Assgn10-Rivers
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Geology
Date
Jan 9, 2024
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docx
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Uploaded by Anthonyreyes1232
GEOL 111G
Learning Assignment 10
Streams, Flooding & Recurrence Intervals
1.
By reading the trendline on the graph, determine the peak discharge expected in a flood
with a recurrence interval of 50 and 100 years. Go to 50 on the x-axis, follow the line until
it intersects the trendline, then to to that point on the y-axis to read the streamflow. Do the
same for the 100-year peak discharge. These are the discharges expected in a 50-year
flood and a 100-year flood.
50-year peak discharge: 31500ft3/sec
100-year peak discharge: 37500 ft3/sec
2.
The annual exceedance probability, P, is the probability that a given discharge will occur
in a given year. It is calculated as the inverse of the recurrence interval, R:
P = 1 / R
Thus, the probability that a flood with a 10-year occurrence interval will occur in any year
is 1/10 = 0.1 or 10%
(0.1 x 100). Using the graph, determine the probabilities that a 50-
year flood and a 100-year flood will occur in any given year.
50-year flood probability (%): 2%
100-year flood probability (%): 1%
3.
The mean annual discharge can be calculated by summing the peak annual discharges
and dividing by the number of years. What is the mean annual discharge for the San
Francisco River? Please show all your work. If doing this within Excel, mark the mean
annual discharge on your spreadsheet in
BOLD GREEN
, to 1 decimal place. On your
spreadsheet select the data in the streamflow column to the first blank at the bottom of
the column. You can either pick out the average from the Sum symbol on Excel or use
the formula bar: =AVERAGE(B3:B32)
Average is 5545.7
GEOL 111G
Learning Assignment 10
Streams, Flooding & Recurrence Intervals
4.
Flood stage, or bankfull stage on the San Francisco river occurs at a discharge of 15000
ft
3
/sec. Looking at your graph, find the flood stage on the y-axis, go to the trendline and
then down to the x-axis and read the number. What is the recurrence interval of such a
discharge according to your graph?
A. 7
5.
Someone has offered to sell you a 4-bedroom, 2-story house with a 2 car garage and
swimming pool on a 1 acre lot on a relatively flat piece of land on the banks of the San
Francisco river for what seems like a great price of $175,000. The last time the house
was flooded it cost $150,000 to repair the flood damage.
a.
Using the data from the graph and the recurrence interval determine how often
this land will flood in a 25-year period.
i.
The recurrence interval will be 2.2
b.
What is the probability that the land and house will be flooded in the first year
that you own it?
i.
P=1/1.5= .66 x 100= 66%
ii.
There is a probability that the land and house will be flooded 66% within
the first year of buying it.
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