Analysis of Particle Transport Using a Particulate Tracer Modeling
Abstract
Understanding the transport of dissolved and particulate materials in the Chesapeake Bay estuary is critical to allocating nutrient and sediment load reduction goals to the seven watershed states. A computer simulation of a particulate conservative tracer was conducted to help determine the transport mechanism. Tracers were loaded daily at the fall-line of Potomac River (a middle Bay's tributary). The settling rate is set at 0.1 m/day, with the assumption of neither scour nor re-suspension of tracer from the bed to allow continuous accumulation of tracers on bed. The low settling rate was used to allow tracer to transport widely in the estuary to provide information on the transport of fine particulates such as dead algae. After the tracers reach the mouth of Potomac River, most of them are further transported into the lower main-stem Bay. Flood tide is the main force for tracers transported north to the upper main-stem Bay and to the upstream of non-source rivers. In the main stem of the Bay, there exist concentration gradients from the Potomac River mouth to the opposite shore (the Maryland and Virginia eastern shore), to the lower Bay, and to the upper Bay. Concentration gradients also exist from the fall-line to the mouth in the source river, and from the mouth to the upstream in non-source rivers. These gradients are usually disturbed across trenches, due to a so-called "trench effect". A trench either deposits more or less tracers than its shallower sides, depending on the trench to be hydrologically landward from the source (i.e., the Potomac fall line) or the sub-source (e.g., the Rappahannock River mouth for the trench in the upstream of Rappahannock River), or hydrologically seaward from the source or the sub-source. Depending on the layer (saline water-rich or fresh water-rich) in which tracers reside and the direction (landward or seaward) along which tracers transport, the transport/deposit of tracer may be favored along trench over its shallower sides, or the reverse. The landward movement of saline water to the upper Bay or upstream of non-source tributaries by flood tide is favored along trenches over their shallower sides of the navigation channel, causing more tracers to deposit on trench than on its shallower sides, as so-called the "landward trench effect". The seaward movement of tracer from source to downstream of the source tributary or the lower Bay causes tracer to deposit more on the shallower sides than on the trench, as so-called the "seaward trench effect". The phenomena of the trench effect are usually disturbed by local bathymetry and freshwater inputs. The lower portion of the upper Bay, which is between north of the Potomac mouth and Patuxent mouth, has the transitional type of trench effects between the seaward and the landward trench effects in this simulation. Detailed comparison of bed tracer across transport channel in various locations will be presented, and the mechanism causing the above phenomena will be discussed.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2002
- Bibcode:
- 2002AGUSM.H41A..24W
- Keywords:
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- 1800 HYDROLOGY;
- 1871 Surface water quality;
- 1878 Water/energy interactions;
- 1899 General or miscellaneous