Document Type


Publication Date

January 1982


Debris flow is a natural phenomenon triggered by special conditions that combine: high intensity rainfall, material available for transport, slopes steep enough to induce flowage, and insufficient protection of the ground by vegetation and/or other erosion control means. These conditions are very common in semiarid and arid regions in Utah, other Western states and many other parts of the globe. Previously, the two models proposed to solve debris flow are the Bingham plastic model and the dilatants model. Both these models depend upon coefficients that are not easy to obtain. Therefore, they are not very useful in practice. According to the field observations and data reported, most debris flows that occur in nature are laminar. The viscosity of these flows has been as large as 600,000 time that of water. Reynolds numbers are less or equal to 500 for these debris flows. Laminar debris flows are the subject of this report. A theoretical model based on the Saint-Venant equations of continuity and motion, together with a modified Chezy equation for defining the energy loss, were found to be suitable to describe debris flow in the laminar range. These equations were solved by numerical methods implemented in a computer program. This report covers only steady by gradually varied debris flow solutions. A formula defining the Chezy coefficient as a function of Reynolds number is proposed. A relationship between the debris flow density and its viscosity is also proposed. These relationships are of necessity based on the limited data available for debris flows. Solutions to four examples are given. The results show that this open channel debris flow model reproduces well debris flows observed in nature. These solutions show that debris flows develop depths greater than water flows. The bed slope is the most important variable that affects the ratio of the depth of debris flow to depth of an equivalent volumetric water flow. For milder slopes this depth ratio exceeds ten. The substantially larger depth of debris flow than of equivalent water flow explains in part why debris flows have been observed to stop flowing, leaving an abrupt wave-shaped form on the landscape.