Dam-Breach Flood Wave Propagation Using Dimensionless Parameters

An analytical model of flood wave propagation is used to study the sensitivity of dam-breach flood waves to breach-outflow hydrograph volume, peak discharge, and downstream-channel bed slope. Dimensionless parameters are identified for discharge and distance along the channel. A dam-breach Froude number is defined to enable analysis through a wide range of site and flow conditions. It is found that, at a certain distance downstream, the attenuated peak discharge is independent of the magnitude of the discharge at the breach site. This attenuated peak discharge is termed the “ultimate discharge,” and the distance associated with it, the “ultimate distance.” These ultimate values are a function, primarily of the downstream-channel bed slope and, secondarily, of the breach-outflow hydrograph volume.     

Investigating the Role of Clasts on the Movement of Sand in Gravel Bed Rivers

The bed morphology of mountain rivers is characterized primarily by the presence of distinguishable isolated roughness elements, such boulders or clasts. The objective of this experimental study was to provide a unique insight into the role of an array of clasts in regulating sand movement over gravel beds for low relative submergence conditions, H/dc<1, and flow depth, H, to the diameter of the clast, dc, a process that has not been studied thoroughly. To assess the role of clasts in controlling incoming sand movement, detailed flume experiments were conducted by placing 40 equally spaced clasts atop a well-packed glass bead bed for replicating the isolated roughness flow regime. The experiments were performed for moderate ( ～ 2.50τcr* where τcr* is the critical dimensionless bed shear stress) and high ( ～ 5.50τcr*) applied bed shear stress conditions, representative of gravel bed rivers. For comparison purposes, experiments were also repeated for nearly identical flow conditions but without the presence of clasts to discern the potential effects that clasts may have on sediment movement and hydraulics within the clast array region and also in the upstream section of the clast region where few observations exist. The experimental results revealed the formation of two distinguishable bed morphological features, namely a funnel shaped “sand ridge” upstream from the clast array region and small depositional “sand patches” around individual clasts. The sand patches were formed in the stoss region of the clasts, which contradicted previous observations of depositional patterns around clasts under high relative submergence conditions (H/dc>1) where, in this case, depositional patches were observed to have formed in the clast wake region. Furthermore, most of the incoming sand was found to be intercepted by the evolving sand ridge upstream from the clast array region with implications in the amount of sand entering the clast array region. The exiting bed-load rate was found to be reduced by a factor of ～ 5.0–20, depending on the prevailing flow conditions when experiments with and without clasts were compared under nearly identical flow conditions. The findings of this research, although limited to the isolated roughness regime, may have significant ramifications in stream restoration projects for the design of engineered riffle sections, which typically consist of an array of clasts installed to improve degraded waterways and aquatic habitat.     

Calculation of Bed Changes in Mountain Streams

Simulation of flow and sediment transport in mountain streams is complicated by the presence of high gradients, abrupt changes in geometry, variations in regime of flow, and large roughness elements. Most of the numerical models to predict aggradation and degradation in alluvial channels have been developed for low-gradient rivers. This paper is devoted to the development of a numerical model to calculate bed elevation and grain size distribution changes in mountain streams where the maximum bed material size is in the range of boulders. An attempt is made to validate the model by using observed field data collected upstream from a small retention dam in a Venezuelan stream. After calibration of the sediment transport equation, reasonable agreement is obtained for the variations in the grain size distribution of the bed-surface material. An additional application is presented in the Cocorotico River, a small mountain stream located in the northwest region of Venezuela, which illustrates the adaptability of the model to handle a case of coarsest-bed-material removal from the active channel and to simulate the armoring process.     

Pollution of Gravel Spawning Grounds by Deposition of Suspended Sediment

Observations have shown that accumulation of fine sediment in the pores of spawning open-work gravel have a detrimental effect on stream biota. The rate of deposition is intimately linked to the concentration of suspended fines near the gravel bed. If interstitial voids in coarse sediment deposits are filled or covered with sand or inorganic fine materials, their habitat value is greatly reduced. In this paper, a simple method is proposed to predict analytically the concentration profile and transport of fine suspended sediment when a steady, uniform suspension flows from a sediment-covered bed to an open-work gravel bed. Comparisons of the analytical model predictions with previous laboratory observations show reasonable agreement. The proposed solution can be used to estimate “clarification distances” for streams carrying fine sediments over open-work gravel beds.     

