Tadao Kakinuma

The motion of coastal groundwater in response to the tide

Abstract Problems of seawater intrusion into confined coastal aquifers under the influence of the tide are analysed with the use of two different models; the freshwater-saltwater interface model and the dispersion model. The freshwater-saltwater interface model, which describes the motion of the groundwater level and the freshwater-saltwater interface in response to the tide, is applied to the confined coastal aquifers in the estuary of the Naka River and Yura district of Gogo Island in Japan. The position of the outlet of the confined groundwater into the sea and also the degree of spreading of the aquifers thickness, which is assumed to vary exponentially with horizontal distance, are estimated. From the non-dimensional analysis based on the dispersion model, it is found that salt water intrudes furthest inland at the time at which the sea level reaches the mean sea level in the ebb tide stage, consistent with the result obtained by the freshwater-saltwater interface model, and the transition zone from fresh water to salt water becomes widest at that time.

The regional unsteady interface between fresh water and salt water in a confined coastal aquifer

Abstract An areal two-dimensional model to describe the unsteady motion of the fresh-salt water interface and the groundwater level in a confined coastal aquifer is first presented and then applied to the confined groundwater in the estuaries of the Naka and Kiki rivers in Japan. Starting from the basic equations of the groundwater flow, two equations are derived; one equation of the effective water level and one equation of the interface. The first equation describes a very rapid variation of the effective groundwater level and the second equation describes a very slow variation of the interface with time. In order to find solutions of these equations under the boundary conditions in practical field problems, the numerical scheme based on Galerkin finite-element technique was employed. Using this numerical scheme, transient positions of the groundwater level and the interface were calculated in the above-mentioned two regions and compared with the available field data. It was found that the calculated results agree rather well with the data.

The behavior of groundwater with dispersion in coastal aquifers

Abstract A three-dimensional steady-state hydrodynamic dispersion model is used to simulate seawater encroachment in the confined aquifers in the estuaries of the Naka and Kiki Rivers in Japan. Two expressions of the dispersion coefficient are considered; one is constant over the entire region of the aquifer and the other is dependent on the flow velocity of the groundwater. The magnitudes of the constant dispersion coefficients in the horizontal and vertical directions, D xx and D zz , as well as the longitudinal and lateral dispersivities, a L and a T , are determined so as to reproduce the regional distributions of salt concentration in the confined aquifers in both estuaries. It is found that D xx = 5 cm 2 s −1 , D zz = 5-0.5 cm 2 s −1 and a L = 1000–1250 m, a T = 100–125 m in the estuary of the Naka River; and D xx = 0.2 cm 2 s −1 , D zz = 0.2–0.02 cm 2 s −1 and a L = 200 m, a T = 200-20 m in the estuary of the Kiki River. Examining the local distributions of the dispersion coefficient computed from the dispersivity and velocity fields of groundwater in both estuaries, the same value as estimated in the analysis with the constant dispersion coefficient is located in the middle layer of the aquifer. In the estuary of the Naka River, the piezometric surface predicted with the dispersion model with the velocity-dependent dispersion coefficient is almost the same as that predicted with the dispersion model with the constant dispersion coefficient and they are 5 10% lower than that predicted with the interface model (Kakinuma et al., 1984). They are, however, about 1.3 times the observed one.

The regional steady interface between fresh water and salt water in a coastal aquifer

Abstract First, a theoretical model is presented which is useful for determining the regional steady interface between fresh water and salt water under the appropriate boundary conditions for both confined and unconfined aquifers. This model is then applied to the confined groundwater in the estuary of the Naka River, Tokushima Prefecture, Japan, and the phenomenon of the seawater intrusion into the aquifer is analyzed. Boundary conditions of the confined aquifer and the parameters necessary for analysis are determined from the data. Calculations using a finite-element method are carried out in various cases, by changing the inflow S R or the total pumping amount Q of the groundwater in this alluvial plain. It is found that the region where salt water is intruding expands rapidly inland when S R approaches Q and vice versa. It is also found that the distributions of chloride ion concentration in groundwater calculated by this model with a simplified assumption are in fairly good agreement with the results observed in deep wells.

