers are replenished with sea water during the flood tide period (Nielsen, 1990). Long-term tidally driven cycles were discovered through studies based on the continuous observation of natural tracers like radon and methane . ...
Submarine groundwater discharge (SGD) is the combination of fresh and saline groundwater flux to marine system through continental boundaries regardless of its chemical composition and factors influencing the flow. We have discussed the SGD studies in the Asian context; SGD has been studied in various parts of Asia, including China, Japan, South Korea, and Southeast Asia. In China, SGD has been studied in several coastal regions, including the Yellow Sea, the East China Sea, and the South China Sea. In Japan, SGD has been studied in the Pacific coast, where it has been found to be an important source of fresh water to the coastal ocean. In South Korea, SGD has been studied in the Yellow Sea, where it has been found to be an important source of fresh water to the coastal ocean. In Southeast Asia, SGD has been studied in several countries, including Thailand, Vietnam, and Indonesia. Recently the SGD studies acquired much development India, the research on SGD in India is limited, and more studies are needed to understand the SGD process, its impact on the coastal environment, and the management strategies, Groundwater extraction for irrigation, industry, and domestic use is increasing in India, which can affect the SGD process in coastal aquifers. Overall, the studies suggest that SGD is an important process in Asian coastal regions, playing a role in the supply of fresh water and the transport of pollutants and nutrients.
... Yet, the temporal variability of sea levels associated with oceanic tides is widespread and the effect of tidally induced sea level fluctuation on the groundwater flow and salinity in coastal aquifers has been examined extensively (Li et al., 2000;Robinson et al., 2014Robinson et al., , 2018Xin et al., 2011Xin et al., , 2014Wilson et al., 2015;Xin et al., 2018;Levanon et al., 2019;Xiao et al., 2019;Yang et al., 2022;Panjehfouladgaran and Mahdi Rajabi, 2022;Suk et al., 2022). All these studies suggested that the coupled groundwater flow and salt transport were significantly altered by periodic tidal fluctuations in both confined and unconfined coastal aquifers, resulting in enhanced dispersion of the freshwater/saltwater transition zone, the formation of an upper saline plume (USP) and the migration of the lower saltwater wedge (SW) towards the seaward side as compared to no-tide conditions (Parlange et al., 1984;Nielsen, 1990;Li et al., 2000;Robinson et al., 2014Robinson et al., , 2018Xin et al., 2011Xin et al., 2022;Das et al., 2021;Fang et al., 2021a;Suk et al., 2022;Yang et al., 2022). A few authors have reported on the effect of physical barriers, such as cut-off wall and subsurface dams, on the groundwater flow and salt transport in coastal aquifer that considered a fluctuating sea level. ...
... Whilst piezometer P4 is located about 100 m from the shoreline, P1 and P5 are more than 500 m away, which evidently exerts a damping effect. In this regard, ref [42] reports that a typical damping distance for the tidal fluctuations in an unconfined aquifer is a couple of hundred meters and [43] notes that the damping rates for tidal fluctuations in phreatic aquifers can be 10-20 times higher than in confined aquifers. Figure 9 displays the temporal distribution of precipitation and its effect on the wetland-aquifer system during the study period. ...
Novel data-acquisition technologies have revolutionized the study of natural systems, allowing the massive collection of information in situ and remotely. Merging these technologies improves the understanding of complex hydrological interactions, such as those of wetland–aquifer systems, and facilitates their conservation and management. This paper presents the combination of UAV technology with water level dataloggers for the study of a coastal temporary wetland linked to an underlying sandy aquifer and influenced by the tidal regime. Wetland morphology was defined using UAV imagery and SfM algorithms during the dry period. The DTM (6.9 cm resolution) was used to generate a flood model, which was subsequently validated with an orthophoto from a wet period. This information was combined with water stage records at 10-min intervals from a network of dataloggers to infer the water balance of the wetland and the transfers to the aquifer. Inflows into the pond were around 6200 m3 (40% direct precipitation over the pond, 60% surface runoff). Outputs equalled the inputs (41% direct evaporation from water surface, 59% transfers into the aquifer). The proposed methodology has demonstrated its suitability to unravel complex wetland–aquifer interactions and to provide reliable estimations of the elements of the water balance.