Scouring at Bed Sills as a Response to Flash Floods

The temporal development of clear-water local scour depth at bed sills in uniform gravel beds is considered. Experiments are presented on the development of scour holes under unsteady hydraulic conditions, with the triangular-shaped hydrographs tested being of different durations and different rates of flow variation. Based on the experimental results and a theoretical framework, a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter. A “flash flood” is here defined as an event for which the scour doesn’t attain its potential magnitude, i.e., the equilibrium value for the peak hydrograph flow rate. This flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream. A quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter. Results show that the ratio between the final scour depth and the potential scour depth at a bed sill for a given hydrograph can be estimated as a function of the identified temporal parameter.     

Stream Classification System Based on Susceptibility to Algal Growth in Response to Nutrients

We developed a stream classification system that is based on stream’s susceptibility to algal growth using a two-step approach. The model portrays algal biomass as a result of stream’s response to nutrient concentrations and the response is governed by various stream factors. In the first step, a nutrient-chlorophyll a relationship was developed to characterize nutrient’s effects on algal biomass. Residuals of the relationship were attributed to stream’s susceptibility to algal growth in response to nutrients and referred to as “observed” susceptibility. In the second step, conditions of other contributing factors were used to explain the variation in the residuals and the developed relationship was used to generate “predicted” susceptibility. Existing data compiled from various monitoring projects of Illinois streams and rivers were used to illustrate the approach. Streams were classified into three (high, medium, and low) categories based on their observed and predicted susceptibility values, respectively. With the available data, the model showed a 40-50% success rate for classifying the streams based on three observed and predicted susceptibility categories. Model entropy also was calculated for selecting the best model. The results show the important role of both nutrients and other contributing factors in explaining the variation of algal biomass. The study also suggests ways to fine tune the model and improve its accuracy, which would make the presented model a more viable tool for stream classification for establishing nutrient criteria to prevent surface streams from eutrophication.     

Modeling the Effect of Water Diversion on the Temperature of Mountain Streams

Water diversion for hydroelectric power generation impacts the temperature of mountain streams. Such changes are estimated by using a coupled one-dimensional dead-zone heat balance model. In very steep river sections, the dissipation of kinetic energy is the dominant heat source. For such streams, water diversion has only a minor effect on water temperature, because dissipation-induced temperature changes are independent of discharge. In contrast, in river sections of gradual slope, the influence by solar radiation, long-wave radiation, and heat exchange with the streambed is stronger. In such cases, a discharge reduction can lead to significant temperature changes. For a small stream in the southern Swiss Alps, model results show that diversion increases temperature by about 3.7 (±0.9)°C in a 21 km long river section under high solar radiation during summer. During a cold winter episode, water temperature is estimated to be about 1.8 (±0.8)°C lower compared to natural conditions. This heat balance model can also be used to simulate the effect of different measures to reduce water temperature changes in affected streams.     

Regime Equations for Natural Meandering Cobble- and Gravel-Bed Rivers

Data obtained from 48 stable reaches of upland rivers in the UK were stratified by stream type to develop regime equations specifically for natural meandering cobble- and gravel-bed rivers: C3 and C4 stream types, according to the Rosgen classification. Multiple regression models were applied to derive equations for reach-averaged values of bankfull width, mean depth, slope, meander arc length and sinuosity in bankfull discharge and associated bed-material load, the caliber of the bed material, bank vegetation density, and valley slope. The equations show that their cross-sectional dimensions are primarily determined by the bankfull discharge, bank vegetation, and bed-material size, whereas their profile and plan form are very strongly influenced by the valley gradient. Although bankfull bed-material load only appears to have a minor influence on channel morphology, its effect is implicit in the value of bankfull discharge because this corresponds to the flow that transports most of the bed-material load. Explanations are given for these results on the basis of processes affecting channel geometry. Comparisons with the regime equations derived more than 20?years ago by Hey and Thorne from the same UK data set indicate that stratification by stream type generates equations that are more consistent; for example, bank vegetation affects all aspects of channel morphology rather than simply channel width, and provides significantly better explanations for channel slope and sinuosity because of the inclusion of valley slope as an independent variable. Their potential for designing river restoration schemes is evaluated against North American data. The equations prove to be comparable to the Hey and Thorne equations for predicting width and depth, but provide a significant improvement for the determination of slope and sinuosity. Although bed-material load was shown, statistically, to influence channel dimensions, numerically its influence is trivial. Removing it from the analysis generates equations that provide the best practical point estimates of channel morphology. Predictions with the simplified regime equations are shown to be comparable to the full equations.     

