Multiple Seepage‐Faces in Tidal Flat With Very Gentle Slopes.

Large‐scale seepage‐faces occur on small‐slope tidal flats. All previous studies assume that the seepage‐face has only one single exit point. Here we show via numerical simulations of tidally‐influenced groundwater flow that, in a two‐dimensional vertical, homogeneous transect of a tidal flat with gentle beach slope of 1‰, multiple seepage‐faces may occur with at most four unsaturated beach surface segments which separate four seepage‐faces. Salinity‐variation induced density‐dependent flow leads to this complex phenomenon. While the seepage‐faces are the groundwater discharging zones on the beach surface, the unsaturated zones are the recharging zones. The whole aquifer beneath the tidal flat is almost occupied by seawater and forms a wall blocking the horizontal seaward discharge of inland fresh groundwater so that inland freshwater discharges mainly occur near the high tide mark. This is in great contrast with the traditional results that inland freshwater discharge occurs mainly near low tide mark. Plain Language Summary: Tidal flat with very gentle‐slopes distributes extensively around the world, for example, the coastline of Bohai, the Yellow Sea of China, and Willapa bay of USA. Due to small slopes, the intertidal zone can be as wide as several kilometers, leading to large‐scale seepage‐faces (saturated beach surface with groundwater efflux) during ebbing and low tides. All previous studies assume that the seepage‐face has only at most one segment, the upper end of which is the exit point of water table and the other end is the intersection point of seawater and beach surface. Here, we found that in a tidal flat with gentle beach slope of 1‰, multiple seepage‐faces may occur with at most 4 unsaturated beach surface segments which separate 4 seepage‐faces during low and rising tides. The locations of these seepage‐faces and adjacent unsaturated zones are quite dynamic due to the complex, dynamic density‐dependent groundwater flow in the aquifer beneath the tidal flat. Seepage‐faces are kind of complex and important boundary for coastal groundwater flow in order to accurately model the groundwater flow and solute transport in intertidal aquifers, the complex nonlinear boundary conditions on the seepage‐faces should be strictly and accurately implemented. This paper made such an attempt. Key Points: Tidal groundwater flow with strict and explicit seepage‐face boundary conditions were numerically simulatedMultiple seepage‐faces were first found to develop on gentle tidal flat with slope around 1‰Multiple seepage‐faces imply multiple local density‐dependent groundwater‐seawater re‐circulations [ABSTRACT FROM AUTHOR]