Journal of Hydraulic Engineering

Present knowledge regarding associations between sediment and riverine aquatic habitats is summarized. Physical processes govern aquatic‐habitat quality and development. Within limits, engineering approaches can be used to evaluate and predict aquatic‐habitat conditions. Stream reaches and the habitats they provide may be categorized by dominant bed‐material type: boulder‐cobble, cobble‐gravel, sand, or fine bed. As the grain size distribution of the bed load approaches that of the bed material, the number of benthic species declines. Sediments provide cover and spawning sites for fish and habitat for fish food organisms. Sediment also serves as an indirect indicator of fish‐habitat quality when it provides a temporal integration of depth and velocity. Major environmental issues associated with sediment transport in rivers include transport of organic sediments, sediment‐water‐quality interactions, deposition of sediments finer than gravel on and within coarser deposits, and filling of low‐velocity areas contiguous to major river channels. Most stream organisms can withstand short‐term exposure to elevated levels of suspended sediment, but chronic exposure is more detrimental.

The Tennessee Valley Authority (TVA) reservoirs were innovatively manipulated to prolong strong stratification to keep a nuclear plant's intake water temperature below safety limits. Thermal stratification in a key reservoir was initially stabilized by changing hydropower peaking operation at both ends of the reservoir to precalculated steady releases. Additionally, a predetermined volume of cold water'was released from a deep tributary reservoir and routed as a density current over a distance of more than 160 mi (257 km) through three reservoirs to the subject plant. A mathematical model capable of simulating the flow and thermal characteristics of the reservoir system, including atmospheric heat exchange, was developed to help identify feasible reservoir operation options and to determine the required volume and duration of the cold‐water release. Kinematic and geometric properties of the cold water traveling through the reservoir system were modeled using theories governing two‐dimensional density currents. Accurate determination of travel time and release date was critical to the arrival of cold water at the plant during the period it was most needed.

Gabions are a common construction material in the African Sahel and elsewhere, especially for spillways. Stepped gabion spillways are structurally stable, resistant to water loads, and efficient energy dissipators, but there are no clear design rules to date. One‐fifth‐scale model tests representing 3–5 m high standard prototypes discharging up to $3m^3\cdot s^{-1}\cdot m^{-1}$ enabled us to establish quantified energy‐dissipation criteria and efficient stilling‐basin design rules, saving 10–30% on basin length as compared with current methods. The different types of flow over the stepped face are also described and analyzed: isolated nappe flow, partial nappe flow, and skimming flow. From a technical point of view, stepped gabion spillways can withstand flows up to $3m^3\cdot s^{-1}\cdot m^{-1}$ without great damage, if setting of gabions complies faithfully with the code of practice. Some advice is given on preventing gabion deformation.

Field measurements of velocity and salinity on a transverse cross section of a straight reach in the upper Conwy estuary have been used to evaluate the temporal and intratidal variations of the transverse and vertical shear induced components of the longitudinal dispersion coefficient. The results yield clear temporal trends and an element of reproducibility between tides. A comparison with earlier published results for a cross section further downstream in the same estuary suggests that changes in the channel width, cross‐sectional shape, and bends probably contribute significantly to a longitudinal variation of dispersion coefficient values. A comparison of the values of the longitudinal dispersion coefficients with those obtained from a solution of governing transport equations emphasizes the need for a better understanding of the spatial and temporal variation of the values of the vertical and transverse eddy diffusion coefficients.

An extension of the double‐sweep technique is presented for solving systems of algebraic equations arising from implicit schemes of the box type for unsteady open‐channel flow. The new technique handles coefficient matrices with a nonzero column of coefficients as well as with coefficients clustered around the diagonal. This arises, for example, when the time step is unknown, or when the spatial location of node points depends upon an unknown distance, such as surge‐front location. Subcritical and zero‐intertia flow, which inherently are governed by a boundary condition at each end of a reach, have typically been treated by a double‐sweep technique. This strategy is preserved with the proposed method, even with a nonzero column of coefficients in the matrix of governing equations. Kinematic‐wave flow and superficial flow are traditionally considered a problem with a marching solution—with boundary conditions picked up at the upstream boundary, and flow variables determined directly in one forward sweep through the computational cells. With the nonzero column of coefficients however, a double‐sweep computational strategy is proposed.

