Lab 3 Instructions

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Geography

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Apr 3, 2024

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RIVERS LAB 3: FLOOD FREQUENCY AND FLOODPLAIN MAPPING IN THE WILLAMETTE VALLEY INTRODUCTION Flooding is and always has been a natural process by which streams remove excess runoff produced by heavy rains. The bank full flood is a common event on most streams; on average, it occurs once a year. Floodplains are the safety valves of rivers where excess water flows during periods of excessive runoff. During a flood a stream may spread laterally and inundate large areas of land with relatively shallow water. Defining the limits of the floodplain is not always a straightforward task. Some prefer to define it on a purely topographic manner (i.e., the distinction between valley slopes and valley floor). Others define it as having hydrologic boundaries and designate the floodplain as the area inundated by a certain event with a given recurrence interval. In this exercise we will see how the hydrologic boundaries of a floodplain can be determined and mapped. This kind of information can then be provided to planning commissions and other groups concerned with making floodplain zoning policies. In this exercise we will use available hydrologic data to assess variations in discharge and flood hazards for the Willamette River at Albany, OR. Study a few maps of the City of Albany at the link below. The floodplain map, historical flood maps, and urban maps will be useful to look over. Pay special attention to the contour intervals, tributary junctions, urbanization, buildings, and road locations. Make a mental note of the nature of the Willamette River through the city and where you would expect the floodplain boundaries to be. https://www.cityofalbany.net/gis/print-maps Good resource with guidelines & information for determining flood frequency: https://acwi.gov/hydrology/Frequency/B17bFAQ.html
RIVERS LAB 3: FLOOD FREQUENCY AND FLOODPLAIN MAPPING IN THE WILLAMETTE VALLEY A. Hydrograph and Rating Curve 1. On the tab labeled “hydrograph” of the Lab 3 data excel workbook you will find discharge data from January 1900 through the present. Create a “smooth lined” scatter plot of the hydrograph (Discharge vs. Time). Format the x-axis to show the dates in ‘month-year’ format. 2. On the tab labeled “Rating Curve” of your excel workbook you will find discharge and stage data from the 1880s through the present decade. Create a rating curve by first adding a “scatter” plot of the discharge (x-axis) and stage (y-axis). Then add a line of best fit. Right click on the data points themselves and select “add trendline” and change the color of the line to make it stand out. Use “logarithmic” and display the equation of the line of best fit and your R 2 value on your graph. You will use this equation in a later part of the activity. **Always include a title, label the axes, (and a legend if you are graphing multiple sets of data). Make sure that your axes are scaled appropriately. B. Flood Frequency Analysis On the tab labeled “Flood Frequency” of your excel workbook you will find a list of the maximum annual discharge at the Albany stream gage from 1861 through the present decade. Note that the “annual” values of discharge are reported for a “water year” rather than a calendar year. The water year begins on October 1 and ends on September 30, and hence encompasses an entire wet season as a single time span. Use this data to prepare a flood frequency curve in the following manner: 1. Use the “sort” function in the data tab in excel to sort the maximum discharge of each year from highest to lowest. Make sure that you sort all columns together, not just 1 individual column. Rank the discharges in the ‘Ranking’ column by assigning relative magnitudes (1=highest discharge and in the case of this dataset, 128= lowest discharge value) ( Hint: look up how to use the ‘RANK’ function ). 2. Determine the “recurrence interval” of each maximum flow according to the following formula: R n m 1 where n = number of years in record & m = flood magnitude ranking *n is determined by the amount of years you have data for, not the amount of years between your first and last data point. For example , if your data set spanned 1900-1999 but you were missing data for 1950 then n=99. If you had data for every year between 1990-1999 n=100. 3. Graph the results by creating a “scatter” scatter plot with recurrence interval (RI) on your X-axis and discharge as your Y-axis. Add a “logarithmic” trend line for the
RIVERS LAB 3: FLOOD FREQUENCY AND FLOODPLAIN MAPPING IN THE WILLAMETTE VALLEY data and display the equation for the line of best fit and your R 2 value on your graph. C. Flood Recurrence and Associated Discharges 1. Extrapolate your flood frequency curve to include the 1000-year flood event. Do this by increasing the maximum value of your X-axis to at least 1000. Format your trendline to extend to a corresponding length by double clocking it and adjusting the forecast under trendline options. Using the equation for the line of best fit displayed on your graph, determine the discharges expected for the 10, 100, 500, and 1000-year events. Enter your results in the discharge column associated with each flood event in the table located on the “Flood Frequency” tab. 2. Now go back to your rating curve from part A and use the equation for the line of best fit displayed on your graph to determine the stage that would be associated with each of these discharges. Enter your results in the same table located on the “Flood Frequency” tab. 3. The gauge heights are given in feet above the gage datum elevation. For Albany, the gage datum elevation is 167.18 feet above sea level. Using this information, determine the elevation of the flood crest for each flood event in Albany. Enter your results in the table. Then fill out column E on your “Flood Frequency” tab. 4. The Willamette River at Albany has the following characteristics: Bank Full 22.3 ft Flood Stage 25.0 ft Major Flood 32.0 ft Calculate the number of times since 1862 that the river has reached each of these stages. You will need to use the ‘COUNTIF’ function, and most of it is provided for you in cell L8. What are their recurrence intervals? Since we don’t have the ability to rank these answers, you should instead divide the number of times it reached that stage by the total number of years of data. Enter your results in the table located on the “Flood Frequency” tab.
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