Lecture Outlines Natural Disasters, 7 th edition Patrick L. Abbott

Floods Natural Disasters, 7 th edition, Chapter 14

Floods Rainfall varies in intensity and duration In small drainage basin, short-lasting maximum floods In large drainage basin, maximum floods for weeks Events of past  forecast of future events –Largest past event likely to be exceeded at some point –Floods of River Arno through Florence

Side Note: A Different Kind of Killer Flood Unusually hot January in Boston caused molasses in heated tank to expand and burst container 2.3 million gallons of molasses flooded out as 9 m high wave Killed 21 people and injured 150 people Many trapped in molasses after it cooled and congealed Figure 14.2

How Rivers and Streams Work Longitudinal Cross Section of a Stream Cross-sectional plot of stream’s bottom elevation vs. distance from source Profile for almost any stream is smooth, concave upward, with steeper slope near source and flatter slope near mouth Base level – level below which stream can not erode –Ocean, lake, pond or other stream into which stream flows Profiles are similar for all streams because of equilibrium processes Figure 14.3

The Equilibrium Stream Streams act to seek equilibrium, state of balance –Change causes compensating actions to offset Factors: –Discharge: rate of water flow, volume per unit of time Independent variable (stream can not control) –Available sediment (load) to be moved Independent variable (stream can not control) –Gradient: slope of stream bottom Dependent variable (stream can control) –Channel pattern: sinuosity of path Dependent variable (stream can control) How Rivers and Streams Work

Case 1 – Too Much Discharge Too much water  stream will flow more rapidly and energetically Response: Excess energy used to erode stream bottom and into banks –Sediment produced by erosion – energy is used up carrying sediment away Erosion of stream bottom results in less vertical drop  flatter gradient  slower, less energetic water flow How Rivers and Streams Work Figure 14.4

Case 1 – Too Much Discharge Too much water  stream will flow more rapidly and energetically Response: Erosion into stream banks creates meandering pattern  longer stream path, lower gradient  slower, less energetic water flow How Rivers and Streams Work Figure 14.7 Figure 14.6

Case 2 – Too much load Too much sediment  stream becomes choked Response: Excess sediment builds up on stream bottom Buildup results in increased gradient  water flows faster and more energetically  can carry away more sediment How Rivers and Streams Work Figure 14.8

Case 2 – Too much load Too much sediment  stream becomes choked Response: Channel pattern becomes straighter  minimum energy needed to flow distance Islands of sediment form within channel, creating braided stream pattern How Rivers and Streams Work Figure 14.9

Case 2 – Too much load Too much sediment  stream becomes choked Response: Similar to stream overflowing and eroding away landslide dam How Rivers and Streams Work Figure 14.10

Graded Stream Theory Delicate equilibrium maintained by changing gradient of stream bottom  graded stream Typical stream: –Too much load, too little discharge in upstream portion  braided pattern –Too much discharge, less load in downstream portion  meandering pattern Change in response to seasonal changes, changes in global sea level, tectonic events How Rivers and Streams Work

The Floodplain Floors of streams during floods Built by erosion and deposition Occupied during previous floods, and will be occupied again in future floods Figure 14.11

Side Note: Feedback Mechanisms Negative feedback: system acts to compensate for change, restoring equilibrium Positive feedback: change provokes additional change, sending system in “vicious cycle” dramatically in one direction –Desirable in investments accumulating interest –Undesirable in debts accumulating interest charges

Flood Frequency Larger floods  longer recurrence times between each Analyze by plotting flood-discharge volumes vs. recurrence interval, construct flood-frequency curve Flood-frequency curves different for different streams Can be used to estimate return time of given size flood –100-year flood used for regulatory requirements, has 1% chance of occurrence in any given year –Difference between yearly probability, cumulative probability Figure 14.12

In Greater Depth: Constructing Flood Frequency Curves For each year’s maximum flood, calculate recurrence interval = (N + 1) / M, where N = number of years of records, M = rank Plot recurrence interval vs. discharge for each year, connect points as best-fit line Longer records of floods  better flood frequency curves Impossible to know exactly when floods will occur, but can predict statistical likelihood over period of time Steps in construction: Record peak discharge for each year, rank years accordingly Figure 14.13

Flood Styles Several causes: Local thunderstorm  flash (upstream) flood lasting few hours, building and ending quickly Rainfall over days  regional (downstream) floods lasting weeks, building and dissipating slowly Storm surge of hurricane flooding coastal areas Broken ice on rivers can dam up, block water flow  fail in ice-jam flood Short-lived natural dams (landslide, log jam, lahar) fail in flood Human-built levees or dams fail in flood

Flash Floods Thunderstorms can release heavy rainfall, creating flash floods in steep topography Flash floods cause most flood-related deaths –50% of flood-related deaths are vehicle-related –Only two feet of moving water required to lift and carry away average car Figure 14.14

Flash Floods Antelope Canyon, Arizona, 1997 Narrow slot canyons of tributaries to Colorado River Thunderstorm releasing rain to form flash flood may occur too far away to hear or see 12 hikers killed by flash flood in 1997 Figure 14.15

Flash Floods Big Thompson Canyon, Colorado, 1976 Centennial celebrations brought thousands to canyon Stationary thunderstorm over area dumped 19 cm of rain in four hours Runoff created flash flood up to 6 m high, 25 km/hr Figure 14.17

Flash Floods Big Thompson Canyon, Colorado, people killed, damage totaling $36 million Figure 14.18

Flash Floods Rapid Creek, Black Hills, South Dakota, 1972 Pactola Dam built in 1952 to give flood protection and water supply to Rapid City, on Rapid Creek  increased development of floodplain Stationary thunderstorm poured up to 38 cm in six hours Canyon Lake overflowed as Canyon Lake dam broke, flooding Rapid City 238 people killed, $664 million in damages Floodplain remains undeveloped  greenbelt –“No one should sleep on the floodway.”

Flash Floods Rapid Creek, Black Hills, South Dakota, 1972 Figure 14.19

Regional Floods Inundation of area under high water for weeks Few deaths, extensive damage Large river valleys with low topography Widespread cyclonic systems  prolonged, heavy rains In U.S., about 2.5% of land is floodplain, home to about 6.5% of population

Red River of the North: unusual northward flow (spring floods) Geologically young – shallow valley Very low gradient – slow flowing water tends to pool As winter snow melts, flows northward into still frozen sections, causing floods Regional Floods

Red River of the North: 1997 record floods: –Fall 1996 rainfall four times greater than average –Winter 1996 freezing began early, causing more ice in soil –Winter snowfalls more than three times greater than average –Rapid rise in temperature melted snow and ice in soil –Floodwaters flowed slowly northward, flooding huge areas of North and South Dakota, Minnesota and Manitoba Regional Floods

Mississippi River System Greatest inundation floods in U.S. Third largest river basin in world Drains all or part of 31 states, two Canadian provinces System includes almost half of major rivers in U.S. Average water flow in lower reaches is 18,250 m 3 /sec Water flow can increase fourfold during flood Regional Floods

Mississippi River System Regional Floods Figure 14.20