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.

Before beginning the lab, examine the topographic map of the area in question, and determine the direction of river flow. Also note the location of Moore Island and Five Mile Point, and other features along the river valley. It will be helpful for you to mark off on your paper copy of the map the ‘river miles’ along the centerline of the river, noting that Five Mile Point is mile 17, Moore Island is Mile 20, and the Kirkwood Bridge is ~ mile 21.5.

Stage-discharge curve: The data set we will use is available from the USGS website (http://water.usgs.gov/). The Excel files (Conklin Gage Data; Conklin Flood Discharge) in the lab #4 folder contain the pertinent data for the zoning study, but you should become familiar with this site and the data available there. Use the stage and 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.

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 1913-1967. Sort this data according to magnitude and calculate the recurrence interval using the formula:

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). Using the recurrence interval plot and the stage discharge 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 linear regression (calculated line of best fit) function to calculate the slope and intercept, and then use those values to calculate the stage for various flood levels.

Flood crest positions: The height of flood crests is determined from field observation of flood damage on buildings, trees and bridge abutments. From this data, flood profiles for the Susquehanna River in the Conklin reach have been constructed as shown on the attached graphic. These profiles can be used to construct profiles for other flood levels, using the elevation of the stream surface at the Conklin gaging station as a guide. On the attached graph the Agnes flood level is plotted from field results, and the profile of the 50 year flood is also shown. Using these profiles, determine the elevation of the river surface at each of the mile markers – and the Conklin gaging station – for each of the flood levels (50 year, Agnes). You may find it useful to construct a table to record this information in a systematic fashion.

Floodplain Zoning Map: The goal of this project is to produce a floodplain zoning map which establishes the extent of expected inundation for the 50 flood, and also shows the extent of the 1973 Agnes floodway. The map files in the Lab 4 folder on the network include the Binghamton East 7.5 minute Quad as a .tif file and a file labeled "Floodway Base Map", which is also a .tif file. You can use either of these as a basemap in ArcMap. The full 7.5 minute file includes UTM co-ordinates, whereas the other file does not. You can use either as the base to create the Floodway Zoning map for this lab, but note that the full .tif file takes a while to load from a Zip disc. In a real circumstance, you would use the full .tif file, and ‘trim’ the file to the area of interest.

1. Use the paper copy of the map and your table of flood level elevations to sketch in the approximate boundaries of each flood zone (50-year, Agnes).

2. Copy the needed image files from the server to your Zip disk. Open ArcMap and add one of the image files as a theme in the view.

3. In ArcCatalogue, add a new data file as shown in the demo. This will be a polygon file. You need not specify the coordinate system since you will be making the new coverage feature on top of the image file layers.

4. In ArcMap, add the new file using add data. This layer should be above the image file data. Enlarge the view to an appropriate scale. You will want to be able to clearly see the contour lines adjacent to the river.

5. Make sure your new coverage feature is selected, and go to the editing toolbox and select begin editing.

6. Use the drawing tool to begin drawing in the polygon which will define the limits of the floodway for the flood level of interest (e.g. Agnes). You will find it easier to draw in shapes successively and use the merge function to link them into a single shape. You can also reshape polygons as you wish using the reshape feature function. Note that a transparent symbol is easier to work with as you are drawing and editing, so that the contour lines under the feature are visible.

7. When you have finished with a polygon, select ‘stop editing’ under the theme menu, and save the theme. Note that until you ‘stop editing’ the polygon can be re-positioned relative to the underlying map. Once you stop editing, the polygon is now fixed in the coordinate space of the underlying coverage.

8. Repeat the same procedures to produce a second feature for the remaining floodway zone. You will find it best the ‘turn off’ the features you are not editing so you can clearly examine the base map.

7. Use the base map and flood zones to produce a map layout of the area.

1. What factors might cause these floodway zones to change over time?

2. Uncle Gilman has proposed opening a miniature golf course business from Packwood, Oregon to the area of old gravel pits on the west side of the Susquehanna just north of Conklin. What would you advise him in terms of the likelihood of flooding in that area?