The role of groundwater outflow in the water cycle of a coastal lagoon sporadically opening to the ocean

SummaryThe water budget of a coastal lagoon, Oikamanai Lagoon, Hokkaido, Japan, sporadically opening to the Pacific Ocean, is estimated by establishing a bathymetric map of high accuracy (0.3 m depth interval), and by monitoring the meteorology, lagoon water level and river stage. The opening to the ocean is produced by incising the sand bar from the overflow and discharge of lagoon water at the lowest site of the sand bar. The overflow results from an increase of the lagoon water level basically by snowmelt or rainfall river runoffs. As a result, the drainage by the opening to the ocean caused the lagoon to decrease the water volume up to more than 96%. The estimate of the water budget at nearly constant water level under closed condition of the lagoon suggests that, as the net groundwater output from the lagoon, the confined groundwater outflow to the ocean across the sand bar prevails. The gravelly confined aquifer was inferred to be at the similar elevation with some thickness along the sand bar about 2000 m long. Meanwhile, during the regrowth of the sand bar at the outlet after the opening, the gravelly sediment first deposited near the sea level. Hence, the spatial distribution of the gravelly aquifer along the sand bar suggests that the whole sand bar was simultaneously broken in the past and then was grown up again.

► We clarify hydrologic features of a coastal lagoon with sporadic outflow to the ocean. ► We estimate the water budget of the lagoon under closed condition. ► Confined groundwater outflow to ocean through the sand bar controls the budget. ► The confined aquifer is likely to exist at the similar elevation in the bar 2000 m long. ► This suggests that the whole bar was simultaneously broken in the past.