Bed‐form data from laboratory experiments using cohesionless, fine silica solids in shallow water flows are analyzed to find dimenionless relationships for geometry and friction factor. With increasing flow discharge over a stationary flat bed, the bed‐form sequence is primary ripples, barchanoid and linguoid ripples (secondary ripples), ripples on dunes (for silt grades only), and a moving flat bed. The primaries develop from the incipient transport stress, otherwise called the primary threshold. By extrapolating dimensionless bedform length and height data to this threshold, it is shown that the geometry is proportional to the depth of the viscous sublayer thickness. The secondaries develop from a secondary threshold, which is an approximate constant stress condition at which incipient suspension also begins. The ripples on dune form is associated with a tertiary threshold for which the dimensionless stress ratio $Y/Y_c$ is about 20. And the moving flat bed form occurs at a transition threshold for which $Y/Y_c$ appears to be a natural constant at about 15. A single friction factor curve is demonstrated for all these data.

Disaggregation models are widely accepted tools for generating synthetic hydrologic time series. Attributes and limitations of such models have been reviewed and discussed by several researchers and practitioners. One shortcoming has been related with the size of the matrices involved and the corresponding number of parameters. The main objective of this paper is to analyze and exploit the potential of disaggregation schemes by steps (stages or cascades). It has been shown that nonsymmetric (NSYM) and a mix of symmetric/nonsymmetric schemes are better than other schemes with regard to the size of matrices and the number of parameters. More specifically, the NSYM stepwise lag‐one and lag‐two models for single‐site, and the lag‐one model for multisite, disaggregation have been described and applied to modeling and generation of bimonthly flows. The results of the case studies indicate that the proposed parsimonious stepwise disaggregation schemes reproduce quite well the intended variance‐covariance properties of the seasonal flows.

Four equations for water flow in unsaturated porous media are compared with the results of several years of data collected in the field. The equations consist of one attributed to Darcy and three developed by Klute, Morel-Seytoux, and Gelhar. The equations require different input parameters, but limiting the input variables to data collectable in a typical soil survey has been attempted. The field data (not collected by the writers) consist of monitoring the downward movement of tritiated water in a research watershed to a depth of 310 in. (787 cm). Soil moisture characteristic curves for the various soil types within the monitored depth are developed by a computer program developed by the writers. Results show that the four equations predict the rate of downward water flow with varying accuracies depending on the soil type within the profile and moisture conditions of the soil. The results are summarized in graphical and tabular form, and conclusions are drawn from the results.

Power laws are widely used in hydraulic engineering because they are simple compact expressions that generate straight lines on log-log paper. The relationship between hydraulic gradient and bulk velocity for one-dimensional non-Darcy flow is often modeled by a power law. Other investigators use a zero-intercept quadratic equation to model the same phenomenon. In this technical note, general expressions are derived to express the coefficients of one form in terms of the coefficients of the other form. Simpler expressions, specific to the study of non-Darcy flow, are also presented.

The Parshall flume is one of the most widely used instruments for measuring water discharge in open channels. However, field observations indicate that because the flume is subjected to field and climatic conditions, it may settle over time. A study was conducted at the Hydraulics Laboratory of Colorado State University to evaluate how settlement affects the accuracy of 1, 2, and 3 in. (2.54, 5.08, and 7.62 cm) Parshall flumes. The Parshall flumes were installed into a recirculating laboratory flume and tested for lateral slope, longitudinal slope, and combined lateral-longitudinal slope settlements. Flume slopes varied ±13% and were conducted for free outfall conditions. The results indicate that Parshall flume ratings can be corrected for lateral, longitudinal, and combined lateral-longitudinal slope conditions. Correction factors are derived for small Parshall flumes for lateral and longitudinal settlement. A procedure is presented to adjust the flow rating as a function of the slope, flow depth, and throat width.

It has been specified in several past publications that the computational time interval should be taken equal to or greater than 2KX to avoid computing negative outflows; K and X are parameters in the Muskingum equations. It is suggested herein that such a specification may need to be imposed only on the first computational time interval; subsequent time intervals can be selected as small as one may practically desire. The first time step, of the order of 2KX, is identified with the time lag parameter previously advocated by the writer. The proposed method of computation is explained by solving a numerical example. Using a time lag of 4.57 hr in this example, the outflow hydrograph is computed with a time step of 1 hr. The computed hydrograph is realistic and satisfies the flow continuity requirement.