4/28/17. Size of Floods - Hydrograph Depicts flood data from gaging station; Hydrographs usually show Q vs. time

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1 GEOL 118 Final Exam Format similar to exams 1 and 2 Covers Lectures #27 40 (Global Climate Change I - Meteorite Impacts III) #35 Floods - II (Size, Prediction, Urbanization, Mitigation) 8:00, Thursday, May 11 th Location TBA Bring Photo ID Optional Question/Answer Session Location and time TBA Official University of Illinois policy will be followed for requests for a conflict final exam time. Specifically, you must have a verifiable official University excuse such as three final exams in 24-hour period to schedule a conflict final exam for GEOL 118. If you have another final exam scheduled for the same time, then it is probably a combined section (or conflict) final exam and you must take a conflict exam in that course in accordance with Student Code. Kangaroo towed to safety through floodwaters near Wellington, Australia, 2011 Size of Floods - Stream Stage One parameter to describe stream flow (flood size): stream stage = water height above the river bottom (somewhat arbitrary) Stage measured at gaging station Simple way of measuring stream stage Size of Floods - Stream Discharge Other parameter describing stream flow (flood size): stream discharge (Q) = volume of water flowing past point over time. Don't measure water volume over time, but velocity and cross-sectional area (width x average depth) Q (discharge) = ft 3 /sec = Velocity x Cross-sectional area ft/sec x ft 2 Velocity sensor attached to wading rod Size of Floods - Stage, Discharge Discharge and stage are related by rating curve (graph that shows specific relation between discharge and stage at given location along river). Depicts flood data from gaging station; Hydrographs usually show Q vs. time Stage is also used Stage vs. time Q vs. time 1

2 Precipitation data can also be shown Time between maximum rainfall and maximum flood stage (or peak Q) = lag time Upstream flood has short lag time, sudden rise and fall of floodwaters. Downstream flood has longer lag time and longer flood duration (maximum Q is usually much greater than upstream unless distance between two areas is small). Upstream flood has short lag time, sudden rise and fall of floodwaters. Downstream flood has longer lag time and longer flood duration (maximum Q is usually much greater than upstream unless distance between two areas is small). Size of Floods - Hydrograph Hydrographs for nearby gaging stations- last 30 days: Middle Fork Vermillion River at Oakwood, IL ~20 miles east of campus Boneyard Creek at Urbana, IL (on U of I campus, just north of Engineering Hall) Clicker Question- Review: What is the discharge of a river or stream? A. It is the depth of the water- the height of the surface above the stream bed. B. It is the velocity of the water moving parallel to the river channel axis. C. It is the amount of water passing through per second. Clicker Question: Why does the Middle Fork River (right) have more rounded hydrograph peaks? A. It has a smaller watershed and therefore changes more slowly. B. It has a smaller watershed and therefore changes more quickly. C. It has a larger watershed and therefore changes more slowly. D. It has a larger watershed and therefore changes more quickly. E. All of the above 2

3 Started here 4/24 Monday Predicting Floods - Flood Frequency Analysis Long-range flood prediction = flood frequency analysis, determine how often you can expect flood of particular size. Uses historic records of flood size (maximum discharge or stage). Annual Peak Discharge of Cedar River at Cedar Rapids, Iowa Predicting Floods - Recurrence Interval Clicker Question: Estimate, roughly, how often this river exceeds 200 cms. A. Once every 2 years B. Once every 8 years C. Once every 50 years D. Once every 150 years Predicting Floods - Recurrence Interval Predicting Floods - Recurrence Interval Flood frequency analysis gives values for flood recurrence interval = average number of years between occurrences of certain size flood. R = (N + 1) M where R = recurrence (interval) N = total number of years of record M = Magnitude (ranking of year under consideration, 1 = biggest, N = smallest) Predicting Floods - Probability, Recurrence Interval Recurrence Interval (R) = average number of years between occurrences of certain size flood, e.g., 100-year flood (very large flood that occurs ON AVERAGE once in 100 years); element of chance involved. Flood Probability (P) = risk (% chance) of certain size flood in any year. Flood probability equals reciprocal of recurrence interval: P = 1 R 100 Recurrence Flood Size Flood Frequency 100-year flood Probability 25-year flood 2-year flood 3

