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 Chehalis Dec 2007 Flood (Seattle Times) Niigata Japan, 1964 liquefaction
Down Stream Floods occur in areas of low relief (floodplains)
http://courses.washington.edu/uwtoce06/puget%20sound%20watershed.jpg
Nooksack River Basin http://wa.water.usgs.gov/data/realtime/adr/interactive/maps/nooksacksc_basin.pdf About 2000 square kilometers or 780 square miles
PNW FLOODS are caused by heavy rains and/or rapid snow melt their severity is controlled by the WATERSHED characteristics.
November, 1990 Flood
November, 1990 Flood January, 2009 Flood
Conditions for Setup for the Nooksack River, November 10, 1990 Flood 1. Heavy Rain 5 inches of rain in Bellingham in 3 days 14 inches fell in the mountains http://seattlepi.nwsource.com/photos/popup.asp?subid=2012&page=2>itle=storm%20of%20december%2c%202006&css=gtitle%2ecss&pubdate=12/19/2006
Pineapple Express: December 3, 2007 http://virga.sfsu.edu/pub/jetstream/jetstream_pac/big/0712/07120318_jetstream_pac_anal.gif
Pineapple Express: January 8, 2009 http://virga.sfsu.edu/pub/jetstream/jetstream_pac/big/0901/09010900_jetstream_pac_anal.gif
Orographic Effect water vapor condenses when it cools which produces rain clouds cool when they expand 70% of the Nooksack basin is steep terrain.
Conditions for Setup for the Nooksack River, November 10, 1990 Flood 2. In November, soils are nearly saturated soils near saturation produce more runoff Hydrograph Q runoff Time
Conditions for Setup for the Nooksack River, November 10, 1990 Flood 3. Rain on Snow John Scurlock
3. Rain on Snow November snow packs are relatively warm. Warm rains release heat into the snowapck causing some snow to melt. Snowmelt produced an additional 2 inches of runoff! Hydrograph Q runoff Time
SNOTEL stations are used to monitor snow depth SNOTEL = SNOwpack TELemetry
NOAA Hydrologic Remote Sensing Center
Q no snow pack so rain falls on exposed bedrock and thin, wet soils Q snow pack Hydrograph Hydrograph Q more volume but less peaked Q Time Time
Conditions for Setup for the Nooksack River, November 10, 1990 Flood 4. High Tide High tide caused the river mouth to be higher (pushes water back on the floodplain). 5. Storm Surge Storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm. http://en.wikipedia.org/wiki/image:surge-en.svg
Nooksack River, November, 1990 Flood
Flood risk questions: 1. When are floods most likely to occur?
Collect the historical peak flows for a river (e.g., Nooksack at Ferndale). Year cfs Year cfs Year cfs Year cfs 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 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 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 1/26/2003 20100 10/21/2003 39900 11/25/2004 42300 1/10/2006 19500 11/7/2006 38100 12/4/2007 21100 11/4/1955 35000 1/17/1974 21800 1/24/1992 18100 12/10/1956 23000 12/21/1974 20800 1/25/1993 19000 1/17/1958 18300 12/3/1975 46700 3/2/1994 18500 4/30/1959 30200 1/18/1977 20600 12/20/1994 21700 11/23/1959 22000 12/3/1977 23900 11/30/1995 47200 1/16/1961 30800 11/8/1978 18800 3/20/1997 38100 1/8/1962 18800 12/15/1979 36400 10/30/1997 17600 11/20/1962 26000 12/27/1980 29700 12/14/1998 24600 11/27/1963 23300 2/15/1982 27200 12/16/1999 22200 1/31/1965 20000 1/11/1983 34200 10/21/2000 14300 12/4/1965 17500 1/5/1984 41500 2/23/2002 30300
Flood risk questions: 1. When are flood most likely to occur? Monthly Occurrence of Peak Flows at Ferndale
Flood risk questions: 1. When are flood most likely to occur? 2. How often do large magnitude floods occur?
A 100-year flood is a large magnitude flood that has a 1% chance occurring in any given year Chehalis Flood, Dec 2007 (Seattle Tiimes photo)
A 100-year flood inundation map is one of the primary mitigation tools that is used to lower the risk of being impacted by a downstream flood
100-year flood risks are determined using historical data and statistical techniques
1. Collect the historical peak flows for a river (e.g., Nooksack at Ferndale). Year cfs Year cfs Year cfs Year cfs 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 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 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 1/26/2003 20100 10/21/2003 39900 11/25/2004 42300 1/10/2006 19500 11/7/2006 38100 12/4/2007 21100 11/4/1955 35000 1/17/1974 21800 1/24/1992 18100 12/10/1956 23000 12/21/1974 20800 1/25/1993 19000 1/17/1958 18300 12/3/1975 46700 3/2/1994 18500 4/30/1959 30200 1/18/1977 20600 12/20/1994 21700 11/23/1959 22000 12/3/1977 23900 11/30/1995 47200 1/16/1961 30800 11/8/1978 18800 3/20/1997 38100 1/8/1962 18800 12/15/1979 36400 10/30/1997 17600 11/20/1962 26000 12/27/1980 29700 12/14/1998 24600 11/27/1963 23300 2/15/1982 27200 12/16/1999 22200 1/31/1965 20000 1/11/1983 34200 10/21/2000 14300 12/4/1965 17500 1/5/1984 41500 2/23/2002 30300
2. Rank the peak flow discharges from highest to lowest. Rank 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.... cfs 57. 17300. 58 16300 59 14300
3. Estimate the exceedance probability P using the ranked values and the Weibull position formula. P = m n + 1 m = rank n = total number of values in this case n = 60
Example: for m = 12 P = m n + 1 m = rank n = total number of values 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 = 1 P Example: for m = 12 where P = 0.20 Return Period = 1 0.20 = 5 years The means that one can expect flood with a peak flow of about 38,100 cfs every 5 years.
Nooksack at Ferndale - Peak Flow Date Water Year Peak Flow Q (cfs) RANK # Peak Discharge (cfs) Exceedence Probability Return 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 (log 10 scale). 80000 70000 Peak Discharge (cfs) 60000 50000 40000 30000 20000 10000 0 1 10 100 Return Period (years)
Estimate the Discharge of a 100-year flood 2. Add a linear trend line to the data and extrapolate out in time. 80000 70000 Peak Discharge (cfs) 60000 50000 40000 30000 20000 10000 0 1 10 100 Return Period (years)
Estimate the Discharge of a 100-year flood 3. Extrapolate out in time (100 years) and estimate the discharge. 80000 70000 Peak Discharge (cfs) 60000 50000 40000 30000 20000 10000 0 1 10 100 Return Period (years)
100-year Floodplain Map
Estimate the stage of a 100-year flood 70000 60000 Rating Curve Discharge (cfs) 50000 40000 30000 20000 10000 0 10 12 14 16 18 20 22 24 26 Gauge Height (feet)
22.76 feet
Mitigation: Structural Techniques 1. Levees are engineered embankments designed to contain the river
Mitigation: Structural Techniques Levees are typically design to contain moderately sized floods levee
Mitigation: Structural Techniques 2. Dams can store and slowly release water storage capacity Δ = monitored release
Mitigation: Levee Setbacks or Removal Allow the river to flood naturally in certain areas. www.skagitcoop.org/.../
Mitigation: Land Purchases
Mitigation: Insurance
Mitigation: Emergency Response
Mitigation: Forecast Modeling