Journal of Hydraulic Engineering

The Research Advocacy Task Committee was formed by the ASCE Hydraulics Division and charged with developing a strategy to be used by the parent Research Committee in advocating support of a vigorous and sustained research effort in hydraulics. In this paper we outline a vision for the future of hydraulic engineering and propose a focus for this discipline. We also outline a strategy to implement the vision, and in doing so explore the relationship between research and practice, as it related to our research agenda, and also the relationship between federal agencies, universities, and the private sector. The need for a long-term investment in research and education, and for a multidisciplinary approach to conducting research is also discussed.

Many existing morphological models are limited by their inability to account for changing channel width through time. In this and a companion paper, the development and testing of a numerical model of river widening are reported. The new model is applicable to straight, sand-bed streams with cohesive bank materials, and nonuniform bathymetry and width in the longitudinal direction. The flow field is obtained by solving versions of the flow resistance, flow momentum, and continuity equations, which account for the influence of gradually varied flow and lateral shear in the near bank zones. Secondary and overbank flows are excluded. Predicted flows are used to account for streamwise and transverse sediment transport fluxes. Numerical solution of the sediment continuity equation allows temporal variations in bed-material size, bed morphology, and bank geometry to be simulated. Channel widening is simulated by coupling bank stability with flow and sediment transport algorithms. A probabilistic approach is used to estimate the longitudinal extent of mass failures within modeled reaches, and mixed layer theory is used to model the transport of the resulting bed and bank material mixture. Assessment of the accuracy of the new model, discussed in the companion paper, indicates that the new model simulations of widths, depths, and deepening rates are within 15% of observed values, but widening rates are underpredicted by a factor of approximately 3.

In this paper the numerical model presented in the companion paper is tested and applied. Assessment of model accuracy was based on two approaches. First, predictions of evolution of a 13.5 km reach of the South Fork of the Forked Deer River, in west Tennessee, were compared to observations over a 24-yr period. Results suggest that although the model was able to qualitatively predict trends of widening and deepening, quantitative predictions were not reliable. Simulated widths and depths were within 15% of the corresponding observed values, but observed change in these parameters at the study sites were also close to these values. Simulated rates of depth adjustment were within 15% of observed rates, but observed rates of channel widening at the study sites were approximately three times those simulated by the model. In the second approach, the model was used to generate relationships between stable channel width and bank-full discharge. The model was able to successfully replicate the form of empirically derived regime-width equations. Simulations were used to demonstrate the model's ability to obtain more realistic predictions of bed evolution in widening channels.

An experimental investigation of local scour at a nonuniform cylindrical pier is presented. A nonuniform pier is one for which the cross-sectional dimension varies over the length of the pier. In this study the pier comprised a cylinder of diameter D founded on a larger cylinder of diameter D *, called the foundation. Besides the parameters that affect scour at a uniform pier, the scour depth at a nonuniform pier is dependent on the ratio D /D* and the depth from the bed level to the top of the foundation. The results are used to define three scour zones, namely: zone 1 where the foundation is below the bottom of the scour hole and does not affect the scour, zone 2 where the top of the foundation is within the scour hole and reduces the scour, and zone 3 where the top of the foundation is above bed level and increases the scour compared to that at a uniform pier. Various design relationships are presented. The concept of an effective pier size ( D _e ) is introduced, and it is shown that use of D _e leads to conservative estimates of scour depth for nonuniform piers.

A practical approach to modeling the hydraulic interaction of a stream and aquifer via streambed leakage is based on the established U.S. Geological Survey (USGS) model, MODFLOW. To represent flood-wave propagation and the associated bank storage, MODFLOW's STREAM module is replaced by the Muskingum-Cunge diffusive-wave-routing scheme. The diffusive wave model closely approximates a dynamic model of a flood wave's speed, shape, and streambed leakage. Because the stream responds more rapidly to disturbances than the aquifer, streambed leakage is calculated at the flood routing time scale in order to properly represent the stream-aquifer coupling. However, both the relative magnitude and timing of aquifer response to a flood wave depend on the strength of this coupling. We find discrepancies in both the flood wave and the streambed leakage when the wave and ground-water motions are evaluated at different time scales. These discrepancies are significant in the case of a strong stream-aquifer coupling, for which equal aquifer and flood-routing time steps may be required. Wave diffusion and bank storage are shown to be comparable in magnitude and should, therefore, be included in stream-aquifer interaction models. Diffusive wave routing more accurately represents wave propagation, bed leakage, and aquifer response if short aquifer time steps are taken, and is preferable to the STREAM module for simulating short time transients. However, the STREAM module is useful for simulating large time frames if accurate modeling of the flood-wave propagation is not required.

In a series of field measurements the mean velocity profiles were measured during the acceleration and deceleration phases of tidal currents off the east and south coasts of England. Salinity/temperature profiles were also measured. The sites have simple geometries, and the beds were covered with sand. The measured velocity profiles consist of an outer region, encompassing 90% of the boundary-layer thickness; an inner region adjacent to the bed; and an overlap region between the outer and inner regions. The velocity profiles in the outer and overlap regions obey the defect law and the usual log-law, respectively. The coefficient of the skin friction was high, mainly due to bed forms.