Kunimitsu Inouchi

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 CHARACTERISTICS OF SCOURING AND DEPOSITING IN FRONT OF VERTICAL BREAKWATERS BY BROKEN CLAPOTIS

The wave motions in front of breakwaters can be classified into three forms; standing wave, breaking clapotis and broken clapotis. Under the action of the three wave motions, the sea bottom in front of a breakwater is scoured, and this will further affect the stability of the breakwater. Our experiments show that the scouring of sandy seabed in front of a breakwater by broken clapotis is more serious than that by the other two wave motions. In this paper, first the scouring and depositing patterns of sandy seabed in front of a vertical breakwater under the action of broken clapotis are investigated experimentally to be classified into three types; DSD, SDSD and DSDS types, i.e. within one wavelength from a breakwater, the scouring (S) and deposited (D) locations are different. Secondly, formation processes of scouring and depositing pattern are probed for those three types. Thirdly, criteria for distinguishing scouring and depositing patterns are given based on the analyses of the scouring and depositing mechanisms and experimental data.

Efficient Allocations of Vertical Turbines for Tidal Power Extractions in the Sea around Islands near Kurushima Straits

It has been confirmed that the energy extraction rate is significantly improved by adopting the flow guide equipment which has curved fins on the circumference of a Savonius-type vertical turbine. In the real field, it would be required to install several turbines in a series, say parallel or tandem allocations, in order to obtain large amount of electric power from tidal currents. In this study, it was presumed that the installation site of the tidal power turbines was limited to the vicinity of coastal area along islands in Kurushima Straits. Under such limitations, it becomes important to know how to specially allocate a group of the turbines to maintain the energy extraction efficiency. In this study, the interaction effect among the specially allocated turbines was examined by both experimental and theoretical studies.

Groundwater salinization caused by excessive pumping during a construction project

本研究は感潮河川近くの建設工事に伴って発生した地下水の塩水化について,まず,その実態を示し,次に数値解析に基づく現状再現によって,塩水化の原因究明を行ったものである。問題となった下水管渠築造工事では地下水位を下げる必要があったので,地下水位低下工法の一つであるウェルポイント工法が採用された。工事は1年6ヶ月にわたって行われたが,工事の後半から現場付近の井戸で高濃度の塩分が観測されるようになり,周辺の地下水が人為的に塩水化された可能性が懸念された。三次元非定常の移流分散モデルを用いた数値解析の結果,現地の塩水化が再現され,原因がこの地下水位低下工法に起因することが特定された。また,問題とする井戸が塩水化されるかどうかは,揚水量も重要な要因であるが,揚水が行われる地点と問題とする井戸の地点との位置関係も重要であることが判明した。さらにこのような観測結果を用いることによって現地の分散長や地下水滴養量の見積もりが可能となることも指摘した。

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.

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.