Lecture 19: Down-Stream Floods and the 100-Year Flood

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1 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

2 Down Stream Floods occur in areas of low relief (floodplains)

3

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

5 PNW FLOODS are caused by heavy rains and/or rapid snow melt their severity is controlled by the WATERSHED characteristics.

6 November, 1990 Flood

7 November, 1990 Flood January, 2009 Flood

8 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

9 Pineapple Express: December 3,

10 Pineapple Express: January 8,

11 Orographic Effect water vapor condenses when it cools which produces rain clouds cool when they expand 70% of the Nooksack basin is steep terrain.

12 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

13 Conditions for Setup for the Nooksack River, November 10, 1990 Flood 3. Rain on Snow John Scurlock

14 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

15 SNOTEL stations are used to monitor snow depth SNOTEL = SNOwpack TELemetry

16 NOAA Hydrologic Remote Sensing Center

17 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

18 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.

19 Nooksack River, November, 1990 Flood

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

21 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/ /14/ /26/ /5/ /5/ /31/ /6/ /26/ /27/ /25/ /24/ /6/ /16/ /11/ /10/ /26/ /21/ /25/ /10/ /7/ /4/ /4/ /17/ /24/ /10/ /21/ /25/ /17/ /3/ /2/ /30/ /18/ /20/ /23/ /3/ /30/ /16/ /8/ /20/ /8/ /15/ /30/ /20/ /27/ /14/ /27/ /15/ /16/ /31/ /11/ /21/ /4/ /5/ /23/

22 Flood risk questions: 1. When are flood most likely to occur? Monthly Occurrence of Peak Flows at Ferndale

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

24 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)

25 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

26 100-year flood risks are determined using historical data and statistical techniques

27 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/ /14/ /26/ /5/ /5/ /31/ /6/ /26/ /27/ /25/ /24/ /6/ /16/ /11/ /10/ /26/ /21/ /25/ /10/ /7/ /4/ /4/ /17/ /24/ /10/ /21/ /25/ /17/ /3/ /2/ /30/ /18/ /20/ /23/ /3/ /30/ /16/ /8/ /20/ /8/ /15/ /30/ /20/ /27/ /14/ /27/ /15/ /16/ /31/ /11/ /21/ /4/ /5/ /23/

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

29 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

30 Example: for m = 12 P = m n + 1 m = rank n = total number of values P = = 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.

31 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 = = 5 years The means that one can expect flood with a peak flow of about 38,100 cfs every 5 years.

32 Nooksack at Ferndale - Peak Flow Date Water Year Peak Flow Q (cfs) RANK # Peak Discharge (cfs) Exceedence Probability Return Period (years) 10/26/ /27/ /10/ /31/ /1/ /31/ /19/ /4/ /10/ /17/ /30/ /23/ /16/ /8/ /20/ /27/ /31/ /4/ /14/ /26/ /5/

33 A 100-year flood is a flood that has a return period of 100 years Return Period = 100 years

34 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) Peak Discharge (cfs) Return Period (years)

35 Estimate the Discharge of a 100-year flood 2. Add a linear trend line to the data and extrapolate out in time Peak Discharge (cfs) Return Period (years)

36 Estimate the Discharge of a 100-year flood 3. Extrapolate out in time (100 years) and estimate the discharge Peak Discharge (cfs) Return Period (years)

37 100-year Floodplain Map

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

39 22.76 feet

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41 Mitigation: Structural Techniques 1. Levees are engineered embankments designed to contain the river

42 Mitigation: Structural Techniques Levees are typically design to contain moderately sized floods levee

43 Mitigation: Structural Techniques 2. Dams can store and slowly release water storage capacity Δ = monitored release

44 Mitigation: Levee Setbacks or Removal Allow the river to flood naturally in certain areas.

45 Mitigation: Land Purchases

46 Mitigation: Insurance

47 Mitigation: Emergency Response

48 Mitigation: Forecast Modeling

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