BEHAVIOR OF RESIDUAL SALTWATER IN A SUBSURFACE DAM OF SALTWATER INTRUSION PROTECTION TYPE

摘要

On islands and peninsulas, water shortage frequently takes place due to the absence of geological conditions suitable for construction sites of surface dams. Therefore, several subsurface dam projects are planned or have already been constructed to meet the water demand. A cut-off wall for preventing saltwater intrusion is usually built in coastal areas. After dam construction, investigations of the movement and the removal of residual saltwater in the storage area are often needed. However, because of difficulties in monitoring, estimation of its behavior below ground is usually not carried out. In this study, experimental and numerical studies were performed to understand the fundamental phenomena of saltwater movement after dam construction. Experimental results demonstrated that residual saltwater in the storage area was gradually flushed out under the condition of freshwater discharge over the cut-off wall. Numerical results showed that circulation flow was created in the residual saltwater region after installation of the cut-off wall. The amount of residual saltwater in the storage area will naturally attenuated because new saltwater from the sea is not supplied. Consequently, residual saltwater was gradually transported with the freshwater flow from the inland area. Therefore, countermeasures to remove residual saltwater are not necessary if the dam can be controlled so as to maintain overflow condition of the freshwater.