The Q4 report “SNL-EFDC Simulations of MHK-Related Changes to Tidal Range and Flushing” reflects significant revisions from the previous version submitted in Q2. It has been revised according to comments received by DOE and an independent reviewer, Dr. Brian Polagye of the University of Washington. The report was augmented with a more thorough description of how the MHK devices numerically withdraw energy (momentum) from the system. Also, a more complete list of assumption that go into the model is now provided along with some discussion of potential environmental consequences. In total, all of the models were updated, rerun, and re-analyzed.
For the tidal reference model, all model runs were redone with the updated version of the MHK module recently improved to yield more accurate power production estimates. All reference model runs are 2D (single layers). Decreases in tidal range due to MHK power generation were estimated. Scoping runs were conducted with a six-layer model to note the effects on power production estimates. As expected, reduced power estimates result because multiple layers afford more opportunity for flow around (above and below) the MHK turbines (no longer is there plug flow through the tidal channel). All water age and e-folding calculations were updated and re-analyzed. E-folding times increase with additional MHK devices because system flushing is decreased. Additional runs were performed where various flow rates of the river into the bay were examined.
For the San Francisco Bay simulation, the entire model was rebuilt starting with a new grid and runs used the new MHK module. The cells are 250×250 m2 (up from 200×200 m2). An updated bathymetry was also applied. The new grid has the ocean tidal forcing boundary much further from the throat of the San Francisco Bay to ensure that boundary effects do not negatively impact the MHK power estimates. In addition, NOAA water-level data were used to drive the model and the modeled water levels inside the Bay were compared to six available data sets to ensure that consistent results were obtained. Deviations are noted and discussed. Decreases in tidal range are noted in the Bay as the number of MHK devices increased. Also, water age and e-folding times were calculated. As MHK devices increase power generation, increased water age and e-folding times are observed.