The water pressure hydraulic grade level (HGL) corresponding to the maximum water surface elevation of the Wachusett Reservoir, feeding the proposed facility by gravity from an overflow elevation of 120.4 m (395 ft Boston City Base Datum, BCB), was selected by the MWRA for design criteria. HGL is used, instead of elevation, data because it is significantly more precise than elevation data. Because 2.31 feet (0.70 m) of elevation translates into 1 psi of pressure (for water), calculating pressure to 1 psi precision requires elevation data that's accurate to roughly 2 feet (0.61 m). Normal elevation data that is accurate to the nearest 10 feet (3.0 m) will result in pressure that is accurate to roughly 4 psi.
Read more about this topic: Metro West Water Supply Tunnel
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