1267 / 2024-09-20 21:25:08

New changes and driving mechanism of salt intrusion in Modaomen estuary

Modaomen estuary; salt intrusion; FVCOM models; tidal oscillation; shear diffusion.

Salt intrusion presents a significant environmental challenge in numerous estuaries around the world. Severe salt intrusion events happened frequently in recent years in Modaomen channel which is the main water supply channel of the Pearl River Estuary. It is important to reveal the mechanisms by which the changes occur for ensuring the safety of water supply in the Guangdong-Hong Kong-Macao Grater Bay Area. We compared the relationship between the runoff and the salt intrusion length in the early decade of this century(2004-2014) and the last five years(2019~2023). The results show that the relationship between the runoff and the salt intrusion length has been changed, and greater flow is needed to control the salt front in the safe position; the response time to the flow has changed also, once the runoff decreased, the salt water intruded faster, the water intake could be affected by salt water with low runoff, even in the summer, which means the estuary mouth does not act as a strong buffer between salt water and fresh water. Two FVCOM models were constructed with different bathymetric and topography around the estuary mouth in 1964 and 2020 to reveal the changes’ mechanism. The results show that because there was a wide estuary bay the average salinity was about 5 SPU lower at the estuary mouth and the salinity decayed along the channel much faster with 1964 bathymetric and topography. Salt was transported mainly by tidal oscillation with 1964 topography while by shear diffusion with 2020 topography, which results the latter had much longer intrusion length. The research results provide scientific support for estuary coastal land planning and the way to ensure water intake safety.