Flooding Exercises Assignment
Name:______________________________
Period:______
Flood frequency information can be determined from knowledge of the peak discharge (highest
discharge) in any given year; provided enough years of information has been collected. This allows one
to relate the expected recurrence interval for a given discharge, and determine the probability that a flood
of a given discharge will occur in any given year. The recurrence interval for a given discharge can be
calculated by first ranking the discharges.
You will be calculating the recurrence interval for flooding on Cazenovia Creek at Ebenezer, NY
(in West Seneca) (see map below). To do this, you need to open the Excel spreadsheet; Caz Creek
Discharge Info. (Your instructor will show you how to open that file.)
Cazenovia Creek Water Gage at Ebenezer, NY (in West Seneca)
Hydrologic Unit Code: 04120103
Latitude: 42°49'47", Longitude: 78°46'31" NAD27
Drainage area: 135 square miles
Gage datum: 604.86 feet above sea level NGVD29
Broad View
Close-Up View
http://waterdata.usgs.gov/ny/nwis/nwismap/?site_no=04215500&agency_cd=USGS&waswidth=0.0234375&zoom=-2
Directions: Excluding the graphing questions (#’s 1-3), answer the questions below on the
Answer Sheet on page 5. You will submit ONLY your completed graph and answer sheet.
1.) Fill in the Rank column. To do this, enter a 1 for the maximum discharge that has occurred during the
65 years of available data. The second highest discharge will be given a rank of 2, etc. with the lowest
discharge given a value of 65. (3 points)
http://nwis.waterdata.usgs.gov/ny/nwis/peak?site_no=04215500&agency_cd=USGS&format=html
JPKreation revision:
Flooding Exercises Assignment: http://www.tulane.edu/~sanelson/geol204/hmwk_5_l06.htm; extracted on 2/17/07
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Flooding Exercises Assignment
2.) After you have filled in the Rank column, you can now calculate the recurrence interval for each peak
discharge. The recurrence interval, R, is given by the Weibull Equation: R = (n+1)/m; where
n is the number of years over which the data was collected (65 years in this case) and m is the rank of
each peak discharge. Use this equation to calculate the recurrence interval for each peak discharge. (5
points)
3.) Next, create and LABEL a graph of discharge (on the x-axis) versus recurrence interval (on the yaxis). Adjust your x-axis to extend to 25,000. Please make the y-axis a logarithmic scale, extending to
1000 years. Once you have plotted the points, insert a “best-fit” logarithmic line through the data
points. BEFORE you print out your completed graph, please ask your instructor if your graph is correct.
(10 points)
4.) By extrapolating your line on the graph, determine the peak discharge expected in a flood with a
recurrence interval of 100 years and 1000 years. What are the discharges expected in a 100 year flood
and a 1000 year flood. (2 points)
5.) From learning about floods in your text and lecture notes, is it possible that discharges associated with
a 100 year flood could occur 5 years apart, 10 years apart, or even one year apart? (1 point)
6.) The annual exceedence probability, Pe, is the probability that a given discharge will occur in a given
year. It is calculated as the inverse of the recurrence interval, R: Pe = 1/R
Thus, the probability that a flood with a ten year recurrence interval will occur in any year is 1/10
= 0.1 or 10%. What is the probability that a 100 year flood will occur in any given year? What is the
probability that a 1000 year flood will occur in any given year? (2 points)
7.) The mean annual peak discharge can be calculated by summing the peak annual discharges and
dividing by the number of years. What is the mean annual discharge for Cazenovia Creek? (1 point)
8.) Flood stage, or bankfull stage on Cazenovia Creek (at this location) occurs at a discharge of 5870
ft3/sec, MODERATE flooding occurs at a discharge of 7800 ft3/sec, and MAJOR flooding occurs at a
discharge of 10,200 ft3/sec.What is the recurrence interval for ALL three flooding events, according to
your graph? (3 points)
9.) There are two homes that interest you, for sale on Orchard Park Rd. in West Seneca. House #1’s
owner has offered to sell you a 4-bedroom, 2-story house (2 baths) with a 2 car garage and swimming
pool on a 1 acre lot (see map on next page) for what seems like a reasonable price of $155,000.
