Cascade of Proppant-Sandwiched Silt Blocks as a Double-Continuum: From Discovery to Mathematical Modeling

A new naturally morphed soil structure has been discovered at 2 out of 33 pedons excavated in Oman, inside and in the vicinity of a flood-protection/recharge dam, subject to a rapid geomorphological changes, caused by sediment loads from a wadi-transported runoff. Low permeable cascades of soil blocks are interspersed by highly permeable fractures filled by a natural “proppant” (sand). Different time scales (ponding with infiltration/seepage—evaporation/transpiration), the pore pressure contrast between fractures and block, capillarity (mostly of blocks), gravity (similar to oil-gas gravity drainage in reservoir engineering of fractured rock), climatological factors and transpiration of the plant roots—all control the topology of water motion in this porous double-continuum medium. Infiltration in layered soils is modeled by the Green–Ampt model with front propagating across an interface between layers of contrasting conductivity, porosity and capillarity. The goal of this work is to explain qualitatively and using a mathematical model the macroscale observations (infiltration) by a pore-scale model.