Measurement of Sediment Release from a Grab Dredge in the River Tees, UK, for the Calibration of Turbidity Prediction Software

摘要

The Dutch Dredging Industry and the Rijkswaterstaat have been engaged in a long term research programme aimed developing reliable, field-calibrated Turbidity Assessment Software (TASS) that can be used to predict rates of sediment release from grab, bucket chain, backhoe, cutter suction and trailing suction hopper dredges. The initial software was produced in Phase 1 of the work, based on reasonably well established concepts and formulations. One of several aims of Phase 2 was to develop and test field measurement protocols that could be used to collect the detailed observational data needed to calibrate the software.The first field test of the protocols was undertaken in the River Tees in May 2000. Measurements were made of the rate of sediment release from the Seal Sands, a small self-propelled grab hopper dredger owned and operated by the Tees and Hartlepool Port Authority Ltd (THPAL). The site was located at the side of the main shipping channel where an accumulation of mud needed to be removed. The measurements were undertaken in several series each of which lasted typically one to two hours and were intended to represent periods during which hydrodynamic conditions and the dredging operation were reasonably consistent.Unfortunately, the thickness of mud was limited and the grab was often filled with a mixture of soft mud and the underlying stiff clay in varying proportions that were impossible to determine. In addition, the site was located on the channel slope with varying water depths and was constrained on one side by a training wall which occasionally prevented the survey boat passing through the plume into clear water. It was also the case that the dredger was not fitted with modern instrumentation that enabled the position of each grab cycle to be recorded. Thus, in several respects, both the dredger and the site were less than ideal for undertaking calibration measurements.However, these deficiencies were not considered to be 'fatal' to the trial as the primary objective was to test the efficacy of the protocol. The difficulties that arose from working in a less than ideal site simply served to demonstrate the need for the protocol. Despite the problems, valuable data were obtained and the 'characteristic' rate of sediment release that was measured is believed to be reasonably reliable. This paper describes the measurements and reviews the results in the context of other measurements of sediment release from grab dredges.