Using Turbidity and Acoustic Backscatter Intensity as Surrogate Measures of Suspended Sediment Concentration. Application to a Sub-Tropical Estuary (Eprapah Creek)
A key element in stream monitoring is the choice of a measuring technique of suspended sediment concentration (SSC). Several studies suggested that turbidity and acoustic Doppler backscattering may be suitable surrogate measures for SSC. A series of new experiments were conducted in laboratory under controlled conditions using water and soil samples collected in a small sub-tropical estuary of Eastern Australia. The tests were conducted with a microADV (16 MHz) system and a YSI6600 probe using two types of sediment material : some fine mud collected on the bed and some slightly coarser material collected on the bank slope. In addition, some experiments were repeated with the creek estuarine waters and with Brisbane tap waters. The best fit relationships were established in terms of the suspended sediment concentration (SSC) as a function of the acoustic backscatter intensity (BSI), the SSC as a function of the turbidity, and the turbidity as a function of the acoustic backscatter intensity. The present results confirmed earlier findings that the relationships presented some monotonic increase. The calibration curves were however affected by the sediment material characteristics and by the water quality. The results indicated that the calibration of an acoustic Doppler system must be performed with the waters of the natural system (creek waters) and with some bed material. Importantly the calibration of an ADV system is specific to the unit itself. Its calibration relationships are functions of the water quality and sediment properties, but also of the intrinsic characteristics of the emitter and receivers. A limited comparison between an ADV (10 MHz) and a microADV (16 MHz) indicated that the newer microADV system could detect significantly more counts per unit volume than the older unit. The results were applied to some earlier field measurements conducted continuously at high frequency for 50 hours each in Eprapah Creek with the same microADV system. For each field study, the instantaneous suspended sediment flux per unit area data showed some high-frequency bursts that were believed to be linked to some turbulent bursting phenomena next to the bed. For each tidal cycle, the suspended sediment flux data were integrated with respect of time. The results yielded a net sediment mass transfer per unit area of about -20 kg/m2 per tidal cycle during the first study conducted mid-estuary and of about -4 kg/m2 per tidal cycle for the second study performed in the upper estuary. That is, the net sediment flux over a full tidal cycle was upstream in average, and the finding was consistent with earlier studies in sub-tropical rivers during dry conditions for a similar tidal range. It must be stressed that the present work highlighted a number of limitations. The present calibration relationships might not be suitable for earlier field studies at Eprapah Creek with different water quality conditions. The calibration curves were also specific to the microADV unit at the time of the tests, and they were developed for a subtropical estuary with relatively low turbidity levels