However, this home is located in 100-year floodplain (a “high-risk” area for flooding). The last time
the house flooded (in 1963) it cost $9,000 (≈$50,000 today!) to repair the flood damage.
a. Using the map on the next page, what is the minimum probability that house #1 will be
flooded in the first year that you own it? (Hint: See #6 above for help.) (2 points)
b. What is the minimum probability that house #1 will be flooded at least once in the first
TEN years that you own it? (2 points)
c. The current annual premium for National Flood Insurance is $370. If your mortgage
company required you to buy Flood Insurance for a 30-year mortgage, how much extra
money would you be paying over the life-time of the loan (30 years) just in flood
insurance? (2 points)
JPKreation revision:
Flooding Exercises Assignment: http://www.tulane.edu/~sanelson/geol204/hmwk_5_l06.htm; extracted on 2/17/07
2
Flooding Exercises Assignment
d. The current average payment for a $50,000 disaster home loan is $311 a month (for 15
years). If your house sustained $50,000 dollars, and you didn’t have flood insurance, how
much money would you pay for the entire loan period of 15 years?
e. Would you still consider buying the house? Why or why not? (5 points)
USGS Water Station
New
Development
House #1
House #2
Mr. K’s
former house
s
Key:
(http://gis1.erie.gov/website/erieny/Viewer.htm)
10.) The 2nd house owner has offered to sell you a similar 4-bedroom, 2-story house (2 baths) with a 2 car
garage and swimming pool on a 1 acre lot (see map on next page) for a price of $175,000. Although it
is not located in 100-year flood plain, it is in a “medium-risk” area:
a. Would you consider buying the house? Why or why not? (5 points)
JPKreation revision:
Flooding Exercises Assignment: http://www.tulane.edu/~sanelson/geol204/hmwk_5_l06.htm; extracted on 2/17/07
3
Flooding Exercises Assignment
Did you know that about 25% of the National Flood Insurance Program’s claims come from
outside high-risk flood areas? (http://www.pueblo.gsa.gov/cic_text/housing/natl-flood/insurance.htm)
b. After learning about the fact above, would you still consider buying the house? Why or
why not? (5 points)
c. If you did buy the house, would you purchase National Flood Insurance for it? (5 points)
11.) Using the same map, look at the location of my house.
a. Would you advise that I buy National Flood Insurance? Why or why not? (5 points)
b. If you owned the home, would you purchase flood insurance? Why or why not? (5 points)
12.) Using the Erie County Internet Mapping System, (located at:
http://gis1.erie.gov/website/erieny/Viewer.htm) find the location of your house.
a. Is your house located in a 100 year flood plain? (5 points)
b. If your house was (or is), would you advise that your parents/ guardians buy National
Flood Insurance? Why or why not? (5 points)
JPKreation revision:
Flooding Exercises Assignment: http://www.tulane.edu/~sanelson/geol204/hmwk_5_l06.htm; extracted on 2/17/07
4
Flooding Exercises Assignment
Name:_____________________
Period:_____
Answer Sheet
Question # and Answer
Graph Points
Possible
Points
15
Points
Earned
4.) 100 year discharge (ft3/sec) =
2
1000 year discharge (ft3/sec) =
1
5.)
6.) 100 year flood possibility =
2
1000 year flood possibility =
7.) Mean annual peak discharge =
8.) a. Recurrence interval for flood stage =
b. Recurrence interval for MODERATE flooding =
c. Recurrence interval for MAJOR flooding =
9.) a.
1
3
2
b.
2
c.
2
d.
2
e.
10.) a.
5
b.
15
c.
11.) a.
10
b.
12.) a.
10
b.
Total Possible Points
72
JPKreation revision:
Flooding Exercises Assignment: http://www.tulane.edu/~sanelson/geol204/hmwk_5_l06.htm; extracted on 2/17/07
5