continued For the Green Line extension beyond Ward Road, says Batta, an engineering firm issued a "no-rise certificate," stating that the extension would not cause a rise in the hundred-year floodway baseline, so mitigation was unnecessary. In the trolley's middle section, between Highway 163 and Ward Road, a one-foot rise in the floodway level did result from the trolley construction. But, says Batta, in that area no flooding of "an insurable structure" in the floodway is ever expected to occur. So no mitigation needs to take place there either.
Still, Batta admits, the city has its requirement that the San Diego River floodway level can't be raised by construction. So it is incumbent on the Metropolitan Transit Development Board, he says, to ask the city council for "a variance" from that regulation.
Randy Berkman thinks FEMA has abdicated its responsibility to force the transit board to mitigate the trolley's effects on the middle section of the river in Mission Valley. Having its own jurisdiction, the transit district does not need to consult the city when making decisions, but it cannot break federal law, says Berkman.
The middle section of Mission Valley that Berkman worries about does have levees on each side of the river channel. And SDSU's Philip Pryde, concerned about the same section, more specifically the section between Highway 163 and Qualcomm Way, notes that during the late 1970s, property owners and the city jointly built a "soft bottom" channel in the area. But in his book, San Diego: An Introduction to the Region, Pryde says, "[That channel] would safely convey a flood the size of the 1980 one, but not the size of the 1916 flood."
A particular danger in the area, and in such other low-lying sections as Grantville, says Pryde, is that rushing floodwater often rips out vegetation and sends it downstream, where it blocks culverts. Last winter a fire department crew found the body of a drowned man in just such a bundle of bushes and small trees logjammed behind Fashion Valley. If blockages occur in the river's flow, serious backflow results, exacerbating any flooding that is already taking place.
Last winter's 22.49 inches of rain recorded at Lindbergh Field was the third-highest annual rainfall in San Diego record books. Annual rainfall has been increasing in San Diego in the past 20 years, says Pryde. But wet winters don't correlate well with flooding. Single-storm intensities are bigger factors in the occurrence of floods, large amounts of rain in short time spans creating the most dangerous situations. And a sudden intense storm is the very thing it would take to cause an already full El Capitan dam to spill over.
According to Pryde, the Arizona and Southern California region has the greatest variability in rainfall patterns of any region in the country. "And it should be emphasized," he writes, "that we have a very poor idea (because of the brevity of historical records) of what the actual hundred-year flood size is for any Southern California river."
Pryde goes on to warn about increasing urbanization "within the San Diego River watershed." Buildings, cement sidewalks, and asphalt streets and parking lots, allowing no soil for seepage, send runoff flowing farther downstream.
"We've built too much in Mission Valley," Pryde tells me. "Earlier building was more flood-conscious." As an example, he cites the I-5 bridge over the 800-foot-wide San Diego River channel to the ocean. Old photos show the bridge's destruction in the 1916 flood. Later "it was designed for a full El Capitan spillway capacity."