4 Predicting Floods - Probability, Recurrence Interval Recurrence interval (R) = average number of years between occurrences of certain size flood, e.g., 100-year flood (very large flood that occurs ON AVERAGE once in 100 years); element of chance involved. Flood Probability (P) = risk (% chance) of certain size flood in any year. Flood probability equals reciprocal of recurrence interval: P = 1 R 100 Misconceptions Year Flood Some mistakenly think that 100-year flood happens regularly every 100 years. Having 100-year flood somewhere doesn t mean it s safe for 99 years from another similar flood. Less confusing to think of risk of flooding in terms of probability. Each year has 1% chance to experience 100-year flood Mississippi River flood near Wyatt, MO Recurrence Flood Size Flood Frequency Probability 100-year flood Very large Rare 100/100 = 1% 25-year flood Large Somewhat rare 100/25= 4% 2-year flood Small Very common 100/2= 50% Predicting Floods - Flood Frequency Analysis Plot of discharge (or stage) vs. recurrence interval can be used to extrapolate to different flood sizes, e.g., discharge values for R > 40 years (dashed line area) can be estimated in graph below Problems with Flood Frequency Analysis Predictions of the frequency of very large floods are often highly uncertain. Why? Due to limited historic data; for example, area may have only 25 years of flood data or even 10 years of data; 100-year flood may have never occurred Many data points for R < 10 years, few data points for R > 10 years Problems with Flood Frequency Analysis Land use changes in drainage basin (dam construction or urbanization) also make it harder to determine size of all floods (large + small). Why? Same rainstorm today vs. 50 years ago will produce very different river discharge values dam construction reduces flooding urbanization increases flooding R = (N + 1) M Clicker Question- In a very old town in Europe, 300 years of flood records show that floods with a stage height of greater than 5 meters happened 30 times. Roughly what is the recurrence interval for a flood of that size? A. 1 year B. 3 years C. 10 years D. 30 years 4

5 Flood Hazard Map Use stage vs. recurrence data and topographic map to plot 50-year floodplain or 100- year floodplain on flood hazard map (area surrounding river up to certain elevation) Clicker Question- Review: Last year a town experienced the 50- year flood. What is probability that the same town will have the same size flood this year? A. Very close to zero. B. 2% chance (1 chance in 50) C. 50% chance Flood Hazard Map Flood hazard maps allow elected officials and city planners to restrict development near rivers. Effects of Urbanization - Worsens Flooding Development of cities around rivers can intensify effects of flooding: more common more severe shorter lag time Large rainstorm in urbanized area will produce worse floods compared to same rainstorm in same area before development. Effects of Urbanization - Impermeable Barriers Urbanization intensifies flooding due to presence of concrete, pavement, and buildings; don t allow water to soak into ground; instead, goes directly and quickly into river due to impermeable barriers. Effects of Urbanization - Floodplain Space Buildings also take up space in floodplain, causing flood levels to rise even higher New York City Guangzhou, China 5

6 Effects of Urbanization - Storm Sewers Storm sewers are underground tunnels that send storm water runoff from streets directly and quickly into local river. Flood Control (Construction) Measures = Channelization, Floodway, Dam/Reservoir, Artificial Levee What are the benefits to society (pros) associated with the above measures? What are the drawbacks to society (cons) associated with the above measures? There are lots of pros and cons. Storm drain receiving rainwater runoff Flood Mitigation in Public Policy - Preserve Wetlands Preserve wetlands (swamps) - excellent locations for rainwater infiltration into ground, rather than running off into river ~50% of world's wetlands have been lost over past 200 years Flood Mitigation in Public Policy - Mapping/Zoning Determine areas of risk and restrict land use (allow only parks, golf courses, agriculture); most effective approach from environmental perspective. Zoning restrictions only apply to new construction Floodplain Regulation Percent of US wetlands lost from 1700s to 2000 Flood Mitigation in Public Policy - Mandatory Insurance Require those who live on floodplain to have insurance (for homes and crops). Offered by Federal Emergency Management Agency since 1950s but not very popular; National Flood Insurance Program is $24 billion in debt, mostly caused by payouts after Hurricane Katrina and Hurricane Sandy; most policy-holders have taxpayer-subsidized rates, recent legislation aims to reduce that problem Flood Mitigation in Public Policy - Mandatory Insurance Problems with National Flood Insurance Program: artificially low insurance rates encourage risky development in coastal areas; flood maps do not accurately reflect risk of flooding, especially with rising sea level expected with global climate change; repeated payments to same highrisk properties in coastal areas - almost $9 billion nationwide since

7 Flood Mitigation in Public Policy - Relocation Give government aid for high-risk communities to move, e.g., Valmeyer, IL Breaking ground in 1994 on new Valmeyer location Valmeyer after 1993 Mississippi flood The new, improved Valmeyer location (400 feet above floodplain!) 7