How the 100-Year Flood Became an Annual Event

To propose that we respond to recent flooding with creek flow monitoring, siren-blaring fire trucks, sandbags, and more excavation and concrete lining of waterways might be laughable were less at stake.

by David Schrom

Was the February 3 overflow of San Francisquito Creek a 100-year flood, an event with a one percent chance of occurring in any year? The smart money says--no. Here's why: flood frequency is defined in terms of past events; floods are generated by interactions between climate and watershed; we're changing both climate and watershed to make flooding here more severe and more common.

Over the last 100 years, we've increased Earth's mean surface air temperature by .5-1.1 degrees Fahrenheit. Warming ocean water has expanded to raise sea levels 4-10 inches. U.S. weather stations show a nearly 10 percent rise in precipitation, most of which has resulted from increased intensity in the most extreme daily rainfalls.

The 2,000 scientists of the Intergovernmental Panel on Climate Change anticipate that during the next century we will warm the atmosphere another 1.8-6.3 degrees Fahrenheit, and that this will be accompanied by a further 6-inch to 3-foot rise in sea levels, and an additional 10 percent increase in precipitation. Overall precipitation gains, as well as increases in frequency and magnitude of extreme daily rainfall events, are expected to be greater in mid-latitude regions (here).

Though a few degrees of temperature and a 10 percent or so rise in precipitation may seem modest, a 7,000-year geological record of overbank floods in the upper Mississippi watershed shows that changes of this size are sufficient to make what had been 500-year floods a frequent occurrence. In low-lying coastal areas (much of Palo Alto), rising sea levels and tidal surges driven by storm winds can combine with intense precipitation to amplify flooding.

In San Francisquito and other local watersheds, impacts of climate change have been, and are continuing to be exacerbated by human alterations to the landscape like those recently approved for the Sand Hill corridor. By felling trees, clearing brush and understory plants, changing slopes, accelerating erosion of topsoils, exposing subsoils on which vegetation has difficulty establishing, covering land with impermeable buildings and pavement, and lining creeks with concrete and timber, we've disrupted a complex ecosystem and degraded critical self-regulatory processes.

Percolation and groundwater recharge are down. Run-off volumes and rates of flow are up. A once-living creek supporting diverse native riparian vegetation and a robust steelhead salmon run is now in many places a weed-filled ditch, largely disconnected from its floodplain, and ever more threatening to our carefully constructed improvements.

To propose that we respond to recent flooding with creek flow monitoring, siren-blaring fire trucks, battery-powered radio giveaways, sandbags, building permit fee waivers, therapy for those who suffer flood damage, and more excavation and concrete lining of waterways might be laughable were less at stake. An economically prudent and ecologically sound course, far more likely to bring benefits to us and our descendants, will include: exiting floodplains; stewarding watersheds to return hydrologic processes to nearer their natural state; and joining with people elsewhere to stabilize global climate.

David Schrom volunteers with Magic, a Palo Alto non-profit, where he applies ecological methods and principles to discern and to further common human interests. For references supporting factual statements in this writing, call 323-7333 or e-mail (david@ecomagic.org.) Magic exists to demonstrate how people can learn and apply methods and principles of ecology to discover and further common interests of humankind.

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