Journal of Hydraulic Engineering

This paper presents a new model (Q2D) for the velocity distribution in a channel cross section for use in estimating discharge. It describes the model and its theoretical basis and presents the results of a case study. The distribution is determined by combining the principle of maximum entropy with a probability distribution obtained by the solution of the Poisson equation over the cross section. The model uses observed depth and velocity in the water column, where an acoustic Doppler current profiler is installed to determine three key flow parameters to obtain velocity and discharge. In addition, if supporting field discharge measurements are available, the model can be further calibrated to account for any asymmetry in the flow. If velocity distribution data exist for the entire cross section, the model can be adjusted to stretch the predicted velocity pattern to better conform to experimental observations. When applied to the Châteauguay River, Quebec, for both ice covered and open water, Q2D predicted 12 gauged discharges with a −4% bias and an average absolute error of 7% prior to calibration. After removing the bias through calibration, the average absolute error was reduced to 5%.

The effect of the flood recession time on the local scour depth at bed sills in gravel deposits is examined. Experiments were carried out to study the development of scour holes under time-varying hydraulic conditions with no upstream sediment feed. Triangular-shaped hydrographs, having recession times up to three times the duration of the rising limb, were used. Traditionally, the peak water discharge in any flood event is used as a design value in estimating the final depth of scour formed by a flood. This approach is overly conservative when the flow hydrograph is steep, i.e., during the occurrence of flash floods. The actual reduction of the scour depth from this estimated value is dependent on both the characteristics of the flood event and the characteristics of the stream. The results show that the maximum potential scour depth can be achieved only for hydrographs with long recession times, while the rate of this process can be estimated as a function of the ratio between a characteristic flood time and the steady-state temporal scale of scour development. A method is proposed for the prediction of the scouring process under unsteady flows in terms of two dimensionless temporal parameters. Results obtained for clear-water boundary conditions can be extended to sediment-supply tests if specific supply input conditions hold.

Fine particle deposition in coarser sediment beds plays an important role in biogeochemical cycling in sediments by (1) supplying organic matter, attached microorganisms, and nutrients and (2) mediating interfacial fluxes through changes in permeability. In some cases, particles could be associated with contaminants or particles could clog pores in the sediment resulting in ecological impacts through changes in porewater chemistry (e.g., dissolved oxygen profiles). We compare two mechanisms for fine particle deposition to permeable sediments. First, for flat beds, particle delivery by sediment grain-scale flows coupled with particle retention within the sediment could result in deposition in excess of the gravitational settling. Second, pressure-driven flows across ripples may deliver additional particles directly into the sediment, although sediment transport can counteract deposition. Flume experiments were used to measure deposition rates over a range of particle and sediment sizes for both flat and rippled beds. Flat bed deposition rates were greater than the particle settling velocities and comparable to the deposition rates to rippled beds under similar flow conditions. An empirical expression based on parameters for flow, sediment, and particle conditions predicts where each mechanism dominates the deposition rate. This result illustrates the potential importance of grain-scale processes for the deposition of fine particles to permeable sediments regardless of bed roughness. Predictions of particle fate, penetration into the sediment, and degradation following the deposition still require a description of bed topography in addition to sediment properties.

Wave damping in vegetation in shallow lakes reduces resuspension and thereby improves the light climate and decreases nutrient recycling. In this study, wave transformation in reed (Phragmites australis) was measured in a shallow lake. Theoretical models of wave height decay, based on linear wave theory, and transformation of the probability density function (PDF), using a wave-by-wave approach, were developed and compared to the collected data. Field data showed an average decrease in wave height of $4–5\mathrm{\%}{\text{m}}^{-1}$ within the first 5–14 m of the vegetation. Incident root-mean-square wave height was 1–8 cm. A species-specific drag coefficient $C_D$ was found to be about 9 (most probable range: 3–25). $C_D$ showed little correlation with a Reynolds number or a Keulegan-Carpenter number. The PDF for the wave heights did not change significantly, but for longer distances into the vegetation and higher waves it tended to be more similar to the developed transformed distribution than to a Rayleigh distribution. Relationships developed in this study can be employed for management purposes to reduce resuspension and erosion.

An experimental study was conducted to investigate the use of a temperature-control curtain in selective withdrawal from a two-layer stratified reservoir. This study focused on the case where cool water at a depth was forced to flow under the curtain. The evolution of the mean flow, the withdrawal water quality, and the mean velocity field were studied using particle image velocimetry and laser-induced fluorescence. Practical relationships were developed for predicting the withdrawal water quality and the interface height as a function of time. The structures of the flow field in both the upper and lower layers are discussed in detail. The flow in the lower layer was dominated by the recirculation eddy induced by the jet flow under the curtain and a relation between the eddy length and the interface height was obtained. Close to the intake, within about $3d$ (where $d=\text{intake}$ diameter), the velocity field can be well described by the potential flow theory. Beyond $3d$ , however, the flow field considerably deviated from the potential flow theory due to the jet expansion and stratification. A general discussion of the results and engineering applications are also provided.

Results of a laboratory modeling study are presented for excluding bed load sediment from a diversion/intake structure on the Rio Grande in Albuquerque, New Mexico. To achieve model similitude, crushed coal was used to model the prototype sediment in a 1:24 scaled model with an exaggerated slope such that shear force is adequately modeled. The Shields parameters and critical Shields parameters were matched between the prototype and the model, resulting in similar grain Reynolds numbers. Twenty-four tests, where guiding walls, submerged vanes, and/or the angle of the intake bay were altered, were conducted for a single river and diversion flow rate to develop the best performing sediment exclusion system at the intake structure. Independent vanes with 45° rotated intake bays were recommended for the most effective sediment exclusion at the intake structure.

The 2003 flood of the Muda River reached $1,340{\text{m}}^3/\text{s}$ at Ladang Victoria and adversely impacted 45,000 people in Malaysia. A flood control remediation plan proposed a levee height based on a 50-year discharge of $1,815{\text{m}}^3/\text{s}$ obtained from hydrologic models. This design discharge falls outside the 95% confidence intervals of the flood frequency analysis based on field measurements. Instream sand and gravel mining operations also caused excessive riverbed degradation, which largely off sets apparent benefits for flood control. Pumping stations have been systematically required at irrigation canal intakes. Several bridge piers have also been severely undermined and emergency abutment protection works were needed in several places. Instream sand and gravel mining activities should be replaced with offstream mining in the future.

Simplified and detailed two-dimensional modeling approaches to transient flows in urban areas, based on finite-volume solution of the shallow water equations, are compared. Through the example of a dam-break flow in a simplified urban district for which accurate laboratory data exist, various methods are compared: (1) the solution of the two-dimensional shallow water equations with a detailed meshing of each street; (2) the use of a porosity concept to represent the reduction of water-storage and conveyance in the urban area; and (3) the representation of urban areas as zones with higher friction coefficient. Accuracy and adequacy of each method are assessed through comparison with the experiments. Among the simplified models, the porosity approach seems to be the most adequate as head losses at the entrance and the exit of the city are considered.