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A. Hydrologic cycle
o The hydrologic cycle is a summary of the circulation of Earth’s water supply
o Processes involved in the hydrologic cycleoPrecipitationoEvaporationoInfiltrationoRunoffoTranspiration
The hydrologic cycle
Sources of Earth’s water
The Three Fates of Precipitation—Where does rain go?
o Evaporation or Transpiration 66% in the US
o Infiltrationo Runoff – the cause of floods
o It is generally of interest to cities to reduce runoff
Civilization exists by geological consent …subject to change without notice.
Will Durant
Floodso Floods = any high flow of surface
waters that overtops normal confinements or covers land
o Floods usually occur when river's channel cannot hold all the water supplied to it by its watershed (the area the river drains).
Fig. 11-2, p.269
Flooding from the Feather River extended far
off to the horizon in some places.
Fig. 12-27, p.313
Floods –o The Worst Geological Hazard
o They are the most costly in terms of life, property, and land
o They can occur almost anywhereo Damage is caused by:
o Erosion by flood waterso Impact of water on structureso Sediment depositiono Contamination of surface waters
o Loss of lifeo 1887 Honan, China 900,000 killedo 1911 Yangtse River 100,000 killedo 1969 Southern California 69 killed
Causes of Floodso High intensity – long duration precipitation
o it is greatly worsened by deforestation, farming, and urbanization
o examplesDuration Depth (inches) Location Date1 minute 1.23 Unionville, MD 195642 minutes 12 Holt, MO 19474 hr, 30 min. 30.8 Smethport, PA 194224 hrs 45.99 Philippines 19112 yrs 1605 Cherraponji, India 1860-61
o Snowmelto Late spring rapid melting often results in problems down river
o Ice Jamso Large blocks of ice clog rivers during spring breakup
o Dam Failureo Baldwin Hills Dam 12/14/63 5 killed http://www.stanford.edu/~meehan/la/baldwin.htmo St. Francis Dam 3/12/28 500 killed http://www.rain.org/~stapaula/StFrancisOther.html
Evaluation of Precipitation
o Depth or magnitude of the rainfallo Area over which the rain fallso Duration of the raino Intensity
Shepherdsville, OhioOhio River
Floods Flood account for over 40% of all
natural disaster deaths worldwide WHY?
Waterheim, GermanyMain River
Dahaka, BangladeshSea Flood
Davenport, Iowa, also on the
Mississippi River floodplain
Fig. 12-15, p.304
Why?
More than half the world’s population lives near large bodies of water
Most rivers burst their banks every two years Severe weather Lack of engineering floodplain design urbanization
D. Recurrence Intervalo The average recurrence interval between major
storms or floods is easily calculatedo a. 10, 100, 1000 year storms for example
o Engineers design for particular recurrence intervalso streets 2-5 yearso debris dams 5 – 25 yearso Upper valley dams 100 yearso Levees 100 yrso Large dams 1000+ years
Flood FrequencyFlood Frequency
o Recurrence Intervalso Analysis
Discharge
o Discharge = amount of water flowing past a point in a given unit of time
o Units of measure o VOLUME/TIME o gal/min o m3/sec o ft3/sec o How is discharge determined?
Cross-sectional area of stream x velocity
Urbanization and its effects on discharge
o Less infiltration o More runoff o More flooding
o Specific causes: oDeforestation oConstruction and paving oChannelization
Rivers do not rise with first rainfall; the thirsty ground absorbs it all.
Seneca
Recurrence IntervalsThe average time interval between the occurrence of two flood event
o Rivers flood regularly, some at frequent interval o Some floods are large, with very high water levelo Small flood are most frequent occurring on the
average every 2 or 3 yearso Large flood are generally less frequent usually
occurring every 10, 20, or 30 years.o There are probabilities not certainties!o There is a 20 % probability that a flood of a given
height –say 3 m above bank – will occur in any one year.
o This chance corresponds to an average time interval (in this case 5 years (20 %=1 in 5) between two floods 3 m height.
What do you think?
