Runoff Analysis of a 100-Year Flood

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Runoff Analysis of a 100-Year Flood

• Creating a Model Using Data from Another Model

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Probing Questions

• Weve looked at the amount of rainfall. Why do
  you think we should follow that with a discussion
  of runoff?
• Where does diffusion fit into this picture and
  why should we be concerned about it?

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Runoff in a 100-Year Storm

• Now you are ready to determine if there is a
  functional relationship between the duration of
  the storm and the amount of runoff that a
  100-Year Storm will produce.

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Runoff Assignment

• To collect your runoff data, you will need to use
  the Estimating Runoff Model that you worked with
  before. You will be using it to determine the
  amount of runoff in 1 hour, 6 hours, 12 hours,
  and 24 hours that a 100-Year Storm will produce
  on three different types of ground cover found in
  a city. For all the cover types, select Poor for
  Hydrologic Condition and A for Soil Condition in
  the model.

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Runoff Assignment Continued

• Use 3 cover types from the list below, selecting
  a different combination of 3 for each watershed.
  You will be using the same two watersheds you
  worked with in the rainfall model.
• If you have time, research each watershed on the
  Internet to locate maps and other types of
  descriptions. This information will allow you to
  determine which of the cover types best describes
  each region.
• Open Urban Areas
• Parking Lots
• Paved Street with Curb
• Paved Street with Open Ditch
• Residential

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Runoff Assignment Continued

• Then follow the same basic procedure that you
  used when comparing the rainfall amounts. Each
  watershed study must include all of the following
  list. It must then be copied and pasted into a
  Word document and turned in or emailed to your
  teacher.

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Runoff Assignment Continued

• 1. Create an Excel spreadsheet for each watershed
  you have been studying, following the sample on
  the next slide.
• a. Collect the data on the amount of runoff
  produced by a 100-Year Storm using the Estimating
  Runoff Model and insert it into Excel.
• b. Average the data from the three cover types
  for each period of time.

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Average Runoff in a 100-Year Flood for a
Watershed with These Cover Types

• Notice that for each period of time, the rainfall
  amount is a constant. The independent variable is
  the type of ground cover.

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Runoff Assignment Continued

• 2. Create a scatterplot graph, following the
  example in the next slide, with
• a. A trendline
• b. A regression equation
• c. An R-squared value (should be between .9 and 1
  to be an accurate model of the data)
• d. The function identified, if there is a
  functional relationship

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Buffalo Bayou Watersheds Runoff Scatterplot,
Trendline and Regression Equation
A Logarithmic Function
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Runoff Assignment Continued

• 3. Briefly analyze the results by doing each of
  the following
• a. Describe the function.
• b. Identify the type of function.
• c. Write it in functional notation.
• d. Describe the transformations of the parent
  function.
• e. Give the domain and range of the graph.
• f. Evaluate the accuracy of the regression
  equation.
• g. Apply the results to the situation/problem.

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Did You Notice A Problem?

• The information on runoff as it applies to
  Tropical Storm Allison is limited. The data we
  have concerning the rainfall amounts has
  durations of 1, 12, and 120 hours, while the
  runoff model will compute only between 1 and 24
  hours. Therefore, the runoff model gave us
  rainfall and runoff amounts for given types of
  cover based on durations of only 2 times that
  were common to our rainfall data on Allison.
  That's not enough to make a reasonable analysis
  of the data.

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A Problem Continued

• However, we can resolve this problem and fill in
  our missing data because we have regression
  equations which we can solve for the unknown
  amounts.
• Lets use our data from the rainfall and runoff
  spreadsheets and paste it into a new spreadsheet
  to help us better see what data we need to
  calculate.

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Assignment

• Open the runoff spreadsheet. Then copy and paste
  into a new spreadsheet the cells giving the of
  hours and the amount of runoff.
• Now open the rainfall spreadsheet and repeat the
  process, using the of hours and the amount of
  rainfall.
• Your spreadsheet should look like the one on the
  following slide.

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Runoff and Rainfall Spreadsheet

• Notice that our times do not match.
• In order to determine if there is a relationship
  between the amount of runoff and the amount of
  rainfall, we must have all our times the same.
• To accomplish this well rearrange our
  spreadsheet. It should then look like the next
  slide.

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Spreadsheet Continued

• You needed to cut and paste the data for 120
  hours and then do the same with the data for 12
  hours. Now it is obvious that we need to use our
  rainfall regression equation to calculate the
  rainfall for 6 hours and 24 hours. We also need
  to use the runoff regression equation to
  calculate the runoff for 120 hours. Complete
  those calculations and add your answers to the
  spreadsheet.

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Creating the Scatterplot

• Now you are ready to create your scatterplot to
  see if there is a functional relationship between
  the amount of runoff and the amount of rainfall.
  As you create your graph, you must decide which
  is the independent variable. Does the amount of
  rainfall depend on the amount of runoff or does
  the amount of runoff depend of the amount of
  rainfall?
• Decide and create your graph. Remember to select
  the independent variable cells first.

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A Linear Function
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Time to Analyze

• When you return to the Allison website, notice
  that the bar graph for each watershed gives the
  rainfall during a 100-Year Storm. It says the
  rainfall is as follows
• 1 hour 5 inches
• 12 hours 10 inches
• 120 hours 17 inches
• How does that compare to our graph? Was it a
  100-Year Storm for the Buffalo Bayou Watershed?
  Why or why not?

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Analysis

• When we ran the runoff model, we used the 100-
  Year Storm as the intensity factor. That means
  that the runoff figures were calculated based on
  that level of intensity of rainfall over each
  specified period of time.
• For example We put in the type of ground cover,
  the hydrologic condition of the soil, the soil
  type, the amount of hours it rained and the
  intensity of a 100-Year Storm. Then the model
  calculated both the amount of rainfall and the
  amount of runoff.

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Analysis Continued

• You now have two set of rainfall data for your
  watersheds.
• Why are the amounts different?
• Which one do you think is more accurate and why?
• In your two watersheds, which do you think had
  the greater impact on water quality as a result
  of diffusion, the amount of runoff or the type of
  ground cover?

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