PROGRESS IN SEDIMENT TRANSPORT THEORY: A RETROSPECTIVE OF THE SOUTHARD ERA

In 1969 Judge R. H. Kroninger (State of California versus E. Righetti et al., 1969) concluded from the testimony of numerous expert witnesses on the impact of redwood logging that "[expert] opinions....are hopelessly irreconcilable on how much and how far solid particles will be moved by any given flow. They are able to agree only that sediment will not be transported upstream." Now 32 years later, after numerical and physical sediment transport modelling confirmed its feasibility, the world's largest hydroelectric dam---the Three Gorges Dam---is being built on the world's third longest river. How did we come so far so fast? This rapid increase in understanding sediment transport was fed by contributions from the MIT sedimentology lab of John B. Southard and students. Southard started his professional career in the early 1970's publishing on the erosion of fine, cohesive marine sediments. By the 1980's his lab was reporting on the deposition of sands and gravels in high speed surges and on autosuspension, thereby significantly contributing to the famous debates over auto-ignition of turbidity currents. By the 1990's Southard and students shifted focus to the vexing problem of sediment entrainment and transport of heterogeneous sizes and densities and to selective sorting. This research drew them into the "equal mobility" wars, out of which came significant advances in our predictive capability of bed material transport. Present sediment transport theory draws heavily upon the Southard canon. Of over 700 citations since 1986, the most heavily cited paper is not on bedforms (surprisingly) but is Southard's 1971 classic on the "Experimental Erosion of Calcareous Ooze." Lest graduate students despair, much work remains to be done. Bedload functions are still prone to order of magnitude errors; present formulations fail in extreme events when much of the sediment transport occurs; suspended load formulations break down at hyper-concentrations; and secondary flows are commonly assumed to be negligible, even though natural channels show significant lateral velocity gradients. For those bold enough to tackle these problems, a good place to start would be just the right blend of theory, experiment, and field observation that is the hallmark of the Southard era.