Sculpting the Earth’s Surface Surface Water &Groundwater

Floods  The Worst Geological Hazard –They are the most costly in terms of life, property, and land –They can occur almost anywhere

Floods  The Worst Geological Hazard –Damage is caused by:  Erosion by flood waters  Impact of water on structures  Sediment deposition  Contamination of surface waters

Floods  Recurrence Interval –Time between floods of the same magnitude –Example: there is 1 in 100 chance that a peak flow of a certain amount (say, 30,000 cf/s) can occur on a river. –Thus, that river is said to have a 100-year interval

Floods  Recurrence Interval –Engineers design for particular recurrence intervals  Streets 2-5 years  Debris dams 5 – 25 years  Upper valley dams 100 years  Levees 100 yrs  Large dams years

Determination of Past Events

Floods  Types of floods –Flash Flooding In Arroyos/washes –River (Regional) Flood –Coastal Flood –Urban Flood –Ice Jam –Dam Failure

Types of Floods  Flash Flooding In Arroyos/washes –Typically caused by:  Intense rainfall  Short period of time  Topography, soil conditions, and ground cover also important

Types of Floods  Flash Flooding In Arroyos/washes –Damage:  Roll boulders  Tear out trees  Destroy buildings and bridges  Scour out new channels  Landslides Sturveyant Falls Trail, San Gabriel Mountains Photo by S. Leyva © 2001

Flood Styles: Flash Floods  What YOU can do: –Know your flood risk ! –Keep your automobile fueled –Store drinking water –Keep a stock of food –Keep first aid supplies on hand. –Keep a NOAA Weather Radio, a battery-powered portable radio, emergency cooking equipment, and flashlights in working order. –Install check valves in building sewer traps to prevent flood water from backing up into the drains of your home.

Types of Floods  River (Regional) Flood –Some floods occur seasonally when winter or spring rains, coupled with melting snows, fill river basins with too much water, too quickly. –Torrential rains from decaying hurricanes or tropical systems can also produce river flooding.

River (Regional) Floods  Upstream floods –generally local in extent & short lag times. –result from intense storms of short duration.  Downstream floods –regional in extent & longer lag times –higher peak discharges. –Result from regional storms of long duration or extended periods of above-normal precipitation.

Flood Styles: Regional Floods  The Great Midwestern Flood of 1993 –Flooding began in the upper Mississippi River Valley –Record flood stage levels recorded –Covered nine states and 400,000 square miles.

32 of the most significant floods of the 20th century

The drainage basin of the Mississippi River

Flood Styles: Regional Floods  The Great Midwestern Flood of 1993 –Abnormally high rainfall in many Midwest states –Ground became over saturated –Streams filled to capacity

Comparison of Average and Observed Monthly Precipitation Totals for the Upper Mississippi River Basin

Flood Styles: Regional Floods

Missouri and Mississippi rivers near St. Louis before (1988) and after peak flood conditions

Floods  Types of Floods –Coastal Flood  Often caused by storm surges.  Can also be produced by sea waves called tsunamis

Floods  Types of Floods –Urban Flood  Roads and parking lots prevent infiltration of water  Urbanization increases runoff 2 to 6 times over what would occur on natural terrain.  Streets can become swift moving rivers, can flood homes and businesses

Floods  Types of Floods –Ice Jam  Floating ice can accumulate at a natural or man-made obstruction and stop the flow of water.

Flood Styles: Dam Failure  St. Francis Dam 3/12/28 –Dream of William Mulholland –200 ft high curved concrete gravity dam –Failed catastrophically at midnight upon first filling

Walking across the top of the St. Francis Dam, looking west, sometime between 1926 and St. Francis Dam shortly before its March 12, 1928, collapse. It is said that this photograph was taken at noon on March 12, However, the caption on the back of the original photo reads: "Taken March 9, 1928, by E.B. (Al) Louden." In any case it is believed to be the last photograph of the St. Francis Dam before it broke at 3 1/2 minutes before midnight on March 12.

This colorized photo shows the St. Francis Dam on its day of formal dedication in May of 1926, almost two years before the failure.

Photo, looking north, shows what was left of the St. Francis Dam shortly after it failed on March 12, 1928.

This colorized photo was taken the day after St. Francis Dam failed.

This picture, taken just after the 1928 disaster,looks towards the southwest. The San Francisquito Fault dips to the northwest.

March 11, 2001: Ruins at the base of the former St. Francis Dam in San Francisquito Canyon. The red speck in the center is a person.

Flood Styles: Dam Failure  St. Francis Dam 3/12/28 –Cause of failure complex  Sespe formation prone to “slaking”  excessive titling when fully loaded  an absence of seepage relief in the dam's sloping abutments  and the partial reactivating of underlying paleo mega-slides within the Pelona Schist.

Flood Styles: Dam Failure  St. Francis Dam 3/12/28 –Some of the most important consequences included  the formulation of the world's first dam safety agency  normalization of uniform engineering criteria for testing of compacted earthen materials  A reassessment of all LADWP dams and reservoirs  the formulation of a state-mandated process for arbitration of wrongful death suits

Flood Styles: Dam Failure  St. Francis Dam 3/12/28 –Excellent book: –The St. Francis Dam Disaster Revisited,edited By Doyce B. Nunis, Jr.

