SFO experiences delays (known as "flow control") in overcast weather when only two of the airport's four runways can be used at a time because the centerlines of the parallel runway sets (01R/01L and 28R/28L) are only apart. Airport planners advanced proposals that would extend the airport's runways by adding up to of fill to San Francisco Bay and increase their separation by up to in 1998 to accommodate arrivals and departures during periods of low visibility. Other proposals included three floating runways, each approximately long and wide. The airport would be required by law to restore Bay land elsewhere in the Bay Area to offset the fill. One mitigation proposal would have the airport purchase and restore the of South Bay wetlands owned by Cargill Salt to compensate for the new fill. These expansion proposals met resistance from environmental groups, including the Sierra Club, fearing damage to the habitat of animals near the airport, recreational degradation (such as windsurfing) and bay water quality.