Lecture 19: Down-Stream Floods and the “100-Year” Flood Key Questions 1. What is a downstream flood? 2. What were the setup conditions that caused the Nov, 1990 Nooksack flood? 3. What is a 100-year flood? 4. How are 100-year flood discharge magnitudes determined? 5. What is a 100-year flood inundation map? 6. Mitigation techniques Niigata Japan, 1964 liquefaction Chehalis Dec 2007 Flood (Seattle Times)
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Lecture 19: Down-Stream Floods and the “100-Year” FloodLecture 19: Down-Stream Floods and the “100-Year” Flood . Key Questions. 1. What is a downstream flood? 2. What were
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Lecture 19: Down-Stream Floods and the “100-Year” Flood
Key Questions1. What is a downstream flood?
2. What were the setup conditions that caused the Nov, 1990 Nooksack flood?
3. What is a 100-year flood?
4. How are 100-year flood discharge magnitudes determined?
5. What is a 100-year flood inundation map?
6. Mitigation techniques
Niigata Japan, 1964 liquefaction
Chehalis Dec 2007 Flood (Seattle Times)
Down Stream Floods occur in areas of low relief (floodplains)
1. Collect the historical peak flows for a river (e.g., Nooksack at Ferndale).
10/26/1945 41600
11/27/1949 27500
2/10/1951 55000
1/31/1952 18300
2/1/1953 19300
10/31/1953 18500
11/19/1954 20700
11/4/1955 35000
12/10/1956 23000
1/17/1958 18300
4/30/1959 30200
11/23/1959 22000
1/16/1961 30800
1/8/1962 18800
11/20/1962 26000
11/27/1963 23300
1/31/1965 20000
12/4/1965 17500
12/14/1966 21400
12/26/1967 23900
1/5/1969 28100
11/5/1969 17300
1/31/1971 38100
3/6/1972 24800
12/26/1972 24800
1/17/1974 21800
12/21/1974 20800
12/3/1975 46700
1/18/1977 20600
12/3/1977 23900
11/8/1978 18800
12/15/1979 36400
12/27/1980 29700
2/15/1982 27200
1/11/1983 34200
1/5/1984 41500
4/27/1985 16300
2/25/1986 29900
11/24/1986 36000
4/6/1988 17700
10/16/1988 21000
11/11/1989 47800
11/10/1990 57000
1/24/1992 18100
1/25/1993 19000
3/2/1994 18500
12/20/1994 21700
11/30/1995 47200
3/20/1997 38100
10/30/1997 17600
12/14/1998 24600
12/16/1999 22200
10/21/2000 14300
2/23/2002 30300
1/26/2003 20100
10/21/2003 39900
11/25/2004 42300
1/10/2006 19500
11/7/2006 38100
12/4/2007 21100
Year cfs Year cfs Year cfs Year cfs
2. “Rank” the peak flow discharges from highest to lowest.
1 57000
2 55000
3 47800
4 47200
5 46700
6 42300
7 41600
8 41500
9 39900
10 38100
11 38100
12 38100
13 36400
. .
. .
. .57 17300
58 16300
59 14300
Rank cfs
3. Estimate the exceedance probability “P” using the ranked values and the Weibull position formula.
P = m
n + 1m = rank
n = total number of values
in this case “n = 60”
P = m
n + 1m = rank
n = total number of values
Example: for m = 12
P = 12
59 + 1 = 0.20
The discharge for “m = 12” is 38,100 cfs. This means that in any given year there is a 0.20 probability or a 20% chance of “peak flow” occurring that will equal or exceed a Q of 38,100 cfs.
4. The exceedance probability can be used to estimate the return period of a certain peak flow.
Return Period = 1P
Example: for m = 12 P = 0.20
The means that one can expect flood with a peak flow of about 38,100 cfs every 5 years.
Return Period = 1
0.20= 5 years
where
Nooksack at Ferndale - Peak Flow
Date Water YearPeak Flow Q
(cfs) RANK #Peak Discharge
(cfs) Exceedence
ProbabilityReturn Period
(years)
10/26/1945 1946 41600 1 57000 0.02 59.00
11/27/1949 1950 27500 2 55000 0.03 29.50
2/10/1951 1951 55000 3 47800 0.05 19.67
1/31/1952 1952 18300 4 47200 0.07 14.75
2/1/1953 1953 19300 5 46700 0.08 11.80
10/31/1953 1954 18500 6 42300 0.10 9.83
11/19/1954 1955 20700 7 41600 0.12 8.43
11/4/1955 1956 35000 8 41500 0.14 7.38
12/10/1956 1957 23000 9 39900 0.15 6.56
1/17/1958 1958 18300 10 38100 0.17 5.90
4/30/1959 1959 30200 11 38100 0.19 5.36
11/23/1959 1960 22000 12 38100 0.20 4.92
1/16/1961 1961 30800 13 36400 0.22 4.54
1/8/1962 1962 18800 14 36000 0.24 4.21
11/20/1962 1963 26000 15 35000 0.25 3.93
11/27/1963 1964 23300 16 34200 0.27 3.69
1/31/1965 1965 20000 17 30800 0.29 3.47
12/4/1965 1966 17500 18 30300 0.31 3.28
12/14/1966 1967 21400 19 30200 0.32 3.11
12/26/1967 1968 23900 20 29900 0.34 2.95
1/5/1969 1969 28100 21 29700 0.36 2.81
A 100-year flood is a flood that has a return period of 100 years
Return Period = 100 years
Estimate the Discharge of a “100-year flood”
1. Plot all the peak flows on the vertical axis (arithmetic scale) versus their respective return periods on the horizontal axis (log10 scale).
0
10000
20000
30000
40000
50000
60000
70000
80000
1 10 100
Return Period (years)
Peak
Dis
char
ge (c
fs)
Estimate the Discharge of a “100-year flood”
2. Add a linear trend line to the data and extrapolate out in time.
0
10000
20000
30000
40000
50000
60000
70000
80000
1 10 100
Return Period (years)
Peak
Dis
char
ge (c
fs)
Estimate the Discharge of a “100-year flood”
3. Extrapolate out in time (100 years) and estimate the discharge.