Floods are natural phenomena, the results of excess runoff. In fact, the floodplain itself is largely a result of deposition when rivers overtop their banks. Floodplains represent a favorable place...

Floods are natural phenomena, the results of excess runoff. In fact, the floodplain itself is largely a result of deposition when rivers overtop their banks. Floodplains represent a favorable place for humans to settle—they are fertile, level, easy to excavate and near water. These characteristics have contributed to increased utilization and urbanization of floodplains, which all-too-often has led to disaster. Despite huge expenditures for structural flood control by dams, levees, and stream channelization, flood damages increase yearly.
In some cases, solutions to the flood “problem” are based on consideration of alternatives other than construction, such as wise management. The aim is to determine the most beneficial use of the floodplain with a minimum of damage, expense and loss of life. To that end the use of flood data—flood recurrence, flood stage and depth of flooding as well as mapping of flood-prone areas—is important. This information provides a basis for wise planning and management of a floodplain. In this lab we will look at flood frequency in part of the Binghamton area, with particular focus on the 2006 and 2011 floods and their apparent recurrence intervals.

Specific Tasks


1. Conklin, NY

From the data of discharge measurements and corresponding stage heights, construct a rating curve for the Conklin gage station and plot a best-fit line.

Rating Curve Data
available in Excel program on Blackboard site

Peak Flow Data
available in Excel program on Blackboard site
A recurrence interval (RI) curve indicates the average return time of a given discharge event–more accurately, the likelihood of occurrence of an event of at least that magnitude in any given year. It is based on measured maximum annual discharges ranked according to magnitude. The list of yearly high flood discharges is listed by
water year–which runs October to September. These are ranked by magnitude in the Excel spreadsheet. The recurrence interval is calculated from RI = (n + 1)/ m where n = total number of years of record and m = magnitude (rank). From these data construct a graph that will look at the variations in flood magnitude over time and recurrence interval. In Excel, plot the ranked discharges on a logarithmic scale (log discharge on the y axis, log recurrence on the x axis). On the probability paper, plot the same data; you can do this by just plotting some (not all) of the data points so you can define a curve that you think reasonably fits the data (i.e. don’t worry about most of the short recurrence interval data). From the graphs you have drawn, estimate the discharge and stage for the 10, 50, and 100 year floods at the Conklin gage station.
Now examine these results in two ways. First, plot the predicted height of the floods on the river longitudinal profile that is attached. The Conklin gage is given a reference elevation of 841 ft. Although the results are not strictly accurate (why not?), you can plot other flood profiles parallel to a known profile (determined, for example, from field examination of flood marks on building, trees, or grass). The 50-yr flood level is plotted on the long profile; plot the 10 and 100 year predicted flood profiles by first plotting the proper levels at the Conklin station then constructing lines parallel to the 50-yr flood level. Second, outline and shade in the area on the topographic map that corresponds to the 100-yr flood you've determined. Give us a legend. How large an area would the 2006 flood have been predicted to have inundated given this kind of analysis (just show where the floodplain would have been inundated in the immediate vicinity of the Conklin gage for this one)?

2. Vestal, NY

We also have an extensive record from the Vestal gage site. The reference elevation for the gage station at Vestal is about 799.2 feet with an 817-ft. bankfull stage. Using the available information, estimate the 100-yr flood event discharge. Again, plot this in Excel as a rank plot and on the attached probability paper as a probability plot. Also, again for the latter, plot enough points to define the graph; you don’t need to plot everything, since you’re doing it by hand. The largest floods of record, in 1936, 2006 and 2011, crested at peak stages of 30.5, 33.66, and 35.26 feet with discharges estimated at 107,000, 119,000, and 129,000 cfs, respectively. The U.S. Army Corps of Engineers estimated a 100-yr flood of 114,000 cfs, corresponding to a stage of about 30 feet above the gage datum, when they computed the flood frequency back in the 1970s.
After doing your work you will have:

1. The relationship between discharge and flood height for Conklin.


2. A log plot of Discharge over Recurrence Interval for Conklin and Vestal


3. Probability plots of the annual flood recurrence curve (given in paper form) for both Conklin and Vestal.


4. A Conklin flooding extent map with the area of the 100 year flood nicely outlined.

Lab Report

For this lab, we’ll have you write an abstract, presentation of results, and an abbreviated discussion of the results. We provide guidelines below for what should be in the results and discussion.

Abstract

1. 
   Summarize the
   purpose, approach, results, and significance
   of your study in one paragraph.

Introduction

1. What is the aim of this investigation? (Match the aim with your conclusions!)


2. 
   Briefly
   describe flood frequency assessment.

Methodology

1. Describe (again, briefly) the data sources


2. Describe (still briefly) the method to estimate 100-year flood

Results

1. Provide and summarize the results (and graphs) from Conklin and Vestal. Be sure to number the figures and refer to them in the text.

Discussion

The discussion must include (but is not limited to):