Lecture 14: Floods
Key Questions1. What is an upstream flood?
2. What is a downstream flood?
3. What were the setup conditions that cause floods?
4. What is a 100-year flood?
5. How are 100-year flood discharge magnitudes determined?
6. What is a 100-year flood inundation map?
Niigata Japan, 1964 liquefaction
Chehalis Dec 2007 Flood (Seattle Times)
An aerial view of the flooded I-5 overpass looking south Flooding in Chehalis (December 04, 2007)
Flooding effects about 75 million people per year
Cowlitz River near Packwood, Washington
Floods are the #1 weather‐related killer in the U.S.
A driver attempts to drive through the flood zone on 171st Street in Woodinville. (December 03, 2007)
In the USA, about 50% of the flood related deaths occur in vehicles.
FLOODS are usually caused by heavy rains and/or rapid snow melt—their severity is controlled by the watershed characteristics.
1. Basin size (area)
2. Topography
3. Drainage dentistry (length of streams per area)
4. Vegetation type and distributions
5. Geology
6. Soil type and thickness
7. Runoff processes
Time
hydrograph
Q
On occasion, there is a flood caused by a dam burst
The March 12-13, 1928 disaster that claimed more than 470 lives.
St. Franics Dam failure, Santa Clarita Valley, CA
On occasion, there is a flood caused by a dam burst
The Teton Dam, 44 miles northeast of Idaho Falls in southeastern Idaho, failed abruptly on June 5, 1976. It released nearly 300,000 acre feet of water, then flooded farmland and towns downstream with the eventual loss of 14 lives, and with a cost estimated to be nearly $1 billion.
On occasion, there is a flood caused by a dam burst
On May 31, 1889, failure of the South Fork dam led to a catastrophe in which 2,209 people died—The Johnstown Flood, PA
Water flows into the Green River from the 48-year-old Howard Hanson Dam (earthen dam) which is upriver from Kent, Renton, Auburn, Tukwila, located in South King County.
On occasion, there is a flood caused by a dam burst
Howard Hanson Dam
Water seeping through Howard Hanson Dam. Engineers are concerned the dam could fail, creating catastrophic flooding in the valley below.
On occasion, there is a flood caused by a dam burst
Upstream or Flash Floods
High intensity and long duration rain
Steep terrain
Exposed bedrock or thin soils
bedrock surfaces in mountain settings have low infiltration rates, hence very rapid surface runoff
Q
Time
Hydrograph
very rapid response
Hortonian overland flow on bedrock
Flood Statistics
Up to 15 inches of rain fell with an average of 10 inches over 60 square miles.
Peak flow on Rapid Creek 50,600 cubic feet per second...more than 10 times greater than the previous flood of record.
During the flood, water rose as fast as 3.5 feet in 15 minutes.
238 people killed.
3,000 people injured.
1,335 homes destroyed.
5,000 automobiles destroyed.
$160 million in total damages (1972 dollars, $644 million in 2002 dollars).
15 of the 23 bridges over Rapid Creek were destroyed.
http://sd.water.usgs.gov/projects/1972flood/photos.html
Rapid City South Dakota, 1972
Flash Flood
Debris Flows and Debris Torrents High intensity and long duration rain
Steep terrain
Loose saturated soils and organic matter (logging debris)
Debris flow on Mt. Baker Highway east of Deming, January 8, 2009
Smith Creek Debris Torrents (similar to flash floods)
The Smith Creek watershed is susceptible to debris torrents because of:
Steep terrain (3000 feet of relief in about 2 miles)
Exposed bedrock and a thin soil cover
Clear-cut exposures
1983 Debris Torrent in the Smith Creek Watershed
Down Stream Floods occur in areas of low relief (floodplains)
Nooksack River at Ferndale, Jan 2009
http://courses.washington.edu/uwtoce06/puget%20sound%20watershed.jpg
http://virga.sfsu.edu/pub/jetstream/jetstream_pac/big/0712/07120318_jetstream_pac_anal.gif
Pineapple Express: December 3, 2007
Tons of earth and vegetation washed away from clear-cut hillsides last week and into Stillman Creek, a tributary of the south fork of the Chehalis River.
December 1-3, 2007 Flood
Acres of timber and debris backed up behind this bridge in the Boistfort Valley, which was inundated by the flooding of the south fork of the Chehalis River.
December 1-3, 2007 Flood
New buildings west of I-5 at Chehalis, including a Wal-Mart (upper right), were inundated by water. Some people believe the development exacerbated the flooding.
December 1-3, 2007 Flood
A Home Depot store, above, along Interstate 5 resembles a floating barge in a photograph taken Tuesday
December 1-3, 2007 Flood
Flooded Chehalis a new automobile dealership, at bottom of photo, is being built just off Interstate 5 on an island of fill in the Chehalis River floodplain. Some nearby stores, including a Home Depot and Wal-Mart.
