Lab 8 - Floods and Flood Hazards

Fall 2023 1 Floods and Flood Hazards Purpose The purpose of this lab is to understand the flood behavior of streams and the purpose of floodplain zoning. These concepts will be illustrated by calculating the frequency of recurrence of various sized floods based on stream gage records and applying this information to planning for future floods. Introduction In the development of this country, people have typically either ignored natural geologic processes because they were not understood, or have tried to force nature to conform to human programs and construction. This approach has frequently led to so-called "natural disasters". Every year, floods result in tremendous property damage and sometimes loss of life. Although most floods cannot be prevented, an understanding of the flooding process can result in planning which can lead to a significant reduction in the detrimental effects to humans. Modern Approach to Flood Mitigation Based on this approach, one method to managing flood hazards is floodplain zoning . Floodplain zoning is based on recognition that: 1) A floodplain was formed by natural stream processes in the first place and will continue to be flooded periodically and, 2) Man’s use of a floodplain should be regulated so that damage from flooding will be minimized or eliminated. Floodplain zoning is the division of a floodplain , or area subject to flooding, into sectors depending on the recurrence interval of different sized floods, as shown in the topographic The above figure shows the floodplain of the river in the center of the image. The dashed lines indicate surface water elevation of floods of different recurrence intervals (i.e., 25-year flood) and sizes. The floodplain has been divided into different zones (A, B, and C) based on the different areas these different sized floods impact. profile below.

Fall 2023 2 In order to reduce hazards from floods, floodplain zoning is a significant factor that is considered when building developments on floodplains. Floodplain Zoning In order to develop floodplain zoning regulations, it is first necessary to know how often different sized floods are likely occur. Generally, progressively larger floods occur with decreasing frequency. The frequency of occurrence of a flood of a given size, or the average number of years between two floods of the same size, is called its recurrence interval . The recurrence interval is determined by past records of a river’s peak discharge each year . Discharge is the volume of water that flows through a river at a given time. Therefore, the peak discharge for any given year indicates the size of the largest flood for that year. For example, a flood of a certain discharge may be found to occur, on average, once in a decade. This is commonly called the "10-year flood". Although past records show a flood of this size occurred about once every ten years, this does not mean that in the future a flood of this size will occur once every ten years, like clockwork. Rather, every year there is a 10% (1 in 10) chance that a flood of this size will occur. Lab Activity One Question 1: For any given year, what is the percent chance that a 25-year flood will occur? [1 point] 50-year flood? [1 point] 100-year flood? [1 point] Question 2: Thinking about probability of the 25, 50, and 100-year floods occurring on the diagram on page 1, which zone should have the greatest building restrictions? [1 point] Although the recurrence interval of different floods are determined by the annual peak discharge record, it is difficult to determine the physical extent of the flood just by the discharge. For each flood of a given recurrence interval and discharge, however, there is an associated gage height , or height of the water surface above a specified point.

Fall 2023 4 The rains pouring down on the eastern slope of Colorado’s Rocky Mountains kept coming, day after day, for an entire week in September. Fueled by tropical moisture drawn north and pinned over the area by a stalled weather pattern, the amount of precipitation between Sept. 9 and 15 in some areas was more than what typically falls in an entire year. The ordinarily staid National Weather Service described it as “biblical.” And weather statistics for this semi-arid region suggest that the odds of such a deluge occurring in this region are about once every thousand years . The resulting floodwaters killed eight people, destroyed more than 1,800 homes and damaged at least 200 miles of roads across 1,918 square miles. On the high plains, more than 48,000 gallons of oil from damaged wells and storage sites washed across the land, mixing in places with runoff from feedlots and sewage plants. The flooding wasn’t unprecedented; an epic event in September 1938 had similar consequences. As in 2013, the ’38 flood was preceded by drought. The contiguous United States has warmed considerably since 1938, and there’s no question that climate change was at play this time, says National Oceanic and Atmospheric Administration scientist Martin Hoerling, who examines links between extreme weather events and climate. Exactly how it did so and to what degree is still a matter of active research. One thing is already clear: A warmer global atmosphere currently holds about 3 to 5 percent more water vapor than it did at the beginning of the 20th century, and that can contribute to heavier precipitation. But during the rains, water vapor over Colorado climbed to a record high — as much as 200 percent above average. Given those numbers, this aspect of climate change likely played a relatively small role in the Colorado disaster. As Hoerling puts it, even without global warming, “nature has a way of delivering. [This article originally appeared in print as "The Colorado Deluge."] Question 3: What is the percent chance that a flood of this magnitude will occur this year? (Hint: the information you need to answer this question is in the article). [1 point] Question 4: When choosing a building site in this location, what things should you consider beforehand? Give at least two answers. [2 points]