Q. What's the probability of a 25 year flooding event occurring each year?A. 1/25= 0.4 *100= 4 %
Q. Given that a 25-year flooding event has a probability of 4 % of occurring each year (1/25), what is the probability that a 25-year flood will NOT occur in a given year?A. 100 - 4 = 96 %
What do you think?
What's the probability a flood with a recurrence interval of 25 years, during a 10 year period?
P(Within 10 years) = 1 - (1 - 1/25)10 = 1 - 0.66 = 0.33 or 33%
Q. What's the probability of there being a 50 year type flood in a 20 year period?
Real World Question!
What is the probability of flooding, 50 years flood, within the 30 years that you are paying on the mortgage?
E. Factors which Have Increased Erosiono Deforestation – results in increased
erosiono Overgrazing – also increased erosiono Urbanization
oMore rapid discharge to streams causes:oMore frequent floodsoHigher flood peaks
oUrban runoff is highly toxic
F. Urbanization and Erosion
o Both natural and urban landscapes have low erosion rates
o Construction as natural lands is urbanized causes high erosion rates
II Formation of Valleys
o River ErosionoRivers cuts their valleys vertically and laterallyo Lateral erosion produces broad valleys, flood
plains, and meandering streams
o Flood Plainso Periodic floods deposit rich soils
o Agricultural production on floods plains is followed by urbanization
oNatural leveeso Forms as floods deposit coarse detritus near the rivero Naturally constraint the river except in the larger floods
Inland flooding can be a major threat to communities hundreds of miles from the coast as intense rain falls from these huge tropical air masses.
Reducing Flood Damageo Two Approaches
o 1. Water Controlo Flood Plain Management
o o 1. Water Control
o Watershed managemento Erosion controlo Reforestation
o 2. Dams and Reservoirso 30% of US reservoir capacity is devoted to flood controlo Small dams are effective in upstream areaso Large dams protect the downstream areas
o May also produce hydroelectric powero Dams have enormous negative environmental impacts
o Channel clearing and dredgingo a. Tends to deepen the channel and increase the
capacityo Channel alteration
o Cutting off meanders tends to steepen gradients and increase downcutting of the channel
o The Mississippi River has been shortened by 100’s of miles
o Diversionso Flood waters are diverted into lakes and flood plains
o Channel stabilizationo Channel banks and floors are paved
o 7. Dikes and leveeso Very common world-wide to protect fertile flood plainso They increase downstream flood peaks by eliminating flood plain
water storageo Failure often results in disaster
Malibu Canyon Dam was built in 1925 in the Santa Monica Mountains
Within 13 years the reservoir was filled with sand. (Photo by John. S. Shelton)
Flood mitigation o Flood control structures
o dams o flood walls
o Channelizationo Human try to control flood waters by making
channels: 1) clear of debris 2) deeper 3) wider, and 4) straighter. Example LA River
o dikes o levees o zoning o prohibition of rebuilding; moving towns o flood insurance
Flood Plain Management
o Flood forecastingo Function of the River & Flood Service of the
US National Weather Service
o Flood Plain Zoning
Case HistoryThe Binational Approach:The issue (s) related to Tijuana river in the US-Mexico
border
o The two countries agreed on a LA-Style project to cement the river channel but US did not complete the project.
o The cement– lined channel project was blocked by environmental activist
o The large concrete channel in the city of Tijuana sends high-velocity floods charging into the subdivisions of southernmost San Diego
View southeast from San Diego, CA into Tijuana, Mexico, Jan 1978Flood waters race through the cement-lined channel in Tijuana (top center of photo)
blasting into the farms and subdivisions in southernmost San Diego.
Flood in San Diego River, Jan 1979
Fig. 11-42, p.290
every year the probability (P) of a Maximum Annual Peak Discharge (we'll call this a flood) with a given recurrence interval (RI) is
P = 1 / RI
From that it follows that the probability of there NOT being a flood within one year isP(NOT) = (1 - 1 / RI)
And finally, the probability of there being a certain size flood in X years is P(Within X years) = 1 - P(NOT in X years) = 1 - (1 - 1 / RI) X
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