Journal of Hydraulic Engineering

Every engineering computation is attended by some degree of uncertainty due to factors not taken into account and limitations of data. As a result, computed values differ from true values in nature, and the size and frequency of these differences determine if any particular estimate is a best estimate. A best estimate of any variable may be defined as one that produces an unbiased least‐squares estimate in the objective variable, defined as the variable that is critical in application. It is not necessarily a least‐squares estimate of that particular working variable. In project design, there is always some probability of overdesign and some probability of underdesign (due to uncertainties), and the consequences can be vastly different. If the objective variable is net expected cost where underdesign and overdesign are each appropriately penalized, minimizing net expected cost will automatically provide an appropriate factor of safety and consequently a best design.

A backward‐facing step sometimes forms in an alluvial river. The step deforms and migrates when the inflow sediment and flow conditions change. A series of experiments were conducted to investigate the migration behavior of a backward‐facing step composed of uniform sediment. According to the experimental observations, the phenomena observed were that the bed material picked up from the upstream reach was deposited in the separation zone at the step and caused the step to migrate downstream. Due to contraction of the flow cross section in the vicinity of the knickpoint, the bed‐shear stress increased locally, which caused scouring. The scouring propagated either upstream or downstream depending on step height, flow conditions, and sediment characteristics. A linear‐wave model was proposed to simulate the process. The model was calibrated and verified with experimental data. Due to the nonuniformity of the flow conditions in the vicinity of the step, the model can only provide qualitative information in this region. Fortunately, the nonuniform‐flow conditions last at most 60 s, and hence the overall simulation results are quite satisfactory.

A new flood‐routing method is compared with the DWOPER model. The new method combines a noninertia (or diffusion model) simplification of the de Saint Venant equation with a statistical procedure. It accounts for the nonlinearity and dispersion of the flood‐wave propagation phenomenon. It also accounts for nonuniform lateral flow. The new method, unlike most hydrologic routing methods, can be used to simulate a variety of flow variables such as water stage, depth, or discharge. The comparison is not only based on the accuracy of both models, but also, and mostly, on their practicality. Both models are compared for accuracy and in aspects such as data requirement, computer running time, actual time needed for calibration.

Water temperatures in natural streams can be modeled using as input air temperature, solar radiation, relative humidity, wind speed, cloud cover and barometric pressure in addition to stream geometry, stream flow, and inflow temperature. MNSTREM is an example of such models that use heat‐budget components to calculate the heat exchange between stream water and the atmosphere. The MNSTREM model has been expanded to include heat flux between streambed and water. The model is sensitive to each of the weather and streambed parameters to different degrees. Sensitivity coefficients represent the change in stream water temperature resulting from a unit change in each of the model parameters. They are calculated by two methods that give same order of magnitude results. To establish the sensitivity of the model to each parameter the mean and/or the standard deviations of each of the weather and streambed parameters is multiplied by its sensitivity coefficient. The sensitivity analysis gives quantitative evidence that stream water temperature is more sensitive to air temperature and solar radiation than to other weather and bed parameters. It also shows quantitatively that daily‐average stream water temperatures are not very sensitive to the streambed thermal conductivity.

Hydrodynamic and hydraulic theories have been developed along two parallel approaches, i.e., the vectorial and variational approaches. Most classical hydraulic theories are based on vectorial approaches. The variational approach is a scalar approach based on the maximization of entropy, minimization of energy, or minimization of energy dissipation rate. A review of variational theories and hypotheses shows that fundamental theories in hydrodynamics and hydraulics derived from vectorial approaches can also be independently derived from variational approaches. A comparison of different variational theories indicates that they are consistent with each other provided that the concepts of entropy and energy are properly defined and correctly applied.

Two series of laboratory experiments were conducted to evaluate the influence of mean flow direction and sampler size (7,6 cm aperture vs 3 cm aperture) upon the efficiency of Helley‐Smith samplers in subcritical flows. The formation of flow circulation zones both inside and outside the sampler was found to occur at divergence angles between the sampler and mean flow of as little as 10°. The creation of these recirculation regions, and especially a zone of separated flow inside the sampler nozzle, is mirrored by a decrease in the sediment sampling efficiency. Additionally, Investigation of the performance of small Helley‐Smith samplers reveals that these trap less sediment per unit width than standard size samplers (7,6 cm). This lower sampling efficiency is linked to the lower protrusion of the sampler into the boundary layer.