... Many theoretical studies on coastal aquifers have been conducted based on the Dupuit-Forchheimer assumption (Bear 1972). Nielsen (1990) derived an expression for time-varying water-table fluctuation under a vertical beach-face condition and presented an approximate method to account for the beach slope effect. Later, the beach slope effect was described analytically by Li et al. (2000a), who proposed a moving boundary method. ...
Groundwater hydrodynamics in the coastal area is significantly affected by tidal fluctuations and other driving factors such as rainfall. A series of analytical studies have been carried out in the past to investigate the water-table dynamics in coastal aquifers under different scenarios. Nevertheless, the horizontal two-dimensional analytical solution of the water-table fluctuation in an unconfined aquifer system under the combined actions of tide and rainfall has not been studied. To this end, a horizontal two-dimensional analytical solution is derived with the Dupuit-Forchheimer assumption by using the Fourier transform method in the present study. Subsequently, water-table fluctuations are estimated based on the present analytical solution. The joint effect of rainfall and tide could lead to complex water-table fluctuations, and the influences of different factors can be quantitatively investigated using the present analytical solution. In addition, the accuracy of the present analytical solution is further validated by numerical models, which indicate that the nonlinear effect could be neglected for a relatively large aquifer thickness, under which the present linearized analytical solution can give satisfying results. The present analytical solution extends the theoretical understanding of groundwater dynamics under the mutual effect of tide and rainfall.
... Worldwide, tidal activities affect the groundwater quality in coastal aquifers, and numerous studies have been documented based on experimental, field, and mathematical simulation modeling. Numerous works have revealed that the fluctuation in tidal drive varies the freshwater head and causes instabilities in the water table (Carr and van der Kamp, 1969;Nielsen, 1990), thus affecting the freshwaterseawater interface (Wang and Tsay, 2001). Such oscillation in the freshwater-seawater boundary roots an influx of seawater into the driving borewells during high tide (Shalev et al., 2009). ...
Around the globe, seawater intrusion in the coastal aquifer is a significant problem. Excessive groundwater extraction because of population growth, industrialization, tourism, and other anthropogenic activities and geogenic processes initiates and accelerates this problem. The contaminated groundwater impacts the health, economic activities, and social and cultural development of coastal regions. This work aims to explore the current status and a holistic comprehending review of geophysical studies applied to delineate the seawater intrusion in the high-quality coastal aquifers in India, as well as its origin and causes, mitigation strategies, and recent advancements in geophysical techniques to access the qualitative and quantitative properties of the complex aquifer system. In the future, it is recommended to do a detailed subsurface imaging of the entire coastal belt of India to decipher the lateral and vertical variation of the lithological conditions and seawater intrusion in space and time with improved/advanced geophysical techniques, which can lead toward sustainable development.
... First, by creating numerical modeling tools that accommodate moving boundaries and flexible meshes, coastal groundwater models will be better able to represent the interactions of surface processes (i.e., erosion, flood inundation, and infiltration) and adequately simulate small-scale vertical SWI phenomena (e.g., density-driven fingering) while maintaining tractable mesh density at depth where processes are slower. Tide and wave overheights are often omitted or treated as constants based on analytical solutions such as that of Nielsen (1990), and thus there is a need to improve the representation of these processes and their variability across a range of conditions in numerical models. ...
