GEOL 101 Name:
(Part of Lab 9) Flooding in Washington State around Election Day, 2006
and Flood Frequency Curve for the Snoqualmie River
Some rivers flooded on or near election day 2006 in Washington State, especially in western Washington. Follow these instructions and answer the questions for an examination of what some of the main western Washington rivers did around that time.
Check out the online articles and photo gallery from the Seattle Times about the early November 2006 floods, at
http://archives.seattletimes.nwsource.com/cgi-bin/texis.cgi/web/vortex/display?slug=flooding10e&date=20061110&query=Flood+photo
and
http://archives.seattletimes.nwsource.com/cgi-bin/texis.cgi/web/vortex/display?slug=flood08m&date=20061108&query=Flood+gallery+Nov+8.
You can see more flood photos by clicking on the photo gallery links in those articles.
Now, go to the National Weather Service satellite imagery at http://newweb.wrh.noaa.gov/satellite/index.php?wfo=otx and click on
1. Western US
o Infrared
§ 16 km
· Animation
1. For the first week of November, these satellite images showed a huge plume of clouds, moisture, and precipitation, originating in the central, tropical Pacific Ocean, and moving up to the Pacific Northwest.
Why is that type of plume of moisture (darkest/brightest colors), that came into the Pacific Northwest to precipitate the floods of November 2006, known as the “Pineapple Express?”
2. Describe the air mass know as the Pineapple express by circling two of its properties:
3. Moist or Dry
4. Warm or Cold
5. Besides huge amounts of rain, what else is it that the Pineapple Express air does in months like November that adds to the amount of water in Pacific Northwest?
Mount Vernon is a town on I-5 in western Washington, about 60 miles north of Seattle.
Look at real-time data from the Skagit River gaging station near Mount Vernon at
http://waterdata.usgs.gov/wa/nwis/uv?cb_00060=on&cb_00065=on&format=gif_default&period=7&site_no=12200500
6. What is the current (i.e. right now) discharge of the Skagit River?
7. What is the flood gage height for the Skagit River? (The flood gage height is listed in the Web site text details at the top, and marked on the gage height graph as a red line.)
Re-set the graphs to cover October 30-November 15, 2006.
8. What was the highest discharge on the Skagit River during November 2006 and what day was it?
9. What was the highest gage height of the Skagit River in November 2006 and what day was it?
10. Was the Skagit River flooding when it was at its highest November 2006?
§ By about how many feet was it above its banks?
Look at the discharge and stage (gage height) of the Snoqualmie River east of Seattle, at Carnation, real-time data online at
http://waterdata.usgs.gov/wa/nwis/uv/?site_no=12149000&PARAmeter_cd=00060,00065
Re-set the graphs to cover October 30-November 15, 2006.
11. Did the Snoqualmie River flood in November 2006?
What day(s)?
How high above flood gage height was it at its maximum in November 2006?
What was the maximum discharge on the Snoqualmie River in November 2006?
Look at real-time data from the Wenatchee River gaging station near Peshastin at
http://waterdata.usgs.gov/wa/nwis/uv/?site_no=12459000&PARAmeter_cd=00060,00065
12. What is the current (right now) discharge of the Wenatchee River?
13. What is the flood gage height of the Wenatchee River?
14. What is the gage height of the Wenatchee River right now?
Re-set the Wenatchee River graphs to cover October 30-November 15, 2006.
15. What was the highest discharge in November 2006?
16. What was the gage height of the Wenatchee River when it was at its highest discharge in November 2006?
17. From the answers to the previous two questions, did the Wenatchee River flood in November 2006?
18. If the Wenatchee River did flood November 2006, was it just a few inches above its banks, or several feet, or how much?
Now conduct a flood frequency analysis for the Snoqualmie River near Carnation (USGS gaging station 12149000) using the data given online at
http://nwis.waterdata.usgs.gov/nwis/peak?site_no=12149000&agency_cd=USGS&format=html
and using the spreadsheet program Excel (required).
It may be easier to get to the URL listed above by going to http://waterdata.usgs.gov/nwis/uv?12149000 and then going to the links labeled “Summary of additional data for this site”/”Peak Streamflow”/”Output Format...Table.”
Copy the complete table, in two blocks, into your Excel spreadsheet, so that there are only four columns with all the data and no empty rows in the data.
Follow the instructor as you label and format your spreadsheet, sort the data, calculate the recurrence intervals of annual peak streamflows, create a flood frequency curve for the Snoqualmie River near Carnation, and fit a line to the data to estimate flood recurrences of up to 100 years.
For a given number of years of recorded annual peak stream discharge (n is the number of years), the recurrence interval (RI, in years), for a discharge of a given rank (m) is
RI = (n+1)/m
The rank of a discharge (m) begins with the greatest discharge in the years of record being assigned rank 1, and then the second-biggest discharge is then rank 2, and so on down through all the years of annual peak discharge in the record.
You will see on the graph that the top five discharges that have been recorded on the Snoqualmie River seem to follow a trend of their own, separate from the lesser discharges.
These top five discharges were all floods, suggesting that the behavior of the Snoqualmie River when it is flooding beyond its banks is different from what controls its behavior when it is confined within its banks and not flooding.
On that basis, the line for extrapolating flood discharges will be fit only to the trend of the five higher discharge values.
As you finish making your Flood Frequency Chart (as an XY Scatter chart), it is
EXTREMELY important to save it as a SEPARATE SHEET.
Answer the following questions:
Remember to use your line that you fit to your graph to get the recurrence intervals you need.
1. What is the “annual peak streamflow?”
2. Are all annual peak streamflows floods?
3. In reference to floods, what is recurrence interval?
4. What are the odds of a 100-year flood happening in a given year?
5. If a flood or peak discharge has a recurrence interval of 100 years, does that mean that the next 100-year flood will happen 100 years after the last one?
6. What rank (number) is assigned to the greatest peak discharge in a stream discharge record?
7. Using the flood frequency curve you plotted for the assignment, estimate the discharge of the 50-year and 100-year floods on the Snoqualmie River near Carnation.
50-year:
100-year:
8. How high above flood stage would the 50-year flood be at that location?
9. How high above flood stage would the 100-year flood be?
10. Is it abnormal for a river to flood?