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Massachusetts Estuary Project(MEP) <br /> Linked Watershed EmbmentModel Peer Review <br /> mass conservation was discussed. One way to quantitatively address this concern is to set the value of the <br /> computed water quality variable to 1.0 at every node of the grid and to attach that concentration to all <br /> inflows and loads. If mass is not conserved there will be deviations during the computations from the <br /> specified value of 1.0. The Panel reconunended to the SMAST and Applied Coastal Research and <br /> Engineering modelers that this computation be made for the Pleasant Bay study. Results from that <br /> recommendation were provided to the Panel. For the 7-day simulation period the maximum error in the <br /> interior of the grid was 0.54 parts per trillion. This substantiates that the Pleasant Bay numerical grid is a <br /> very good representation of the estuary. <br /> In the review of the SMAST approach,the Panel was instructed to focus on the Pleasant Bay and Bourne <br /> Pond studies. The numerical grid covering Bourns Pond was constructed in 1999 and visually appears to <br /> perhaps need additional grid elements. The coarseness of the existing grid is likely related to the need to <br /> minimize computing time with the computer resources that were available in 1999. Since the <br /> computation of TMDLs in the Cape Cod estuaries and bays is an ongoing process,the Panel recommends <br /> that as the process proceeds for Bournes Pond,the mass conservation check outlined above be applied to <br /> help guide additional grid refinement. <br /> Are computational tinges excessive? Generally speaking,numerical models based on the finite element <br /> solution method can be computer intensive. However,the simulation periods for the Cape Cod estuaries <br /> and bays are relatively short,i.e.,days rather than months or years,so the computational time is not <br /> considered a detriment. Thus,this is not considered a weakness of employing the RMA2 and RMA4 <br /> models. <br /> Key Issue 2-Should the RMA2 Simulations Include Sub-Tidal Events as well as Purely Tidal <br /> Conditions? <br /> The simulations for Pleasant Bay and Bournes Pond were for 7 and 5 days,respectively. The water <br /> surface elevation boundary condition for each contains only tidal fluctuations. If sub-tidal events,such as <br /> set ups and set downs of the water surface due to the passage of fronts,were also contained in the <br /> boundary conditions,there would be increased tidal flushing. However, as noted by Mr.Paul Stacey of <br /> the Panel, in the determination of TMDLs the conservative approach is taken. Thus, simulations that only <br /> contain tidal fluctuations are the appropriate approach. <br /> Key Issue 3-How are Culverts in Pleasant Bay Handled in RMA2? <br /> Applied Coastal Research and Engineering responded that the culverts at Frost Fish Creek are crushed <br /> and partly blocked. They are modeled in RMA2 through the specification of an extremely high friction <br /> coefficient. There are also culverts at Muddy Creek. In the existing model they are also represented by a <br /> high friction coefficient. However,since Muddy Creels is part of an estuarine and salt marsh restoration <br /> project,plans call for replacing the existing.culverts with a much larger culvert or perhaps abridge. As <br /> part of the restoration study,the grid resolution for Muddy Creek was increased,resulting in the use of <br /> more normal friction coefficients. <br /> Key Issue 4-Is the RMA2 Model`Fell Corroborated? <br /> As noted above,the Panel was instructed to focus on the Pleasant Bay and Bournes Pond applications. In <br /> these applications,measured water surface elevations on the ocean boundary were applied. After varying <br /> the bottom friction and eddy viscosities,the computed water surface elevations in the interior were then <br /> compared to collected field data in two ways. First there was a visual comparison of the computed and <br /> observed elevations. Next, a harmonic analysis of the measured and computed elevations was conducted <br /> to yield the major harmonic constituents of the tide. Both the amplitude and phase of each measured and <br /> computed constituent were then compared. The Panel considers the resulting comparisons for both <br /> Pleasant Bay and Bourns Pond to be excellent. <br /> December 30,Z011 <br />