1. FLOOD CONTROL PROBLEMS IN SOUTHERN 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 , 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:
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

26. 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

27. 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

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