Flood Frequency and Floodway Limits

Geology 101 –Lab #5

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 cross- sections (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 Y- variable, 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:

n + 1 Ri = 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.

Elevation of Elevation of Elevation of Elevation of Flood Discharge Stream Stream Stream Surface Stream in cubic Surface – Surface - - Valley School Surface - ft/sec Conklin Gage Berkalew Is 5-Mile Pt.

10 year

50 year 100 year

2006 76800

Agnes 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 cross- section 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