Societal Responses to Floods  Hard Responses –Dams –Levees –Sandbagging  Soft Responses –Watershed Management  Zoning and Land Use  Insurance  Erosion Control –Forecasting

Dams Created by S. Leyva © 2006

Levees

Levees

Urbanization and Floods  Channelization –The Extreme Approach: Los Angeles –The Binational Approach: Tijuana and San Diego –The Uncoordinated Approach: San Diego –The Hit-and-Miss Approach: Tucson

Flood Control Problems In Southern California

The Need For A Comprehensive Flood Control Program  The Area Involved in Potential Flooding –The coastal megapolis from San Diego to Santa Barbara –This area includes 3 major mountain ranges

The Need For A Comprehensive Flood Control Program  Climatic Characteristics –Rainy season – October to April

The Need For A Comprehensive Flood Control Program –Storms moving inland must rise over rugged mountains  Record storms in the San Gabriel Mtns –26” in 24 hours January, 1943 –1” in 1 minute April 5, 1926 –3.02” in 3 minutes  Debris flows and mudflows are generated and dump their deposits on the alluvial fans

The Need For A Comprehensive Flood Control Program  Fatal Storms , 1916, 1921, 1926, 1927, 1934, 1938, 1943, 1952, 1962, 1969, 1976, 1978, 1983  1914 –Los Angeles Basin population ~700,000 –19+ inches of rain in four days in the San Gabriel Mountains –Resulted in floods causing $10 million in damage.

The Need For A Comprehensive Flood Control Program  1934 –"New Year's Day Flood" in the La Canada Valley  40+ people died  400 houses destroyed & damaged streets, bridges, and highways –Causes:  fire and lack of heavy rainstorms in the years before the flood

The Need For A Comprehensive Flood Control Program  The 1938 storms were the greatest in 70 previous years –Hit entire area from SD to SB with 36” rain in 6 days –1000 cfs/sq mile of runoff –87 lives and $78 million –Data gathered is the basis of our flood control program

The Need For A Comprehensive Flood Control Program  Department of Public Works of Los Angeles County –Flood control is their responsibility –The LA County Flood Control District first formed in 1915

Flood Control Problems In Southern California  Uncontrolled Mountain Runoff –Construction of flood control dams within the mountain ranges  Dams are designed to take instant peak water flow  They conserve storm water for public use

Flood Control Problems In Southern California  Uncontrolled Mountain Debris Flows –Controlled with debris dams  354 debris dams in LA County  Designed to catch large amounts of sediment  Must be cleaned out when 25% full

Flood Control Problems In Southern California  Uncontrolled Mountain Debris Flows –Factors determining the type and magnitude of debris flows  Drainage area characteristics  Canyon slopes and stream gradients  Type and degree of rock weathering  Type and density of ground cover  Duration and intensity of the storm  Recent Fires

Flood Control Problems In Southern California  Uncontrolled Alluvial Cone Runoff –Channels have to be constructed from top to bottom –Water is saved for public use

Flood Control Problems In Southern California  Uncontrolled Upper Valley Runoff –Flood regulating reservoirs serve 2 purposes  Catch and control runoff from rain falling downstream from mountain controls  Store water released from the mountains –These reservoirs often help recharge groundwater supplies

Flood Control Problems In Southern California  Uncontrolled Lower Valley Storm Flow –Controlled by fully lined reinforced concrete channels  Controls high velocity flow  Prevents scour

Flood Control Problems In Southern California  Uncontrolled Local Street Runoff –Storm drains and tunnels connect to major flood control facilities –The total length of all LA County channels of all types is 2500 miles

Flood Control Problems In Southern California  Problems –It probably can not handle a 100 year flood –Disposal of debris from debris dams is a problem

Water Conservation  Water is saved for Public Use –Done in mountain and upper valley reservoirs and lower valley drainages –Water may be directed to spreading grounds  Recharges groundwater supplies

Water Conservation  Importance of this Function –Groundwater is cheaper than imported water –Groundwater is not subject to Earthquake emergency problems

Santa Ana River Flood Control Project  This is a Local, State, and Federal Effort –Cost will be $1.1 billion  Part of a $16.3 billion federal package  $822,000,000 federal funds  $268,000,000 state and local funds

Santa Ana River Flood Control Project  Largest CA river basin south of the Sierra Nevada  River has breached its levees many times –1862 – large flood, small population –1938 – smaller flood, larger population

Santa Ana River Flood Control Project  Santa Ana River Drainage Basin –These are the projects owned and operated by the Los Angeles District Corps of Engineers in the Santa Ana River Basin –Seven Oaks Dam Seven Oaks DamSeven Oaks Dam –San Antonio Dam San Antonio DamSan Antonio Dam –Prado Dam Prado DamPrado Dam –Carbon Canyon Dam Carbon Canyon DamCarbon Canyon Dam

Santa Ana River Flood Control Project  Project Protects 90 miles along Santa Ana River –Will handle a ‘worst in 170 years’ flood –This area is regarded by the Army Corp of Engineers as the worst flood hazard west of the Mississippi  Serious flood could cause 3000 deaths  Could cause $14 billion damage in Orange County alone

Santa Ana River Flood Control Project  Project includes: –550' long Earthfill dam 4 miles above Mentone –Raise the Prado Dam 30 feet

   0.html 0.html 0.html  

~ End ~