Rivers and Flooding Lab Alexi Lindsay

GEOL121 Name Alexi Lindsay Lab Section 16 Rivers and Flooding Lab Learning Objectives After completing this lab, you should be able to: 1) Design an experiment to learn more about river morphology and behavior. 2) Calculate the recurrence interval and probability of a given discharge. 3) Read and interpret a flood plain map to assess flood risk. Introduction to Rivers and Flooding Humans have inhabited areas adjacent to rivers for millennia. Traditionally people have settled along floodplains because of the availability of fertile soils for agricultural purposes. As population increases, and urban areas grow, development along flood plains changes the way rain and snow infiltrate the ground, and runoff flows across the surface. Habitation next to rivers also puts people at risk for flood hazards. Floods are the third deadliest natural hazard in the United States, with only tornados and lightning strikes ranking higher. In this lab, you will be evaluating river forms and processes, and flood potential in Fort Collins and on the CSU campus using discharge data to calculate flood recurrence intervals and examining floodplain maps. We live in the Cache la Poudre drainage basin, with the Cache la Poudre River flowing from its headwaters in Rocky Mountain National Park, down Poudre Canyon and through Fort Collins. The Poudre River joins the South Platte River east of Greeley, Colorado, and the South Platte River joins the North Platte River in Nebraska, to form the Platte River, which eventually flows into the Missouri River, and finally the Mississippi River. Channel planform describes the two-dimensional pattern of the river channel as viewed on a map or an aerial photograph. The planform of a river reflects sediment available to be transported and stored, and how the energy of the flowing water interacts with the channel boundaries. Many types of channel planform exist, but the two most common planforms are meandering and braided rivers . The Poudre River is a meandering river through Fort Collins, though it has a straight planform where it is confined by rock walls in Poudre Canyon. Many boaters and fishermen are interested in the amount of water flowing within the Poudre River in order to evaluate recreational uses. We talk about the volume of water moving down a river over a specified time interval as discharge ( Q ) which is calculated using the Continuity Equation as follows: Q = w * d * v or Q=vA , where w is average width of flow (m), d is average flow depth (m), and v is average velocity (m/s), and A is channel cross sectional area (m 2 ). The units of discharge are m 3 /s or ft 3 /s.

To describe the characteristics of floods, we discuss the flood magnitude (discharge or size of the flood, in ft 3 /s), the duration (how long it lasted), and how common or rare it is ( recurrence interval and annual probability ). Flood recurrence intervals ( RI ), describe the average number of years between successive floods of a given size. The recurrence interval is a useful way to describe how often we might expect a flood of at least a certain size. For example, a 100-year flood is a flood of a given size that happens once in 100 years on average. The probability of a flood event of a given discharge occurring during a particular year is called the annual probability . Annual probability is related to recurrence interval by the following equation: P=1/RI where P = probability (between 0-1), and RI = recurrence interval A low P or probability (or percentage) indicates that the flood will occur less frequently (high magnitude floods are more rare) and a high P or probability indicates that it will occur frequently (low magnitude floods are more common). As you can imagine, this information is important for engineering purposes, as well as zoning for urban development along rivers. For example, many engineered structures such as culverts or bridges are typically constructed to handle the 100-year discharge for the river they span. The probability of the 100-year event occurring during any given year is 1/100 or 0.01 or 1% (low probability of occurrence). Additionally, recurrence intervals are used to develop floodplain zoning maps in towns to inform homeowners of flood risks and the need to purchase flood insurance. Increasing development within urban areas may change the magnitude, duration, and probability of flooding. As the density of houses and impervious roadways continues to increase in Fort Collins, think about how this might change the flood hazard. Urbanization played a large role in the destructiveness of the 1997 Spring Creek flood.

Part 2: Discharge Variability and Flooding Recurrence Intervals Variability in the discharge of a river through time contributes to the form and characteristics of that river. Flooding constitutes increases in discharge beyond what the banks of a river can handle. The figure below shows daily discharge measurements in 2021 for the Cache la Poudre River gaging station in Old Town. The gaging station is operated by the US Geological Survey, and can be accessed via the web at: http://waterdata.usgs.gov/nwis/uv?06752260 1) Describe the changes in discharge of the Poudre River through the year. When was discharge the highest? Lowest? What factors might contribute to the variability you observe? The moment in time when the discharge was the highest was in the months of June, July, and August

during the summer period and its lowest was during September/October. Factors that might contribute to the variability might be amount of rain during those periods.