Fall 2023 5 Lab Activity Two In this activity, you will use a record of the Dunne River’s annual peak discharge to determine the recurrence interval of each flood. You will then construct a flood frequency graph to use to predict the size of the 25, 50 and 100-year floods of the Dunne River. In order to determine the recurrence interval of each flood complete the following instructions: 1. From the peak discharge data from the Dunne River in the table below, rank each year's peak discharge. The rank of the maximum discharge for all years = 1; the second highest = 2; and so on. [10 points] 2. Calculate the total number of years in the flood record by subtracting the oldest year from the youngest year in the record. 3. Using the ranks and the following equation, determine the recurrence interval for each year's flood. [10 points] 𝑡 = ? + 1 ? Where: t = recurrence interval n = number of years in the flood record m=rank Peak Discharge (CFS) Rank ( m ) Recurrence Interval (t) 1973 13,500 1972 37,000 1 21 1971 18,500 1970 11,100 1969 16,200 1968 13,450 1967 22,900 1966 12,250 1965 10,000 1964 25,500 1963 14,500 1962 9,000 20 1.05 1961 19,400 1960 13,000 1959 10,100 1958 18,800 1957 16,000 1956 10,800 1955 13,410 1954 16,010 Total years: n =

Fall 2023 7 Question 1: Based on your graph on the previous page, what is the approximate discharge ( in CFS ) of the Dunne River 10-year flood? [1 point] 25-year flood? [1 point] 50-year flood? [1 point] 100-year flood? [1 point] Lab Activity Three In this activity, you will use your data from Lab Activity One as well as provided data of Dunne River discharges and the corresponding gage heights to determine the floodplains of the 10, 25, 50 and 100-year floods. The table below shows measured discharges of the Dunne River and the corresponding gage height for each discharge: Gage Height (ft) Discharge (CFS) 21 85,000 20 65,000 17 32,000 16 24,000 15 21,000 14 16,500 13 13,400 12 10,800 11 9,000 10 7,000 9 5,000 8 2,100 7.5 500 Note: The gage height for this river is measured in feet above the river’s bottom. Using this data, plot discharge versus gage height on the graph on the following page and connect your data points with a curve (Note: the y-axis is again in CFS x 1000, but the axes are linear scale). [10 points]

Fall 2023 8

Fall 2023 10 Based on the elevations the Dunne River 10, 25, 50, and 100-year floods flow at, map the floodplains of each of the four floods on the topographic map below. Use a different color for each floodplain and make sure you label which size flood is represented by each color. Include a legend for the map. [8 points] LEGEND

Fall 2023 11 Lab Activity Four In this part of the lab you will act as a city planner to determine the best locations of features for a new development to minimize flood damage. Each zone has a certain price: - 10-year flood: $1,000 - 25-year flood: $10,000 - 50-year flood: $20,000 - 100-year flood: $50,000 - Greater than 100-year flood: $90,000 For each feature, write which floodplain zone (10, 25, 50, or 100-year) you would build on, the price, and why . You may build outside of the floodplain if there is land available. Remember: cities are always looking for ways to save money; it is more likely that your plan will be approved if it is cost-effective. [3 points each] 1. Playground Price _____________ 2. Hospital Price _____________ 3. Elementary school Price _____________ 4. Cemetery Price _____________ 5. Nuclear Power Plant Price _____________ 6. Moto-cross bike park Price _____________ 7. McDonald’s Price _____________ 8. YOUR house Price _____________