CSULA - Geology 357: Flooding

A. Water Basics
- 1. Sea water = 97.2% of world's water
- 2. Fresh Water 2.8%
  - a. Polar ice and glaciers 2.15%
  - b. Groundwater 0.62%
  - c. Lakes and Rivers 0.01%
B. The Three Fates of Precipitation
- a. Infiltration
  - 1) The movement of water into rocks or soil through cracks and pore spaces
- b. Runoff
  - 1) Water that flows over the land
  - 2) It is generally of interest to cities to reduce runoff
- c. Transpiration/Evapotranspiration
  - 1) The release of water vapor to the atmosphere by plants
  - 2) 66% in the US
C. Evaluation of Precipitation
- 1. Depth or magnitude of the rainfall
- 2. Area over which the rain falls
- 3. Duration of the rain
- 4. Intensity
D. Recurrence Interval

1. The average recurrence interval between major storms or floods is easily calculated
- a. Is based on the probability that the flow will be equaled or exceeded in any given year.
- b. 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.
- c. Thus, that river is said to have a 100-year interval.
2. Engineers design for particular recurrence intervals

1. Deforestation - results in increased erosion
2. Overgrazing - also increased erosion
3. Urbanization
- a. More rapid discharge to streams causes:
  - 1) More frequent floods
  - 2) Higher flood peaks
- b. Urban runoff is highly toxic

A. River Erosion
- 1. Rivers cuts their valleys vertically and laterally
- 2. Lateral erosion produces broad valleys, flood plains, and meandering streams
- 3. Vertical erosion produces narrow valleys, rapids and waterfalls.
B. Flood Plains
- 1. Periodic floods deposit rich soils
  - a. Agricultural production on floods plains is followed by urbanization
- 2. Natural levees
  - a. Forms as floods deposit coarse detritus near the river
  - b. Naturally constraint the river except in the larger floods

B. Causes of Floods

1. High intensity - long duration precipitation

b. Examples:

4. Dam Failure

PRESENT STATUS OF DEATH & INJURY CASES ST. FRANCIS DAM DISASTER 15-Jul-29
Total Number of Persons Killed	306
This number includes the identified and missing dead on which claims were presented, and a few who were known to be killed but left no heirs. It does not include the unidentified bodies, of which there are ...	64
Number of Persons Injured	66
Number of Settled Death & Injury Claims	352
Number of Unsettled Death & Injury Claims	38
Total Amount of Death & Injury Claims	$3,674,207.56
Amount of Settlement of Above Claims	$915,751.74
Total Amount of Unsettled Death & Injury Claims	$1,376,251.35
The above amount was filed by 12 claimants, and covers 21 deaths and 4 injuries.
Number of Claims which have gone into Litigation	35.00
The above claims were filed by 10 claimants.
Entire Families Wiped Out	20
Number of persons reported missing, but on whom no inquiries were received or claims filed	67

from: http://www.scvhistory.com/scvhistory/stfrancis-claims071529.htm

4) Cause of failure complex
- a) Sespe formation prone to "slaking"
- b) Excessive titling when fully loaded
- c) An absence of seepage relief in the dam's sloping abutments
- d) And the partial reactivating of underlying paleo mega-slides within the Pelona Schist.
5) Some of the most important consequences included
- a) The formulation of the world's first dam safety agency
- b) Normalization of uniform engineering criteria for testing of compacted earthen materials
- c) A reassessment of all LADWP dams and reservoirs
- d) The formulation of a state-mandated process for arbitration of wrongful death suits
6) Excellent book:
- a) The St. Francis Dam Disaster Revisited edited By Doyce B. Nunis, Jr.

A. Provides flood insurance at affordable rates
B. But it also Requires:
- 1. Stringent control on new construction in flood plains
- 2. Comprehensive flood management plans