Hydrology and suspended sediment transport in the Richmond River catchment and estuary, NSW, Australia

Shahadat Hossain

doi:10.25918/thesis.313

Catchment and estuary hydrology and sediment transport, and catchment and estuarine water and sediment budgets were studied for two hydrological years (1994-96) for the Richmond River catchment, northern New South Wales (NSW), Australia. The average annual runoff from the Richmond River catchment is about 389 mm from an average of 1343 mm of rainfall, and shows a higher coefficient of variation (Cv=0.65) than catchments in other parts of the world (Cv=0.43). The catchment runoff volume from different parts of the catchment varies significantly (665 to 126 mm) due to catchment rainfall distribution, topography and forest coverage. The catchment water budget shows higher evapotranspiration losses from the southeastern part of the catchment because of flat topography and large forest coverage. Water exchange between the estuary and the continental shelf shows that during most of the year the estuary is dominated by tidal variation due to very small freshwater input from the catchment. The average water exchange though the mouth during low flows varies between 8 and 28x 106 m3 /tide during neap to spring tidal cycles. In contrast, during floods the estuary is flushed fresh at the mouth when freshwater input from the catchment exceeds 80-90xl06 m3/day. Suspended sediment concentrations during the dry months of the year are low (< 20 mgL-1) across the catchment due to very low flows. Most of the suspended sediment is exported from the catchment during floods, and sediment export from the cleared areas of the catchment is more than 4 times greater than from the forest covered areas of the catchment. Suspended sediment export from the Richmond River catchment has increased about 4.6 times since European settlement due to 54% forest clearance. Net erosion within the Richmond River catchment varied more than threefold from the dry year (1994-95) to the wet year (1995-96) and the corresponding sediment delivery ratios vary from about 0.14 to 0.29 respectively. Suspended sediment export from the Richmond River catchment varies from 5.5 to 40 t/km2, which is similar to other average Australian catchments. However, most of the sediment (72-91%) from the Richmond River catchment is exported during less than 5% of the year. Suspended sediment concentration in the Richmond River estuary remained very low (<22 mgL-1) under normal tidal variation because of very small sediment inputs from the catchment and the absence of tidal asymmetry. The estuary remains well mixed during the dry season and receives net sediment input from the continental shelf. In contrast, during the wet season, half of the flood borne sediment is trapped in the estuary during minor floods (1:2 year). In contrast, all flood borne sediment is exported to the continental shelf during moderate floods (1:5 year). Sedimentation in the lower estuary is mainly due to marine input during the dry season, and sedimentation in the upper estuary is mainly due to fluvial input during the wet season. Annually, sedimentation in the estuary was higher during the dry year (1994-95) than in the wet year (1995-96) because the trapping capacity of the estuary decreases with increasing flood magnitude. The estuary erodes if the catchment discharge exceeds 11OOx 10 6 m 3. Fluvial sediment input is the major component (90-99%) of the Richmond River estuarine sediment budget. Long term sedimentation rates suggest that a large deposition in the upper estuary has occurred in recent times possibly as a consequence of forest clearance. In contrast, sedimentation rates in the lower estuary, which is dominated by the marine input of sediment, remain low. The residence time of the fluvial sediment in the estuary is short due to flushing through the mouth, which maintains a low sedimentation rate that is less than half that of large estuaries in northern America and western Europe.

Hydrology and suspended sediment transport in the Richmond River catchment and estuary, NSW, Australia

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Hydrology and suspended sediment transport in the Richmond River catchment and estuary, NSW, Australia

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