An irrigation-induced debris flow in northern New Mexico

Describes investigations of a 10 000 m³ (12 000 cu yd) debris flow that mobilized from coarse-grained Holocene stream terrace deposits near Cordova, New Mexico. Field evidence suggests that mobilization occurred very near the main scarp - which intersects a steeply dipping, clayey paleosol directly beneath the terrace - and that the debris was flowing before it reached the nearby Rio Quemado. Recompacted stream terrace samples were contractive in two out of three direct shear tests, and we calculated the approximate mobility index of the terrace sediments to be greater than unity. If they were completely saturated, therefore, the terrace sediments could have been mobilized from a small debris slump without the addition of any water. In order to investigate this possibility, we performed a series of limited equilibrium stability analysis and found that a phreatic surface coincident with the ground surface would have been necessary in order for sliding to occur. We conclude that the failure was probably due to the unprecedented development of a perched water table above the paleosol, which caused sliding and subsequent mobilization. The two most likely sources of water were seepage from an irrigation ditch above the failed slope and over-irrigation of the terrace, which was being used as a pasture. -from Authors