Activity 5 PDF
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Texas A&M University *
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Course
101
Subject
Geology
Date
Apr 3, 2024
Type
Pages
7
Uploaded by KidYak4086
GEOL 101: Activity 5—Rivers and Floods 25 points
The goal of this exercise is to show you how to use public tools that can help you determine stream flow, flood recurrence intervals, and flood hazard maps. Part 1: Brazos River discharge and flood recurrence intervals We will use river gauge data maintained by the United States Geological Survey (USGS) for the Brazos River at SH21 near Bryan, Texas. Upload the Brazos River discharge data: 1. Go to: https://waterdata.usgs.gov/tx/nwis/inventory?site_no=08108700 and click on “Annual Statistics.” 2. Click on the box for “discharge, cubic feet per second,” box for “use incomplete data for statistics calculation,” and box for “tab-separated data.” Press submit and download data. 3. Open downloaded data (called “annual”) with notepad or any txt-reading application. Manipulate the data on Excel: 4. Copy and paste the data from notepad into an Excel spreadsheet. Once in excel delete all the data except for the last two columns, i.e., annual peak discharge and year. 5. Sort the annual peak discharge data on the Excel spreadsheet from largest to smallest (descending order), according to the peak annual discharge and assign a rank in the next column ranking the highest with a number 1, then 2 for the next largest, etc. 6. Calculate the recurrence interval of each annual peak discharge using the flood frequency equation, R
=(N+1)/M
, where N
= the number of annual peak stream discharges in the data base, M
is the magnitude rank and R
= the calculated recurrence interval.
7. Calculate the natural logarithm of the recurrence interval (we take the natural logarithm because the recurrence interval varies logarithmically with peak annual discharge) in the next column. 8. Plot the annual peak discharge values versus the natural logarithm of the recurrence interval values, i.e., ln(R), with ln(R) on the X-axis and peak discharge on the Y-axis. a.
Highlight the ln(R) values and annual peak discharge values. b.
Under “Insert” select scatter plot under “charts.” c.
Scatter plot should pop up. d.
Change chart title to "Flood Discharge Graph for Brazos River near Bryan, TX.” e.
X-axis label should be "natural logarithm of recurrence interval." f.
Y-axis label should be "Discharge (cfs)", cubic feet per second. 9. Plot the “best fit” trend line for the relationship between annual peak discharge and ln(R). a.
Click on any of the data points in the chart. b.
Click on the + sign in the upper right corner. c.
Click a check on the boxes for “axes, axis titles, chart title, grid lines, and trend line. d.
Double-click on the trendline and a “format trendline” box should pop up. e.
At the bottom of the “format trendline” box click a check mark on the boxes for “display equation on chart” and display “r-squared value on chart.” Part 2: Tasks to perform and questions to answer. Task 1. (12 pts) Submit your Brazos River spreadsheet with plot as part of this activity. Question #1.
(1 pt) a) What is the r-squared value for your “best fit” line? Does this r-squared value give you confidence in interpolating values in your graph? Using your predictive “best fit”
line, what is the magnitude of the discharge of the 20-year flood for the Brazos River based on the graph created? R^2 = 0.9791 This R^2 value does give us confidence in interpolating values because the R^2 value is close to 1. y=5403.2(Ln(20))-204.22= 15982.32 b) (1pt) In 2016, the 30-year flood occurred at this location in the Brazos River. Why? What happened in 2016? Question #2. (1 pt) The beauty of the best fit line and its equation is that you can forecast what the discharge is for floods for which you have no data (i.e., extrapolation of the best fit line in a region for which you have no peak discharge data). Forecast what the discharge of the 100-year flood would be at this gauging location. Can you think of any uncertainties in trying to forecast the discharge of the 100-year flood? What additional information would help make you more confident in the forecast?
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Question #3.
(1 pt) Assume that flooding of the Brazos River at this location in Bryan can occur when discharge is greater than 10,000 cfs. What is the recurrence interval (R) for this sized flood? Question #4. (1 pt) What are hydrogeologic factors that influence the relationship between discharge and recurrence interval of floods in a region along a river?
Part 3: Flood Hazard Maps Large scale flooding hazards are managed by the Federal Emergency Management Agency (FEMA). N
avigate to www.fema.gov and at the top click “Disasters & Assistance.” What are some current disasters FEMA is helping to handle? N
ow click on “Floods & Maps.” What are some things FEMA handles in this regard? Now we want to look at flood hazard maps to see the extent of flooding in a given area. Flooding events can be quite major and cause both loss of life and extensive damage to property. FEMA produces flood insurance rate maps (FIRMs) that can help homeowners, business owners, and insurance agencies determine flooding risks. These maps can be found on the FEMA page: https://
msc.fema.gov/portal/advanceSearch •
Choose a state, county, and community you are interested in. o
This may be your hometown or somewhere you would like to move one day. •
Click "
Search
" o
Search results will pop up with various files •
Click "
Effective Products
" then "
FIRM panels
" •
Each line is a floodplain map, and all maps for that community show up here •
Rather than downloading, click "view"
•
In this activity we will look specifically at a location in Harris County (Houston) near Galveston Bay. Here is the direct link to the FIRM we will use: https://
map1.msc.fema.gov/firm?id=48201C1080M (Zoom further out or further in if it is hurting your eyes). Use the map and legend to answer the following: 1.
(2 pts) Calculating probabilities of flood risk and the discharge. The most commonly reported flood size is a 100-year flood, also referred to as a __
base flood
_ by the Federal Emergency Management Agency (FEMA). A 100-year flood means there is a 1/100 risk that such a flood will occur in a given year. To calculate %-risk (Annual Exceedance Probability), you then multiply by 100%: e.g. 1 / 100 = 0.01 * 100% = 1% chance of that size flood occurring in a given year. Now you try: a. What is the % risk of a 50-year flood occurring? 1/50 = .02*100= 2%
b. a 5-year flood? 1/5=.2*100=
20%
c. a 500-year flood? 1/500=.002*100=
0.2% 2.
(1pt) What communities are shown on this map? Harris County, City of Houston, City of Pasadena, City of Taylor Lake Village 3.
(1pt) When was this map last revised? January 6, 2017
4.
(1pt) Areas shaded on this map and denoted with “Special flood hazard areas” are those with what annual flood risk percent? __________________ 1% annual chance
5.
(1pt) These areas are labeled with “Zone AE” or “Zone VE.” What do these zones indicate? AE: Base Flood Elevations determined
VE: Coastal flood zone velocity hazard (wave action); Base Flood Elevations determined 6.
Areas of moderate wave action are mapped in locations like this near coastlines. According to FEMA, areas of moderate wave action “mark the inland limit
… where wave
heights can be between 1.5 and 3 feet during a base flood event.” (1pt) How are these areas indicated on this map? These areas are indicated by lines with triangles.
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(1pt) If sea level rise continues, how will this hazard change? If the sea level rises, there will be higher waves. The hazard will increase. 7.
(1pt) How are areas where building is restricted indicated on this map? Why does this area extend further from the river channel in some locations as compared to others? Areas where buildings are restricted are indicated by the dashed line. These areas extend further from the river channel in some locations because some areas of the river are more prone to flooding.