reproduced the phenomenon of interest over a given hydrograph as observed on the prototype,
it will also reproduce the future response of the river over a similar range of conditions.
The mobile bed models are more difficult to design and their theory is significantly more
complicated as compared to clear water rigid bed models. However, many successful
examples of their use are available. In general, important river training and control works are
invariably studied on physical models. The interpretation of results from a mobile bed model
requires a basic understanding of the fluvial processes and some experience with such
models. Even in the many cases where it is only possible to obtain qualitative information from
mobile bed models, this information is of great help in comparing the performance of different
designs.
Adoption of a particular method for estimating river response depends on quality and
availability of data as well as the engineer's experience. More information on physical modeling
can be found in HEC-23 and reports by Gessler (1971), Yalin (1971), Shen (1979), and
Richardson et al. (1987, 1989). Additional applications of physical model studies of alluvial
channel flow at highway crossings can be found in: "Hydraulic, Erosion, and Channel Stability
Analysis of the Schoharie Creek Bridge Failure, New York," (Richardson et al. 1987); "Flume
Modeling Experimental Plan for the Replacement of the Herbert C. Bonner Bridge," (Parsons
Brinkerhoff Quade and Douglas, Inc. 1996); "Laboratory Report of the Acosta Bridge Scour
Study," (Stein, S.M. et al. 1990); "FHWA Hydraulics Laboratory and Partners Perform Scour
Evaluation for Woodrow Wilson Bridge" (Jones, J.S. 1999); and "Local Pier Scour Model Tests
for Jensen Beach Bridge," (Sheppard, D.M. 1999).
5.6.2 Computer Modeling
The design engineer's interest in alluvial river response is generally focused on anticipating
how the river bed and water-surface elevations will change if an existing stable or equilibrium
situation is perturbed. This perturbation may be the occurrence of an unusually large annual
flood that temporarily scours the bed and banks to accommodate the higher flow before
returning to normal conditions. The perturbation may also be a permanent change in river
discharge and sediment supply caused by upstream regulation of flows, or a change in channel
geometry resulting from bank stabilization or channelization. The first type of perturbation can
often be simulated using a physical scale model. Although problems arise with interpretation of
the results, physical models in the hands of experience modelers can yield valuable information
on local scour and deposition around structures. However, the sheer expense and space
requirements of physical scale models generally disqualify them for simulation of long-term,
large-scale river bed response to the second type of perturbation. This is where numerical,
computer-based models, which can simulate both short- and long-term response, find their
natural area of application. Another area where prediction of long-term response is desired is
river stabilization and river restoration design.
Numerical models of alluvial river response are the natural outgrowth of rigid-boundary,
unsteady flood propagation models that have proven so useful in engineering design. These
unsteady flow models have succeeded because they are based on mathematical descriptions
that incorporate all the important physical processes involved and use reliable, carefully
implemented numerical methods to obtain approximate solutions to the appropriate
partial-differential equations. However, alluvial river-response models have not enjoyed the
success of their rigid-boundary cousins, precisely because of the weaknesses in our
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