Journal of Hydraulic Engineering

The objective of this paper is to identify, collate, and define the procedures required to evaluate the performance of an analytical or numerical surface water model. The essential elements are: Identification of the problem; relationship of the model to the problem; solution scheme examination, model response studies, model calibration; and model validation. Codification is not attempted, rather literature examples are used to define the techniques that have been used to address each of the elements above. Emphasis in the six elements is placed on moving the evaluation of models, particularly those in journal publications, towards more quantitative or objective measures of calibration and validation. Also recognized, however, is that such objective measures can only be meaningful when placed in context with all the performance elements.

A classification scheme for the regimes of steady flow over a fixed two‐dimensional obstacle is developed and tested experimentally. The results are compared with those derived from towing tank experiments. The two approaches differ, since the towing tank experiments do not always reach steady state. However, both approaches predict that two stable states may exist for the same external conditions. The existence of a hysteresis loop is demonstrated experimentally, but its extent is not as large as predicted. This discrepancy occurs because the theory implicitly assumes that the obstacle and any hydraulic jumps in the flow are of negligible length.

Dimensional analysis is used to obtain asymptotic solutions that describe plume behavior in a two‐layer ambient fluid. A single dimensionless parameter is found that measures the ability of the density discontinuity to stop plume rise. Experiments show the range of values of the parameter that define conditions for weak and strong density discontinuities. Constants are determined from the experimental data, which allow quantitative prediction of some spreading layer characteristics.

The influence of surface waves on the dilution of buoyant discharges is investigated. The relevant dimensionless variables are derived. In the case of stagnant unstratified environments, it is shown that the dilution depends on six dimensionless variables. An experimental program is conducted to determine the relationship between surface wave parameters and the dilution relative to the no‐wave case. The dimensionless variables kept constant in the study are assigned values that are characteristic of operating ocean outfalls and are not wave‐related. The results indicate that in shallow outfalls, in a typical persistent wave environment, the surface dilution is significantly higher than for the no‐wave case. In particular, it is verified that, for an outfall in a depth of 7.7 m with persistent waves whose amplitude is 30 cm and whose period is 5 s, the surface dilution is twice that for the nowave case. This difference is quite important to the economics of shallow outfalls, especially with the gentle offshore slopes found in the southeastern United States.

Tropical rivers often transport a heavy bed load and have a high silt content, and their surface waters carry leaves, twigs, and an occasional tree or branch. Extracting reasonably clean water for a high head power plant usually requires the installation of sand traps, sediment sluices, and leaf skimmers. This paper describes the work associated with the development and hydraulic model testing of a unique gathering tube intake built into the diversion dam of the 112 MW, 702 ft (214 m) head hydropower development at Andekaleka on the island of Madagascar. River water is extracted from the relatively clean mid‐flow level, above the bed load and below the contaminated surface waters. Results from an inspection after two years of operation are presented, along with a comparison of model and prototype performance.

This paper describes a two‐dimensional numerical model of thermal discharge in coastal regions. The model uses potential flow theory to calculate the ambient velocity field of the coastal water. Using the calculated velocity field, the heat transport equation is solved to define the excess temperature field. The finite difference method for arbitrary boundary shapes is used to solve the stream function for the current velocity calculation and the two‐dimensional heat transport equation for the excess temperature calculation. To illustrate the computational procedure, the model is applied to the case of a recently developed industrial complex in the Red Sea region. The power and desalination plants of the industrial complex require a large amount of seawater for cooling purposes. The used cooling water is discharged into the Red Sea. Hypothetical values for the various parameters are used in this case study. The model is run on a minicomputer, a VAX‐11/730. The case study demonstrates the usefulness of the stream function to obtain the velocity field. It also demonstrates that the model can provide a first approximation of the temperature field. Therefore, the use of the model is justified where field data are not readily available.

A recent report of the Interagency Advisory Committee on Water Data found that there is “no current procedure for assigning an exceedance probability to the probable maximum flood (PMF)… in a reliable, consistent, or credible manner.” This conclusion was used as justification for continuation of the current, quasi‐deterministic, PMF‐based spillway design methods used by all federal agencies. This is despite criticism by both researchers and practitioners that PMF‐based methods tend to lead to a false sense of security and to misallocation of resources for dam safety improvements. As an alternative to perpetuation of the status quo, the writers outline four general areas in which research should be promoted for improved estimation of extreme floods, as well as research aimed at development of a method for incorporation of risk information into spillway design. The writers believe that if the federal action agencies were to promote research in the areas suggested, the current stagnation that has set in would be broken, and the self‐fulfilling prophecy that there are no alternatives to current practice could no longer be justified.

The pressure oscillations within a two-dimensional, rectangular cavity have been explored. The flow was subsonic and the chord-to-depth ratio of the cavity ranged between 0.25 and 1.25. The experimental setup, comprised a plexiglass channel-cavity model, was inserted within the test section of a wind tunnel. The moveable walls of the cavity were adjusted for various cavity geometries. A pressure transducer was flush mounted along the upstream wall of the cavity and was used to monitor, with the help of an oscilloscope, the pressure fluctuations within the cavity. Within a Strouhal number range of 0.4 - 2.4, periodic pressure fluctuations as high as 50% of the free-stream dynamic pressure were observed. The modulation of the external flow is influenced by the cavity which acts as a simple harmonic oscillator with a certain natural frequency. As the velocity of the external flow is increased gradually, the frequency of the pressure fluctuations experiences step changes, shifting to higher or lower frequency levels. Of the two parameters describing the cavity geometry and their effects on pressure oscillations, the cavity length appears to influence the frequency, and the cavity depth effects the amplitude. The maximum amplitude of the pressure oscillations within the cavity were observed to occur under mode 2 and for a cavity geometry corresponding to a chord-to-depth ratio of about 0.5.

The applicability of Einstein's and Colby's sediment transport formulas is tested with flume data of total bed sediment discharge in clay suspensions. The contact bed sediment discharge calculated using Einstein's formula remains practically unchanged after correction for the fluid viscosity and density of fine sediment mixtures. The increased total bed sediment discharge in hyperconcentrated flows is, therefore, attributable to the suspended sediment discharge. The results of Colby's method are similar to those of Einstein without correction.