Lecture 14: Floods. Key Questions

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Lecture 14: Floods Key Questions 1. 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?

1 Lecture 14: Floods Key Questions 1. 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? Chehalis Dec 2007 Flood (Seattle Times) Niigata Japan, 1964 liquefaction

2 Flooding effects about 75 million people per year An aerial view of the flooded I-5 overpass looking south Flooding in Chehalis (December 04, 2007)

3 Floods are the #1 weather related killer in the U.S. Cowlitz River near Packwood, Washington

4 In the USA, about 50% of the flood related deaths occur in vehicles. A driver attempts to drive through the flood zone on 171st Street in Woodinville. (December 03, 2007)

5 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 hydrograph Q Time

6 On occasion, there is a flood caused by a dam burst St. Franics Dam failure, Santa Clarita Valley, CA The March 12-13, 1928 disaster that claimed more than 470 lives.

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.

7 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, 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.

8 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

9 On occasion, there is a flood caused by a dam burst 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. Howard Hanson Dam

10 On occasion, there is a flood caused by a dam burst Water seeping through Howard Hanson Dam. Engineers are concerned the dam could fail, creating catastrophic flooding in the valley below.

11 Upstream or Flash Floods High intensity and long duration rain Steep terrain Exposed bedrock or thin soils

13 Hydrograph Q very rapid response bedrock surfaces in mountain settings have low infiltration rates, hence very rapid surface runoff Time

14 Hortonian overland flow on bedrock

Flood Statistics Up to 15 inches of rain fell with an average of 10 inches over 60 square miles. Rapid City South Dakota, 1972 Flash Flood Peak flow on Rapid Creek 50,600 cubic feet per second.

15 Flood Statistics Up to 15 inches of rain fell with an average of 10 inches over 60 square miles. Rapid City South Dakota, 1972 Flash Flood 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.

16 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

17 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

18 1983 Debris Torrent in the Smith Creek Watershed

19 Down Stream Floods occur in areas of low relief (floodplains) Nooksack River at Ferndale, Jan 2009

21 Pineapple Express: December 3,

22 December 1-3, 2007 Flood 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.

23 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.

24 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.

25 December 1-3, 2007 Flood A Home Depot store, above, along Interstate 5 resembles a floating barge in a photograph taken Tuesday

26 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.

27 December 1-3, 2007 Flood An exit sign is seen on the submerged I-5 freeway in Chehalis

28 December 1-3, 2007 Flood A closed Interstate 5 is shown running directly into the flooding Chehalis River near Centralia

29 December 1-3, 2007 Flood A Centralia neighborhood that runs next to I-5 which is fully submerged at the right.

30 December 1-3, 2007 Flood Airplanes were moved to high ground at Chehalis Airport.

31 Nooksack River Basin About 2000 square kilometers or 780 square miles

32 Pineapple Express: January 8,

34 Rainfall recorded at the Cedarville stream gauge (5.5 inches) Bellingham had 3.15 inches Mount Baker had 7.90 inches

35 Real-time stream gauges and historical discharge data are the most used tool for mitigating downstream floods.

36 Peak: 1/7/ :00 hr

37 Peak: 1/8/ :30 hr

38 Nooksack River at Nuggents Corner late January 7, 2009

59 Real-time stream gauges and historical discharge data are the most used tool for mitigating downstream floods.

60 November, 1990 Flood

61 November, 1990 Flood January, 2009 Flood

62 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

63 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

64 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

65 NOAA Hydrologic Remote Sensing Center

66 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.

67 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.

68 Nooksack River, November, 1990 Flood

69 Flood risk questions: 1. When are floods most likely to occur?

70 Real-time stream gauges and historical discharge data are the most used tool for mitigating downstream floods.

71 1. Collect the historical peak flows for a river (e.g., Nooksack at Ferndale). Year cfs Year cfs Year cfs Year cfs 10/26/ /27/ /10/ /31/ /1/ /31/ /19/ /4/ /10/ /17/ /30/ /23/ /16/ /8/ /20/ /27/ /31/ /4/ /14/ /26/ /5/ /5/ /31/ /6/ /26/ /17/ /21/ /3/ /18/ /3/ /8/ /15/ /27/ /15/ /11/ /5/ /27/ /25/ /24/ /6/ /16/ /11/ /10/ /24/ /25/ /2/ /20/ /30/ /20/ /30/ /14/ /16/ /21/ /23/

72 Flood risk questions: 1. When are flood most likely to occur? Highest risk months are Nov, Dec, & Jan Monthly Occurrence of Peak Flows at Ferndale

73 Flood risk questions: 1. When are flood most likely to occur? 2. How often do large magnitude floods occur?

74 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

75 How to determine the discharge of a 100-year flood

76 1. Collect the historical peak flows for a river (e.g., Nooksack at Ferndale).

77 1. Collect the historical peak flows for a river (e.g., Nooksack at Ferndale). Year cfs Year cfs Year cfs Year cfs 10/26/ /27/ /10/ /31/ /1/ /31/ /19/ /4/ /10/ /17/ /30/ /23/ /16/ /8/ /20/ /27/ /31/ /4/ /14/ /26/ /5/ /5/ /31/ /6/ /26/ /17/ /21/ /3/ /18/ /3/ /8/ /15/ /27/ /15/ /11/ /5/ /27/ /25/ /24/ /6/ /16/ /11/ /10/ /24/ /25/ /2/ /20/ /30/ /20/ /30/ /14/ /16/ /21/ /23/

78 2. Rank the peak flow discharges from highest to lowest. Rank cfs

79 3. Estimate the exceedance probability using the ranked values and the Weibull position formula. P = m n + 1 m = rank n = total number of values in this case n = 61

80 2. Rank the peak flow discharges from highest to lowest. Rank cfs

81 Example: for m = 15 P = m n + 1 m = rank n = total number of values P = = 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.

82 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 = 1 P Example: for m = 15 where P = 0.24 Return Period = = 4.13 years The means that one can expect a flood with a peak flow of about 36,000 cfs every 4.13 years.

83 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

84 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).

85 Estimate the Discharge of a 100-year flood 2. Add a linear trend line to the data and extrapolate out in time.

86 Estimate the Discharge of a 100-year flood 3. Extrapolate out in time (100 years) and estimate the discharge.

87 100-year Floodplain Map

88 Estimate the stage of a 100-year flood Rating Curve Discharge (cfs) Gauge Height (feet)

89 22.76 feet

February 1996 rain-on-snow flood: looking to the westnorthwest across the Nisqually River delta from I-5. Pat Pringle photo.

February 1996 rain-on-snow flood: looking to the westnorthwest across the Nisqually River delta from I-5. Pat Pringle photo. Big Thompson Canyon, east of Estes Park, Colorado; bouldery deposits of July