Lab 10 Course Notes - GEOG 265 The Water Planet...

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GEOG 265 – The Water Planet Laboratory 10: Predicting 100 Year Floods 1. LEARNING OBJECTIVES Learn the terms and processes related to “100 year floods.” Calculate and graph recurrence intervals as a means for predicting and examining flood  frequency over time. Learn how human intervention affects flood frequency. 2. BACKGROUND INFORMATION The population in the Puget Sound is growing rapidly and humans have made many changes to  rivers and drainages.  This activity investigates recurrence intervals, 100-year floods, and  changing flood frequencies for two watersheds in the state of Washington.   A flood is an event in which the water flowing through a channel (such as a stream or river)  exceeds its bankfull capacity.  A definition that includes lakes or other non-flowing water is an  event in which water exceeds its normal encasement and covers land not normally inundated.  Floods are by categorized by their discharge (volume of water in the stream) and by their  frequency.  However, these two types of categorization are interrelated: the frequency of a flood  is determined by analyzing discharge over a period of years.   The flood  recurrence interval  (return period) for a stream is the probability that a flood of a  given magnitude will be equalled or exceeded in a given year. A flood having a recurrence  interval of 10 years has a 10% chance of recurring in any year; a  100-year flood  has a 1%  chance of recurring each year. Please read the following USGS pamphlet for further discussion  of this topic: http://pubs.usgs.gov/gip/106/pdf/100-year-flood_041210web.pdf The 100-year flood, as determined by this type of flood frequency analysis, serves   as a legal  definition of areas that are likely to be flooded.  If someone chooses to purchase a home in the  100-year floodplain, they must obtain flood insurance provided by the federal government. Gauges placed along a stream monitor discharge at several locations. 1
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The USGS and other organizations collect data from gauging stations and use it to determine  the frequency of flooding along streams. Estimates of  flood frequency  are more accurate with a  long record (many years) of discharge records.  The flood frequency is typically expressed as a 
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