Pore water pressure

About: Pore water pressure is a research topic. Over the lifetime, 11455 publications have been published within this topic receiving 247670 citations. The topic is also known as: pwp.

Dominant particle support mechanisms in debris flows at Mt Thomas, New Zealand, and implications for flow mobility

Abstract: Within zones of little or no deformation by internal shearing in debris flows at Mt Thomas, about two-thirds of the weight of large particles is supported by buoyancy and about one-third by static grain to-grain contact. In boundary shear zones of low velocity flows and in high velocity, turbulent debris flow, grain-to grain contact is replaced by turbulence and dispersive pressure. Cohesive strength of the clay + silt + water interstitial fluid provides less than 2 % of the force keeping particles larger than 1 cm gravel in suspension. Excess pore pressure is generated in the interstitial fluid by the weight of coarse particles suspended in the slurry. According to Coulomb strength theory, pore pressures measured in these debris-flow slurries reduce the shear strength of the material to less than 10 % of what it is in the unsaturated state. The excess pore pressures are slow to dissipate because of the small connections between pore spaces that result from the extremely poor sorting of the debris and the presence of silt and clay in the pore fluid. Maintenance of sufficient pore space to trap fluid and facilitate flow on low-gradient slopes may be accomplished by dilatancy and subsequent partial liquefaction of the debris during shear.

Interplay of stream-subsurface exchange, clay particle deposition, and streambed evolution

Abstract: [1] Hyporheic exchange (mixing of stream water with pore water beneath the stream) is responsible for the transport of many ecologically relevant substances across the stream-subsurface interface. This process is dependent on both the streamflow conditions and sedimentary properties, but the complex nature of fluvial systems has presented a barrier to the development of mechanistic understanding of the dynamics of stream-subsurface interactions. This work presents the results of a controlled study to examine in detail the relationship between stream-subsurface exchange fluxes, delivery of suspended sediments to the hyporheic region, fine particle accumulation in the streambed, and alteration of sedimentary properties. Laboratory flume experiments were used to observe kaolinite clay deposition in a sand bed and the resulting alteration of hyporheic exchange fluxes. Solute and suspended sediment exchange with clean sand beds is predicted well by a fundamental model for bed form-driven advective pumping exchange. However, substantial accumulation of clay in the bed causes an alteration of the pore water flow environment, which reduces both water flux across the stream-subsurface interface and subsequent particle deposition. Measurement of bulk solute exchange and direct observation of clay accumulation in the bed both indicate that transported fine particles are preferentially removed near the stream-subsurface interface. Clogging of inflow regions produces heterogeneous subsurface clay deposits even when the bed is initially homogeneous. This behavior is also predicted by the fundamental colloid pumping model. These results contradict the accepted view that suspended sediments will generally deposit in the deepest regions of the bed and thus clog the bed from the bottom upward. Our results indicate that clogging of the streambed surface will often isolate deeper regions of the bed from the streamflow, so that even relatively low amounts of suspended sediments can substantially degrade streambed habitat.

Effects of wave forcing on a subterranean estuary

Abstract: Wave and tide are important forcing factors that typically co-exist in coastal environments. A numerical study was conducted to investigate individual and combined effects of these forces on flow and mixing processes in a near-shore subterranean estuary. A hydrodynamic model based on the shallow water equations was used to simulate dynamic sea level oscillations driven by wave and tide. The oscillating sea levels determined the seaward boundary condition of the coastal aquifer, where variably-saturated, variable-density flow was modeled. The simulation results showed that waves induced an onshore upward tilt in the phase-averaged sea level (wave set-up). The resulting hydraulic gradient generated pore water circulations in the near-shore zone of the coastal aquifer, which led to formation of an upper saline plume (USP) similar to that due to tides. However, mixing of recirculating seawater in the USP with underlying fresh groundwater was less intensive under the high-frequency wave oscillations. In the case of combined forcing, wave-induced circulations coupled with the intra-tidal flows strengthened the averaged, circulating pore water flows in the near-shore zone over the tidal period. The circulating flows increased exchange between the subterranean estuary and ocean, contributing 61% of the total submarine groundwater discharge for the simulated condition in comparison with the 40% and 49% proportions caused by the same but separate tidal and wave forcing, respectively. The combined forces also created a more extensive USP with the freshwater discharge zone shifted further seaward. The freshwater flow paths in the intertidal subterranean estuary were altered with a significant increase of associated transit times. The interplay of wave and tide led to increased mixing between discharging fresh groundwater and recirculating seawater. These results demonstrated the complexity of near-shore groundwater systems and have implications for future investigations on the fate of land-sourced chemicals in the subterranean estuary prior to discharge to the ocean.