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9-foot [2.74-m] SUPER TIDE AT DRAWBRIDGE

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The objective of our Hidden Ecologies Don Edwards San Francisco Bay National Wildlife Researve Drawbridge Site Study is to follow Salt Marsh evolution in two salt ponds in the Drawbridge area. The levees impounding salt water in these ponds were breached in the spring of 2006. Development of a healthy Salt Marsh depends on the relationship between tide and elevation.

Notice that on Sunday, September 10th, the tide exceeded 9-feet at 3:20 PM. Cris Benton, three members of his class, and Wayne Lanier were there to greet it. Such super tides do not occur often, and are even more rare in the afternoon when they can be observed by aerial photography.

This is how the tidal surge looked at the breach between the pond and Coyote Slough shortly before 3:20 PM. Click on the black triangle to play the video.

Notice that the speed of the incoming flow was greater than the rate at which I panned the camera. By observing the video with a stopwatch, I estimated that it took about 15-seconds from the time incoming water reached the breach to the time it exited the breach. The breach was approximately 10-meters across by 15-meters wide. The tidal height above datum was about 2.7-meters. That represents a flow of about 1,600-cubic meters of water per minute into the pond, or almost 600-thousand cubic meters of water during the 6-hour period from low tide to high tide.

Our interest is in mapping those areas above the mean high tide that were covered by a super tide. Land with elevation above any super tide will not remain salt marsh. Land covered by a super tide can either become salt marsh or mud flat. The land covered by a supertide that is at or above the elevation of the mean high tide will form salt marsh. Land below that elevation remains mud flat. Mapping the tidal boundaries, then, is the first step in following salt marsh evolution.

To be Continued…

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