Water exchange and material exchange through a strait due to tidal flow

The process of material transport through a strait due to tidal flow is modeled, and then the differences between various concepts of tidal exchange which have been used hitherto are pointed out using this model. In particular, the exchange of water itself and the exchange of material should be distinguished even in the case where the material of interest is carried by the water,i.e., the material and water move as one body. Further, the physical meaning of “tidal trapping” (Fischeret al., 1979) is discussed by using the model in this paper. The relationship between the exchange ratio for the water itself (r) and the phase lag (δ) of material concentration to the tidal stream in a section of the strait, which is an important factor in tidal trapping, is obtained as follows: $$\delta = \tan ^{ - 1} \left( {\tfrac{1}{r} - 1} \right)$$ Observational results at Lake Hamana (Shizuoka Pref) and at Kabira Cove (Okinawa Pref.) support the validity of the above relationship.     

Steady critical discharge rates from vertical and horizontal wells in fresh–saline aquifers with sharp interfaces

In this study, the critical (or maximum) discharge rates before saline water enters a well were determined for vertical and horizontal wells in a freshwater aquifer which is separated from a static saline aquifer by a sharp interface. Flow around the well was solved by integration of a point sink solution along the well axis, and both the critical discharge rate and critical interface rise were determined through a comparison of the heads and vertical gradients at the saline–fresh water interface. The rates were determined for vertical and horizontal wells with various lengths and depths for different aquifer salinities. Results were generalized by drawing dimensionless type curves. The results showed that the dimensionless total critical discharge rates are higher for the longer horizontal wells and longer vertical wells with a certain bottom depth, and they almost linearly decrease with well depth at rates of 0.7–0.9. For the dimensionless well length of 0.2, the dimensionless total discharge rate of a horizontal well is about 0.1 more than that of a vertical well with the same length and well-top depth. Also, the critical discharge rates per unit length of well are inversely proportional to well length and remarkably higher for shallower wells. Additionally, the critical pumping rate is proportional to the salinity difference of the aquifers. These results were confirmed by comparison to existing solutions for vertical wells with dimensionless lengths of 0.2, 0.5 and 0.6, and for critical interface rises in the range of 0.75–0.9.     

A transfer pathway modeling template for radiological environmental risk assessment

As radioactive human doses are calculated through food chain and exposure pathways, it is important to take a pathway analysis to determine all pathway contributions for further radiological environmental risk assessment. A challenge of contaminant fate and environmental pathway analysis is to handle a large number of environmental components and to assign proper calculation models to linked component pairs. This paper presents a model template called transfer pathway model template (TPMT) that stores transfer models to environmental class pairs. When a site-specific calculation scenario is built, a source–receptor transfer matrix (SRTM) is defined by the scenario. Then the calculation loops through all source–receptor component pairs in SRTM and apply proper models stored in TPMT to calculate the transfer factors. TPMT can be used as the component pathway verification template and model allocation template. This structure is used in an environmental risk assessment (ERA) application called IMPACT that has been applied for a number of nuclear power plants and uranium mines in Canada.     

Parameters affecting the occurrence of quicksand and the drying up of large diameter wells that gain water from the bottom: a case study from Iran

Large diameter fully cased wells that gain water from the bottom are often dug in sandy and collapsible aquifers. They have cylindrical vertical walls lined with brick or concrete. The well bottom is partially filled with aquifer material through which the flow is vertically upward. When the vertical hydraulic gradient reaches a critical value, quicksand occurs and the well structure can be destroyed. Another difficulty encountered is drawdown in the wellbore and the drying up of the well. To overcome these problems, the flow around and beneath these wells is numerically simulated. The simulation results are used to investigate the effect of well and aquifer parameters on quicksand and drawdown. For practical purposes, the dimensionless drawdown-time and dimensionless vertical gradient-time curves are developed. It was found that the ratio of filling material thickness to well radius affects the shape of these type curves. The type curves may be used to predict the time after pumping commences when quicksand occurs and the well dries up. They are also useful to design the safe pumping rate and duration as well as the optimum well radius. These are demonstrated by analyzing the pumping test data from a case study in the arid Chah Kutah region, southern Iran.     

Steady flow to a partially penetrating blind‐wall well in a confined aquifer

Wells in aquifers of loose collapsible sediment are cased so that they have a blind wall and gain water only from the bottom. The hydraulic gradient established at the bottom of these wells during pumping brings the aquifer materials in a quicksand state, which may cause abrasion of pipes and pumps and even the destruction of well structure. To examine the quicksand occurrence, an analytical solution for the steady flow to a partially penetrating blind‐wall well in a confined aquifer is developed. The validity of the proposed solution is evaluated numerically. The sensitivity of maximum vertical gradient along the well bottom in response to aquifer and well parameters is examined. The solution is presented in the form of dimensionless‐type curves and equations that can be easily used to design the safe pumping rate and optimum well geometry to protect the well against sand production. The solution incorporates the anisotropy of aquifer materials and can also be used to determine the hydraulic conductivity of the aquifer. Copyright © 2012 John Wiley & Sons, Ltd.     