SUSPENDED SEDIMENT CONCENTRATION IN THE SURF ZONE

The Labrador Sea Extreme Waves Experiment (LEWEX), is an international basic research programme concerned with full-scale measurements, analysis, modelling and simulation in test basins of 3-dimensional seas. The research is carried out in order to assess the significance of 3-dimensional sea states in engineering applications. The first phase of the programme full scale wave measurements in the North Atlantic Ocean was performed at a site and time that had a high probability of encountering severe sea states. The present publication shows examples of measured bi-modal directional sea spectra obtained with the WAVESCAN buoy and directional sea spectra measured with an airborne Synthetic Aperture Radar (SAR). Directional spectra of gravity waves are obtained with the SAR both in open waters and below an ice cover. Further work is needed in order to verify SAR-measurements with in-situ observations. In-situ measured directional spectra are also compared with hindcast spectra from the 3G-WAM model. Hindcast significant wave heights were found to be lower than the in-situ measurements.The littoral drift model developed at DHI and ISVA, see Deigaard et al. (1986b) has been extended to include the effects of the irregularity of the waves, of a coastal current and a wind acting on the surf zone. Further, a mathematical model to simulate the near-shore current pattern along a barred coast with rip channels has been developed. The influence on the littoral drift of the irregularity of waves, wind, coastal current, and rip channels is discussed. It is concluded that irregularity of waves and presence of rip channels must be considered while coastal current and wind action are of minor importance.At present, the Prodhoe Bay oil field in Alaska contributes a substantial amount of the domestic oil production of the United States. Oil is also expected to be present on the continental shelf of Alaska, and it is estimated that approximatedly 28 percent of the total U. S. reserve is located beneath the shallow ice covered seas of the Alaskan continental shelf. To expolre and to exploit these oil rich resources, engineers are confronted with hostile oceanographic conditions such as high tides, waves, strong currents and sea ice. The same area is also rich in fishery resources. Being one of the most productive fishing grounds in the nothern Pacific, the potential ecological impact due to an oil spill is of a major concern. This paper describes the methologies used for the development of a modeling system for the oil risk analysis. The system is designed with generality in mind so it can be used for other coastal areas. The development of three dimensional models used in the modeling system described here have been published in the earlier International Coastal Engineering Conferences (Liu and Leendertse, 1982, 1984, 1986) and a report published recently by RAND (Liu and Leendertse, 1987). In the oil-spill risk analysis, these three dimensional hydrodynamic models are coupled to a two-dimensional stochastic weather model and an oil weathering model.The two projects (LUBIATOWO 79 and LUBIATOWO 86) were aimed at study pore pressure behaviour in natural sand bed in the coastal zone of the Baltic sea under the action of storm waves. During both projects, the wave induced pore pressures at the various levels in the sea bed were measured. The collected data were used to verify the applicability of the various theoretical approaches. In the conclusion, the range of the application of the particular analytical method is given.The rapid recession of the shingle bank of Hurst Beach (up to 3.5m/yr) makes it an excellent natural laboratory for the study of the factors which influence the stability of shingle beaches. Studies have included: the significance of long period, high energy, swell waves the classification and quantification of overwash processes run-up and seepage characteristics the effect of settlement of the underlying strata and the implications for practices in shingle nourishment. The studies have revealed the distinctive character of shingle beaches as compared with the more fully researched sand beaches. More detailed research on shingle beaches is justified particularly in relation to (i) the run-up characteristics including its interaction with swash cusps and (ii) the influence of the subsidiary sand fraction on the beach characteristics.Environmental assessment, engineering studies and designs were completed for a new 26.5 m3/s seawater intake system in the Persian Gulf. The original intake facility consisted of a curved, 60m breakwater with one end attached to the shoreline, a settling basin immediately adjacent to the shoreline and dredged to a maximum depth of approximately 5m, and a pumphouse structure located on shore such that the seaward wall formed one side of the settling basin. The facility located on an island in the Gulf, which served multiple seawater uses, had experienced both structural and operational problems, the latter consisting principally of excessive ingestion of sediment and seaweed. These factors plus the requirement for additional demands for seawater beyond plant capacity caused the owner to initiate a study of alternative intake systems, produce a design for the most effective solution and construct the new intake system.A two-dimensional wave prediction model suitable for use on personal computers is described. The model requires the twodimensional time-dependent wind field as input. Output consists of wave height, wave period, and wave direction estimates at all grid points on a computational grid representing an enclosed or semi-closed basin. Model predictions compare favorably with observations from a wave research tower in Lake Erie. A formula is provided to estimate how long a model simulation would take on a personal computer given the surface area of the computational domain, the grid size, and the computer clock speed.Cullera Bay is a neritic ecosystem placed on the Spanish Mediterranean Littoral largely influenced by the Jucar River, that brings about lower salinities than surrounding waters, and broad variations of its values. An extensive research, with 9 samplings throughout the year, was carried out, measuring both physical and chemical parameters, and the planktonic communities. The trophic status of the ecosystem, the spatial and temporal variations of the nutrients and the planktonic communities were studied, evaluating the influence of the river loads and the littoral dynamics. Some essential basis to allow a suitable emplacement of waste waters disposals along the Valencian littoral are set up in order to minimize the gradual eutrophication of this coast.In the last two years a whole of studies was realized in order to determine precise solutions to the regeneration of Villajoyosas beach, in the Spanish mediterranean coast. Investigations were carried out to the surrounding coastal areas based in field investigations and laboratory analyses of the beaches materials.

