Journal of Hydraulic Engineering

The search for physically based distributed runoff models has been underway for more than half a century. A great deal of progress has been made and we have a much better understanding of hydrological processes today than engineers and scientists did in the 1930s. Physically based models are being used more frequently in engineering practice, but questions are being raised regarding their “physical basis” and accuracy, and it has been suggested that simpler models are superior. Selected papers presenting tests of physically based models or comparisons with simpler models are reviewed. It is shown that, on a relatively small scale, these papers are overly pessimistic due to problems of hydrologic measurement and interpretation. However, there are great difficulties involved in scaling up to larger watersheds and we must realize that significant uncertainties are involved in predicting surface runoff.

The conceptual analogy between the impulse response function and the unit hydrograph is applied for the identification of a transfer function model for real-time flood forecasting. The real-time corrections of flood forecasts are obtained from probability distributions of observed and forecasted (without real-time corrections) flows approximated by exponential distributions. The method adds considerable smoothing to error terms. The flood event is defined as the amount of total flow in excess of an alarm level of the flood over its duration. The correlation between the flood flow and its duration is considered to estimate the joint probabilities of exceedance (JPE) of designed flood flow over specified duration using the joint distribution of the flow and duration, given by the Gumbel's bivariate exponential distribution (second type). It is discussed that the univariate analysis (i.e., flow alone) underestimates the exceedance probability of flood flow. The study shows that the JPE increases with the increase of correlation between the flood flow and its duration. The radar-derived rainfall data in the Irwell basin, U.K., are applied in a case study. The method of JPE can be extended to link flood forecasting and flow-duration profile for effective management of flood risk areas.

Unsteady flow properties are experimentally studied in an open channel with a rough bed. A recently developed acoustic Doppler velocity profiler (ADVP) is used to obtain instantaneously the flow profiles. From these measurements, using the Fourier components method, the mean velocities, the turbulence intensities and the Reynolds-stress profile, are obtained. Thirty-three different hydrographs are experimentally investigated. The experimental results show several things. First, unsteady flow can be characterized by the overall unsteadiness parameter, Γ_HG, and by the longitudinal pressure-gradient parameter, β_u. Second, the hydrographs investigated are complete dynamic waves, where the time sequence of U , Q , and D is respected, resulting in the looping rating curves. Third, the distribution of the horizontal velocities, ū ( y ), is given by the log law with B _r ≅ 8.5 ± 15% in the inner region, being almost independent of the unsteadiness, and by the Coles' law in the entire region where the wake-strength parameter, Π, depends (inconclusively) on the β_u-value. Fourth, the distribution of the vertical velocities, $\overline{v}$ ( y ), being almost always negative, could be explained with a theoretical relation. Fifth, during the passage of a hydrograph, the turbulence intensities (root-mean-square values) and the Reynolds stress in the rising branch are larger than in the falling branch. Finally, using the equation of Reynolds, a theoretical expression for the Reynolds-stress distribution could be obtained, which compares reasonably well with the measured data.

This article discusses the most common types of spillway gates including radial gates, vertical lift gates, and flap gates including inflatable gates, and their application. Design considerations for each type are explained. Design and application considerations for different types of spillway gate operators are also included. Items covered for gates include gate geometry, gate proportions, location of gate hoists, location of gate trunnions, trunnion anchorage arrangements, gate seals and sealing arrangements, use of dogging devices, wave deflectors, flow splitters, corrosion protection including use of cathodic protection, and ice problems associated with spillway gates. Items covered for hoists include selection criteria for hydraulic and mechanical hoists, functional requirements of the hoists, use and type of position indicators, normal speed of opening and closing, use of automatic controls, the need for maximizing the minimum opening, and automatic stopping of the gates at set intervals to prevent unintentional spilling. Permissible leakage rate through the gates is also included.

A field method for measuring the flow drag force on individual boulders on a riverbed is identified and tested. Drag is determined from the momentum deficit in the boulder wake, obtained by measuring the flow velocity in a traverse across the wake. A field test yielded a mean drag coefficient of 0.36, which is low compared with flume measurements of 1.1–1.5 for cubes. This may be the result of higher velocity water near the riverbed being sucked into and reenergizing the wake. Further tests are needed to define the conditions in which the method can be used and to indicate how well the assumptions behind the method can be accommodated in a natural flow environment.

A number of predictive bed-load sediment transport equations are rated against a unique set of field data collected by automatic slot samplers during flash floods in a desert wadi. The Meyer-Peter and Müller equation is shown to perform well, providing a median ratio of calculated to observed ( C /O) bed-load flux of 1.18. The Bagnold equation is shown to underpredict considerably, with a median C /O of 0.44. The Parker equation performs better, though it still underpredicts with a median C /O of 0.78. The apparent success of the Meyer-Peter and Müller formula is attributed to the ready supply of sediment to the channel system in desert and semidesert environments. This ensures that the channel bed remains unarmored, in contrast to supply-limited, armored, perennial rivers of humid zones. It also ensures that bed-load flux responds to changing hydraulic conditions in a comparatively simple fashion.