Analytical solutions for the capture zone of a pumping well near a stream

The capture zone for a fully penetrating well in an aquifer with regional flow to a stream boundary under steady-state conditions was delineated using complex algebra and image well theory. Regional flow in the aquifer was allowed to take different directions relative to the stream axis. Two critical pumping rates, Q _C1 and Q _C2, produce three capture-zone pattern scenarios: (1) at low pumping rates (Q?<?Q _C1) water is solely withdrawn from the aquifer and no water from the stream enters the aquifer, (2) at medium pumping rates (Q _C1?<?Q?<?Q _C2) a portion of stream water enters the aquifer but it is not captured by the well, and (3) at high pumping rates (Q?>?Q _C2) pumped water is supplied from both the aquifer and the stream with different proportions. For the second and third scenarios, the stream length interval through which stream water enters the aquifer was determined and found to be more sensitive to pumping rate as the regional flow direction approaches the stream axis. The portion of pumped water supplied by the stream was determined in the third scenario. Finally, the capture-zone asymmetry with respect to its axial line was delineated.     

Steady flow rate to a partially penetrating well with seepage face in an unconfined aquifer

The flow rate to fully screened, partially penetrating wells in an unconfined aquifer is numerically simulated using MODFLOW 2000, taking into account the flow from the seepage face and decrease in saturated thickness of the aquifer towards the well. A simple three-step method is developed to find the top of the seepage face and hence the seepage-face length. The method is verified by comparing it with the results of previous predictive methods. The results show that the component of flow through the seepage face can supply a major portion of the total pumping rate. Variations in flow rate as a function of the penetration degree, elevation of the water level in the well and the distance to the far constant head boundary are investigated and expressed in terms of dimensionless curves and equations. These curves and equations can be used to design the degree of penetration for which the allowable steady pumping rate is attained for a given elevation of water level in the well. The designed degree of penetration or flow rate will assure the sustainability of the aquifer storage, and can be used as a management criterion for issuing drilling well permits by groundwater protection authorities.     

Water exchange in Kabira Cove,Ishigaki Island

The mechanism and rate of water exchange were investigated in Kabira Cove, Ishigaki Island, in the southernmost part of Japanese islands, near Taiwan. During observations in the summers of 1976 and 1977, a larger proportion of the salt transport into the bay was derived from the so-called “tidal trapping effect”. In the latter period of observation carried out after heavy rain brought by a seasonal typhoon under annual mean tidal conditions, the turnover time, i.e. the scale of replacement of the whole bay water with the open sea water, is estimated to be 3.6 days. Based on these observational results, a concept of the tidal trapping due to coupling of the actions of tides and buoyancy in a vertical two-dimensional field with a sill at the bay mouth is proposed. Considering the topographical, hydrometeorological and geographical conditions of the cove, it is inferred that this water exchange process tends to be formed in Kabira Cove in summer except during neap tides.     

Year-to-year change in water exchange characteristics in a semi-enclosed bya,Lake Hamana

The year-to-year change in characteristics of water exchange between Lake Hamana, a semi-enclosed bya, and the adjacent open sea is investigated.The destruction of the bay mouth by a typhoon in 1953 and subsequent stabilization work on the bay mouth from 1954 to 1973 resulted in an increase in the tidal prism volume of the bay (Mazda, 1983). In the present paper, a simple model has been constructed in which the magnitude of water exchange depends on the tidal prism, and using this model, the year-to-year increase in salinity of the bay water after 1953 can be well explained. Consequently, it can be said that the salinity increase after 1953 is a result of a progressive increase in water exchange caused by successive changes in topography of the bay mouth.The extent of water exchange in Lake Hamana, which varies seasonally, has increased gradually since 1953, and became stable after about 1967. For instance, at present the turnover time of the bay for exchange with open sea water reaches a maximum (2.9 months) in January and a minimum (0.9 month) in October, while in 1955 it is estimated to have been about 2.5 times that at the present time.     

Vertical two-dimensional mechanism of tidal exchange in a bay with a sill-entrance

For a vertical two-dimensional field with a sill at a bay entrance, the tidal exchange mechanism is discussed.The schematic model is proposed as follows. The tidal trapping effect which is detected at the entrance section,i. e., the material transport due to the phase difference between the tidal periodic constituent of material concentration and tidal current at the entrance section, results because the oscillatory tidal flow at the sill entrance induces a gravitational flow along the sill slope inside the entrance. Accordingly, the tidal trapping effect depends largely upon the difference in water density between the bay and open sea and the density stratification in the bay.This model is supported by the observations at Kabira Cove (Okinawa Pref.) and Lake Hamana (Shizuoka Pref.) in 1976 through 1984. In addition, based on this model, in the case of Lake Hamana, the activity of the tidal exchange is inferred to change seasonally.