NA1-San Francisco Bay, California, USA

In the early January of 1982, a disastrous rainstorm struck the San Francisco Bay region, triggering thousands of debris flows and other shallow landslides across the region, causing many millions of dollars in property damage and 25 deaths. The 1982 storm in the San Francisco Bay region made debris-flow advisory system seem like an urgent necessity. The Landslide Working Group at the USGS, decided to begin with the concept of a ‘threshold’ – that a critical amount of rainfall is required to trigger debris flows on susceptible slopes.

1 The setting

During the 1980s, meanwhile, the NWS had made two key advances that furthered the development of the debris-flow warning system: (1) It refined its procedures for preparing quantitative precipitation forecasts (QPF) throughout northern and central California and (2) coordinated the development of the Automated Local Evaluation in Real Time (ALERT) system, a network of radio-telemetered rain gauges across the San Francisco Bay region. When a storm approached the San Francisco Bay region, the local NWS Forecast Office would attempt to make a quantitative forecast of the storm rainfall. Then the USGS Landslide Initiation and Warning Project in Menlo Park would perform a manual comparison of the observed and forecast rainfall to the estimated thresholds for debris flow initiation. Finally, both groups worked together to assess the probable hazard from debris-flows, so that appropriate public statements could be issued.

The ALERT system was designed to collect automatic measurements of high-intensity rainfall at remote locations and transmit these data to central receiving stations for observation and analysis in a near real-time environment. By 1995, there were more than 60 rain gauges in the ALERT network in the San Francisco Bay region (Fig. 30).

Figure 30: Map of the ALERT network in the San Francisco Bay region in 1992

At the local NWS Forecast Office, the lead forecaster prepared the Quantitative Precipitation Forecast (QPF). The QPF, issued twice daily, estimates the amount of rainfall expected in each of four 6-hour periods, for the following 24 hours throughout northern and central California. In evaluating the chance for an approaching storm to trigger hazardous debris-flow activity, two thresholds had to be considered: (1) the accumulation of antecedent seasonal rainfall and (2) the combinations of rainfall intensity and duration forecast for the approaching storm. In order to evaluate the seasonal progress of soil moisture towards the antecedent rainfall threshold, the USGS Landslide Working Group installed and monitored shallow piezometers at the La Honda study area, which served as a benchmark site for the region. USGS technicians assumed that the soil moisture had reached the antecedent threshold when the piezometers first responded strongly to storm rainfall, generally within a few weeks after the winter solstice. Once the antecedent threshold for soil moisture was exceeded, subsequent storms were evaluated as they approached to see if the intensity and duration of the expected rainfall would be sufficient to trigger debris flows. The 1986 debris-flow warnings were based on the empirical rainfall thresholds determined by Cannon and Ellen (1985). By 1989, the pair of cumulative rainfall/duration relationships was developed for a spectrum of size and frequency of debris flows described above (Wilson et al., 1993).

2 The modelling

Cannon and Ellen (1985) developed thresholds for the San Francisco Bay region, using data from the January 1982 storm and several other major storms. Initially, Cannon and Ellen (1985) separated the historical rainfall data into two groups on the basis of whether the mean annual precipitation (MAP) in the area of the rain gauge was above or below 660mm (26 in). They found that abundant debris-flow activity in the more humid upland areas required storm rainfall with a minimum duration and average intensity of 4 hours at 15 mm/hr (0.6 in/hr), 12 hours at 10 mm/hr (0.4 in/hr), or 20 hours at 8 mm/hr (0.3 in/hr). The Cannon and Ellen (1985) threshold formed the basis for the debris-flow warning system in the San Francisco Bay region when it was initiated formally in February 1986, by 1989, the USGS developed a pair of cumulative rainfall/duration relationships for a spectrum of size and frequency of debris flows (Wilson et al., 1993). The pair of relationships between the duration and cumulative amount of peak rainfall, together, outlined a spectrum of debris-flow activity (Figure 11.a). The lower, ‘safety’threshold was adapted from Wieczorek’s (1987) threshold for the initiation of individual debris flows in the La Honda study area to represent a rainfall level below which significant debris flow hazards were considered unlikely. The upper, ‘danger’ threshold was adapted from the threshold of Cannon and Ellen (1988), and was intended to represent a rainfall level above which abundant debris flows are likely to occur across broad areas in the San Francisco Bay region.

The relationship between rainfall, soil moisture and slope failure in climates with a strongly asymmetric distribution of rainfall through the year, such as the Pacific coast of California, creates an additional complication, the so-called ‘antecedent condition’, that has important implications for the operation of a landslide warning system. In the San Francisco Bay region, the rainfall and evapotranspiration cycles are about six months out of phase, leading to significant seasonal variations in soil moisture (Fig. 31.b). There is a period in which positive pore pressures may be formed and intense rainfall can trigger debris flows and this period, in a “typical year”, begins in late December and extends through late March.

Figure 31: a) Rainfall/debris-flow thresholds determined for La Honda, California. There is slight chance of significant debris-flow activity below the Safety threshold, a likelihood of damaging debris flows above the Danger threshold. B) Variations in rainfall, evapotranspiration and soil-moisture content in a typical year on a hillslope in the Santa Cruz Mountains.

