Single alternating density currents during initial impoundment of estuarine reservoirs

To explore the occurrence and effects of single alternating density currents during initial impoundment of estuarine reservoirs, we conducted flume experiments to simulate the exchange of salts between sloped sediments and water. We monitored the conductivity profiles of sediment porewater and the water overlying the sediments. Thereby, we analyzed the spatio-temporal distribution of salts, and computed the total salt mass per unit area(SPA) of the benthic boundary layer(BBL) in different zones. The results show that the inflows infiltrate and flow through the surface sediment in the shallow zone. The salt concentration of sediment porewater in the shallow zone was significantly lower than that of water in the middle and profundal zones at the same depth. The SPA in the BBL and salt concentration of water near the sediment-water interface in the profundal zone at the zero hour were both the highest among the three zones. These evidences indicate that density currents moving down sloped sediments are generated and then may have resulted in the accumulation of salts in the profundal zone. After the flume is filled with water, SPA in the BBL and salt concentration near the sediment-water interface in the profundal zone rapidly decrease. Salts reach equilibrium quickly between the water in the profundal zone and the porewater of surface sediment in the shallow zone. These facts indicate that the currents originate in the profundal zone and flow into the sediments of the shallow zone. The salts so released from the sediments in the shallow zone eventually go back into the same sediments. To prevent the salts from going back into the sediments and continuing to affect the salinization of the reservoir, the accumulated salty water in the profundal zone should be discharged regularly after the estuarine reservoir is impounded.