Response of freshwater transport during typhoons with wave-induced mixing effects in the Pearl River Estuary, China

We examined the response of freshwater transport in the Pearl River Estuary (PRE) to typhoons in 2017 to investigate the effects of wave-induced mixing under extreme conditions using a 1D-3D coupled river-ocean model with a wave-induced mixing module. The results show that wave-induced mixing trapped more freshwater in coastal regions by narrowing the plume width and increasing the freshwater thickness. The freshwater volume in the coastal region increased by 10%–60% and the flushing time increased from 5–42 days to 10–50 days. The trapping effect of wave-induced mixing was modulated by alongshore wind, wave and river flow, and it was sensitive to the initial freshwater thickness before typhoon. Cross-shore and eastward freshwater transport was substantially altered during typhoons by wave-induced mixing, whereas westward freshwater transport exhibited only minor changes. It was found that wave-induced mixing caused a change in vertical/horizontal density gradient, and thereby altered the freshwater volume, ocean current and freshwater flux. The effects of wave-induced mixing on the plume structure and flushing time scale were further investigated by applying a salinity coordinate system. The results show that wave-induced mixing significantly increased/reduced the plume area and freshwater volume in the far field (26–32)/near field (19–26) during typhoons. As a consequence, the flushing time in the near field/far field was reduced/increased. Shorter flushing times inside the estuary and longer flushing times on the shelf induced by wave-induced mixing have important implications for the fate of nutrients, sediments, and other pollutants. This study highlights that wave-induced mixing effects are important in modulating freshwater transport and flushing time scales, which are of great significance in regional coastal management to protect the marine environment.