Abstract
A data set was acquired on a shallow mudflat in south San Francisco Bay that featured simultaneous, co-located optical and acoustic sensors for subsequent estimation of suspended sediment concentrations (SSC). The optical turbidity sensor output was converted to SSC via an empirical relation derived at a nearby site using bottle sample estimates of SSC. The acoustic data was obtained using an acoustic Doppler velocimeter. Backscatter and noise were combined to develop another empirical relation between the optical estimates of SSC and the relative backscatter from the acoustic velocimeter. The optical and acoustic approaches both reproduced similar general trends in the data and have merit. Some seasonal variation in the dataset was evident, with the two methods differing by greater or lesser amounts depending on which portion of the record was examined. It is hypothesized that this is the result of flocculation, affecting the two signals by different degrees, and that the significance or mechanism of the flocculation has some seasonal variability. In the earlier portion of the record (March), there is a clear difference that appears in the acoustic approach between ebb and flood periods, and this is not evident later in the record (May). The acoustic method has promise but it appears that characteristics of flocs that form and break apart may need to be accounted for to improve the power of the method. This may also be true of the optical method: both methods involve assuming that the sediment characteristics (size, size distribution, and shape) are constant
Highlights
Suspension, transport, and deposition of sediments in rivers, bays and estuaries are processes of critical importance to understand the overall condition of complex and often highly variable water systems
This study focuses on the evaluation of backscatter obtained from a 6 MHz ADV to estimate suspended sediment concentration (SSC) that was separately estimated using an optical turbidity proxy
Given a suspension of uniformly sized particles that combine to form flocs, one would observe an increase in water clarity and a corresponding reduction in turbidity, despite the constant suspended sediment concentration
Summary
Suspension, transport, and deposition of sediments in rivers, bays and estuaries are processes of critical importance to understand the overall condition of complex and often highly variable water systems. When an acoustic instrument is calibrated for a water system in which suspended sediment properties (size, size distribution, density, shape) display little time dependence, no additional sensor is needed to measure SSC. SSC is typically measured either via direct methods, such as obtaining water samples in situ and processing to determine sediment content, or via a different indirect method or tool, such as an optical turbidity sensor.
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