A numerical model was developed to investigate salinity distribution in the Yura Estuary, a micro tidal estuary in Japan. The model results show that the salinity distribution as represented by salt wedge intrusion agreed well with field observations. In addition to the seasonal variation, the salt wedge responds over short time scales according to the flood events. The retreat of the salt wedge is dependent on the scale of the river discharge; the salt wedge moved back and disappeared from the estuary when over250 m3·s-1 of fresh water was discharged from the estuary and it takes ~11 days for salt wedge to recover from the fresh water discharge event. The Yura Estuary has on average three floods during summer, this coincides with when phytoplankton is most productive in the river and indicates that the short temporal variations in the river discharge has important effects not only on the hydrodynamics, but also on the ecosystem in the estuary.
Estuaries are formed around the river mouths, where lighter fresh water meets denser sea water. There is a great deal of variety in mixing processes and salinity distributions, affected by the balance between tides and river discharges [
Fresh water is utilized for irrigation and drinking purposes in many rivers. Therefore, salt wedge intrusions have been studied for management of estuarine water quality [
Compared to American and Southeastern Asian continental countries, Japan is characterized by a relatively narrow strip of mountainous land. This implies that the root of its rivers bear a strong altitude gradient from the origin to the mouth, leading to short residence time of rainwater, as well as strong and rapid variations in river discharge [
The Yura River is a 146 km long and flows into Wakasa bay, Sea of Japan (
The typical tidal range of the Yura Estuary is less than 0.5 m, and it is thus classified as a micro tidal estuary. Therefore the effects of tidal currents on the physical conditions are negligible. The Yura Estuary enters the salt wedge regime during low discharge periods, with a maximum salt wedge length of 18 km from the river mouth [
The salt wedge dynamics were calculated by Delft3DFlow [
and the coastal area from the Japan Coast Guard. The model system has 14 σ-levels in the vertical; five layers of 3%, three of 5%, four of 15%, two of 5% from the surface to bottom. The horizontal grid scale ranges from 15 m by 200 m in the river to 150 m by 200 m in the sea. The vertical eddy viscosity and diffusivity are calculated by a k-ε model. The horizontal eddy viscosity and diffusivity are calculated by a 3D-turbulence closure model [
The salt wedge intrusion was simulated for two years; from April 2006 to March 2008. The model was spun up and reached steady state after one month, and then real calculation started with observed boundary conditions. Calculated salinity distribution and salt wedge length by the model were compared with those obtained by field observations [
In order to study the response of the salt wedge to variations in river discharge, the river discharge condition was changed while the boundary conditions of temperature, salinity and sea level were kept constant. With the intention of reproduce the summer salinity distribution, the
salinity and temperature of the river open boundary were set to 0 and 15.4˚C respectively, and the sea side boundary conditions were set to 33.4, 17.6˚C and 0.2 m for salinity, temperature and sea level condition respectively. The river discharge (Q) of a flood event was empirically calculated as follows;
where Q0 represents maximum discharge (m3·s–1) and T is date from the flood event (day). Equation (1) is derived from typical flood event conditions and river flow decreases gradually without any increasing of the river discharge after the flood.
First, the model was run for one month with a constant river discharge (15 m3·s–1) in order to obtain a steady state as representative for dry season. The second stage of the experiment was forced with the steady state as the initial condition and then the river discharge was changed according to Equation (1). Eight flood scenarios of Q0 equals 50, 100, 150, 200, 250, 500, 1000 and 1500 were conducted. The minimum river discharge was set to 15 m3·s–1 Examples of variation of discharge are 50, 150, 250, 500 and 1000 m3·s–1 are shown in