Geology 101 –Lab #5
Flood Frequency and Floodway Limits
Introduction: Zoning of floodplains adjacent to streams is dependent on the use of
historical hydrographic data to calculate the probability of floods of particular
magnitudes. Floodplain zoning places limits on development and land use, determines
land value, and establishes eligibility for insurance. Accurate characterization of
floodplain limits is critical, and to generate useful maps requires statistical analyses of
stream discharge data, determination of floodway extent for floods of a determined
magnitude, and generation of maps that can be used for planning purposes. In this lab we
will develop floodway limits for an area along the Susquehanna River near Binghamton,
New York. This area suffered major flood damage during hurricane Agnes in 1972 and
again during an intense rainfall event in late June and early July of 2006.
Before beginning the lab, examine the topographic maps of the area in question, and
determine the direction of river flow. Also note the location of the Conklin Gaging
Station, Berkelew Island, Moore Island and Five Mile Point, and other features along the
river valley. Note that in ‘river miles’ Five Mile Point is mile 17, Moore Island is Mile
20, and the Kirkwood Bridge is ~ mile 21.5.
Part A:
Examine the two versions of the topographic maps for the Susquehanna River upstream
and downstream of the Conklin gaging station. Locate the three areas for the crosssections (Berkelew Island, Valley School, Five Mile Point).
1. What direction does the Susquehanna River flow in the map area?
How do you know?
2. What is the gradient of the river channel in the map area, in ft/mile?
3. Note that the 1961 map is pre-Agnes; the other map (1988) is post-Agnes. Briefly
describe examples (3) of changes in the river morphology between 1961 and 1988.
a.
b.
c.
Part B: Stage-discharge and recurrence intervals
1. Use the maps attached to the lab handout to draw topographic profiles perpendicular
to the river at the following locations: Berkelew Island (just downstream of the Conkin
Gaging Station), Valley School and Five Mile Point. You will need to highlight the
contour lines and examine contour line trends carefully. It is important that these profiles
are drawn accurately. Note that the contour interval is 10 feet. Use the 900 ft. contour
line as the beginning and ending boundaries of each profile. Use a vertical (elevation)
scale of 1” = 10’.
Determine the river bed elevation at each of the three locations from the longitudinal
channel profile. Show this elevation on each of your topographic cross-sections.
Stage-discharge curve: The data set we will use is available from the USGS website. The
Excel files (Conklin Gage Data; Conklin Flood Discharge) is in the Lab 5 folder on the
desktop of the computers in Lathrop 305 and 401, and in the network folder
(Fileshr01/labsandclasses/geology/geology101)
1. Use the stage discharge data to plot a stage-discharge curve. Note that the elevation
of the gage (the ‘0’ datum) for the Conklin Gaging Station is 840’ MSL. You will find it
helpful to plot the actual elevations of the gage heights when using this plot later on in
the lab. Plot these data as an X-Y plot in Excel. Plot the elevation (gage height) as the Yvariable, and discharge as the X variable. The best-fit line through these data is called a
rating curve. Use the trendline function to establish the equation for the rating curve.
Note that a logarithmic function has a slightly better R2 value than the straight line
function.
What is equation of line for the rating curve?
What is the R2 value?
What is the significance of this value?
Why do you think a logarithmic function fits the data (slightly) better than a straight-line
function?
2. Flood recurrence interval: A recurrence interval curve is used to determine the expect
time recurrence of a given discharge. The Conklin Flood Discharge Excel data file
contains yearly high flood discharges for the years 1913-1905. Sort this data according
to magnitude and calculate the recurrence interval using the formula:
Ri =
n+1
m
Where n = number of years of record; m = magnitude (rank order, 1 = highest).
Construct a plot of recurrence interval vs. discharge for this data. You will probably want
to use a semi-log plot (recurrence interval vs. log of discharge). Plot discharge as the
Using the recurrence interval plot and the rating curve for the Conklin Station, determine
the stage (stream surface elevation at the Conklin Station) for the 10, 50 and 100-year
floods. You may want to use Excel’s trendline function to calculate an equation of line,
and then use that equation to calculate the stage for various flood levels.
Part C: Flood crests and floodway zoning: In addition to data from gaging stations,
the height of flood crests is determined from field observation of flood damage on
buildings, trees and bridge abutments. During the Agnes Flood, the Conklin gage did not
record the maximum stage and discharge, so field observations were used to determine
the Agnes level. The flood profiles for the Susquehanna River in the Conklin reach have
been constructed as shown on the attached longitudinal channel profile. The Agnes
profile can be used to help construct profiles for other flood levels, using the elevation of
the stream surface at the Conklin gaging station as a guide. On the attached longitudinal
profile the Agnes flood level is plotted from field results. Using this profile, determine
the elevation of the river surface at the position of each of your cross-sections – and the
Conklin gaging station – for each of the flood levels (10 year, 50 year, 100 year, 2006,
Agnes). Sketch in these river levels on the longitudinal profile. Also record the data in
the table below.
Flood
Discharge
in cubic
ft/sec
10 year
50 year
100
year
2006
Agnes
76800
Elevation of
Stream
Surface –
Conklin Gage
Elevation of
Stream
Surface Berkalew Is
Elevation of
Stream Surface
- Valley School
Elevation of
Stream
Surface 5-Mile Pt.
For the 2006 flood, the peak discharge recorded was 76800 ft3/sec. What was the
corresponding stage (elevation) for the Conklin gage and the other points downstream?
Fill in the table with the appropriate elevations.
Estimate the recurrence interval for the 2006 flood using your plot of discharge vs.
recurrence interval.
____________years
Estimate the recurrence interval for Agnes.
____________years
3. Plot the flood levels for the 10 and 100 year floods on each of your cross-sections.
Also plot the Agnes and 2006 flood levels. Sketch in the floodway limits for the 10 and
100-year floods on the detailed maps, using the elevation of the flood on your crosssection as a guide to the elevations. This is the methodology used to determine floodway
zones for planning purposes, and eligibility for federal flood insurance programs.
4. Uncle Gilman opened a miniature golf course in area of old gravel pits on the west
side of the Susquehanna just north of Conklin. Describe what happened to his business
during the flood of 2006.
Be sure to hand in:
Data plots (stage-discharge; recurrence interval-discharge)
Longitudinal profile with flood elevations plotted
Topographic cross-sections (3) with flood elevations plotted
Maps (3) with flood limits drawn
Completed table and answers to questions