Generation and origin of persistent high excess pore water pressure in debris flows

Due to building up excess water pressure, debris flows differ markedly from other water-sediment mixtures in mountain basins. Although a number of mechanisms are known to create excess water pressure, persistent high excess water pressure develops in static-quasistatic debris-flow slurries when possible triggers such as saturation, contraction, and Reynolds stress are absent. This suggests that there exist more fundamental mechanisms for the creation and persistence of high excess water pressure. Using 12 clastic materials with four maximum diameters up to 40 mm from three typical debris flows in Southwest China, we make comparative measurements of excess water pressure, gravity drainage, and decanting of water on 12 pairs of debris-water mixtures. It is found that saturation is not a mechanism for excess water pressure generation, which can be ascribed to the heterogeneity of saturated sediments. Mixing undoubtedly creates persistent high excess water pressure in the mixed saturated sediments. Thorough mixing causes the homogenization of water-saturated sediments through random grain rearrangements. The particles in the homogeneous debris-water mixtures stay suspended in water, resulting in excess water pressure derived from particle weight. More significant is the integration of clasts and water over the timescales for debris-flow durations promoted by mixing, which leads to the persistence of high excess water pressure. The development of persistent high excess water pressure in sediment-water mixtures is controlled by external factor-mixing, and internal factors, including sorting of sediments and water content of mixtures.