Morphometry and stratification of the Bents Basin scour pool, Nepean River, NSW

Abstract

The Bents Basin scour pool is a large hole eroded into the bed of the Nepean River where it flows through the Lapstone Structural Complex at the downstream and of the Bents Bsin sandstone gorge. The scour pool has a surface area of 17500m², a volume of 157400m³, a mean depth of 9.0m, a maximum depth of 22.8m and is an oval-shaped cone. While the capacity-inflow ration is only 0.00054 and indicated that pool volume is very small in relation to the mean annual flow, the scour pool is not flushed for time period of up to 12 months long. Thermal stratification was measured between Mid-January and March 1996 when the thermocline was usually at a depth of 10m. Temperature differences of between 9.6 and 14.7C were measured between the surface and the bottom at times of stratification. Density differences of 2kg/m³ were calculated as a result of thermal stratification. Oxygen stratification was measured between January and April 1996 when the oxycline varied between depths of 7 and 12m. Anoxic conditions were usually recorded below the oxycline. Oxygen stratification resulted in a large increase in total phosphorus concentrations, due to the release of sediment-bound phosphorus. Cation concentration did not vary with depth. Datalogging on 6-7 March 1996 proved that both thermal and oxygen stratification persisted throughout the night. Stratification was recorded between mid-January and March/April 1996 because climatic and hydrologic conditions were not capable of generating sufficient turbulence to mix and hence destratify the scour pool. Current water quality monitoring programs on the Nepean River by government agencies are flawed because they do not sample the whole water column, only surface waters. As a result, these programs present a biased assessment of current conditions during summer.