Mechanism of Water Exchange at the Sediment-Water Interface in a Tidal Estuary

摘要

Tidal variations of groundwater salinity and seepage flow through a stream bed were observed in a tidal estuary. The processes of fresh water intrusion and salinity recovery in the estuarine bed due to floods were analyzed using observed data in relation to the circulation between stream water and a wide-area groundwater table. The seepage flow varies with tidal variations of the stream stage on the order of 10~(-3) cm/s, whereas the groundwater salinity decreases only at the surface of the estuarine bed. Fresh water intrusion to the estuarine bed during flooding depends on the seepage flow caused by the hydraulic gradient between the stream stage and the wide-area groundwater table, and the fresh water intrudes into the estuarine bed during flooding to a depth twice that of tidal intrusion during non-flood events.