DEBRIS FLOWS AND CONSTRICTIONS: A COMPARISON OF HYDRAULIC THEORY AND DEBRIS FLOW OBSERVATIONS

Between source and final destination, many debris flows enter and exit constrictions (e.g. the submarine Saharan debris flow; subaerial Kolka, Russia; Pandemoniaum Creek and Zymoetz, Canada; Huascaran, Peru). The behavior of pure water flowing through constrictions is well understood: subcritical flow accelerates through constrictions and decelerates smoothly as it exits. If the constriction parameter* is equal to or less than (very) roughly 0.8, flow enters the supercritical regime. Backwaters form upstream of the constriction, water accelerates through the constriction under so-called “critical conditions,” and decelerates through both smooth expansion and hydraulic jumps downstream of the constriction. Dry granular flows show a semi-quantitatively similar behavior. To see if these ideas could be extended to wet granular flows, i.e., debris flows, we conducted a literature survey to find the best-documented debris flow-constriction settings. Unfortunately, (a) many constriction settings are complicated by the presence of a turn in flow direction (that is, a corner), and (b) a lack of real-time observations of the event. Preliminary analysis indicates that constriction geometries are conducive to supercritical flow conditions (constriction parameters are equal to, or even significantly less, than those required). Behaviors documented or inferred are: Froude numbers exceeding unity; extreme erosion upstream of and through the constriction; inferred hydraulic jumps; pseudo-backwater formation. Our preliminary conclusion is that upstream of a constriction there are qualitative analogies to hydraulic flow (superelevation, flow acceleration, erosion), and that the “jamming” of the larger particles in constrictions and formation of debris ponds behind a constriction are qualitatively analogous to the occurrence of critical conditions and backwater formation in flows of pure or weakly-sediment-laden water flows. To a first approximation, the jamming of particles in constrictions in debris flows is analogous to the jamming phenomenon in dry granular flows. However, in wet debris flows, jamming is often accompanied by dewatering, conditions that preclude carrying the hydraulic analysis downstream in many settings.

* Constriction parameter=constriction width÷a typical upstream width.