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5 <br />from Redwood Shores Lagoon is limited by the short duration of tide levels low enough to <br />permit gravity outflow. The two pump stations, when properly operated, can increase outflow to <br />the level required to maintain adequate flushing of the lagoon as a whole. <br />1.4.1 Summer Flushing Rates <br />Current lagoon operating procedures call for maximum summer lagoon levels at 99.0. If the <br />lagoon level were to drop to 98.5 during low tide and rise back to 99.0 over a day, the flushing <br />rate would be approximately 1/12; that is, the volume of the lagoon would be exchanged once <br />every twelve days. A study of lagoon water circulation conducted 1994 concluded that the <br />existing lagoon system as a whole has a summer flushing rate of 1/14. However, individual <br />segments of the lagoon such as the southern end and the two internal water bodies between Marlin <br />Court and Davit Lane, experience poor circulation because of lesser exchange rates. <br />Only 10 percent of the tides have LLW at 98.0 or higher. Therefore, with the maximum lagoon <br />level at 99.0, gravity outflow from the lagoon is possible during the low part of practically <br />every tide cycle. Adding the effect of the pump stations, it can be concluded that with proper <br />lagoon operation acceptable overall lagoon flushing rates may be attained throughout <br />summer. However, localized low circulation areas may experience lower exchange rates. <br />1.4.2 Winter Flushing Rates <br />Maximum lagoon levels in the winter are kept at elevation 97.0 to reserve storage for storm <br />runoff. To assure some exchange of the lagoon water during this period, the lagoon level <br />must be lowered to less than 97.0 and then allowed to fill back to elevation <br />97.0. Fifty percent of the tides at Belmont Slough have LLW at or above elevation 97.0; therefore, <br />no gravity outflow is possible during these tides. LLW between 97.0 and 96.5 occurs for <br />approximately 20 percent of the tides and will permit gravity outflow, but not for a long enough <br />period of time to lower the lagoon level to 96.5. This places a bigger burden on the pumps if <br />overall lagoon flushing rates <br />are to be maintained. As a result, water circulation and quality is better in the northeast half of <br />the lagoon due to the consistent operation of the pump stations. In contrast, circulation in the <br />southern half of the lagoon which relies only on the existing gravity outlet is typically not as <br />good. <br />l.5 BENEFICIAL USES <br /> <br />The beneficial uses of Redwood Shores Lagoon include: <br />o Feeding and resting habitat for waterfowl and migratory birds; <br />o Estuarine aquatic habitat; <br />o Sport fishing; <br />o Aesthetic enjoyment; <br />o Boating; and <br />o Swimming. <br />6.3.A. - Page 170