Saltwater Dynamics in Brackish River Reaches

Unlike lakes and reservoirs, rivers have continuous water flow. As a result, river environments can undergo rapid and significant changes driven by water movement. In addition, complex three-dimensional flow structures develop due to geographical features such as variations in river channel cross-sections and river meandering. Therefore, understanding spatial and temporal variations in water flow is essential for investigating environmental phenomena in rivers.

The animation above shows simulation results for a river estuary (brackish-water region). In estuaries, seawater intrudes into rivers and may travel upstream along the riverbed for several tens of kilometers. Within these intruding saline water masses, water exchange with the upper freshwater layer is limited, potentially leading to a decrease in dissolved oxygen concentrations. Therefore, understanding the behavior of intruding saltwater is extremely important for evaluating environmental conditions in estuarine regions.

To investigate saltwater dynamics in river channels with complex topography and three-dimensional flow structures, we applied TITech-WARM, a three-dimensional parallel flow simulation model developed in our laboratory. Through these simulations, we obtained fundamental insights that can contribute to maintaining healthy aquatic environments.

The animation below shows an example of a simulation investigating the deposition of organic sludge near a river mouth. The red bar graph at the bottom represents the amount of sludge deposited on the riverbed.

Organic sludge transported from upstream tends to settle and accumulate when it enters the estuarine region, where the water becomes deeper and the riverbed slope becomes gentler. However, when river discharge increases due to rainfall or other factors, stronger currents can transport the deposited sludge downstream along the riverbed. As a result, sludge is thought to accumulate beneath the oxygen-poor salt wedge.

Under oxygen-deficient conditions, organic sludge is decomposed by anaerobic bacteria, producing gases as byproducts that can cause unpleasant odors.