I. THE NEED FOR A COMPREHENSIVE FLOOD CONTROL PROGRAM

• A. The Area Involved in Potential Flooding
  
  • 1. The coastal megapolis from San Diego to Santa Barbara
    
    • a. Inland to the San Andreas Fault
      
  • 2. This area includes 3 major mountain ranges
    
    • a. This entire area adjacent to the mountains could be subject to flooding if it weren't for the flood control system
      
• B. Climatic Characteristics
  
  • 1. Rainy season - October to April
    
  • 2. Storms moving inland must rise over rugged mountains
    
    • a. Record storms in the San Gabriel Mtns
      
      • 1) 26" in 24 hours January, 1943
        
      • 2) 1" in 1 minute April 5, 1926
        
      • 3) 3.02" in 3 minutes
        
    • b. Debris flows and mudflows are generated and dump their deposits on the alluvial fans
      
  • 3. Fatal Storms - 1914, 1916, 1921, 1926, 1927, 1934, 1938, 1943, 1952, 1962, 1969, 1976, 1978, 1983
    
  • 4. 1914
    
    • a. Los Angeles Basin population ~700,000
      
    • b. 19+ inches of rain in four days in the San Gabriel Mountains
      
    • c. Resulted in floods causing $10 million in damage.
  • 5. 1934
    
    • a. "New Year's Day Flood" in the La Canada Valley
      
      • 1) 40+ people died
        
      • 2) 400 houses destroyed & damaged streets, bridges, and highways
        
    • b. Causes:
      
      • 1) fire and lack of heavy rainstorms in the years before the flood
        
  • 6. The 1938 storms were the greatest in 70 previous years
    
    • a. Hit entire area from SD to SB with 36" rain in 6 days
      
    • b. 1000 cfs/sq mile of runoff
      
    • c. 87 lives and $78 million
      
    • d. Data gathered is the basis of our flood control program
      
• C. Department of Public Works of Los Angeles County
  
  • 1. WEB Page http://ladpw.org/services/water/
    
  • 2. Flood control is their responsibility
    
  • 3. The LA County Flood Control District first formed in 1915
    
  • a. After the fires of 1913 and the floods of 1914
    

II. FLOOD CONTROL PROBLEMS IN SOUTHERN CALIFORNIA

• A. Uncontrolled Mountain Runoff
  
  • 1. The basic control is construction of flood control dams within the mountain ranges
    
    • a. Dams are designed to take instant peak water flow
      
    • b. They conserve storm water for public use
      
  • 2. Big Tujunga Dam - typical flood control dam
    
    • a. This is one of 20 such dams in LA County
      
      • 1) 15 LA County Dams
        
      • 2) 5 Army Corps of Engineers dams
        
    • b. 251' high variable radius arch dam
      
    • c. Inner gorge is 160' deep eroded in granodiorite
      
  • 3. Financing construction costs
    
    • a. Originally property taxes
      
    • b. Presently considerable aid from state & Federal governments
      
• B. Uncontrolled Mountain Debris Flows
  
  • 1. Debris flows formed below the dams that could come from canyon mouths into urban areas
  • are controlled with debris dams
    
    • a. 354 debris dams in LA County
      
  • 2. Effect of Fires
    
    • a. A fire that burns 15,000 acres can cause half million cubic yards of debris to move in the next storm
      
    • b. Debris flow will consist of incinerated material, clay, sand, and boulders
      
  • 3. Factors determining the type and magnitude of debris flows
    
    • a. Drainage area characteristics
      
    • b. Canyon slopes and stream gradients
      
    • c. Type and degree of rock weathering
      
    • d. Type and density of ground cover
      
    • e. Duration and intensity of the storm
      
• C. Uncontrolled Alluvial Cone Runoff
  
  • 1. Channels have to be constructed from top to bottom
    
  • 2. Water is saved for public use
    
• D. Uncontrolled Upper Valley Runoff - Hanson Dam example
  
  • 1. Flood regulating reservoirs serve 2 purposes
    
    • a. Catch and control runoff from rain falling downstream from mountain controls
      