Low‐elevation coastal areas are increasingly vulnerable to seawater flooding as sea levels rise and the frequency and intensity of large storms increase with climate change. Seawater flooding can lead to the salinization of fresh coastal aquifers by vertical saltwater intrusion (SWI). Vertical SWI is often overlooked in coastal zone threat assessments despite the risk it poses to critical freshwater resources and salt‐intolerant ecosystems that sustain coastal populations. This review synthesizes field and modeling approaches for investigating vertical SWI and the practical and theoretical understanding of salinization and flushing processes obtained from prior studies. The synthesis explores complex vertical SWI dynamics that are influenced by density‐dependent flow and oceanic, hydrologic, geologic, climatic, and anthropogenic forcings acting on coastal aquifers across spatial and temporal scales. Key knowledge gaps, management challenges, and research opportunities are identified to help advance our understanding of the vulnerability of fresh coastal groundwater. Past modeling studies often focus on idealized aquifer systems, and thus future work could consider more diverse geologic, climatic, and topographic environments. Concurrent field and modeling programs should be sustained over time to capture interactions between physical processes, repeated salinization and flushing events, and delayed aquifer responses. Finally, this review highlights the need for improved coordination and knowledge translation across disciplines (e.g., coastal engineering, hydrogeology, oceanography, social science) to gain a more holistic understanding of vertical SWI. There also needs to be more education of communities, policy makers, and managers to motivate societal action to address coastal groundwater vulnerability in a changing climate.
Freshwater lenses and their freshwater–saltwater transition zones are affected by climate change. Both sea-level rise and groundwater recharge influence freshwater volume and transition zone thickness. This study used a semi-generic approach to investigate climate change effects on freshwater lenses: a hypothetical island cross-section was combined with real-world boundary conditions. Sea-level projections including tides and storm surges, annual mean sea-level rise data, and monthly recharge projections of several climate models of the German barrier island Norderney in the North Sea were used to evaluate changes in freshwater lens and transition zone size between 1971–2000 and 2071–2100. Firstly, impacts of sea-level and recharge boundary conditions were investigated on islands of different widths. Secondly, a multi-parameter study was conducted focussing on variations of several relevant hydrogeological parameters. Results showed that it is very likely but not certain that freshwater lens volume and depth will decrease and transition zone thickness increase as a consequence of climate change. Model predictions revealed a strong dependency on the employed climate models and to a lesser extent on the hydrogeological parameters, at least for the parameter ranges used in this study. Of all hydrogeological parameters tested, the largest effects were caused by the hydraulic conductivity and its anisotropy. Furthermore, the study showed that boundary conditions have larger impacts on smaller islands. These results illustrate the importance of using projections from climate models in a sufficiently high resolution. Furthermore, their uncertainties and changes in variability of boundary conditions should be considered in studies about climate change impacts on freshwater lenses.
Coastal flooding transforms barrier island morphology and rapidly salinizes freshwater lenses that support island populations and ecosystems. Climate change is expected to increase coastal flood risks, and understanding future island vulnerability requires understanding erosion and salinization processes and their feedbacks. This study investigates how island morphology and groundwater salinity distributions on Hog Island, Prince Edward Island, Canada, responded to high water levels during post‐tropical storm Fiona (24 September 2022), the costliest hurricane to make landfall in Canadian history. Island morphology was monitored with drone‐based LiDAR, and beach groundwater dynamics were investigated with frequency‐domain electromagnetic geophysics surveys and monitoring wells. Comparing pre‐storm and post‐disturbance data revealed high dune scarping that thinned the ocean‐side foredune by 12.3 m on average and reduced the total island volume by 12%. Beach groundwater levels and electrical conductivity increased by up to 2 m and 19 mS cm⁻¹, respectively, and the freshwater lens was lost under the eroded foredune. Measurements 9 months after the storm revealed early‐stage recovery of the foredune; however, high dune scarping prolonged recovery, and the island volume only increased by 1%. Without a stable ocean‐side foredune, the landward extent of recurring coastal flooding increased and limited freshwater flushing and aquifer recovery. Results indicate that rapid erosion from extreme coastal storms shifts coastal boundaries, salinizes formerly freshwater resources, and limits freshwater recovery. These findings emphasize the importance of understanding the often‐overlooked interconnections between coastal flooding, erosion, and groundwater salinization to effectively manage coastal resources in an age of environmental change.
