Transport of Suspended Solids in the Lower Fox River

Abstract

The general purpose of the present study was to study the transport of the sediments and associated contaminants in the lower Fox River and from the Fox into Green Bay. For this reason, a numerical model of the transport and fate of suspended solids in the lower Fox River has been developed and verified. The model consists of a two-dimensional, vertically-integrated, time-dependent hydrodynamic and transport model coupled with a three-dimensional, time-dependent model of the sediment bed and its properties. Settling speeds and sediment resuspension parameters needed in the model were determined from laboratory and field tests. In the description of the transport of suspended solids, three components of solids are considered, i.e., fine (zero settling speed), medium (moderate settling speed affected by flocculation), and coarse (large settling speed). It is assumed that the sediment bed is layered in the vertical direction. The properties of each layer depend on time after deposition (and therefore with depth) and composition (relative fractions of medium and coarse sediments), and the number of layers and their thicknesses can be arbitrarily specified at the beginning of the calculation. The thickness of the surface layer changes with time depending on the rates of resuspension and deposition. Calculations were made for steady flows at high, medium, and low flow rates as well as for real, time-varying flow events. In particular, two flow events were modeled in detail, the first from 22 May to 20 June 1989 (this included a once in a 10 year high flow as well as moderate to low flows) and the second from 24 March to 10 April 1989. For these events, calculated sediments concentrations at the river mouth were compared with observations. Good agreement between the calculations and observations was obtained, thereby validating the model and the description of the physical processes implied in the modeling. In particular, the presence and effect of an easily resuspendable surficial layer was demonstrated.