Examining thin layer cap behaviour in a freshwater industrial harbour

Abstract

A sediment remediation project in Hamilton Harbour, Lake Ontario, Canada, proposes to use a thin layer cap for managing residual contamination from dredging as well as areas of marginal contamination not included in the dredge plan. Bottom shear stress (BSS) from weather, as well as propeller wash, have the potential to severely erode thin layer caps. The proposed thin layer cap was examined with respect to the BSS expected in Hamilton Harbour over a 6-month period. BSS from vessels was estimated using published equations and BSS from weather using the MIKE 3 model. Critical shear stress for the capping sand was estimated using established equations and an annular flume. Estimated BSS from marine vessels was found to exceed the critical shear stress in most cases. However, the extent of impact was difficult to determine and was mitigated to some extent by short vessel passage times, slightly differing routes and further sediment movement caused by weather. The average estimated BSS from weather for the study period was found to be below the critical shear stress for the capping sand, but the maximum BSS exceeded for a small area of the site. Apart from the shipping routes, the majority of the capping area should remain well below the critical shear stress. However, consideration of a larger grain size of capping material for the shallow areas should be considered. This would be of value for future storm conditions which might exceed what was experienced during the 6-month study period.