Salt Balance Evolution during the Monsoon Onset: a Case Study in the Pearl River Delta (China)

Salt intrusion is a widespread and dramatic problem in many deltas, despite mitigation efforts. For example, in the Pearl River Delta (PRD), where rapid growth and urbanization have resulted in the world’s largest megalopolis, salt intrusion now threatens the freshwater supply for over 40 million people during the dry season. Our understanding of three-dimensional salt transport and balance remains limited, particularly for transient conditions such as the onset of monsoon. This in turn severely limits the efficacy of estuarine management. We present a numerical study of salt transport in the Modaomen branch of the PRD, spanning dry season conditions through the onset of monsoon.

We implemented the FVCOM (Finite Volume Community Ocean Model) to South China Sea and the entire PRD. The grid resolution ranges from 20km at the open ocean to 20m inland. Tides, salinity and temperature are introduced at the offshore open boundary and daily measurement data are used to specify freshwater inputs. The bathymetry is generated from a set of surveys performed in 2008 allowing accurate representation of the delta at the time of the simulation from October 2007 to September 2008. This has allowed us to explore how salt transport evolves throughout the dry season and the monsoon for the first time.

Numerical results provide a detailed picture of the 3D salt dynamics and show temporal variability at different scales. We focus on the fortnightly salt intrusion modulation responding to spring neap variations in stratification. We identify different periods in the evolution of salt dynamics. During the dry season, the estuary is partially mixed; it then reaches a salt wedge state after a transition period linked to the onset of the monsoon. Our results will help determine which conditions favour or inhibit the development of salt intrusion and the sensitivity to changes in those conditions. This knowledge will aid with estuary management and will help to identify mitigation measures.