Historic comparison
Comparison of floods is a favorite topic for the layman, with superlatives casually thrown around. The so-called 100-year flood is also a frequent topic for discussion. But the December 31, 2005 flood was remarkable, no matter how it is compared, and there are many lessons to be learned.
My own assessment of the “historic” nature of the New Year’s Eve Flood, is attempted using two different methods for comparison. The first is to compare the FEMA-defined extended floodplain—the so-called 500-year floodplain—to the maximum elevation attained on December 31st. The second is to compare discharge readings of a select gauge on the Russian River near Guerneville.
The comparison of the FEMA-defined extended floodplain is shown in the accompanying map. As it can be readily seen, areas in the south, near the Meadowlane Ponds, and near Rohnert Park and Cotati, are all much wider in extent than previously thought. The size of this floodplain, 611 acres, is significant.
In the northern part of the watershed, the December 31st floodwater extent falls short of the FEMA-defined boundary.
For comparing historic discharge readings, the Russian River gauge near Guerneville was chosen because it is the only one that has a continuous multi-decadal record (still it should be noted that the gauge was moved to a new site in 1963, making strict comparison somewhat inaccurate.)
|
Water year |
Event date |
discharge (ft3/sec) |
stage (feet) |
|
1940 |
02/28/40 |
88400 |
|
|
1941 |
04/05/41 |
48100 |
|
|
1942 |
02/06/42 |
67800 |
|
|
1943 |
01/22/43 |
69200 |
|
|
1944 |
03/04/44 |
32000 |
|
|
1945 |
02/01/45 |
34600 |
|
|
1946 |
12/28/45 |
56800 |
|
|
1947 |
02/12/47 |
23600 |
|
|
1948 |
03/24/48 |
23400 |
|
|
1949 |
03/12/49 |
41400 |
|
|
1950 |
02/05/50 |
44900 |
|
|
1951 |
12/04/50 |
53600 |
|
|
1952 |
12/28/51 |
41300 |
|
|
1953 |
01/10/53 |
52200 |
|
|
1954 |
01/17/54 |
59900 |
|
|
1955 |
12/06/54 |
13500 |
19.32 |
|
1956 |
12/23/55 |
90100 |
49.7 |
|
1957 |
02/25/57 |
45800 |
33.77 |
|
1958 |
02/25/58 |
68700 |
42.95 |
|
1959 |
02/16/59 |
48900 |
35.98 |
|
1960 |
01/31/60 |
63100 |
40.8 |
|
1961 |
01/31/61 |
33100 |
29.73 |
|
1962 |
02/13/62 |
57400 |
28.91 |
|
1963 |
02/01/63 |
71800 |
43.7 |
|
1964 |
01/21/64 |
33400 |
29.88 |
|
1965 |
12/23/64 |
93400 |
49.6 |
|
1966 |
01/05/66 |
77000 |
45.28 |
|
1967 |
01/21/67 |
68400 |
42.45 |
|
1968 |
01/30/68 |
40600 |
32.22 |
|
1969 |
01/14/69 |
68600 |
42.52 |
|
1970 |
01/24/70 |
72900 |
43.95 |
|
1971 |
12/04/70 |
59800 |
39.33 |
|
1972 |
12/27/71 |
8990 |
14.71 |
|
1973 |
01/12/73 |
62800 |
40.56 |
|
1974 |
01/17/74 |
74000 |
43.18 |
|
1975 |
02/13/75 |
67300 |
37.97 |
|
1976 |
03/01/76 |
5260 |
11.74 |
|
1977 |
03/16/77 |
1370 |
7.5 |
|
1978 |
01/17/78 |
65200 |
40.73 |
|
1979 |
01/11/79 |
26200 |
24.93 |
|
1980 |
01/14/80 |
59700 |
37.46 |
|
1981 |
01/28/81 |
35200 |
27.95 |
|
1982 |
12/20/81 |
67200 |
40.09 |
|
1983 |
01/27/83 |
71900 |
41.63 |
|
1984 |
12/25/83 |
55200 |
35.79 |
|
1985 |
02/08/85 |
28500 |
25.1 |
|
1986 |
02/18/86 |
102000 |
48.56 |
|
1987 |
03/13/87 |
26000 |
24.34 |
|
1988 |
01/04/88 |
35300 |
28.35 |
|
1989 |
03/19/89 |
23800 |
25.42 |
|
1990 |
01/14/90 |
18000 |
20.34 |
|
1991 |
03/04/91 |
48500 |
33.32 |
|
1992 |
02/20/92 |
28000 |
25.29 |
|
1993 |
01/21/93 |
55100 |
37.79 |
|
1994 |
02/20/94 |
14700 |
19.43 |
|
1995 |
01/09/95 |
93900 |
48.01 |
|
1996 |
02/05/96 |
49200 |
35.21 |
|
1997 |
01/01/97 |
82100 |
44.99 |
|
1998 |
02/03/98 |
56100 |
38.66 |
|
1999 |
02/09/99 |
37800 |
31.86 |
|
2000 |
02/14/00 |
37900 |
31.89 |
|
2001 |
03/05/01 |
24700 |
24.15 |
|
2002 |
01/03/02 |
44000 |
33.43 |
|
2003 |
12/16/02 |
57600 |
36.48 |
|
2004 |
02/18/04 |
63400 |
38.17 |
|
2005 |
12/28/04 |
21900 |
22.78 |
|
2006 |
01/01/06 |
86000 |
44.26 |
|
2007 |
02/11/07 |
29800 |
26.42 |
At the extreme, there have only been five recorded floods—since recordkeeping began at that station in 1940—that have exceeded the volume of water recorded during the New Year’s Eve Flood of 2005. These occurred on January 9, 1995; February 18, 1986; December 23, 1964; December 23, 1955; and February 28, 1940.
Some historic floods were more severe than recent floods because we now have a series of reservoirs that can hold water back for later release. They include:
- Santa Rosa Creek Reservoir (a.k.a. Spring Lake)—which retains water diverted into it from Santa Rosa Creek—was completed in 1964.
- Matanzas Creek, Piner (on Paulin Creek), and Brush Creek reservoirs were constructed in the late 1960s.
- The Warm Springs Dam, which retains water in Lake Sonoma, was completed in 1983.
- Since the completion of these flood control projects only two events have exceeded the intensity of the December 31, 2005 event.
Perhaps more informative than superlatives is the periodicity of floods of similar size. I’ve chosen the value of 68,800 ft3/sec (discharge) as one such threshold. This represents all floods at 80% or more of the size of the New Year’s Eve Flood. These have occurred 13 times in the past 69 years (water years 1940, 1943, 1956, 1963, 1965, 1966, 1970, 1974, 1983, 1986, 1995, 1997, 2006), a periodicity of once every 5 years.
I’ve also chosen the value of 77,400 ft3/sec as a threshold. This represents all floods at 90% or more of the size of the New Year’s Eve Flood. These have occurred 7 times in the past 69 years, (water years 1940, 1956, 1965, 1986, 1995, 1997, 2006) a periodicity of once every 10 years.
The periodicity of the New Year’s Eve Flood itself, at the threshold of 86,000 ft3/sec, which has occurred 6 times in the past 69 years (water years 1940, 1956, 1965, 1986, 1995, 2006), is once every 12 years.