Permeability is an important parameter in determining flow equations and beach sediment transport. This article evaluated studies of permeability and hydraulic conductivity in beaches, analyzing the performance and structure of research through a systematic review of the literature. The database consisted of 51 articles taken from the Web of Science platform. Data analysis was compiled using the Tree of Science, VOSviewer and Altrimetric Bookmarklet tools. The results showed that the main source of dissemination of the articles was the "Journal of Coastal Research" and that the countries with the most publications and collaborations were the USA and England. The connection networks show low collaboration between the different research subgroups, but good links between authors from the same group. Gear Masselink was the most influential author on the subject. The data from Altimetric Bookmarklet shows that the topic does not receive much social attention. The results of this review have positioned permeability in coastal research and pointed out gaps, mainly the lack of in situ permeability measurements, which still need to be investigated in the different branches of research.
Small‐island populations disproportionately rely on fresh groundwater resources, which are increasingly threatened by salinization from changing ocean and climate conditions. This study investigates island groundwater dynamics and salinization over multiple timescales in response to marine, atmospheric, and morphologic drivers. To address this, new geophysical and hydrological datasets were collected on a remote sand island in the Northwest Atlantic Ocean between 2020‐2022 and compared to historical baseline data from the 1970s.
Data reveal saltwater intrusion due to multi‐decadal erosion, seasonal climate patterns, tidal forcing, and episodic flooding from Atlantic hurricanes. Long‐term dune erosion has caused the freshwater lens to thin and become asymmetrical; however, a lagged groundwater response causes the freshwater lens to be in disequilibrium with present island morphology. The maximum vertical lens thickness is seasonally constant, but the lateral transition zone along the coast thickens and moves seaward in spring when the water table is high from precipitation and frequent beach flooding. Groundwater level and electrical conductivity along low‐lying beaches significantly increase following Atlantic hurricanes due to seawater flooding, and reach a maximum of 1.93 m above sea level and 38 mS/cm, respectively. While groundwater levels recover quickly, conductivity (salinity) remains elevated due to the short intervals between winter flood events that outpace freshening from meteoric recharge. Results emphasize the importance of multi‐temporal groundwater dynamics and feedbacks between coastal flooding, erosion, and salinization. Field‐based studies considering multiple drivers and timescales of saltwater intrusion are critical for understanding and managing coastal freshwater resources in an age of rapid environmental change.
Seepage face and the corresponding exit point (EP) contribute to the sophisticated groundwater hydrodynamics in the coastal aquifer. Several analytical solutions have been presented to estimate the EP movement. Nevertheless, previous theoretical studies are all limited to the Darcy flow assumption which is not applicable under the wave induced swash condition. In this study, a more general condition taking into account the non-Darcy flow effect with the Forchheimer formula was considered. Applying the v-transform method, a new analytical solution was derived to represent the swash induced EP movement under the wave action. Subsequently, detailed analyses of the EP movement characteristics were conducted, upon which its temporal variation features, as well as the influence of the non-Darcy flow effect and beach slope, were scrutinized. In addition, a decay parameter is used to describe the further reduction of the aquifer storage coefficient resulting from the truncated capillary fringe. By inter-comparing the available experimental data with the newly derived analytical solution, the optimal decay parameter was found to be 0.46. These new findings are of great importance to extend our understandings of the wave induced groundwater hydrodynamics in the swash zone.
Hydraulic conductivity is one of the most challenging hydrogeological properties to appropriately measure due to its dependence on the measurement scale and the influence of heterogeneity. This paper presents a comparison of saturated hydraulic conductivities (K) determined for a quasi-homogeneous coastal sand aquifer, estimated using eight different methodologies, encompassing empirical, hydraulic and numerical modeling methods. The geometric means of K, determined using 22 methods, spanning measurement scales varying between 0.01 and 100 m, ranged between 3.6 and 58.3 m/d. K estimates from Cone Penetration Test (CPT) data proved wider than those obtained using the other methods, while various empirical equations, commonly used to estimate K from grain-size analysis and Tide-Aquifer interaction techniques revealed variations of up to one order of magnitude. Single-well tracer dilution tests provided an alternative for making preliminary estimates of K when hydraulic gradients were known. Estimates from the slug tests proved between 1.2 and 1.6 times larger than those determined from pumping tests which, with one of the smallest ranges of variation, provided a representative average K of the aquifer as revealed by numerical modeling. By contrast, variations in K with depth could be detected at small scales (~ 0.1 m). Hydraulic Profiling Tool (HPT) system data indicated that K decreases with depth, which was supported by the numerical model results. No scale effect on K was apparent when considering the ensemble of results, suggesting that hydraulic conductivity estimates do not depend on the scale of measurement in the absence of significant aquifer heterogeneities.