Model analysis of seepage of seawater in the saturated - and unsaturated-zone having impermeable stratum.

This paper deals with the phenomena of seawater intrusion into the phreatic aquifer with an impermeable layer, by using a hydrodynamic dispersion model in both saturated and unsaturated domains. The numerical solutions by the finite-element method are obtained for various locations of an impermeable layer. The main results are as follows : Salt water under an impermeable layer contacted with the sea intrudes further inland with the lengthening/lowering of the impermeable layer. In the case of the lower impermeable layer, salt water intrudes inland around the inner edge of the impermeable layer. The pattern of the concentration distribution becomes close to the one obtained in the case of nonimpermeable layer with separating from the sea and shortening of the impermeable layer.

Model analysis of the transport of solutes in saturated and unsaturated domains.

This report deals with the problem of contaminant transport in the saturated and unsaturated domains of phreatic aquifer. Numerical solutions by the finite-element method are obtained for fertilizer dissolution and seawater intrusion. The main results are as follows : 1) In fertilizer dissolution from the center of the aquifer surface, since the dispersion coefficient becomes smaller, the pattern of the steady state concentration distribution becomes more eccentric towards the outlet of the aquifer. When the dispersion coefficient increases or the precipitation intensity decreases, the concentration distribution changes more slowly to the steady state distribution. 2) With seawater intrusion, since the dispersion coefficient becomes smaller, the pattern of the steady state concentration distribution in the saturated domain chages from strong to moderate mixing, and a circulating current develops near the outlet of the aquifer.

The motion of coastal confined groundwater in the presence of various patterns of pumping. (II). Numerical analysis by steady state dispersion model.

A two-dimensional steady state hydrodynamic dispersion model is used, with the fresh-salt water interface model, to investigate the motion of the coastal confined groundwater in the presence of various pumping patterns. Two expressions of the dispersion coefficient are considered; one is constant over the entire region of the aquifer, and the other is dependent on the flow velocity of the groundwater. The main results are as follows : 1) As the groundwater is pumped more inland, salt water intrudes more inland, whereas the pumping in the lower layer of the aquifer near the coast is effective to prevent the seawater intrusion; 2) in the upper layer pumping, the fresh-salt water interface model serves as a convenient method for approximate analyses and predictions of seawater intrusion into the coastal confined aquifers.

Studies on the motion of ground water with dispersion in coastal aquifers. (III). Dispersivities in Naka River and Kiki River estuaries.

A three-dimensional steady state hydrodynamic dispersion model with the velocity-dependent dispersion coefficient was used to simulate seawater encroachment in the confined aquifers in the estuaries of the Naka River, Tokushima Prefecture, and the Kiki River, Ehime Prefecture. The main results are as follows: 1) The longitudinal and lateral dispersivities are 1, 000 to 1, 250 m and 100 to 125 m in the estuary of the Naka River, respectively. In the estuary of the Kiki River, the corresponding values are 200 m and 200 to 20 m, respectively. 2) Examining the local distribution of the dispersion coefficient values computed from the dispersivity and velocity fields of groundwater, the same value as estimated in the analysis with the constant dispersion coefficient (KAKINUMA et al., 1985) is located in the middle layer of the aquifer. 3) In the estuary of the Naka River, the piezometric surface predicted using the dispersion model with the velocity-dependent dispersion coefficient in almost the same as the one predicted with the dispersion model with the constant dispersion coefficient. They are 5 to 10% lower than the one predicted with the interface model (KAKINUMA et al., 1984), but about 1.3 times the observed one.

The motion of coastal confined groundwater in the presence of various patterns of pumping. I Numerical analysis by fresh-salt water interface model.

The motion of the coastal confined groundwater in the presence of various pumping patterns was investigated by numerical analyses, based on one and two-dimensional , unsteady fresh-salt water interface models. Approximate equations describing the temporal variation of groundwater level and freshsalt water interface were utilized in the numerical computations together with appropriate boundary conditions. The results were as follows : 1) Groundwater level varied very quickly in response to sudden change in pumping amount, whereas the interface varied very slowly ; 2) movement of the interface was faster when the groundwater was pumped in the fresh water region as compared to that in the saline water region ; 3) movement of the interface was faster when retreating toward the sea as compared to when it was advancing inland ; 4) artificial recharge through wells in the fresh water region and the saline water region was effective to retreat the toe of the interface and to decrease the thickness of salt water layer, respectively ; 5) in the two-dimensional model, results similar to those in the one-dimensional model were obtained for movement of the interface in the presence of various pumping patterns ; 6) characteristic features of the two-dimensional model in the presence of areal distribution of pumping points were also found, e. g., the speeds of interface toe movement close to and far from pumping points were considerably different.