Pool Quality Index: New Method to Define Minimum Flow Requirements of High-Gradient, Low-Order Streams

High-gradient (>1%), low-order streams, characterized by hydraulically nonuniform and heterogeneous channels, represent a problem for the most widely employed habitat-based in-stream flow methods (IFIM-PHABSIM). In a nonuniform high gradient and turbulent channel, as low-order streams usually are, the classical 1D hydraulic modeling, ordinarily employed by in-stream flow models to simulate the changes in fish habitat with the flow, could be questionable, if not completely inapplicable. Channel morphology in fact plays a major role in association with hydraulics in determining the abiotic environments (biotopes) in which aquatic communities live. Particularly, in low-order river systems, different channel form features shape the biological community that can be hosted in a certain biotope. For this reason, the link between morphology and discharge is important when evaluating possible impacts of flow reduction on aquatic organisms. To represent the relationship between hydraulics and channel morphology quantitatively, a hydraulic diversity concept has been adopted. Studies from the literature have revealed that, in a regulated river, a decrease of the environmental variability including hydraulic diversity quite often resulted in a downstream decrease of the macro-invertebrate diversity, which can consequently affect fish biomass. These considerations create the ground for a hydraulic diversity-discharge–based in-stream flow method with the aim to promote high community diversity in a low-order regulated stream. A statistic ordination technique (correspondence analysis) applied to 370 hydraulic sections helped to identify four main morphological units (pools, deep pools, and slow and fast riffles) in terms of hydraulic diversity. In each morphological unit, the hydraulic diversity-discharge relationship was investigated and modeled by means of best-fit regression curves. Combining the hydraulic diversity-discharge curves from different morphological units (pools and riffles), a simplified model of the stream [pool quality index (PQI)] was obtained. This model has been applied to make recommendations for the minimum flow requirements in six low-order river sites. PQI recommendations were consistent with hydrology and other hydrology-based in-stream flow methodologies. Finally, a multiple regression model indicated that in 12 low-order stream sites a good deal of the variability of macro-invertebrate diversity is explained by the availability of hydraulic environments modeled by means of PQI curves. In conclusion, given the encouraging cues about the ecological meaning of PQI and the possibility to overcome difficulties typically encountered by other methods in the low-order river modeling, PQI can be considered a valid alternative for assessing the in-stream flow needs of low-order streams.     

Reproduction of Hysteresis in Rating Curves

The well-known Jones formula has long been used to convert a stage hydrograph to a discharge hydrograph under many unsteady flow situations. The logic behind the formula can be explained with the use of the approximate convection–diffusion equation in its development. However, its applicability criterion has not yet been quantified. Assessment of the applicability of the Jones formula is studied herein. In addition, two variations of the Jones formula have been developed: one, incorporating the inertial forces of the one-dimensional momentum equation and the other, incorporating a refined estimate of ?y/?x, the longitudinal gradient of the water depth, accounting approximately for the parabolic variation of the water surface. The suitability of these three formulas in converting a given hypothetical stage hydrograph to a discharge hydrograph is assessed under varying channel conditions. The study shows, that the Jones formula and its variants developed in this study, indicate a limit of ∣(1/S0)?y/?x∣<0.5, where, S0 is the bed slope, for their successful application. Further, it is found that the formula developed considering a refined estimate of the longitudinal gradient of the water depth performs better than the other two formulas.     

Numerical Morphological Modeling of Open-Check Dams

Open-check dams are built in mountain streams to control sediment transport during a flood. Sediment passes through them at the lowest discharges, whereas deposition occurs during the highest discharges. Open-check dams are currently designed based mainly on construction experience. Modeling of hydraulics and bed morphology in check dams involves mixed flows (supercritical and subcritical) as well as discontinuities such as hydraulic jumps. In this paper an unsteady coupled numerical mobile-bed model that can tackle rapid varying flows and discontinuities is applied. The numerical technique is based on the classical staggered grids and implicit integration schemes, together with a proper mass and momentum balance. The 1D numerical model is successfully verified with experimental data of slit-check dams. The applicability of the model in the design of open-check dams is also illustrated.     