The erratic distribution pattern of precipitation, demographic pressures and increased anthropogenic activity have taken a toll of the hydrological environments of Goa. The declining trend and degradation of groundwater is a cause of serious concern that needs urgent attention.Three lithological types characterize the groundwater aquifers in the State, namely (i) the lithomarges beneath the laterite duricrust, (ii) sandy alluvial-colluvial deposits of the coastal plains that abut against the lithomargic aquifers and (iii) weathered and fractured country-rock metasediments/granitic gneisses. The first is an unconfined aquifer, the second is a semi-confined aquifer whereas the third is a confined aquifer. The presence of intercalated clays/lithomarge horizons, at places, gives rise to sub-artesian conditions.The annual precipitation in Goa varies from 3000 to 3800 mm. The water potential of the State is placed at 85.70 Mm3 of which nearly 40% goes as runoff. Irrigation accounts for 146.50 Mm3 of which 112.50 Mm3 is contributed by surface water and 34.00 Mm3 from groundwater. The water requirement of the State is 1329 Mm3. In comparison with the water level of the previous decade (2005–2015), a declining trend of watertable is noticed in the northern, central and western parts of the State. The eastern talukas namely Bicholim, Sanguem and Valpoi, which host the iron ore mining belt, exhibit recession in watertable due to dewatering of the mines. Positive impact on the groundwater table is also observed in some parts of these talukas as a result of secondary recharge due to the return flow. Increased anthropogenic activity has had an impact on the groundwater quality whereas excessive withdrawal has resulted in saltwater intrusion in coastal areas.The chapter reviews the experiences and strategies of efficient utilization of water resources and recommends actions for mitigation to control overconsumption of the resource from increased demographic pressures. Efficient ways of rainwater harnessing and water conservation initiatives are discussed to relieve pressure on the groundwater resources and to keep pace with the increasing demand for ensuring sustainability of this fundamental component of the economy.KeywordsSurface waterGroundwaterAquiferSalt water intrusionRain water harvestingArtificial recharge
Coastal beaches are the most important part of the marine environment and are highly influenced by the interactions taking place between groundwater and surface water of any form. Generally, the purity index of the groundwater is based on the concentration of Total Dissolved Solids (TDS) in it. Analytical solutions are the best tools for groundwater flow and transport modeling. However, contaminant transport along the sloping coastal beaches with complex boundaries cannot be addressed with available analytical solutions. In the present study, new analytical models are developed for groundwater flow and contaminant transport for sloping coastal beaches with continuity and tidal boundary conditions. To assess the performance of the new analytical solution and to validate the range of aquifer parameters, numerical simulation is performed using Du-Fort Frankel (DFF) Scheme. Numerical experimentation is carried out using the Tchebycheff and L2 norms. It is observed that the new analytical solution for the contaminant transport gives acceptable results over the wide range of the aquifer parameters. To show the effectiveness of the developed models, two case studies from Indian coastal aquifers namely, Kalpakkam, Tamil Nadu, and Bhavnagar, Gujarat are considered. The profiles of the contaminant concentration are obtained to study the TDS behavior along these complex coastal beaches in the spatiotemporal directions. The results are compared with numerical model results and found to be satisfactory. The combined effect of the initial rainfall and rainfall decay constants showed a significant impact on the concentration of TDS. TDS concentrations are observed to be varying highly with the variation in the tidal constituents and bed slope.
