摘要:

This paper revises the framework for incorporating the risk of catastrophic failure in the evaluation of new and existing high‐hazard dams. The paper demonstrates that by including both losses to structures and lost flood control benefits, the current method of computing risk costs introduces some measure of double counting. Several rebuilding scenarios are discussed and related to the loss of flood control benefit

ISSN:0043-1397    

DOI:10.1029/WR025i009p01931

年代:1989

数据来源: WILEY

摘要:

A composite analytical model is developed for analyzing the results of pumping tests where the influence of well bore storage and a skin region of finite thickness are present at the pumping well. The solution of the boundary value problem for dimensionless drawdown in the pumping well, skin region, and formation is derived using the Laplace transform method. The solution is verified by comparison to solutions of pumping test problems with well bore storage only, with a composite formation only, and with well bore storage and infinitesimally thin skin. Type curves obtained by numerically inverting the solution for drawdown in the formation are used to illustrate the influence of well bore storage, the effect of skin region characteristics, and the effect of radial distance. These show that the influence of a finite thickness skin of reduced permeability is clearly identifiable over a fairly broad range of radial distance from the pumping well when well bore storage effects are minimized. Conversely, the effects of finite skin of enhanced permeability are more easily identified where the influence of well bore storage is greater. In both cases the type curves are uniquely defined provided that the skin region is of nonzero thickness. Type curves obtained for the solution for drawdown in the skin region are used to illustrate the effect of outer or far‐field boundary conditions. These type curves show that early time data not influenced by well bore storage effects are required to detect the presence of outer boundaries of reduced permeability. Drawdown data at late time, although less influenced by well bore storage effects, are subject to nonuniqueness with regard to the characteristics of the skin and formation regions. Outer boundaries of enhanced permeability are identified only at early time and are almost entirely masked by well bore storage at later tim

ISSN:0043-1397    

DOI:10.1029/WR025i009p01937

年代:1989

数据来源: WILEY

摘要:

An improved air pycnometer method was used to examine the water content dependence of trapped‐air volumes in two repacked, nonswelling soils. Trapped‐air volumes were determined at a series of hydrostatic equilibrium stages which were attained during water pressure‐controlled wetting and drying cycles over a range of 0 to −10 kPa for a sand and 0 to −20 kPa for a loam. Small pressure perturbations, between 0.2 and 0.6 kPa, were used in the air pycnometer method. Volumes of trapped air obtained at each hydrostatic equilibrium stage were independent of perturbation level and remained relatively constant over the time required to make repeated determinations. In contrast with most of the results obtained in previous studies, which often showed irregular relations, in this study the volume fraction of trapped air was found to be a regular, monotonically increasing (though possibly hysteretic) function of water content. For the soils studied, the function definitely exceeded zero only at water contents greater than 70% of saturation. However, during the initial drying from complete water saturation, the volume fraction of trapped air was virtually zero. Air trapping influenced the water retention curves significantly only at water contents higher than about 60% of saturation. Except at zero water pressure, however, not all of the differences between the initial and the other drying retention curves were accounted for by observed differences in trapped‐air volumes. Air trapping was not required for the onset of hysteresis in the water retention relation for the cases studied, i.e., when drying‐to‐wetting reversals were imposed at about 27% and 40% of saturation for the sand and loam soil

ISSN:0043-1397    

DOI:10.1029/WR025i009p01947

年代:1989

数据来源: WILEY

摘要:

To improve understanding of hysteretic air permeability relations, a need exists for data on the water content dependence of air permeability, matric pressure, and air trapping (especially for wetting‐drying cycles). To obtain these data, a special instrument was designed. The instrument is a combination of a gas permeameter (for air permeability determination), a suction plate apparatus (for retentivity curve determination), and an air pycnometer (for trapped‐air‐volume determination). This design allowed values of air permeability, matric pressure, and air trapping to be codetermined, i.e., determined at the same values of water content using the same sample and the same inflow‐outflow boundaries. Such data were obtained for two nonswelling soils. The validity of the air permeability determinations was repeatedly confirmed by rigorous tests of Darcy's law. During initial drying from complete water saturation, supplementary measurements were made to assess the magnitude of gas slip. The extended Darcy equation accurately described the measured flux gradient relations for each condition of absolute gas pressure tested. Air permeability functions exhibited zero‐permeability regions at high water contents as well as an abruptly appearing hysteresis at low water contents. Measurements in the zero‐permeability regions revealed that the total amount of air in general exceeded the amount of trapped air. This indicates that the medium' s air space is partitioned into three measurable domains: through‐flowing air, locally accessible air (i.e., air accessible from only one flow boundary), and trapped air. During repeated wetting and drying, the disappearance and reappearance of air permeability coincided closely with the reappearance and disappearance, respectively, of trapped air. The observed relation between critical features of the air permeability functions and those of the air‐trapping functions suggest that water‐based blockages play a significant role in the disruption of gas‐phase connectivity and in preventing air flow, and must be considered in any effectual model of air per

ISSN:0043-1397    

DOI:10.1029/WR025i009p01959

年代:1989

数据来源: WILEY

摘要:

