Journal of Hydrologic Engineering

Kinematic wave theory is widely used in modeling a variety of hydrologic processes. Results of applying the kinematic wave overland flow solution using different time steps showed that the conventionally used stability criterion known as the Courant condition fails to give a time step estimate that ensures stable and accurate numerical solutions. Accordingly, a new accuracy-based dynamic time step estimate for the one dimensional overland flow kinematic wave solution is developed. The newly developed dynamic time step estimates are functions of the mesh size, watershed slope, roughness, excess rainfall, and time of concentration. The new criteria were developed by comparing the consistent formulation of the Galerkin-Crank Nicholson numerical solution of the kinematic wave equation to the characteristic method-based analytical solution, using different time steps and meshes. For each simulation, characterized by boundary and initial conditions and mesh size, an optimal time step that integrates the problem within 5% error was determined. The series of mesh sizes and corresponding optimal time steps were used to develop the dynamic time step. The time step criteria were tested on a variety of problems, including a steady state and time varying rainfall scenarios, and proved to be adequate for accurate and stable results within an efficient computational time. The criteria can be easily integrated in flow routing models to select the optimal time step with minimal user input.

In this paper, an attempt has been made to predict river flow in meso-scale basins using the general circulation model generated atmospheric forcings by coupling a land surface model and a river routing model. Several versions of the variable infiltration capacity model were used as the land surface model, including one that incorporates a double parabolic curve to describe the infiltration capacity of the soil. In the runoff routing model, which uses a much finer grid, backward distribution of observed river discharges to upstream cells as well as flood plain inundation are introduced. The proposed models and the procedures are applied to two major river basins in the Southeast Asian Region—the Mekong and the Chao Phraya. The Mekong is an international river that runs through China, Myanmar, Lao, Thailand, Cambodia, and Vietnam, and the Chao Phraya River is contained within Thailand. Despite the limitations in the data availability, the results of daily river discharge predictions seem to be reasonable in the scale concerned.

A typical flood frequency analysis is based on gauged annual maximum discharges. One assumption behind the analysis is that the measured discharge signal is stationary. The validity of this assumption can be difficult to establish, particularly where urbanization has occurred within the gauged watershed, altering the response of the affected watershed to precipitation. This alteration can produce a nonstationary streamflow signal that can be significant, depending on the percentage of the watershed altered. As urbanization increases, peak discharges are shown to increase, producing a positive trend in the annual maximum series. Urbanization occurring during and/or after the gauging period is quantified using spatially and temporally distributed land use data. Three statistical tests (a parametric t-test on the slope of the linear relationship between the flood series and time and two nonparametric tests: the Kendall’s Tau and the Spearman Rank Correlation) are performed on both the annual maximum discharge and annual maximum discharge-precipitation ratios series to test for trends or nonstationary signals corresponding to periods of urbanization. A case study suggests that the ratios are more effective than the discharges alone for identifying nonstationarity resulting from urbanization. In addition, relationships between measures of urbanization and the presence or absence of significant trends in the discharge series are presented.

A methodology for determining reservoir stage-frequency relations is described that includes the following three component parts: (1) adjusting several-decade long historical sequences of daily streamflows for the effects of urbanization; (2) simulating reservoir system operations; and (3) performing a frequency analysis of the resulting series of maximum annual water surface elevations. Analyses are presented for the Addicks and Barker Reservoirs, which are operated by the U.S. Army Corps of Engineers to reduce flooding along Buffalo Bayou in Houston. Extensive urbanization occurring throughout the floodplain and watershed during the past 50 years includes development adjacent to reservoir lands upstream of the dams. Operating policies, including both allowable release rates and timing of gate operations, are demonstrated to significantly affect reservoir stage-frequency relations. The effects of different operating practices are shown to become more pronounced with increased urbanization.

The runoff curve number (CN) is a widely used input to hydrologic designs. It is treated as a constant even though it is a random variable. The antecedent moisture condition CN adjustment is the only interval estimate available, but it is of questionable origin and not recommended for use. A general method is provided herein for developing confidence intervals for CNs. The procedure is illustrated for curve numbers to be used for designs reflecting annual maximum design storms. The quantity 100-CN was found to fit the gamma distribution, which was used to develop confidence intervals for CNs from 65 to 95. The confidence intervals can be used to assess the variation in CN. A hypothetical case is used to demonstrate the variation in the design of a detention basin due to variation in table CNs. Confidence intervals are also provided for the retention parameter, S.

