# As a watershed manager working for a county at the base of the Rocky Mountains, you are charged with calculating a flood recurrence interval, so that county residents who live near streams have some understanding of the potential frequency of large flood events

# Geog Lab #6

**Lab #6: Risky Waters: Determining Flood Risk**

**Overview**

All streams generate an average annual flow, but over time, most streams exhibit a wide variation in streamflow from very low stages to large floods. On September 12, 2013, heavy precipitation in Rocky Mountain National Park sent floodwaters roaring down the Big Thompson River and its tributaries (Figure 1). The epic flooding damaged or destroyed the major roadways along with hundreds of homes and businesses in this narrow canyon (Figure 2).

**Figure 1 Big Thompson Rover- Little Thompson River watershed in relation to cities and highways**

Steep slopes and sparse vegetation combined with heavy rain have lead to repeated flooding in the Big Thompson River canyon and communities downstream: A flash flood in 1976 killed 144. In 2013, 8 died as floods caused $2 billion in property damage. The average number of years between floods of a certain size is the recurrence interval. These intervals are based on the magnitude of the annual peak flow, which occurs once a year. However, the actual number of years between floods of any given size varies because of highly variable weather patterns, plus human impacts such as deforestation, urbanization, dam building, and the planting of crops can affect runoff. The goal of this lab is to understand the concept of a stream flood recurrence interval.

Watch the prelab video at https://mediaplayer.pearsoncmg.com/assets/secs-christopherson-gc1-prelab-10

**Figure 2: Deadly topography**

**Application**

As a watershed manager working for a county at the base of the Rocky Mountains, you are charged with calculating a flood recurrence interval, so that county residents who live near streams have some understanding of the potential frequency of large flood events.

**Objectives**

· Outline a drainage basin.

· Calculate a recurrence interval using actual stream data.

· Apply these data to predict flood risk.

**Grading**

Grading for the lab will be as follows: Remember the point score for the questions shows how many sentences or elements should be included in the answer. Answers will be graded on accuracy, completeness, spelling and grammar. As always, make sure your sentences are complete and your paragraphs are well organized.

· Part A Written component 70%

· Part B Online component in Mylab Mastering 20%

· Neatness, spelling and grammar 10%

**Note on Academic Honesty:** The written and online components for this week’s lab are to be done on your own and not copied from any other sources.

With the lab, you must hand on a separate page, the following words “*This work, including the on-line component is entirely my own. I did not copy the whole or any part of my answers to this this lab from anyone else, from a web site, from a book, or from any other source.*” Underneath these words, **type your name and date**.

**Part A (70%)**

*Draw and Calculate the following:*

1. Your first exercise is to outline, with a colored pen or pencil, the drainage basin for the Big Thompson River (Figure 3, p6). Note that in a few places (red marker), some of this has been done for you. Next, mark the following: (12 pts)

a. headwaters

b. interfluve

c. mouth, or location where the river flows onto the Great Plains

2. The next step is to calculate the flood recurrence intervals and rank flood events by their maximum discharge and their recurrence intervals. From this data, it is possible to calculate the probability of the flood recurring. Table 1 (p7) lists nine years of maximum daily streamflow stage (cfs) for the Big Thompson River. (12 pts)

a. In the third column, rank this data by year from the largest amount (rank of 1) to the smallest (rank of 9).

b. In the fourth column, calculate the probability, in percent, of the flood recurring using the formula: rank/( n + 1 ) × 100 ; where n = the number of years with available data

c. In column five, rank the recurrence intervals. This is determined by the following equation: RI = (N + 1 )/M Where: RI = the recurrence interval N is the number of years the floods are on record M is the rank of the flooding event (largest flood M = 1) *Because there are nine years of data, this calculation becomes 9 + 1, divided by the rank number you assigned*

*Analyze and Conclude*

3. In your completed data table, how is the recurrence interval related to the rank and probability of each year’s maximum discharge? (4 pt)

4. Based on your answer to question 3, what maximum discharge might residents along the Big Thompson River typically expect? Explain. (4 pts)

5. Does knowing the recurrence interval enable you to predict floods on the Big Thompson River during the next nine-year period? Explain why or why not. (6 pts)

6. In many areas of the world, accurate steam recurrence interval data now extend over one century. After a record (rank 1 flood) in these areas, the media often refer to it as a “once in a century flood,” or a “100-year flood.” In your job as a watershed manager, does this mean that prospective home and business owners are safe from another event of that magnitude for more than the average human lifetime? Explain. (6 pts)

7. Looking at Figure 3 on the next page, describe the difference in flow characteristics discussed in Section 10.5 (in your text) for stream water flowing in the middle of the canyon and the same water as it flows out of the mountains and across the Great Plains just below the Rocky Mountains. (6 pts)

8. Hypothesize how and why the following landscape changes might influence future floods in the lower sections of the Big Thompson River watershed: (6 pts)

a. More urbanization

b. A large forest fire

c. A series of upstream dams

**Part B (20%)**

Part B of this lab is online and continues to the impact of rivers on the landscape. Log in to Canvas to complete the online portion of this lab in the MyLab Mastering section.

**What to Hand in**

Upload your completed Part A to Canvas by the due date including the completed Table 1 and Figure 3. You may attach digital photos of the marked up map or scan it and email it. Please type out Table 2. Make sure the file(s) name has your name on it and lab number.

Include this honesty statement, date and sign (just type your name).

*This work, including the on-line component is entirely my own. I did not copy the whole or any part of my answers to this this lab from anyone else, from a web site, from a book, or from any other source.*

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