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FLOOD CONTROL PROBLEMS IN SOUTHERN CALIFORNIA

Explore the flood control problems in Southern California and the need for a comprehensive program. Learn about the area at risk, climatic characteristics, historical storms, and the existing flood control system. Discover the role of the Department of Public Works in Los Angeles County and the challenges in controlling mountain runoff, debris flows, and local street runoff. Discover the potential shortcomings of the current flood control system in handling a 100-year flood.

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FLOOD CONTROL PROBLEMS IN SOUTHERN CALIFORNIA

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  1. FLOOD CONTROL PROBLEMSINSOUTHERN CALIFORNIA

  2. The Need For A Comprehensive Flood Control Program • The Area Involved in Potential Flooding • The coastal megapolis from San Diego to Santa Barbara • Inland to the San Andreas Fault • This area includes 3 major mountain ranges • This entire area adjacent to the mountains could be subject to flooding if it weren't for the flood control system

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

  4. 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

  5. The Need For A Comprehensive Flood Control Program • Fatal Storms - 1914, 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.

  6. 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

  7. 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

  8. The Need For A Comprehensive Flood Control Program • Department of Public Works of Los Angeles County • WEB Page: http://ladpw.org/services/water/ • Flood control is their responsibility • The LA County Flood Control District first formed in 1915 • After the fires of 1913 and the floods of 1914

  9. Flood Control Problems In Southern California • Uncontrolled Mountain Runoff • The basic control is 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

  10. Flood Control Problems In Southern California • Uncontrolled Mountain Runoff • Big Tujunga Dam - typical flood control dam • This is one of 20 such dams in LA County • 15 LA County Dams • 5 Army Corps of Engineers dams • 251' high variable radius arch dam • Inner gorge is 160' deep eroded in granodiorite

  11. Flood Control Problems In Southern California • Uncontrolled Mountain Runoff • Financing construction costs • Originally property taxes • Presently considerable aid from state & Federal governments

  12. Flood Control Problems In Southern California • Uncontrolled Mountain Debris Flows • Debris flows formed below the dams that could come from canyon mouths into urban areas are controlled with debris dams • 354 debris dams in LA County • Designed to catch large amounts of sediment • Must be cleaned out when 25% full

  13. Flood Control Problems In Southern California • Uncontrolled Mountain Debris Flows • Effect of Fires • A fire that burns 15,000 acres can cause half million cubic yards of debris to move in the next storm • Debris flow will consist of incinerated material, clay, sand, and boulders

  14. 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

  15. 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

  16. Flood Control Problems In Southern California • Uncontrolled Upper Valley Runoff - Hanson Dam example • 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

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

  18. 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

  19. Vast Flood Control System has Problems • It probably can not handle a 100 year flood • Public should be as concerned about this as a great Earthquake

  20. Vast Flood Control System has Problems • Disposal of debris from debris dams is a problem • Average annual disposal is 450,000 cu. yds at $8-10/cu yd (1990) • Material is hauled to disposal beds in the mountain canyons • Disposal space is running low • County will use 81 acre gravel pit near Irwindale • Should last 50 years

  21. Water Conservation Is An Important Function Of The Entire System • 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

  22. Water Conservation Is An Important Function Of The Entire System • Importance of this Function • Groundwater is cheaper than imported water • Groundwater is not subject to Earthquake emergency problems

  23. 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

  24. 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

  25. 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 • San Antonio Dam • Prado Dam • Carbon Canyon Dam

  26. 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

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

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