Rivers and Floods Packet
EE 105 Lab #4 Rivers and Floods PART A. TROPICAL STORM ALBERTO & THE FLOOD OF 1994 GEORGIA & ALABAMA HOW MUCH RAIN? Examine Figures A and B. Figure A provides a brief summary of the wet season of 1994. Notice the “contour lines” on Figure B, indicating the amount of rain between July 1-7, 1994. Question 1. a) How much total rainfall for Montezuma in the seven-day period? b) Calculate average rainfall per day. (for the 7-day period) Question 2. Compare this to the normal amount of precipitation for Montezuma. Calculate the daily average for July using the normal precipitation records, then quantitatively compare it to the daily average you determined for question 1b above. Normal precipitation records (in inches) for 1961-1990 June July August Montezuma, GA 3.58 4.30 3.63 Los Angeles, CA 0.03 0.01 0.15 Sacramento, CA 0.12 0.05 0.07 Orlando, FL 7.32 7.25 6.78 Question 3. Use the topo map of Montezuma (Figure C). Locate the gaging station along the east side of the Flint River near the center of the map, between the towns of Montezuma and Oglethorpe. The gaging station is located at an elevation of 262 feet, and the river is considered to be at flood stage when it is 20 feet above this level (282 feet). The old flood record was 27 feet above the gaging station, but the July 1994 flood established a new record at 35 feet above the gaging station, or 297 feet above sea level. Trace the imaginary contour line for this elevation on both sides of the Flint River on the map. The line will be just below the 300-foot contour line that is already drawn on the map. Label the area within these contours as a Flood Hazard Zone. Question 4. Assess the damage caused by the 1994 flood as follows: List the specific features/structures that were underwater in Montezuma during the flood. (consider — highways, roads, schools, cemeteries, railroads) Figure B X = Montezuma, GA Part B. Forecasting Floods We would obviously like to be able to predict exactly when and where a disaster will strike. Unfortunately, the natural systems associated with natural disasters are highly complex, commonly exhibiting chaotic (unpredictable) behavior, and individual occurrences of a given hazard are impossible to forecast with any precision. However, it is often possible to say something about the likelihood of an event occurring over a given span of time. While statistical forecasts won’t provide warnings about when a looming disaster will occur (e.g., allowing for evacuation), they are extremely valuable when it comes to building structures to withstand natural disasters. For example, when building a bridge over a river it is useful to know the most extreme currents the bridge is likely to experience over its lifetime so that it can be designed to be both safe and economical. In the US, bridges are typically designed to withstand what are known as 100-year flood events, which is to say flood conditions that occur on average once every 100 years on a given river. (100 years is known as the return period, or recurrence interval, of the flood.) In this exercise, you will learn how to determine the 50-year and 100-year flood conditions for a river by constructing a flood-frequency curve for the Connecticut River which is the largest and longest river in New England. You will be using real data provided by the US Geological Survey (USGS) from stream gage 01170500, located on the Connecticut River near Montague City, MA in this exercise. For those who are interested, the actual data (along with data for other river systems), along with additional details about the station, can be obtained online through the USGS website at http://waterdata.usgs.gov/ma Table 1. Annual peak data for USGS Stream Gage 01170500 (Connecticut River) from 2000-2009. Year Peak Flow (cfs) Peak Height (ft) 2000 69100 26.04 2001 93200 30.62 2002 73200 26.95 2003 81000 28.4 2004 86700 29.45 2005 92600 30.51 2006 121006 35.04 2007 91100 30.25 2008 82200 28.62 2009 74200 27.07 1. Using data from Table 1, list annual peak flood heights from greatest to smallest for the 2000-2009 period in the table below. Height (ft) Rank Probability (%) Recurrence Interval (yrs) Log10(RI (yrs)) 1 2 3 4 5 6 7 8 9 10 2. For each annual peak condition in the table above, calculate the annual probability, recurrence interval, and log10 of the recurrence interval and enter values in the table. Useful equations: Probability (%) = (rank/total number of years of data) x 100 Recurrence Interval (yrs) = (total number of years of data + 1) / Rank 3. Make a plot of the Annual Peak Flood Height vs. log10 of the Recurrence Interval for the data in the table above using graph paper. Draw a “best fitting” line through the data points. 4. Use the best-fitting line from your plot to predict the water height associated with 50- and 100-year flood conditions at this location. Annotate your graph to illustrate your approach and enter your predicted values in the table below. Predicted Height (ft) 50-year flood 100-year flood
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