An evaluation of seabed stability, liquefaction and development of fluid muds during dredging and disposal was carried out in the Miramichi Inner Bay over a three-week period during July 1993. The objective of the study was to determine if dredging and disposal activities cause instabilities that lead to enhanced sediment erodibility and resuspension. Measurements of sediment pore pressures, erosion thresholds, erosion rates, and settling rates were made at a number of sites representing the natural seabed, the navigation channel, and an experimental disposal site prior to and after disposal of spoils. The effect of ship passage on channel stability and sediment resuspension was also evaluated during passage of two commercial freighters.

Sediments in the Bay are characterized by high organic contents and minimal consolidation and are very close to liquefaction. Measurements of sediment settling rates, however, indicated that in most locations there is little potential for generation of fluid mud layers. Erosion thresholds were lowest at the Channel sites and were typical of fluidized gels. Erosion thresholds of dredge spoils were initially low, but quickly (within 60 hrs) increased to exceed those of the original dredge materials. This strengthening, however, was restricted to the surface 2 mm of sediment only. Immediately below this depth the strength of the disposed material remained extremely low, and was even in a fluidized state. Monitoring of in situ pore pressures at depths up to 50 cm before and after disposal of spoils indicated that spoils deposition has no effect on the long term stability of underlying sediments.

Continuous monitoring of suspended sediment concentrations indicated little evidence of either naturally occurring fluid mud layers or that dredging and disposal activities result in chronically high suspended sediment concentrations outside of the immediate area of the Channel where dredging is taking place. On two occasions high suspended sediment concentrations (>1000 mg-1), lasting for periods of up to six hours, were detected near the Experimental disposal site. It was not clear if the source of these sediments was recently deposited spoils or river inputs resulting from high tides and strong precipitation events.

Ship passage produced elevated suspended sediment concentrations (>2000 mg-1) at a site approximately 100 m north of the Channel, but these lasted for only a short period (less than 15 min). A more significant impact of ship passage was the generation of short term cyclic excess pore pressures by the wake which is a major factor leading to reduced stability of the Channel margin. Ship passage effects, however, may be relatively minor compared to the potential effects of sustained loading during extreme storm events.