Data on California's Extreme Rainfall from 1862- 1995 By Jim Goodridge PO Box 970 Mendocino CA 95460 707 937 4709 June 29,1996 Prepared for: 19% CALIFORNIA WEATHER SYMPOSIUM Theme: "A prehistoric Look at California Rainfall and Floods" Sierra College Science Center, 5000 Rocklin Road, Rocklin, CA 95677 Introduction Prehistoric storms sculpted Earth's landscape and left evidences of their effects every where we look. Prehistoric storms eroded 10,000 feet of the top of the Sutter Buttes and filled the Sacramento Valley on the west side three miles deep with alluvial sediments. Among the condominiums of Incline Village they left boulders that could only have been deposited between houses by debris flows. Debris flows generally have the density of newly mixed concrete and move at about the speed of a freight train. Debris flows are a major force in sculpting Earth's land surface. All rainfalls when they hit the ground pick up minerals and modify the landscape. Significant sever storms can be quite localized and of short duration. They often fall between the measurements from our rain gage network and even now might still qualify as prehistoric. Sometimes we can find the sudden peaks in the stream flow records to tell us of their existence; if we are lucky. This is an account of some of the measured storms within the memories of recent California inhabitants. Some of these storms have remarkably large return periods (Map 1). They are not limited to the windward slopes like winter time orographic storms, (Map 2) they occur randomly distributed geographically as well as by season. The data base of this study consisted of rainfall depth duration frequency tables for 3100 California rainfall records. These are public domain records representing 105,000 station years of record. This data base is available on a data exchange basis. It can be supplied to cooperators who send a lOOmb iomega Zip drive disk in a Macintosh format. When estimating the frequency of large storms, there are many ways to estimate return periods. A uniform method as was used in the 1978 Bulletin 195 of the Department of Water Resources was used through out this study for internal consistency. Undocumented storms have been a problem in recent times. These are the storms without quantitative records of their existence. An example is the flood of 1850 when Sacramento was swamped by over-flow from the American River. Sacramento was apparently founded on a river bank in the dry season. There were no rain records in the water shed to indicate the source or magnitude of the flood water in the early unprotected city. Now we are hopefully entering a new era when large rainfalls will no longer escape detection by a radar. Hopefully they will be observed and recorded along with storm size and distribution. There will still be a concern with maintaining a historic continuity with the older studies for use in engineering applications. A 1000 year rainfall is one that occurs on the average once in a thousand years at a given site. This is an event in which the maximum storm rainfall is approximately five standard deviation above the average annual maximum event. With a 1000 rain gages we should expect an average of one 1000 year rainfall each year; if rainfalls were independent events- which they are not. A single storm can effect many rain gages therefore the measurements are not independent. I have found only 46 storms which could be classified as 1000 year events in just over 147 years, based mainly on the daily rainfall readings. If hourly records were considered there would be many as the short duration extremes seem to be much more variable than the once a day rainfalls. The 1000 year one day rainfall expressed as a percent of the mean annual precipitation varies from 15 percent in the north west comer of the State to 165 percent in the south east comer (Map 3). Tropical storms can be a factor in studying the distribution of sever storms especially in the Desert Southeast, by bring heavy summer time rainfalls. Tropical cyclones are formed from the connective dissipation of heat; as expressed by the sea surface temperature (SST). The Pacific Ocean off Mexico's South Coast is the region of origin of summer tropical storms effecting California rainfall. The 40 year trend in annual numbers of tropical cyclones matches the trend in SST quite well (Figure 1). The rainfall climate of California has become more variable in the last 90 years revealing a greater flood hazard than was known when Folsom Dam was design 50 years ago. The number of 1000 year storms has increased with a similar trend. To illustrate the increasing variability a 10 year running average of the coefficient of variability (CV) was calculated using 73 rain records averaged together to form a wetness index for the years 1883 to 1995. The CV was calculated for each year covering the previous 10 years. This CV was plotted along with the decade totals of the number of 1000 year storms (Figure 2). There was a good relationship, indicating a consistency in the increased number of 1000 year storms along with in the increasing variability of annual total rainfall in the past 100 years. Table 1 summarizes the data of 46 events which can be classed as 1000 year storms of the past 133 years. Table 2 is a summary of flood damage and the number of lives lost, it shows that the most costly storm of California's history occurred on March 10,1995 with almost a billion dollars in flood related damage. These values were from the State Office of Emergency Services and miscellaneous souses such as newspaper accounts. They were not necessarily comparable and are not corrected to 1996 dollars. One conclusion that can be made from this Storm Damage in California is that 1 rain station reporting the highest ever rainfall correlates with about 5 million dollars in flood damage, based on storms of the last 15 years. This is an updated version of the paper offered here two years ago. The data base has been expanded and there have been two of California's most costly storms in terms of 1996 dollars, have occurred since the last report. The main deference beside the expanded data base is the up date to include the great storms of 1995. The storm maps showing lines of equal return period which were included in the proceedings of this conference two years ago are not repeated Maps 4 and 5 for the storms of 1995 are included in this study. Here is a brief description of historic storms of California based mostly on the daily observations of volunteer weather observers. HISTORIC 1000 YEAR STORMS OF CALIFORNIA Storm of December 23 to January 21, 1862 The flood of 1862 we know to have been real, even with inadequate coverage of rain records, because the size of the temporary lake that formed in the Central Valley .The Central Valley reportedly swelled up to a size rivaling that of Lake Superior before draining off into San Francisco Bay. William H. Brewer (1930) of the Whitney California Geological Survey wrote from San Francisco on Sunday, January 19, 1862, "The amount of rain that has fallen is unprecedented in the history of the state.----The great central valley of the state is under water - the Sacramento and San Joaquin valleys - a region of 250 to 300 miles long and an average of twenty miles wide, a district of five thousand or six thousand square miles, or probable an area of three to three and a half millions of acres!" Brewer writes of the Central Valley on February 9,1862, " Nearly every house and farm in this immense region is gone. There was such a body of water-250 to 300 miles long and 20 to 60 miles wide, the water ice cold and muddy-that the winds make high waves which beat the farm homes in pieces". On October 4,1861 the Red Bluff Independent reported: "Six months have elapsed since rain has fallen at Red Bluff. This has been the hottest, driest season since California became a State. On Sunday something happened in Red Bluff which nobody has expected or wanted. Dust blew in every crack. It came in showers, people breathed gallons of dust and grit every time they opened their mouths." On November 7,1861 the Independent wrote " The mortality of cattle was high, they were starving." The Red Bluff Independent states that on December 10,1861 the drought was over and flood damage was extensive. The Red Bluff Beacon reported; "Even though California received tremendous damage, Oregon suffered more, The Williamette Valley completely overflowed and a town was swept away. Crescent City California was nearly swept away." A Belgian miner Jean -Nicolaus Perot (1985) left the gold fields to settle at Portland Oregon in time to witness the flood there. He writes, 'The peaceful Wllamette became, by the fifth of December, an impetuous torrent; leaving its bed, it upset and carried away the establishments which bordered its bank. It was, for two days, a curious and heart-rending spectacle: the river was covered with strays of all kinds, trees, animals, fences, provisions, houses, sawmills, flour mills all that was floating pell-mell, and passed before Portland with a speed of three leagues an hour." Rainfall was recorded at only a few stations in the lower elevations in 1862. The heaviest rains were recorded at San Francisco where 28.25 inches occurred in 30 days. This was 6.48 standard deviations above the mean rainfall for 30 consecutive days with a return period of 37,000 years.