Water Level Control and Tidal Flows

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At the Glen Echo Inlet station, the average oxygen level for 20 days when the tide gates were left open was 4.62 ppm. When the gates were shut during high tides, oxygen levels were only 3.2 ppm.

The Relationship Between Tidal Restriction and Proposed Wetland Construction: According to a professional hydrological study (Abbe and Haltiner, 1992) conducted in conjunction with the Lake Merritt Enhancement Plan “a functional marsh (emphasizing mid-and high-marsh vegetation) can be created with the existing system hydraulics. The limited tide range restricts marsh restoration, but does not prevent it; however, the extended periods during which the lake levels are kept high or low are of greater concern.” This study concluded that:

“If a marsh is created, gate closures and extended periods of artificially high and low water levels should be minimized.”

Current Location and Design of Flood Control Station are Out-Dated: Designed in the late 1960’s, the County Flood Control Structure has become out of date. If redesigned and re-built at the 12th Street dam, the following benefits could occur:

• Water withdrawal from the Lake during tide gate closure could occur from the bottom of the Lake, thus reducing the layer of stagnant water
• Flood control could be improved to include events larger than the 25 year storm
• The channel upstream of 7th Street could be opened to natural tidal action
• Computer modeling could be used to refine decisions as to when tide gates should be closed and more accurate electronic (not paper) tide gauge charts could be used
• Movement of marine life to and from the Lake could be improved by eliminating the trash rack bars across the channel.

Summary
History of Tidal Flow Limitation: Lake Merritt is a natural tidal estuary whose connections to the sea have been drastically limited, both structurally and operationally. In decades past, this limitation has led to water quality problems, fish kills and limits on the types of life found in the Lake. Operational limits to natural tidal flows were reduced in the 1990’s after studies recommended a more natural tidal regime. Next >>

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