Date Annual Peak Dis- charge (Q) (ft 3 /s) Rank (m) (years) Recurrence Interval (RI) 6/18/95 3500 8 5.13 6/16/96 2670 18 2.28 6/9/97 3290 12 3.41 6/4/98 959 33 1.24 4/30/99 7710 2 20.50 5/6/00 785 36 1.14 5/30/01 713 40 1.03 5/31/02 751 38 1.08 5/30/03 2100 21 1.95 7/1/04 997 31 1.32 6/4/05 1720 26 1.58 10/30/05 1210 29 1.41 6/13/07 933 34 1.20 6/7/08 1830 23 1.78 6/22/09 1790 25 1.64 6/12/10 4570 6 6.83 6/9/11 3220 13 3.15 7/30/12 764 37 1.11 9/13/13 8140 1 41 5/31/14 5860 4 10.25 3) Plot the RI vs. the Discharge values from the table above on the semi-log graph paper provided on the next page. Logarithms are used to show several orders of magnitude of data. Draw a best-fit line to your data. Here is a resource explaining how to plot points: https://serc.carleton.edu/mathyouneed/graphing/plotting.html And this page describes how to draw a best fit line: https://serc.carleton.edu/mathyouneed/graphing/bestfit.html

Part 3: Examining Flood Maps Obtain the Fort Collins Flood Risk Map . You can also view a GIS version of this map at: https:// www.fcgov.com/utilities/what-we-do/stormwater/flooding/floodplain-maps-documents 1) What are the flood risk categories shown on the map legend? What are the flood recurrence intervals associated with each flood risk designation? The flood risk categories shown on the map legends seem to be all around the poudre river, the flood recurrence intervals are high, medium, floodway, 100 year, 100/500 year. 2) Reflecting on the RI calculations and discharge data in Part 1, why do you think the 500- year floodplain is wider than the other floodplains? I think the 500 year floodplain is a lot wider than the others as 500 years is a long time away and during those years there could be a significantly “good chance’ of a flood occurring in the city of Fort Collins. 3) Find W. Elizabeth St. on the map, just west of Shields. Why do you think Elizabeth Street is marked as a flood zone on the map? Elizabeth street is marked as a flood zone as it’s connected to a couple local canals in the area. 4) What happens when the water from W. Elizabeth enters campus? Does it get blocked like the map may suggest? When the water from Elizabeth enters campus, it runs right through into Moby. 5) Examine the CSU Floodplain Map. Where are the most flood-prone places on campus? Are they the areas you would have expected? Why or why not? The most flood prone places on campus is the lagoon pound outside the LSC, I would definitely expect that area to be prone as it’s a flat surface area and it already has a body of water next to it.

6) Locate your place of residence on the floodplain map. (If you don’t live on the map, pick a location you go to frequently.) Do you live in a flood zone? If so, what is the risk? If you live on campus, do think that your dorm is susceptible to flood waters? In which RI floodplain do you live? I don’t live in a flood zone, I live in a low risk area. 7) Locate the intersection of N. Lemay and E.Vine Dr. If you were to buy property here, would it be in a floodplain? If so, do you think flood insurance is required? What are some of the risks you may assume as a homeowner in a floodplain? It would be in a moderate flood plain, I wouldn’t say flood insurance is required even though it is a flatter surface of land and the poudre is nearby but I wouldn’t see it as a high risk. 8) If you lived on a floodplain with a 500-year RI, would you buy flood insurance? What is the probability that the flood would occur in a given year, and damage your house? Based on this probability, if a flood did occur while you lived there, would this be an acceptable risk to you? I would definitely buy flood insurance if I lived in a 500-year RI, the probability may not be the next year but it’s the build up that I would be more worried about, I lowkey don’t mind the flood, house is a house just need insurance and take high precautions before flood happens. 9) The flood recurrence intervals that you see on these maps are determined using data from stream gauges, just as you did in Part 2. Are there any reasons you can think of why the likelihood of floods in the future may be different than the likelihood determined from past data? Our climate continues to change and so does the precipitation of rain and other occurrences, past data has been relatively accurate to predict future occurrences but it’s not 100 % guaranteed.