A model that explicitly accounts for multiple sources of nonequilibrium is presented. The multiprocess nonequilibrium (MPNE) model is specifically formulated for cases where nonequilibrium is caused by a combination of transport‐ and sorption‐related processes. Sensitivity analyses were performed to delineate the conditions under which the MPNE model reduces to the bicontinuum and to the local equilibrium models. These conditions are strongly controlled by the magnitude of the dimensionless rate parameters. Performance of the MPNE model was evaluated with several published data sets. An empirical regression equation was successfully used to estimate values for sorption rate constants required in the MPNE model. The MPNE model, with values for all parameters obtained from independent sources, predicted behavior exhibited by the data. This provides support for the validity of the conceptualization upon which the model is ba

ISSN:0043-1397    

DOI:10.1029/WR025i009p01971

年代:1989

数据来源: WILEY

摘要:

The small perturbations method is applied to a two‐dimensional flow problem due to a steady but nonuniform infiltration into the soil and a given head at the lower boundary. The perturbation solution is compared to an analytic solution for periodic distributions of the infiltration. Good accuracy is obtained by only two terms of the expansion. The first term is simply the vertical flow with no lateral terms. In the second term the horizontal flow divergence depends parametrically only on the horizontal distribution and the zero‐order approximation, i.e., the vertical flow. For deep lower boundaries the naive perturbation expansion leads to some secular terms that made the convergence poorer. The uniform asymptotic expansion is then used and shown to give an improved accuracy in one term only. The perturbation method is extended to nonperiodic distribution, in fact, any distribution. It may be used for two‐ and three‐dimensional problems that evade any analytic solutions. One has to solve numerically or analytically only one dimensional and ordinary linear equations for any number of approximations. The problem is singular in that it accepts at this stage only impermeable vertical boundaries or symmetrical lines. However, this covers a very important class of practical p

ISSN:0043-1397    

DOI:10.1029/WR025i009p01989

年代:1989

数据来源: WILEY

摘要:

Snow falling in high‐elevation forests is an important source of water for much of the arid Southwest. Snowpack conditions in Arizona and New Mexico differ from those in more northern regions, because of the variability in annual accumulations and the intermittent melting throughout the winter season. Snow management research in Arizona and New Mexico over the past 25 years has indicated the possibilities of increasing snowmelt water yields through forest management activities. The effects of management can be predicted from forest inventory data. Other research has resulted in the development of snow‐runoff forecasting procedures and computer simulation models of forest snowpack dynamics, as well as increasing the general knowledge of snow hydrol

ISSN:0043-1397    

DOI:10.1029/WR025i009p01999

年代:1989

数据来源: WILEY

摘要:

A “regionalization” methodology is presented that uses a conceptual model of long‐term water quality responses to acidic deposition (MAGIC) in a Monte Carlo simulation framework to reproduce observed distributions of water quality variables derived from regional surveys. The methodology is applied to data from a 1974 survey of 700 lakes in southernmost Norway to produce a calibrated model for the region. Predicted water quality in 1986 agrees well with results from a resurvey of 300 lakes. The model indicates that significant acidification occurred in the decades preceding the survey. The simulated effects of the 5% deposition decrease over the period 1974–1986 are very small but the effects of a 30% decrease in 26 years are readily apparent. Simulations for a 30% reduction indicate little improvement in water quality in the future. Simulations for a 50% reduction and for a 70% reduction in deposition indicate substantial and continuing improvement in water quality. The regionalization approach is a new and potentially useful method for modeling impacts of atmospheric acidic dep

ISSN:0043-1397    

DOI:10.1029/WR025i009p02009

年代:1989

数据来源: WILEY

摘要:

A two‐dimensional hydrodynamic and numerical model for overland flow is developed in this paper. The model allows spatial variations in hillslope physical characteristics, including surface roughness, infiltration, and microtopography. The accuracy of the model is tested by comparison with characteristic‐based solutions and experimental data. The results of these tests indicate that the model is accurate and has good stability and convergence properties. The application of the model to two‐dimensional overland flow on hillslopes is demonstrated by applying it to surfaces with spatially variable roughness, infiltration, and microtopography. These simulations show that microtopography is the dominant factor causing spatial variation in overland flow depth, velocity, and dire

ISSN:0043-1397    

DOI:10.1029/WR025i009p02019

年代:1989

数据来源: WILEY

摘要:

The analysis of large river networks obtained from digital elevation models has given insight into the variation of channel slope with scale. Investigators have recently suggested that channel slopes are self‐similar with magnitude or area as a scaling parameter. Our data indicates otherwise; in particular, the variance of channel slope is larger than that predicted by simple self‐similarity. This suggests multiscaling. The scaling exponent for the standard deviation is approximately half the corresponding exponent in the relationship of the slope mean to magnitude or area. A model for channel slopes based on a point process of elevation drops along the channel is presented. The model reproduces observed multiscaling properties when the density of elevation increments is related to area (or magnitude) as

ISSN:0043-1397    

DOI:10.1029/WR025i009p02037

年代:1989

数据来源: WILEY