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Wallowing in Mud Bath

"Grey, slimey stuff," up to 65 million cubic yards of it, is how one scientist described the Bay mud that might need to be scooped up off the bottom to make way for a proposed new runway for the San Francisco airport. Add another 95 million cubic yards (mcy) of less slimey stuff, needed to provide firmer footing and a higher elevation for a possible 1,200 acre new runway area, and you have some mind-numbing mud math that has scientists worried about impacts on sediment dynamics in the Bay. If this solid-liquid balance gets out of whack, shorelines may retreat, newly restored wetlands may languish underwater, and steelhead may suffer and stray.

So just how much mud-moving is too much? "We asked ourselves are these numbers big or small?" says the U.S. Geological Survey's David Schoellhamer of some of the back-of- the envelope calculations he and other top scientists made during an October 20 brainstorming session. The session, organized by the National Oceanic and Atmospheric Administration and co-sponsored by local regulators faced with a potential permit for the biggest Bay fill in 30 years, aimed to list research questions for a forthcoming environmental impact analysis of various options for new runways to be built on fill, pilings or floats.

In the case of the fill option, Schoellhamer concluded the numbers are "very big" when compared to the 4-6 mcy now dredged from the Bay every year to keep ships and boats from running aground. And the grey matter swirling around this option doesn't stop there. Based on the sediment deposition history of San Francisco's current runways, Bay tides and currents could import another 30 mcy of mud into the low-wave energy areas between and behind the new structures, says Schoellhamer. Scribbling on a different envelope, hydrologist Philip Williams put the figure at closer to 50 mcy.

Shifting so much mud and sand around, not to mention mitigating for the runway fill with restoration of vast acreages of below-sea-level wetlands, could create some sizable new sediment sinks in the Bay. "There's a dynamic equilibrium in the Bay between sediment supply and erosion and deposition," says Schoellhamer. "Any new sinks will tend to fill up with sediments that have to come from somewhere."

According to other USGS scientists, the Bay only gets about 8-10 mcy of new sediment from its rivers and watershed every year, and this number has decreased due to more dams and less mining upstream. Partly as a result of such declines, San Pablo Bay has become erosional rather than depositional in the latter half of this century. So what does this mean for the airport project?

"You're basically adding a lot of sediment sinks in the context of decreasing sediment supply and increasing Bay floor erosion. The question becomes, is there enough material for all this?" says Schoellhamer. "The answer is you get a higher likelihood of erosion." Out comes that envelope again. Say the airport creates a 10 mcy sediment sink (airport- related sinks could add up to 14 times that amount if fill material is "borrowed" from the Bay itself), and assume the material is going to come from erosion of the Bay floor. Areas most likely to erode are the parts less than two meters deep, about 250 square miles. Take 10 mcy off the top of these areas and you get a half inch drop in Bay bottom level, estimates Schoellhamer. With a little more math (based on a natural 1:1000 slope that evolves wherever land and water meet, noted by Phil Williams), a half inch drop could result in a 40-foot retreat in shoreline or mudflat - another "big" number. "If there's a lot of erosion, we may be losing just those habitats we're trying to preserve and increase," says Schoellhamer. "Sediment is the basic building block of the marshlands people want in the Bay."

So why not take that 65 mcy of grey slimey stuff from the airport and put it in our wetland projects to bring them up to sea level? "We may end up valuing dredged material a lot more as material for wetland creation because of declining sediment inputs,"says Phil Williams. If there isn't enough mud to go around, we could end up with a Bay ringed with lakes, not pickleweed and cordgrass, especially if levees are breached to allow water into huge new areas proposed as airport mitigation sites, such as Cargill's 20,000 acres of salt ponds. "Such a mitigation could turn out to be a bigger hydrodynamic issue than the runway fill itself, by causing major changes to the tidal prism south to the Dumbarton Bridge," says Stanford's Steve Monismith, who also attended the October 20 event.

Some of the possible hydrodynamic effects intrigued Monismith and colleague Jeff Koseff, experts in modelling estuarine water circulation. Working with a graduate student over the past year, they plugged one of the proposed new runway configurations into a South Bay circulation model to see what might happen to tidal flows. They found that an eddy formed around the tip of the runway (see below), which took water from the main channel and pushed it into the shallows, and vice versa. "How far that eddy goes out into the main channel could influence flow exchange between the North and South Bays," says Monismith.

But it was fish not flows that dropped a bombshell at the October 20 panel, at least according to Cal Fish & Game's Chuck Armor, who was surprised to find the airport didn't even have endangered species issues on its radar screen. Armor says steelhead and salmon migrate down the west side and channel of the Bay to spawn in Peninsula streams. Armor "strongly suspects" that the steelhead passing the airport are of the recently listed California Coastal variety, and that the chinook could be among several listed or candidate runs. If a federal agency (such as the FAA) is involved in a project that might disturb or destroy federally-listed or candidate species, there's a rule that candidate fish must be extended the same protections enjoyed by listed fish, says Armor. Needless to say, mega mud movements and clogged gills could prove unhealthy for fish. "Sediment isn't very sexy, but it could end up being a pretty ugly issue for the airport in the long run," says Armor.

Other issues worrying the panelists included loss of shorebird habitat (those shrinking mudflats), toxics stirred up by shoreline rearrangements, and incursions on some of the best halibut fishing shoals in the Bay, to name only a few. A summary of the panel's suggested research questions is now being drafted by moderator Dr. Jerry Schubel of the New England Aquarium. But given the airport's projected two-year turnaround for an EIR on the street, who knows whether the science used to justify it will manage to leap off the backs of envelopes or sink under its own weight. Contact: Dave Schoellhamer or Stephen Monismith.

Contact: (650)723-4764

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