Beaches along the coastal town of Thoothukudi, Veerapandiyapattinam, Kallamozhi and Kulasekharapatnam of Tamil Nadu state, India, were identified for conducting field experiments to understand the tide and wave-induced hydraulic dynamics on the beach groundwater. The stress due to the pressure radiation along the floor of the surf zone is dynamic and consequent to the tide and wave conditions above the surf zone floor. The wave breaks when the trough of the wave amplitude touches the ocean floor resulting in the dissipation of kinetic energy that is directly proportional to factors related to tidal heights, ocean floor slope conditions, ocean floor characteristics and the dynamics of wind and wave energies. To understand the influence of surf zone parameters on the beach groundwater, three piezometric wells were installed perpendicular to the seashore at a distance of 5 m each in the experimental sites to record the beach groundwater fluctuations. A newly built chart recorder was used to continuously record the coastal groundwater fluctuations. The time lag was determined by comparing the sea tide levels with the levels of the beach groundwater in the piezometric wells. The tidal and wave conditions contribute the maximum to the beach groundwater fluctuations and the time lag with the distances from the runoff line of the beach. The time lag in beach groundwater fluctuations between the piezometric wells perpendicular to the coastline is due to the cumulative effect of the surf zone conditions and the nonlinear impermeable beach groundwater table boundary conditions. Hourly data of varying water levels were extracted from piezometric well charts that were later compared with the hourly tidal fluctuations. The porosity and permeability of the beaches were calculated through soil samples of each piezometric well for determining seawater intrusion and beach groundwater discharge rate. The influence of the surf zone stress radiation on the beach groundwater can be understood by comparing the varying energy levels of the surf zone owing to the combined impacts of tides and waves with the fluctuations and the time lag in fluctuations between the evenly spread piezometric wells.
Hydraulic conductivity (K) is one of the most challenging hydrogeological properties to appropriately measure due to its dependence on the measurement scale and the influence of heterogeneity. This paper presents a comparison of saturated hydraulic conductivities determined for a quasi-homogeneous coastal sand aquifer, estimated using eight different methodologies, encompassing empirical, hydraulic and numerical modelling methods. The geometric means of K, determined using 22 methods, spanning measurement scales varying between 0.01 m and 100 m, ranged between 3.6 and 58.3 m/d. K estimates from Cone Penetration Test (CPT) data proved wider than those obtained using the other methods, while various empirical equations, commonly used to estimate K from grain-size analysis and Tide-Aquifer interaction techniques revealed variations of up to one order of magnitude. Single-well tracer dilution tests provided an alternative for making preliminary estimates of K when hydraulic gradients were known. Estimates from the slug tests proved between 1.2 and 1.6 times larger than those determined from pumping tests which, with one of the smallest ranges of variation, provided a representative average K of the aquifer as revealed by numerical modelling. By contrast, variations in K with depth could be detected at small scales (~0.1 m). Hydraulic Profiling Tool (HPT) system data indicated that K decreases with depth, which was supported by the numerical model results. No scale effect on K was apparent when considering the ensemble of results, suggesting that hydraulic conductivity estimates do not depend on the scale of measurement in the absence of significant aquifer heterogeneities.
Tidal analysis is an aquifer scale approach that is a low-cost alternative to pumping tests for assessing aquifer hydraulic parameters without groundwater extraction. Many analytical solutions may be used to assess aquifer head fluctuations in response to tidal fluctuations. Nonetheless, they are rarely used in practice. Aside from that, most analytical solutions are based on a conceptual model that typically consists of an unconfined aquifer and a confined aquifer separated by an aquitard, where hydraulic head fluctuations in the unconfined aquifer part are commonly neglected. Additionally, the confined aquifer short response time to sea-level fluctuations cannot rely on the hydraulic connection of the confined aquifer through the aquitard. As a consequence, when analytical solutions are applied to real-world cases, the hydraulic diffusivity is overestimated. In this study, we investigate through different numerical simulations the fluctuations of the phreatic surface by considering the delayed yield. Numerical results demonstrate that the mechanical effect generated by the load over the bottom of the sea due to sea-level fluctuations is a key factor when determining hydraulic aquifer parameters. We further show that in multilayer systems, head fluctuations in various aquifer layers can cause interferences and, consequently, increased attenuation of the tidal signal, resulting in an overestimation of the inferred hydraulic diffusivity. Our results provide guidance on how to properly reproduce tidal responses in coastal aquifers.
