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

Abstract

The 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 (0.3m 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 grave-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.

Water and heat budgets in a coastal lagoon controlled by groundwater outflow to the ocean

“…This supports that the confined groundwater outflow occurs by passing through the gravelly aquifer inside the sand bar. This is because such gravelly deposits are distributed along the sand-bar base in expose to the lagoon bottom (Chikita et al, 2012).…”

“…However, the opening in mid-July is artificially done by an excavator at the lowest point of the sand bar, in order to pick up rarely large corbicula (Corbicula japonica; shell size, 45 mm or around). The openings make the lagoon drain more than 90 % in volume within a day (Chikita et al, 2012). The frequency of the openings depends on the drainage area and lagoon water volume, which decide the residence time of lagoon water, related to material cycles and the ecosystem in the lagoon and back marsh.…”

Groundwater Leakage and River Runoff in a Catchment Influenced by Tectonic Movement

“…The Oikamanai River is a main influent river of the Oikamanai Lagoon in the Tokachi coastal region of southeastern Hokkaido, Japan (Figure 1; [15] [16]). The river catchment (42˚33'46'' to 42˚40'40''N, 143˚21'47'' to 143˚28'36''E; altitude, 6 m to 330 m asl) upstream of site R1 has the area of 62.6 km 2 , the mean slope angle of 34.7˚ (gradient, 0.692) and the mean riverbed gradient of 0.033 (Figure 1).…”

“…Meteorological data were obtained at site M (altitude, 6 m asl; rainfall and air temperature) near the Oikamanai Lagoon [15] [16] at 4.0 km south-southeast of site R1, at site P (altitude, 142 m asl; rainfall and air temperature) near the water divide of the Oikamanai catchment, at site F (altitude, 34 m asl; air temperature, relative humidity and rainfall) (Figure 1), and at the Taiki Aerospace Research Field (altitude, 21 m asl; rainfall, air temperature, solar radiation, and wind velocity) 10.5 km south-southwest of site R1, and at Taiki town (altitude, 85 m asl; snow depth, rainfall, air temperature, and wind velocity) 18.9 km southwest of site R1. The distinction between rainfall and snowfall in the catchment was made by the air temperature of 0˚C at site M.…”

Future of Coastal Lagoons in Arid Zones under Climate Change and Anthropogenic Pressure. A Case Study from San Jose Lagoon, Mexico

“…Coastal lagoons are shallow water bodies with the influence of marine water and a particular ecology [2,3]. Freshwater inflow can come from in situ rainfall, runoffs, and horizontal groundwater flow [3][4][5]. In the case of the San Jose Lagoon, the groundwater flow plays an important role in the water and mass budget [5].…”