Dynamics, flushing and trapping in Hinchinbrook channel, a giant mangrove swamp, Australia

Abstract

Hinchinbrook Channel is a 44-km long tidal channel connected at its northern and southern ends to the continental shelf of the Great Barrier Reef. The channel drains 164 km 2 of mangrove swamps. It also drains the Herbert River. The dynamics within the channel and the flushing of the fringing mangrove swamps were studied both in the field and using numerical models. The dominant currents are tidal semi-diurnal and are due to two nearly identical tidal waves entering the channel at its norhtern and southern openings and meeting in the middle. The mangrove swamps contribute significantly to the tidal dynamics. When the Herbert River discharge is small, net residual currents are negligible even in the presence of wind and the residence time of water is about 2 months. Flushing of the fringing mangrove swamps is largely controlled by the trapping effect in the mangrove swamps, and is slow with an e -fold residence time of about 54 days. On the northern end, the mangrove-fringed channel is wide and shallow, and a barotropic, coastal boundary layer is formed, due to shallow water effects and trapping in mangrove swamps. This results in a long-term (2 weeks) trapping of water along the mangrove-fringed coast in calm weather. Thus, coastal waters are continuously exchanged and mixed with mangrove swamp waters, but mixing between coastal and offshore waters is much slower.