The Loma Prietan
April/May 1999
Airport Update: Sensitive Bay Hydrology Could Be Disturbed By New Runways
by Richard Zimmerman
Many elements of San Francisco
International Airport's (SFIA) dreams of
expansion into the Bay have raised concerns among environmentalists. One question raised is, What would the impact of such a large fill be on the hydrology of the South Bay?
Hydrology is the study of watersheds and water movement. No scientist contacted for this article would hazard a guess as to the possible outcome of SFIA's planned expansion - everyone called for the need for more study. All the scientists were, however, in agreement on one point: the hydrology of the San Francisco Bay would be altered by the proposed runway additions.
"Even minor changes within an ecosystem can lead to major problems." wrote the San Francisco Estuary Baylands Ecosystem Goals Project in a recent report. Each of the two preferred plans put forth by the Airport in their latest feasibility study calls for filling over one thousand acres of wetlands. "There is only one Bay," added David Lewis, Executive Director of Save The Bay "and it should not be any smaller."
Moon Struck
Tides, the periodic rise and fall of the water in the sea, are produced by gravitational forces between the moon, sun and earth. The point of the maximum force on earth moves as the moon, sun and earth revolve. The sea moves in response. Thus the tides have a period and magnitude determined by the revolving solar system bodies.
As the water rises along the coast, it is forced through the narrow Golden Gate into the Bay. When the ocean ebbs, water flows out of the Bay and this basic action
flushes it.
The amount of water flowing in and out of the Bay, the tidal prism, is tremendous. Within the Bay, the tidal current slows as the water spreads out. Generally the currents are stronger in the deeper channels and less in the shallows.
In the northern reaches of the San Francisco Bay, the tidal wave moves into the Delta - which provides up to 90% of the fresh water to the upper bay - and dissipates. In the southern reach, however, the flood tide "rebounds" off the southern shore and interacts with the next wave.
This produces a standing wave that oscillates slowly back and forth in the lagoon-like South Bay. A simple test can demonstrate the South Bay's action. Hold a pan half full of water and slosh the water back and forth. If you time your sloshings right, you'll create a wave that moves back and forth.
Though the Bay is much more complicated than our pan, the pan model does imply correctly that there is less water movement in the South Bay than in North. Ralph Cheng of the USGS concurs. Water residence times, he noted, can be several months in the summer.
That's fresh water?
Although recent studies have confirmed that water from the Delta does flow into the South Bay in the summer, treated sewage from local communities is virtually the only source of fresh water. As long ago as 1985, the South Bay received 76% of waste water dumped into the Bay, said T.J. Conomos of the USGS.
The construction of Anderson Dam cut off a major fresh water input to the South Bay, according to Conomos. The only times that Coyote Creek contributes fresh water are during El Ninő-like years.
The lack of fresh water sources leads to higher salinity in the South Bay. Increased evaporation in the summer can increase salinity to the point where it could be higher than the ocean, says Conomos.
South Bay Needs More Study
The combination of reduced fresh water flows and naturally limited tidal movement illustrate that the delicate ecosystem of the San Francisco Bay is vulnerable to changes in its hydrology.
As Phil Bobel, manager of Environmental Compliance for Palo Alto noted, "Putting in new fill could conceivably change the flushing action in the South Bay."
Scientists agree that much work and research remains before the Estuary is understood. "Large gaps remain in our understanding of how the Bay functions and the contribution of natural variability to those functions," say Cloern and Nichols of the USGS.
"The Bay ecosystem is an extremely complex yet delicate natural system about which scientists really know very little. We cannot forsee all of the implications of seemingly minor changes..." summed the S.F. Estuary Baylands Ecosystem Goals Project Report.
References
"San Francisco Bay, The Urbanized Estuary." T.J. Conomos, Ed. Pacific
Division, American Association for the Advancement of Science, 1979
"Hydrobiology -Temporal Dynamics of an Estuary: San Francisco Bay."
Cloern, James and Nichols, Fredric, Ed. Dr W. Junk, Publishers. 1985
"Baylands Ecosystem Habitat Goals" San Francisco Bay Area Wetlands
Ecosystem Goals Project, 1999