Regional frequency analysis based on the index flood method has been utilized to analyze short duration annual maximum precipitation (AMP) for Northeastern Colorado. An extraordinary storm and flood that occurred in the City of Fort Collins, Colorado, in July 1997 prompted the study. The main objective of the study was to develop regional frequency curves that may be useful for that part of Colorado. In addition, we wanted to answer the question whether the City’s design criteria for storm drainage as of 1997 was adequate, and to determine to what degree the referred storm was indeed an extraordinary storm. We utilized up-to-date technology that has been widely popularized in literature, with some minor modifications. We conclude that the regional growth curves developed may be useful for the subregions specified. However, they should be used with caution for sites located near the boundaries of subregions close to the Colorado Front Range. This has become evident in determining the precipitation quantiles for Fort Collins, for which it was necessary to further narrow down the subregions. The study also showed that the city’s storm drainage design criteria were underestimated. Furthermore, the 1997 2 and 3 h storms appear to be 100 yr+ events, while the return period of the 6 h storm may have been of the order of 400 years. While the study provided us with some practical answers, it nevertheless also brought a number of questions concerning the assumptions underlying the index flood approach and several steps regarding the definition of regions and testing criteria thereof. These are discussed at length in the final section of the paper.

During the 1980–1999 period, many regions in the central and southern Great Plains experienced the longest and strongest increase in average annual precipitation of the century. The size of the increase ranged from 6 to 12% of mean annual precipitation and from 25 to 60% of interannual precipitation variability. Precipitation increased for dry, average, and wet years, though not equally for each category. Generally, precipitation in very wet years increased less than in average and dry years. The probability of occurrence of dry years was greatly reduced compared to earlier in the century, whereas the probability of average years remained about the same and the probability of wet years increased. The observed decade-scale precipitation increase is attributed to a reduction in the number and severity of dry years, as well as to an increase in the number of wet years, though very wet years did not increase as much. The seasonal distribution of the increase in precipitation showed no statistically significant prevalence during any one particular month, yet qualitative considerations suggest that early summer and autumn months capture more of the annual precipitation increase. In the northern and northwestern Great Plains, a similar precipitation increase, but of smaller proportions, was observed during the 1990–1999 period.

Bank storage is a process in which volumes of water are temporarily retained by alluvial stream banks during storm events and gradually released to partially sustain baseflow. This process has important hydrologic and ecological implications. In this paper, analytical solutions are developed for routing stream flow, lateral stream-aquifer interactions, and aquifer storage. In effect, the stream-flow routing Muskingum method is modified for bank storage. The analysis is based on one-dimensional lateral groundwater flow in semiinfinite homogeneous unconfined aquifers, which are in contact with streams through semipervious bed sediments. Impulse response and unit step response functions are derived for the stream-aquifer system, using Laplace transformations. These response functions relate stream outflow, stream-aquifer flow, bank storage, and cumulative reach discharge volume to discrete-time inflow hydrographs through convolution integrals. The impulse response function decreases with increasing aquifer hydraulic conductivity at earlier times, but increases with the conductivity and persists at later times. The unit step response function decreases with aquifer conductivity uniformly in time. The dependence of stream flow and bank storage on aquifer hydraulic conductivity, streambed leakance, stream width, and aquifer diffusivity is investigated. The analysis is extended to discrete input data, and modification of the methodology to route discrete-time inflow hydrographs of general form is achieved, using discrete-time kernels. The presented analysis establishes the time domain for validity of the analytical solutions in terms of the Muskingum parameters and an aquifer-related parameter.

Naturally fractured rock is idealized as a double porosity medium which is composed of homogeneous fractures and blocks. The groundwater flow is Darcian toward a fully penetrating large diameter well, from which the water is withdrawn at a constant rate. For such a configuration of well and flow characteristics, the necessary continuity expressions are written for the blocks and fractures separately in addition to the fracture as well as the block-to-fracture flow laws. The combination of these expressions and solution of the resulting partial differential equation by means of Boltzmann transformations lead to type curve expressions, which give the drawdown variations with time. These curves have initial straight line portions that are steeper than a singly porous medium case and therefore indicate the natural fracturation within the aquifer. The application of the derived type curves is shown through matching a procedure to actual field data.