Reference no: EM132864858

CEE 4554 Natural Disasters

The objective of this homework is to familiarize you with the computation of flood frequency analysis and to introduce you to topics related to FIRMs (Flood Insurance Rate Maps).

Question 1. Data interpretation using scatter plots snd Weibull recurrence interval plots.

In 1972 torrential rains developed over the Black Hills, producing a record flow on Rapid Creek and flooding Rapid City. 238 people died as a result of the flood. The 1972 storm and resulting flood was a Black Swan.

• Go to: USGS Water Data for the Nation obtain instantaneous peak annual streamflow data for the gaging station RAPID CR AT RAPID CITY SD (Station No. 440509103143101) for the full period of record, 1905 on. These are the maximum flows in cubic ft per second (cfs) measured in each year.
• Copy-paste the table into a spreadsheet. Alternatively, you can download the data as a tab-separated file and open it in a spreadsheet.
a. Scatter plot. Create a scatter plot to get a feel for the data with years along the bottom and flows in cfs on the vertical axis. Label axes and add a title.
b. Weibull recurrence interval plot. We will graph the data using a very simple Weibull Rank approach to see what we can discern about flood predictions at this location.
• Copy the data into another pair of columns. Sort the data on peak flow (and sort the years along with the flows), putting largest flow at the top. Add a column with sequential numbers for rank m: 1 for the largest flow, then 2, 3, 4, etc. The smallest peak flow (at the bottom) will have rank equal to the number N of measurements.
• In the next column compute the quantity R = (N+1)/m. This is the Weibull plotting position equation, which is sometimes used for flood frequency analysis, and which we will use here to estimate the recurrence interval for each peak flow.
• R in the equation above is the estimated recurrence interval for the flow with rank m. In the Weibull Rank approach, for a given number N of measurements (years), the maximum peak flow has recurrence interval equal to N+1 in years (for the max flow, rank m = 1).
• Plot Peak Flow (vertical axis) against Recurrence Interval (horizontal axis). Use logarithmic scales on both axes. Add a title and label the axes.
c. Estimate peak flow for the 100-year flood on the Weibull plot (use a power law trend line).
d. Estimate the recurrence interval for the 1972 flood (use a power law trend line).
• Are you confident in your answers to (c) and (d) using the Weibull approach for Rapid Creek? Why / Why not?

Question 2. Rapid City Flood - Flood frequency analysis using Log Pearson Type III Methodology

The Log Pearson Type III statistical distribution has gained wide acceptance for modeling peak stream discharge versus frequency and is recommended by the US Water Resources Council Bulletin #17B for flood-frequency analysis. The Log-Pearson Type III method for statistical analysis of flood data applies to just about any series of natural floods. Extensive experience with the Log Pearson Type III (LPT3) methodology has shown it to be a reliable tool for flood frequency predictions. Here we will apply it to the Rapid City data set.

Develop a spreadsheet for applying the LPT3 methodology to the Rapid City data. Step-by-step instructions for doing this are given in the handout: Log Pearson Type III Distribution and Procedure. A more detailed explanation is given in the original reference: Bulletin #17B for flood-frequency analysis.
• Cleaning the data. There is a gap in the data from 1906 until 1943. The simplest way to deal with this is to delete the 1905 and 1906 rows and consider the dataset as starting in 1943. Do this. Then apply the house cleaning described in the handout (Step 2). Because of the difference between the "water year" and the calendar year, some years there are two readings, with no readings the following year. For example, 1955 has two readings and 1956 has none. To deal with this discrepancy, reassign e.g. the second 1955 reading to 1956. You should end up with 1 data point per year, beginning in 1943. This is the dataset we will work with.
• Skew value. One of the parameters for the LPT3 distribution is the skewness. Normally the skew is calculated (for 25 ≤ N ≤ 100) as a weighted combination of station skew Cs and map skew Cm (see handout). For this exercise we will take the skew value as the station skew (ignoring the contribution of map skew). The station skew is calculated from the data.
• Frequency factor table (K-values). When using the K-factor table, it is usually best not to interpolate. Just use the Cs value in the left column closest to your station skew.
a. Complete the Table below for Recurrence Intervals (Return Periods) as given

b. Plot Peak Flow (CFS) (vertical axis) against Recurrence Interval (yr) (horizontal axis).
Use logarithmic scales on both axes. Add a title and label the axes
c. Estimate return period for the 1972 flood. Show graphically on the plot, and label the result.

Discuss any differences between your result in (c) and the return period estimated from the Weibull plotting position formula. In which result do you have more confidence? Why?

Question 3. FIRMS - Special Hazard Zones

We now consider flood hazard for a proposed industrial facility for the chemical company MOPITUP Inc (which manufactures cleaning agents). The proposed location (square marked X on the map in Figure 1) is immediately north of the intersection of 2000 Omaha St (at the intersection of Omaha St and Mountain View Rd) in Rapid City S. Dakota.

A FIRM (Flood Insurance Rate Map) is the official map of FEMA special hazard areas and risk premium zones. FIRMs can be purchased from the FEMA Map Services Center Web site. On FIRMs, Special Flood Hazard Areas and high risk zones (zones starting with the letters A or V) are darkly shaded; low to moderate risk zones (starting with the letters B, C, or X) are lightly shaded or not shaded at all.

We will only look at maps that become part of the FIRM. To create this map for the proposed MOPITUP location, go to FEMA's Map Service Center and enter the address in the Search bar (as shown in Figure 2). When you enter an address, it is usually best to omit St. Av, N, S, etc.

a. Create a flood map roughly centered on location X (Mtn View/Omaha). Annotate the map to show the location of the proposed facility.

b. Is X within a SPECIAL FLOOD HAZARD AREA INUNDATED BY 100-YEAR FLOOD? If so, what is the FIRM zone designation? [note that the Base Flood is the 100-year flood]

c. Are there any flood-related problems with the proposed location? Explain.

d. Explain what is a floodway and indicate whether the proposed location is in a floodway. If not, are there any potential dangers related to the location with respect to the floodway?

What is your recommendation concerning the location proposed for the industrial facility, considering what you have learned about the site and the history of Rapid City SD? Put your recommendations in your submittal memo, as if they were directed to the president of MOPITUP, inc.

Attachment:- Natural Disasters.rar