Quantification of Deep Pore Water Flow in Intertidal Sand Flats Using High-Resolution Pore Water Pressure and Temperature Measurements and Numerical Modeling

Intertidal sand flats are subject to periodical advective pore water flow induced by hydraulic gradients established during low tide, when the water level falls beneath the sand flat surface. Seawater which is introduced into the sediment during high tide, circulates through the sand body towards the low water line where it is discharged again to the open water column. This pore water flow drives biogeochemical processes within the sediment on tidal as well as seasonal scales and affects transformation, removal and release of nutrients and trace metals.

We tried to quantify pore water advection and associated water discharge from a tidal sand flat margin located in the German Wadden Sea by using the hydrological model SUTRA-MS and in situ measurements of pore water pressure and temperature, which were recorded by data loggers at 10 min time intervals. The model was run with real time sea level and sea temperature data collected at a nearby continuous time series station. Finally, the model was tested and calibrated by comparing measured and modeled temperature and pore water pressure trends in the sand flat.

Initial results demonstrate that pore water discharge is most pronounced at and below the moving shoreline. Based on our model we calculate a discharge of 0,25 to 0,36 m³ per meter of shore line per tide, which translates into 74 to 106•10⁶ L pore water discharging each tide, assuming a tidal flat shore line of 295 km for the entire east frisian back barrier area. Our results significantly exceed previous estimates, that are based on different modeling approaches and show the significance of deep advection for permeable coastal sands affected by tides.