Incorporating a Semi-Empirical Point Source Discharge Mixing Model into a 3D Nested-Grid Hydrodynamic Model to Examine Residence Time Scales on the Shallow Shelf of a Large Lake
Abstract
Cayuga Lake is long (65km), narrow (3km), and deep (130m max). A shallow shelf (<6m deep) extends 2km north from the southern end. Concern about phosphorous and sediment loading has initiated an effort to understand the physical processes controlling water residence time on the shelf. Of particular interest are the influence of a deep lake cooling facility that discharges onto the shelf and internal waves impacting the shelf from the lake basin. To assess the impact of various physical processes on residence time we have employed the three-dimensional hydrodynamic model Si3D, forcing a fine-grid (25m x 25m x 0.1m) simulation of the shelf with output from a coarse-grid (125m x 125m x 0.1m) simulation of the entire lake, and developing an empirically-based near-field point source mixing model for the cooling facility outfall. Si3D solves the continuity equation, the Reynolds-averaged Navier-Stokes equations (employing the hydrostatic assumption for pressure and an eddy viscosity closure for Reynolds stresses), the transport equation for temperature (employing an eddy diffusivity closure), and an equation of state. Vertical eddy viscosity/diffusivity are represented using level 2.5 Mellor-Yamada. Horizontal eddy viscosity/diffusivity are typically set to constants, but we use these to enforce the correct near field dynamics of the cooling facility discharge. The cooling facility outfall is a 23m-long array of nozzles pointed north and discharging with sufficient velocity to induce significant back entrainment (order 10x the 1.5m3/s discharge) and rapid lateral diffusion of momentum and scalars for hundreds of meters downstream. Once the negatively buoyant effluent has lost sufficient momentum, it plunges and spreads under the influence of both momentum and gravity. In the near field, we specify horizontal eddy viscosity/diffusivity within Si3D such that evolution of momentum and scalar fields match semi-empirical predictions for a jet in shallow water with friction. Empirical constants are calibrated using data from a field gridding study. The calibrated model is used to examine the relative influence of the cooling facility and internal waves on shelf-wide residence time. We also investigate the influence of the near-field mixing model on predictions of residence time for cooling facility effluent.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMEP53F1046C
- Keywords:
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- 1862 Sediment transport;
- HYDROLOGYDE: 1902 Community modeling frameworks;
- INFORMATICSDE: 3022 Marine sediments: processes and transport;
- MARINE GEOLOGY AND GEOPHYSICSDE: 4568 Turbulence;
- diffusion;
- and mixing processes;
- OCEANOGRAPHY: PHYSICAL