The measurement of changing water levels across a tropical carbonate
beach profile over three tidal cycles has provided basic data for the
development of a predictive mathematical model of the fluctuation in the
level of a beach water table. The model is based on partial differential
equations governing transient, one-dimensional movement of groundwater
through porous media. A finite difference algorithm for the digital
computer was developed to solve the equations. Beach homogeneity and
nonlinear boundary conditions imposed by tidal fluctuations were assumed
in these calculations. Field measurements of water table fluctuations in
Galleon Beach, Grand Cayman Island, show that the mathematical model
simulates this system within the limits of accuracy of the experimental
measurements. A method for in situ determination of the
permeability-porosity ratio is proposed and demonstrated for the beach
studied. The mathematical model of the beach water table is used to
develop an accurate method of calculating inflow and outflow at the
beach-ocean interface.
Observations of two-dimensional variations in groundwater level on beach profiles at South Beach, Wollongong, support and extend observations previously reported. Time-series curves showing water-level change at individual wells along the profiles are markedly asymmetrical and their ranges of oscillation are dependent on tidal range and distance landward of the beach face. The asymmetry is attributed to filtering processes at the beach face and in the beach, that separate the various tidal constituents. Tidally induced groundwater changes are superimposed on a three-dimensional water-table surface that is tied to the beach morphology and to groundwater recharge from the backshore zone. The three-dimensional flow net is raised and lowered as the tide rises and falls. although with some time lag between change in level of ocean water and groundwater response. The groundwater rise begins earliest in shoreline embayments and spreads landwards and outwards to higher water-table surfaces near shoreline salients and in the backshore zone. Groundwater responses, therefore. differ on the salients and in the embayments: a landward water-table slope prevails in the embayments and a seaward slope characterizes the salients.
For unsteady free surface flow in a rectangular dam a simple integral
relation which connects the pressure integral with the movement of the
free surface is derived. In the case of steady periodic movement driven
by oscillating reservoir levels the time average of the pressure
integral can be found, which leads to an average discharge formula which
is a generalization of the Dupuit-Forchheimer discharge formula. For
periodic fluctuations of groundwater in a coastal aquifer in response to
tidal oscillations a corollary is that the equilibrium water level far
from the sea is equal to the root mean square of the sea level,
independent of the validity of the Dupuit-Forchheimer assumptions.
A general integral relation in steady periodic nonlinear diffusion is established through a Kirchhoff transformation. The time average e of the transformed concentration e satisfies Laplace's equation and may therefore be evaluated readily. Results for e not only serve as simple and exact checks on detailed numerical solutions of the nonlinear diffusion equation but also provide, immediately and exactly, the principal information often sought about these solutions. The results are especially simple in many steady periodic nonlinear diffusion problems where the time average of the flux density is everywhere zero and i!j is constant. The method is applied to examples in fluid mechanics and geophysics: spatially periodic laminar boundary layers, tidal influence and seasonal stream effects on groundwater levels, and soil temperature waves.
A Hele-Shaw experiment which models groundwater flow is described. The experiment confirms the Philip prediction that in steady periodic nonlinear diffusion the root mean square of the oscillation, rather than the mean is constant. For the weakly nonlinear case examined, however, it is found that the logarithm of the amplitude of the oscillation still diminishes in proportion to , so from a practical viewpoint the hydrological problem could be treated as a linear one provided the displaced mean were taken into account. The anomalous appearance and subsequent decay of harmonics in the initially sinusoidal wave are discussed.
Ebene nichtstatinare Grundwasserabflusse mit freier OberflacheRep
- T Dracos
The proof of the correctness of Dupuits formula in the case of unconfined seepage (in Russian)
- Charnyi I. A.