At the local NWS Forecast Office, the lead forecaster prepared the Quantitative Precipitation Forecast (QPF). The QPF, issued twice daily, estimates the amount of rainfall expected in each of four 6-hour periods, for the following 24 hours throughout northern and central California. In evaluating the chance for an approaching storm to trigger hazardous debris-flow activity, two thresholds had to be considered: (1) the accumulation of antecedent seasonal rainfall and (2) the combinations of rainfall intensity and duration forecast for the approaching storm. In order to evaluate the seasonal progress of soil moisture towards the antecedent rainfall threshold, the USGS Landslide Working Group installed and monitored shallow piezometers at the La Honda study area, which served as a benchmark site for the region. USGS technicians assumed that the soil moisture had reached the antecedent threshold when the piezometers first responded strongly to storm rainfall, generally within a few weeks after the winter solstice. Once the antecedent threshold for soil moisture was exceeded, subsequent storms were evaluated as they approached to see if the intensity and duration of the expected rainfall would be sufficient to trigger debris flows. The 1986 debris-flow warnings were based on the empirical rainfall thresholds determined by Cannon and Ellen (1985). By 1989, the pair of cumulative rainfall/duration relationships was developed for a spectrum of size and frequency of debris flows described above (Wilson et al., 1993).

3 The warning strategy

Observed rainfall amounts, combined with rainfall forecast, were compared to the warning thresholds (Figure 11) to determine the level of hazard and the type of public statement to be issued. Both the NWS and the USGS participated in this phase of operation. Storms with peak rainfall periods that fell below the lower threshold (‘safety’) were considered unlikely to trigger hazardous debris flows and generally required no statements. For storms with rainfall levels just above the lower threshold, brief statements were sometimes added to an NWS ‘Urban and Small Streams Flood Advisory’, warning motorists that roadways may be obstructed by rockfalls or debris flows. If rainfall was forecast to approach the upper threshold, a Flash-Flood/Debris-Flow Watch was issued, advising people living on or below steep hillsides, or near creeks, to stay alert and be prepared to evacuate, as debris flows were a strong possibility during the watch period. Storms that exceeded the upper threshold could trigger numerous, massive debris flows leading to loss of life and substantial property damage. Therefore, when rainfall was observed to exceed the upper threshold, or if reports of significant debris-flow activity were received, the strongest statement – a Flash-Flood/Debris-Flow Warning – was issued. Sample texts for these debris-flow statements were prepared, with wording agreed upon by both the USGS and the NWS, so that timely, informative advisories with complete, relevant information could be issued with a minimum of preparation time.

References

Cannon, S.H. and Ellen, S.D., 1985, Rainfall conditions for abundant debris avalanches, San Francisco Bay region, California, California Geology, 38(12), 267–272.

Cannon, S.H. and Ellen, S.D., 1988, Rainfall that resulted in abundant debris-flow activity during the storm, in S.D. Ellen and G.F. Wieczorek (eds), Landslides, Floods, and Marine Effects of the Storm of January 3–5, 1982, in the San Francisco Bay region, California: US Geological Survey Professional Paper 1434, 27–34.

Wieczorek, G.F., 1987, Effect of rainfall intensity and duration on debris flows on central Santa Cruz Mountains, California, in J.E. Costa and G.F. Wieczorek (eds), Debris- Flows/Avalanches: Processes, Recognition, and Mitigation, Geological Society of America, Reviews in Engineering Geology, vol. 7, 23–104.

Wilson, R.C., Mark, R.K. and Barbato, G., 1993, Operation of a real-time warning system for debris flows in the San Francisco Bay area, California, in H.W. Shen, S.T. Su, and F. Wen (eds), Hydraulic Engineering ’93: Proceedings of the 1993 Conference, Hydraulics Division, American Society of Civil Engineers, San Francisco, CA, 25–30 July 1993, vol. 2, 1908–1913.

Wilson, R. C. (1997a). Daily rainfall along the U. S. Pacific Coast appears to conform to a square-root normal probability distribution, in Isaacs, C. M., and Tharp, V. L., eds., Proceedings of the Thirteenth Annual Pacific Climate (PACLIM) Workshop, April 14-17, 1996, p. 19-32.)

Wilson, R. C. (1997b). Operation of a landslide warning system during the California storm sequence of January and February 1993, in Larson, R. A., and Slosson, J. E., eds., Storm-induced geologic hazards: Case histories from the 1992-1993 winter in Southern California and Arizona, Geological Society of America, Reviews in engineering geology, v. XI, p. 61-70.

Wilson and Jayko (1997). Preliminary Maps Showing Rainfall Thresholds for Debris- Flow Activity, San Francisco Bay Region, California. U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY.

Wilson (2004). The Rise and Fall of a Debris-flow Warning System for the San Francisco Bay Region, California. Landslide Hazard and Risk Edited by Thomas Glade, Malcolm Anderson and Michael J. Crozier © 2004 John Wiley & Sons, Ltd ISBN: 0-471-48663-9