LAR 512 – Fall 2002
PROBLEM #18 – DRAINAGE BASIN DISCHARGE
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Once again let’s use the Falling Creek Watershed near
Drewrys Bluff, Virginia. Go to the USGS
website http://waterdata.usgs.gov/va/nwis/nwis and proceed through the following steps: (1) go to surface water data, (2) go to
stream flow data, (3) check the site number box and hit the submit button, (4)
put the site number 0238500 in the box and hit the submit button, and (5) then
hit the “Available data for this site” drop down menu and go to “Surface-water:
Peak streamflow”. This will produce a
graph of the highest instantaneous peak flow for each year. Then under “Output formats” hit “Table” to
obtain the numeric instantaneous peak flows for each year.
- Arrange
the discharge rates in a table in ascending order, starting with the
smallest discharge rate. In the
second column, rank the discharge rates from the smallest having a rank of
1, to the largest having a rank of 21.
In the third column, compute a “Recurrence Interval”. Compute the “Recurrence Interval” using
the following formula:
T = 100
(m / n + 1) , where:
T = Recurrence
Interval (years)
n = number of values, in this case 21 values
n + 1 = number of values +1, in this case 22
m = rank number
assigned to the discharge rate
- Plot
the data on a log-log graph with the x-axis being both the “Exceedance Probability”
and the “Recurrence Interval (years)” and the y-axis being “Annual Maximum
Discharge (cfs)”. See Figure 10-13
of Dunne and Leopold. Draw a line
that best fits the data points to create a “Flood Frequency Curve”.
- Now
calculate and plot a 90-percent confidence band, using Table 10-12 of
Dunne and Leopold. Each discharge
rate is multiplied by the appropriate upper 90% coefficient for record of
21 years. You will need to
interpolate between the 20 and 30 year data, as well as, between the appropriate
coefficients (e.g., between 100 and 10 year recurrence interval). Plot the upper points on your log-log
graph and draw a best fit line through these points. Plot the lower points on your graph and
draw a best fit line through these points.
- Now
using the graph you have constructed, determine the discharge rate range
(90% confidence) for a 2-year flood event? __________ maximum cfs and __________ minimum cfs.
- Do
the same for a 10-year flood event?
__________maximum cfs and __________ minimum cfs.
- Do
the same for a 25-year flood event?
__________maximum cfs and __________ minimum cfs.
- Do
the same for a 50-year flood event?
__________maximum cfs and __________ minimum cfs.
- Do
the same for a 100-year flood event?
__________maximum cfs and __________ minimum cfs.