Comparison of engineering options for seawater intrusion control at river mouths

Seawater intrusion into rivers through river mouths can cause salinization of affected farmland and threaten water supply. A number of engineering options for seawater intrusion control are compared in this paper in terms of their effectiveness, feasibility, by-effects, operational reliability, risks it may induce. These engineering options include dam with gate, multiple-gate barrier, submerged weir or dam, submerged movable rubber dam, sluice gates at outlets of tributaries. Three-dimensional numerical model was employed to predict effectiveness of submerged weir or dam in seawater intrusion control. The simulation was conducted with Modaomen Waterway, one of the main outlets of Pearl River, China. It demonstrates that a submerged dam 3m under average water surface in dry season and constructed at around 4km to the exit of the waterway can reduce salinity of 1.5-2.0⁰/₀₀ and 1.2⁰/₀₀ at the two water intakes 3km and 10km upstream of the submerged dam, respectively. Salinity contours of 0.5⁰/₀₀ and 2.0⁰/₀₀ draw back by 1.3km and 2.5km, respectively, due to the proposed submerged dam. It is concluded that submerged weir or dam is more suitable for medium and small rivers in order to increase chance of taking fresh water from the main stream of the river during seawater intrusion, while in order to prevent farmland from being intruded by seawater, the best way is to construct sluice gates at outlets of tributaries.