GEO 109 Hmwk 6 Followup to Flood Frequency exercise.
Name:
Available: Feb. 26. Read all of the instructions. Print out and fill in data table and graph.
Due: March 6 in class or in my office
Introduction
The population in the Puget Sound is growing rapidly and humans have made many changes to rivers and
drainages. This homework follows up on the flood frequency exercise we did in class. Here you will
examine a river (Mercer Creek) near the Green River, and see the changes it has undergone over time.
Gauges placed along a stream monitor the discharge (volume of water in the stream) at several locations.
From the peak annual flood data, the recurrence interval is calculated and plotted on a flood frequency
graph. The line on a flood frequency graph allows geologists to estimate the average number of years that
will elapse until a flood of a particular magnitude reoccurs. Flood frequency graphs are used in flood
prediction. The 100-year flood, as determined by this type of flood frequency analysis, serves as a legal
definition of areas that are likely to be flooded. If someone chooses to purchase a home in the 100-year
floodplain, they must obtain flood insurance provided by the federal government.
1.
For each dataset provided for Mercer Creek, rank the peak flood discharge in order of magnitude,
starting with 1 for the largest and ending with 11 for the smallest. CONSIDER THE 1957-1967 DATA
SEPARATELY FROM THE 1979-1989 DATA. Write these results in the “Rank” column of the table.
2.
Use the formula (T= (n+1)/m) to determine the recurrence interval of each of the 11 floods in each of
the two data sets. N is the number of years of record (here n=11). M is the rank of the flood. The
results should be recorded in the “Recurrence Interval” column of the table.
3.
For the 1957-1967 data, plot the discharge and recurrence interval for each of the 11 floods. Use the
graph paper provided. Using a ruler, draw a best-fit straight line through the data points. (If you do not
know how to draw a best-fit line, ask your instructor.) The line should be extended all the way to the
right side edge of the graph.
4.
Now plot the discharge and recurrence interval for each of the 11 floods from 1979-1989 on the same
graph paper. Using a ruler, draw a best-fit straight line for this data. The line should be extended all
the way to the right side edge of the graph.
Questions
Grading: 4 points for calculations of rank and recurrence interval
5 points for the graph
1. (3) Based on your data, what is the predicted discharge for a 100-year flood? To find this
information, you must read the value from your graph where it intersects the 100 yr
recurrence interval line.
Data Sets
Predicted discharge for a 100-year flood
Mercer Creek – Data Set 1 (1957-1967)
Mercer Creek – Data Set 2 (1979-1989)
345
1220
2. (3) How do your predictions for the river for the two time periods compare to each other.
Remember you looking at the for the same river ,just two different times. Describe it in words.
The two time periods look totally different. The 1957-1967 has a lot less 100 year flood
predicted discharge then the 1979-1989. The period in the 80’s increases its predicated 100 year
discharge for a flood about 4 times.
3. (3) Suggest possible human activities in the watershed that could have caused the differences in
predicted floods that result from the two sets of data for your river.
Human activities that might have cause the watershed to act this way are things like global
warming. Global warming can change weather behavior and result in more waterfall. Also the area
might have water diverted from other areas that increase the amount of water the river has in it.
4. (2) What information do you need to know if you are about to buy a house that is located adjacent
to, but just outside the 100-year floodplain?
I would want to know what is causing such a high increase in the flood basin because in
another decade or so the house I bought might be in the floodplain. I would also want to know what
they are doing about the increase.
5. (2) It is possible to calculate the probability (or chance) that the annual maximum flood will
equal or exceed a given discharge within any single year. This is called the annual probability of
excedence, P, and it is the reciprocal of T (the recurrence interval). Written as a formula:
P
1
T
Use the formula to calculate the following:
What is the probability in any given year that the stream discharge will exceed the discharge of the
100-year flood recurrence interval?
The probability would be 1/100 that the discharge of the flood will exceed the predicated
100 year flood discharge in a given year.
Year
1957
Mercer Creek – Data Set 1
Peak
Rank (1 =
RecurFlood
greatest)
rence
Discharge
interval
180
9
1.33
Mercer Creek – Data Set 2
Peak
Rank (1 = Recurrence
Flood
greatest)
Interval
Discharge
1979
518
5
2.4
Yea
r
1958
238
2
6
1980
414
7
1.71
1959
220
4
3
1981
670
2
6
1960
210
5
2.4
1982
612
4
3
1961
192
7
1.71
1983
404
8
1.5
1962
168
10
1.2
1984
353
9
1.33
1963
150
11
1.09
1985
832
1
12
1964
224
3
4
1986
504
6
2
1965
193
6
2
1987
331
10
1.2
1966
187
8
1.5
1988
228
11
1.09
1967
254
1
12
1989
664
3
4