    • b. Store water released from the mountains
      
  • 2. These reservoirs often help recharge groundwater supplies
    
• E. Uncontrolled Lower Valley Storm Flow
  
  • 1. Controlled by fully lined reinforced concrete channels
    
    • a. Controls high velocity flow
      
    • b. Prevents scour
      
• F. Uncontrolled Local Street Runoff
  
  • 1. Storm drains and tunnels connect to major flood control facilities
    
  • 2. The total length of all LA Cnty channels of all types is 2500 miles
    

I. The Need For A Comprehensive Flood Control Program | II. Flood Control Problems In Southern California | III. Vast Flood Control System has Problems | IV. Water Conservation Is An Important Function Of The Entire System | V. Santa Ana River Flood Control Project | Links | top | Geol 357 Home

III. VAST FLOOD CONTROL SYSTEM HAS PROBLEMS

• A. It probably can not handle a 100 year flood
  
  • 1. Public should be as concerned about this as a great Earthquake
    
• B. Disposal of debris from debris dams is a problem
  
  • 1. Average annual disposal is 450,000 cu. yds at $8-10/cu yd (1990)
    
  • 2. Material is hauled to disposal beds in the mountain canyons
    
    • a. Disposal space is running low
      
  • 3. County will use 81 acre gravel pit near Irwindale
    
    • a. Should last 50 years

I. The Need For A Comprehensive Flood Control Program | II. Flood Control Problems In Southern California | III. Vast Flood Control System has Problems | IV. Water Conservation Is An Important Function Of The Entire System | V. Santa Ana River Flood Control Project | Links | top | Geol 357 Home

IV. WATER CONSERVATION IS AN IMPORTANT FUNCTION OF THE ENTIRE SYSTEM

• A. Water is saved for Public Use
  
  • 1. Done in mountain and upper valley reservoirs and lower valley drainages
    
  • 2. Water may be directed to spreading grounds
    
    • a. Recharges groundwater supplies
      
• B. Importance of this Function
  
  • 1. Groundwater is cheaper than imported water
    
  • 2. Groundwater is not subject to Earthquake emergency problems
    

I. The Need For A Comprehensive Flood Control Program | II. Flood Control Problems In Southern California | III. Vast Flood Control System has Problems | IV. Water Conservation Is An Important Function Of The Entire System | V. Santa Ana River Flood Control Project | Links | top | Geol 357 Home

V. SANTA ANA RIVER FLOOD CONTROL PROJECT

• A. This is a Local, State, and Federal Effort
  
  • 1. Cost will be $1.1 billion
    
    • a. Part of a $16.3 billion federal package
      
    • b. $822,000,000 federal funds
      
    • c. $268,000,000 state and local funds
      
• B. Project Protects 90 miles along Santa Ana River
  
  • 1. Will handle a worst in 170 years flood
    
  • 2. This area is regarded by the Army Corp of Engineers as the worst flood hazard west of the Mississippi
    
    • a. Serious flood could cause 3000 deaths
      
    • b. Could cause $14 billion damage in Orange County alone
• B. Largest CA river basin south of the Sierra Nevada
  
  • 1. River has breached its levees many times
    
    • a. 1862 - large flood, small population
      
    • b. 1938 - smaller flood, larger population
      
• C. Santa Ana River Drainage Basin
  
  • 1. These are the projects owned and operated by the Los Angeles District Corps of Engineers in the
  • Santa Ana River Basin
    
  • 2. Seven Oaks Dam
    
  • 3. San Antonio Dam
    
  • 4. Prado Dam
    
  • 5. Carbon Canyon Dam
    
• D. Project includes:
  
  • 1. 550' long Earthfill dam 4 miles above Mentone
    
  • 2. Raise the Prado Dam 30 feet

I. The Need For A Comprehensive Flood Control Program | II. Flood Control Problems In Southern California | III. Vast Flood Control System has Problems | IV. Water Conservation Is An Important Function Of The Entire System | V. Santa Ana River Flood Control Project | Links | top | Geol 357 Home