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