Mechanisms for Onshore Sediment Transport in Boston Harbor

Lead Pi: Ole Madsen · 8/1995 - 7/1997

Project Personnel: Eric Adams

Project number: 1995-RC-40

Objectives:Our objectives are to develop models for and test two hypotheses for mechanisms for net onshore transport of suspended sediments into Boston Harbor. The two hypotheses are: [Wave tide geometry interaction] During conditions with onshore winds, wave action is significant outside the harbor whereas the protection offered by islands results in less pronounced wave action inside the harbor. Considering the response characteristics of the suspended sediment, concentration in the water column results in a larger transport into the harbor during flood than out of the harbor during ebb tides. [Wave wind bathymetry interaction] A typical harbor entrance consists of a deep navigation channel and shallower portions. Local winds produce downwind flow in the shallow and upwind flow in the deeper portions. Thus, onshore winds will create onshore flow in the shallow portions, which have more deposited sediments available for transport than the deep portion. Adding to this increased agitation of bottom sediment by larger waves during onshore winds suggests a net onshore flow of sediment. Methodology:Through the use of simple, yet realistic models for suspended sediment transport by the combined effects of wind waves, wind induced currents and tidal currents at typical harbor entrance channels the two hypothesized mechanisms for net sediment transport will be investigated. Initially, highly simplified scenarios and models will be developed to capture the essence of the two mechanisms in order to assess their potential importance. This will be followed by the development of improved (more complex) models and their applications to realistic scenarios representing Boston Harbor.Rationale:Rates of sediment accumulation in Boston Harbor exceed the rate sediment loading known sources (rivers, treatment plant outfalls, CSOs, etc). Now that most contaminated sources to the harbor are being/have been eliminated the major question is what will happen to existing contaminated sediments? For example, will cleaner sediments from offshore help to bury the contaminants or dilute their concentrations, thereby making it easier to dispose of sediments from maintenance dredging and eliminating the need for remedial dredging? The answers strongly depend on the topic addressed by this proposal.