Aquifer Response to Sinusoidal or Arbitrary Stage of Semipervious Stream

Analytical expressions for the aquifer responses, viz., groundwater head, rate of flow and cumulative volume of flow, to a generalized sinusoidal stage of semipervious streams considering the stream boundary resistance, are derived. The analytical aquifer responses to a linear stream stage and to a typical analytical flood wave that was used by Cooper and Rorabaugh, are also derived. For a zero-stream resistance, the aquifer responses converge to those for a fully penetrating stream. Also, two analytical methods, a “ramp kernel method” and a “Fourier series method,” for obtaining the aquifer responses to an arbitrary temporal stage of sempervious stream, are developed. The analytical expressions of the ramp kernels for different aquifer responses are developed. The ramp kernel method is found superior to the conventional convolution that uses numerical integration or pulse kernels for obtaining the convolution integral. In the Fourier series method, the aquifer responses to sinusoidal stage are used along with Fourier series. The results obtained using both methods are in close agreement. The new methods are also applicable to fully penetrating streams by assigning a zero value to the stream resistance.     

New Method for Meander-Loop Cutoffs

When a reach of a meandering river becomes very sinuous, and thereby causes significant problems for navigation and flood discharge, cutoff works in the narrow-neck portion of the river bend may be carried out. It is common practice to excavate a small slightly bending cutoff channel, referred to as pilot channel, which is gradually scoured and enlarged toward the concave bank by the river flow. In the design of the Wujiadu cutoff in the Caoe River (already constructed in 1998), the authors proposed a new method for constructing meander-loop cutoffs. Using this technique, the new cutoff channel on the concave bank side and the flood protection embankments are formed during construction, allowing the nonprotected convex bank and the bed of the new cutoff channel to be scoured and be enlarged by flood. Thus, the amount of excavation of the cutoff works can be significantly reduced and the construction period can be largely shortened. In this way, the lines of the new cutoff channel may be unified with the overall river alignment. This paper presents the design philosophy and major points of the method for meander-loop cutoffs.     

Volume Conservation in Variable Parameter Muskingum-Cunge Method

The simple Variable Parameter Muskingum-Cunge (VPMC) method is still frequently used for flood routing. However, difficulties arise in the selection of an appropriate “reference” discharge for evaluating the routing parameters and in the small volume loss that can occur. Several commonly used schemes for the VPMC method are compared through a series of numerical experiments that cover different channel bed slopes and different space∕time steps. The tests show that 4-point schemes are better than 3-point schemes, that a certain amount of volume loss (up to 8%) still occurs in all schemes, and that an empirical relationship exists between the volume loss and channel bed slope (S). A new scheme for the VPMC method is presented, with the routing parameters (c and D) being modified to take into account the longitudinal hydrostatic pressure gradient term. This scheme improves the routed hydrographs, not only with regard to the sensitivity of the outflow peak for given space and time steps, but also with regard to volume loss, typically less than 0.5% even for a channel with S = 0.0001.     

Modeling the Evolution of Incised Streams. III: Model Application

Incision and the ensuing widening of alluvial stream channels represent important forms of channel adjustment. Two accompanying papers have presented a robust computational model for simulating the long-term evolution of incised and restored or rehabilitated stream corridors. This work reports on applications of the model to two incised streams in northern Mississippi, James Creek, and the Yalobusha River, to assess: (1) its capability to simulate the temporal progression of incised streams through the different stages of channel evolution; and (2) model performance when available input data regarding channel geometry and physical properties of channel boundary materials are limited (in the case of James Creek). Model results show that temporal changes in channel geometry are satisfactorily simulated. The mean absolute deviation (MAD) between observed and simulated changes in thalweg elevations is 0.16?m for the Yalobusha River and 0.57?m for James Creek, which is approximately 8.1 and 23% of the average degradation of the respective streams. The MAD between observed and simulated changes in channel top width is 5.7% of the channel top width along the Yalobusha River and 31% of the channel top width along James Creek. The larger discrepancies for James Creek are mainly due to unknown initial channel geometry along its upper part. The model applications also emphasize the importance of accurate characterization of channel boundary materials and geometry.     

