High Mobility Of Subaqueous Debris Flows And The Lubricating-Layer Model

Abstract

Abstract Numerous field cases illustrate the puzzling tendency of subaqueous debris flows to flow with much less resistance than their subaerial counterparts under otherwise comparable conditions. Subaqueous debris flows may attain stupefying runout distances of hundreds of kilometers, even on very gentle slopes. In order to understand the role played by water in flow enhancement, laboratory experiments were performed to investigate the flow and depositional geometry of subaqueous gravity flows with clayey slurries. Basal pressure measurements (pore and total pressures) and velocity profiles from high-speed videos confirm previous interpretations that clay-rich debris flows develop a rigid front riding on top of a thin lubricating water layer. Water penetrates underneath the front of the debris flow and mixing occurs at the basal surface of the debris flow. This effect sharply enhances the mobility of the front and stretches the sediment main body, in agreement with field observations. Possibilities for numerical simulations are discussed and applied to idealized cases and to the giant Storegga slide in Norway. Introduction and background Subaqueous slides and debris flows can be extremely mobile, reaching at times runout distances of several hundred kilometers (Hampton, Lee and Locat, 1996; Locat and Lee, 2002; Elverhøi et al., 2002). This high mobility is remarkable, considering that the characteristic slope angles in the submarine environment are very small, almost always less than 5° and often less than 1°. In contrast, subaerial debris flows have on average shorter runout distances for comparable release volumes, despite much higher slopes. According to the general principles of soil mechanics, most of the subaqueous debris flows should not move at all because remolded shear strengths measured for submarine clays are often far higher than the shear stresses on such gentle slopes (Elverhøi et al., 2002). Evidently, a major role in the flow enhancement must be played by the ambient water, which in some way increases debris flow mobility, even though it causes much larger resistive drag forces than air and reduces the effective gravitational acceleration. The problem of the dynamics of subaqueous debris flows is far from being understood quantitatively, especially concerning the interaction of the deforming material with the ambient water (Elverhøi et al., 2002). Debris flow dynamics not only represents an important question in marine morphodynamics, but has also practical applications to hazard assessment for human settlements around the coastlines, for marine installations offshore and for the generation of tsunamis. Quantitative studies of submarine debris flow dynamics are likely to increase in importance in the near future, as oil and gas exploration and exploitation require increased human presence and activity in offshore areas prone to sliding. The questions addressed in this work may eventually also lead to useful insight into the possible differences in the roles that turbidity currents and debris flows play in the transport of massive deposits of clay and sand in deep water environments (Stow and Mayall, 2000; Shanmugam, 2000). Subaqueous debris flows vary in composition (from clay and silt to sand, gravel or even rock slabs). However, in the present study we restrict ourselves to clay-rich soils, where the percentage of clay is at least comparable to the one of sand. Typically, each component of the material (clay, sand, and water) share 1/3 of the volume.