Potential sources and magnitude of errors associated with the measurement of suspended sediment concentration

Dredging activities cause the mobilisation of sediments from the bed of water bodies, often resulting in the suspension of sediment into the water column in the vicinity of the activity. Sediment release to the aquatic environment and the effects of this release are often the prime environmental concern associated with dredging. As a consequence the accurate monitoring of suspended particulate matter (SPM) concentrations is of considerable importance to the industry. The three techniques most widely used for the measurement of SPM associated with dredging are: - Water sampling; - Use of optical (e.g. optical backscatter) sensors (OBSs); and - Use of Acoustic Doppler Current Profilers (ADCPs). The commonly used approach of water sampling combined with optical sensor measurements is particularly important, as: - The techniques are well established; - Widely used; - Relatively simple and inexpensive; and - Provide a means of calibration for other techniques (e.g. ADCP measurements). Although versatile and in widespread use there is often little consideration of the accuracy (or potential lack of accuracy) in the application of OBS and water sampling methods. This is important information for the design and implementation of very many dredging monitoring studies, the costs of which can be exceptionally high. This paper describes the potential error sources and magnitudes associated with water sampling and the use of OBSs for the measurement of suspended sediment concentrations