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Earth Science - Lecture 34 Notes.docx

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Earth Sciences
Sergei Basik

November 22, 2013 FLOOD FREQUENCY AND FLOOD RELATED HAZARDS Flood Frequency Recurrence Interval  Recurrence interval: o Represents frequency of a natural disaster as an average time between past events o Helpful when natural events are random o Also called return time  Flood frequency is recorded as recurrence interval: o Average time between floods of a given size o Larger flood discharges on given stream have longer recurrence interval between floods  Calculated recurrence interval depends on: o Total number of years in flood o Record of rank of flood  Any new larger flood reduces rank and thus reduces recurrence interval  For a given-size flood, recurrence interval is: o T = (n+1)/m o T = recurrence interval o n = total number of years in record o m = rank of flood (largest flood = 1; second largest flood = 2, etc.) Ex. If the largest flood on a rivet at a single location was in 1997 and there are 87 years of records, what is the recurrence interval for the 1997 flood? m 1997= 1 n = 87 T1997= (87+1)/1 = 88 years Ex. If in 2007 a slightly larger flood occurs, what is the corrected recurrence interval for 1997? m 2007= 1 and m1997= 2 T1997= (97+1)/2 = 98/2 = 49 years Problems with Recurrence Intervals  Data for recurrence interval must cover interval long enough to be representative  Assumes that upstream conditions were similar through time: o Climate changes o Anthropogenic factors (urbanization, dams, deforestation, etc.)  All floods plotted should originate from similar causes for random distribution of flood sizes November 22, 2013 100-year Floods  100-year flood used to establish regulations for buildings near streams  100-year flood has 1% change of happening in any given year (including after a similar event)  Term 100-year flood is misleading because it leads people to believe that it happens only once every 100 years  Better term would be: “1-in-100 chance flood”  100-year floodplain: o Area likely to be flooded by largest event in 100 years (on average) o Based on extrapolation from few large recorded events o Does not account for probable changes from upstream alterations to drainage basin (human activities) Paleoflood Analysis  Short record of stream flow data is major problem  Use physical evidence of past flooding that is preserved in geologic record to reconstruct approximate magnitude and frequency of major floods  Estimated paleoflood magnitude or flood height  Critical info on minimum hazard of past flood Early post-flood evidence (Paleoflood markers):  Nature and magnitude of flood is most obvious immediately afterward  Useful features include features indicating height, velocity and size of flood: o High-water marks o Cross-sectional area o Mean flood depth o Estimated water velocity o Mean flow velocity o Discharge Dendrochronological method (tree ring damage):  Trees may preserve effects of damage from flood and indicate number of years since damage  Height on tree indicates minimum hei
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