Drawdown of urban drain trenches triggering 2-D transient seepage in soil massifs subject to managed aquifer discharge: sandbox experiments, analytical and HYDRUS2D modeling

Managed Aquifer Discharge (MAD) through trenches lowers the water table of ‘shallow perched aquifers’ in urban areas (Muscat-Oman). Drawdown of the water table in periodic trenches or a single trench is experimentally studied in a sandbox, and modelled numerically (by HYDRUS2D) and analytically. Evaluation of transient flow fields of pore pressure and hydraulic gradients is necessary for assessing the stability of trench slopes to seepage-induced erosion. A controlled emptying of trench’s water level is examined. An analytical solution is obtained for an unconfined seepage at an early stage of drainage. Numerical simulations are conducted for early and late stages of seepage towards a partially filled trench in soils with capillarity. For a steady-state Darcian flow limit, comparisons of HYDRUS results with the Charny analytical formula for capillarity-free soils are done. The flow rates, isobars, isotachs and streamlines are determined, with assessment of zones of high hydraulic gradients susceptible to seepage erosion.