Dynamics of the turbidity maximum zone in a micro‐tidal estuary: Hawkesbury River, Australia

Abstract

Bed sediment, velocity and turbidity data are presented from a large (145 km long), generally well‐mixed, micro‐tidal estuary in south‐eastern Australia. The percentage of mud in the bed sediments reaches a maximum in a relatively narrow zone centred ≈30–40 km from the estuary mouth. Regular tidal resuspension of these bed sediments produces a turbidity maximum (TM) zone in the same location. The maximum recorded depth‐averaged turbidity was 90 FTU and the maximum near‐bed turbidity was 228 FTU. These values correspond to suspended particulate matter (SPM) concentrations of roughly 86 and 219 mg l−1, respectively.Neither of the two existing theories that describe the development and location of the TM zone in the extensively studied meso‐ and macro‐tidal estuaries of northern Europe (namely, gravitational circulation and tidal asymmetry) provide a complete explanation for the location of the TM zone in the Hawkesbury River. Two important factors distinguish the Hawkesbury from these other estuaries: (1) the fresh water discharge rate and supply of sediment to the estuary head is very low for most of the time, and (2) suspension concentrations derived from tidal stirring of the bed sediments are comparatively low. The first factor means that sediment delivery to the estuary is largely restricted to short‐lived, large‐magnitude, fluvial flood events. During these events the estuary becomes partially mixed and it is hypothesized that the resulting gravitational circulation focuses mud deposition at the flood‐determined salt intrusion limit (some 35 km seaward of the typical salt intrusion limit). The second factor means that easily entrained high concentration suspensions (or fluid muds), typical of meso‐ and macro‐tidal estuaries, are absent. Maintenance of the TM zone during low‐flow periods is due to an erosion‐lag process, together with a local divergence in tidal velocity residuals, which prevent the TM zone from becoming diffused along the estuary axis.