December 1-3, 2007 Flood
An exit sign is seen on the submerged I-5 freeway in Chehalis
December 1-3, 2007 Flood
A closed Interstate 5 is shown running directly into the flooding Chehalis River near Centralia
December 1-3, 2007 Flood
A Centralia neighborhood that runs next to I-5 which is fully submerged at the right.
December 1-3, 2007 Flood
Airplanes were moved to high ground at Chehalis Airport.
December 1-3, 2007 Flood
http://wa.water.usgs.gov/data/realtime/adr/interactive/maps/NooksackSC_basin.pdf
Nooksack River Basin
About 2000 square kilometers or 780 square miles
http://virga.sfsu.edu/pub/jetstream/jetstream_pac/big/0901/09010900_jetstream_pac_anal.gif
Pineapple Express: January 8, 2009
Bellingham had 3.15 inchesMount Baker had 7.90 inches
Rainfall recorded at the Cedarville stream gauge (5.5 inches)
Real-time stream gauges and historical discharge data are the most used tool for mitigating downstream floods.
Peak: 1/7/2009 20:00 hr
Peak: 1/8/2009 18:30 hr
Nooksack River at Nuggents Corner late January 7, 2009
Real-time stream gauges and historical discharge data are the most used tool for mitigating downstream floods.
November, 1990 FloodJanuary, 2009 Flood
Conditions for Setup for the Nooksack River, November 10, 1990 Flood
1. Heavy Rain (Pineapple Express)5 inches of rain in Bellingham in 3 days
14 inches fell in the mountains due to the orographic effect
http://seattlepi.nwsource.com/photos/popup.asp?SubID=2012&page=2>itle=Storm%20of%20December%2C%202006&css=gtitle%2Ecss&pubdate=12/19/2006
soils near saturation produce more runoff
Q
Time
Hydrograph
Conditions for Setup for the Nooksack River, November 10, 1990 Flood
2. In November, soils are nearly saturated
runoff
3. Rain on SnowNovember 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!
Q
Time
Hydrograph
runoff
NOAA Hydrologic Remote Sensing Center
Q
Time
Hydrograph
Q
Time
Hydrograph
snow packno snow pack so rain falls on exposed bedrock and thin, wet soils
more volume but less peaked
Q Q
Conditions for Setup for the Nooksack River, November 10, 1990 Flood
4. High TideHigh tide caused the river mouth to be higher (pushes water back on the floodplain).
5. Storm SurgeStorm 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?
Real-time stream gauges and historical discharge data are the most used tool for mitigating downstream floods.
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
Year cfs Year cfs Year cfs Year cfs
Flood risk questions:
1. When are flood most likely to occur?
Monthly Occurrence of Peak Flows at Ferndale
Highest risk months are Nov, Dec, & Jan
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 flood that has a 1% chance of occurring in any given year
Nooksack River in Whatcom County, Jan 9, 2009
How to determine the discharge of a “100-year flood”
1. Collect the historical peak flows for a river (e.g., Nooksack at Ferndale).
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
Year cfs Year cfs Year cfs Year cfs
2. “Rank” the peak flow discharges from highest to lowest.
Rank cfs
3. Estimate the exceedance probability using the ranked values and the Weibull position formula.
P = m
n + 1m = rank
n = total number of values
in this case “n = 61”
2. “Rank” the peak flow discharges from highest to lowest.
Rank cfs
P = m
n + 1m = rank
n = total number of values
Example: for m = 15
P = 15
61+ 1 = 0.24
The discharge for “m = 15” is 36,000 cfs. This means that in any given year there is a 0.24 probability or a 24% chance of a “peak flow” occurring that will equal or exceed a Q of 36,000 cfs.
4. The exceedance probability can be used to estimate the return period of a certain peak flow (i.e., how often can we expect a certain magnitude flood?)
Return Period = 1P
Example: for m = 15 P = 0.24
The means that one can expect a flood with a peak flow of about 36,000 cfs every 4.13 years.
Return Period = 1
0.24= 4.13 years
where
A 100-year flood is a flood that has a return period of 100 years
OR a probability of 0.01 of occurring in any given year
OR a 1% chance of occurring in any given year
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).
Estimate the Discharge of a “100-year flood”
2. Add a linear trend line to the data and extrapolate out in time.
Estimate the Discharge of a “100-year flood”
3. Extrapolate out in time (100 years) and estimate the discharge.
100-year Floodplain Map
Estimate the stage of a “100-year flood”
0
10000
20000
30000
40000
50000
60000
70000
10 12 14 16 18 20 22 24 26
Gauge Height (feet)
Dis
char
ge (c
fs)
Rating Curve
22.76 feet
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