Sequential grouping of pure-tone percepts evoked by the segregation of components from a complex tone.

A sudden change applied to a single component can cause its segregation from an ongoing complex tone as a pure-tone-like percept. Three experiments examined whether such pure-tone-like percepts are organized into streams by extending the research of Bregman and Rudnicky (1975). Those authors found that listeners struggled to identify the presentation order of 2 pure-tone targets of different frequency when they were flanked by 2 lower frequency “distractors.” Adding a series of matched-frequency “captor” tones, however, improved performance by pulling the distractors into a separate stream from the targets. In the current study, sequences of discrete pure tones were substituted by sequences of brief changes applied to an otherwise constant 1.2-s complex tone. Pure-tone-like percepts were evoked by applying 6-dB increments to individual components of a complex comprising harmonics 1–7 of 300 Hz (Experiment 1) or 0.5-ms changes in interaural time difference to individual components of a log-spaced complex (range 160–905 Hz; Experiment 2). Results were consistent with the earlier study, providing clear evidence that pure-tone-like percepts are organized into streams. Experiment 3 adapted Experiment 1 by presenting a global amplitude increment either synchronous with, or just after, the last captor prior to the 1st distractor. In the former case, for which there was no pure-tone-like percept corresponding to that captor, the captor sequence did not aid performance to the same extent as previously. It is concluded that this change to the captor-tone stream partially resets the stream-formation process, and so the distractors and targets became likely to integrate once more. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Rapid Calculation of Oxygen in Streams: Approximate Delta Method

The “approximate delta method” is a simple procedure for simultaneous calculation of the stream reaeration coefficient, primary production rate, and respiration rate from a single-station stream diurnal profile of dissolved oxygen (DO). It approximates the exact graphs of results for the “delta method” reported in 1991 by Chapra and Di Toro by means of simple logistic curve-fitting approximations. The necessity of reading graphs or of obtaining numerical solutions is thereby avoided, so making it suitable for inclusion in a decision support system, particularly for streams reaeration coefficients less than 10?day?1 and for moderate photoperiods (10–14?h). Worked examples are given for streams in the USA and in New Zealand. Results are used to show that the constellation of parameters for the three fundamental processes is much more important than their individual values in calculating diurnal DO profiles. Independent measurement of the reaeration coefficient enhances the utility of the method, by enabling separate calculation of production and respiration rates.     

Sediment Pickup on Streamwise Sloping Beds

This paper presents an experimental investigation on noncohesive sediment pickup under a unidirectional steady-uniform stream flow on streamwise steeply sloping (down slope and adverse) sedimentary beds. The characteristic parameters affecting the sediment pickup, identified based on the physical reasoning and dimensional analysis of the sediment particle movement under stream flow, are the transport-stage parameter, particle parameter, and geometric standard deviation of sediment particles. A large number of experiments (426 runs) were carried out in two long rectangular ducts (closed-conduit flow) with nine types of sediments (six uniform and three nonuniform sediments), having a variation of bed slope from ?15° (adverse slope) to 25° (down slope). In an open channel flow (laboratory flume study), the uniform flow is a difficult, if not impossible, proposition for a steeply sloping channel and is impossible to obtain in an adversely sloping channel. To avoid this problem, the tests were conducted with a closed-conduit flow. Measurements included flow discharge and sediment pickup rate. The bed shear stress for a particular run was computed considering side wall correction. The experimental data were used to determine the equation of sediment pickup function through a regression analysis. The equation is adequate to estimate sediment pickup not only on horizontal and mild slopes but also on steep and adverse slopes.     

Grazer Control of Stream Algae: Modeling Temperature and Flood Effects

A computer model for epilithic algae and grazer biomass in streams is modified to better predict the effects of temperature and is calibrated for diatoms and mayflies. Mayflies are predicted to maintain low diatom biomass provided that (1) temperatures remain within their preferred range (10–20°C); and (2) mayfly populations are not adversely affected by floods. Algal blooms are predicted to occur in mayfly-dominated streams above 20°C—temperatures common in pasture streams over summer. We hypothesize that mobile bed streams are susceptible to blooms during summer low flows following floods because (1) they usually lack temperature tolerant snail grazers; and (2) mayfly recovery lags behind algal regrowth, and there is a short period when algae escape from “top-down” grazer control.