CORPS EVALUATION OF FLUID MUD MEASUREMENT AND THE DEFINITION OF NAVIGABLE DEPTH

摘要

The U.S. Army Engineer Research and Development Center (ERDC) has responsibility under the Innovative Technologies (IT) Focus Area of the Dredging Operation and Environmental Research (DOER) Program to identify and evaluate innovative dredging and dredged material management technologies. An IT work unit, Fluid Mud Measurement and the Definition of Navigable Depth, will identify, demonstrate, and evaluate both established and emerging fluid mud measurement technologies, and also evaluate existing navigable depth definitions and project management practices. Results from these evaluations will be used to improve Corps fluid mud characterization capabilities, develop criteria for defining the limiting depth for safe navigation in areas with fluid mud, and formulate engineering recommendations for potential Corps implementation of a navigable depth policy. Fluid mud can be characterized as suspensions with density gradations that range from slightly greater than that of the overlying water in its upper layers, to that of stiff, dense lower layers. Fluid mud densities can range from 1050 to 1350 g/1, with concentrations ranging from 50 to 500 g/1 or 2-13 percent solids by volume, and consist of silt and clay-sized material with clay minerals and organic material (Teeter 1997). Furthermore, navigable depth is defined as the limiting depth in fluid mud areas at which vessels can navigate freely, without serious loss in speed or maneuverability and without incurring damage to hull or hull appendages (PIANC 1982). No standardized method exists in the Corps to survey fluid mud. The Corps Engineer Manual 1110-2-1003 Hydrographic Surveying (2001) states "when the upper sediment layer is not well consolidated, the three major depth measurement methods used in the Corps (sounding pole, lead line, and acoustic echo sounding) will generally not correlate with one another, or perhaps not even give consistent readings from one time to the next when the same type of instrument or technique is used." This ambiguity in determining depth has hindered Corps-wide optimization of maintenance dredging in fluid mud areas. This paper describes the work unit and results of investigations to date. While various technologies are currently being identified, a preliminary field demonstration of an acoustic characterization system is described to illustrate just one of the types of fluid mud measurement systems that the work unit may ultimately select for detailed evaluation. The SILAS software suite (developed by Sterna Survey Services of The Netherlands) was demonstrated in the Gulfport, Mississippi, navigation channel in 2002. This software samples the unaltered return signal from a low frequency hydrographic survey depth sounder and, with calibration input from an acoustical probe with a known signature, finds the variances present along the signal path and purportedly converts those changes into density and viscosity information using onsite calibration factors.