ENVI 485 02/15/07
• INTRO. TO NATURAL HAZARDS &
DISASTERS
• STEAMS AND FLOODING
• Case Study 1
Phase 1: Emergency
(days)
•
•
•
•
Normal activities stop
Search and rescue
Emergency shelter/feeding
Capital damaged or destroyed
Phase 2: Restoration
(weeks-months)
• Normal activities return but at minimal
levels
• Restoration of urban services
• Return of refugees
• Capital patched
•
•
•
•
•
Phases 3 & 4:
Reconstruction I & II
(months-years)
Normal activities return to predisaster levels
Capital rebuilt (replaced)
Activities improved and developed
Capital improved beyond predisaster levels
Disaster preparedness & response improved
Effects of Selected Hazards in the U.S.
Hazard
Deaths/yr.
flood
86
occurrence
Catastrophic
influenced by potential
humans?
yes
H
earthquake
50+
yes
H
landslide
25
yes
M
volcano
1
no
H
coastal
erosion
expansive
soils
0
yes
L
0
no
L
Effects of Selected Hazards in the U.S.
Hazard
Deaths/yr.
hurricane
55
occurrence
Catastrophic
influenced by potential
humans?
no
H
tornado
218
no
H
lightning
120
?
L
drought
0
?
M
freeze
0
no
L
Rivers and floods
Stream/river
–any body of water that flows in a channel
Floods
–any high flow of surface waters that overtops
normal confinements or covers land normally dry
–most devastating of all geologic agents -in loss of
life
Extent of a floodplain
Longitudinal Profile & Gadient
Drainage basin and cross sections at the headwater and near
base level
Sediment in Rivers
• Stream total load
 Bed load: Coarse particles moving along the
bottom of river channel
 Suspended load: Accounts for about 90% of its total load
 Dissolved load: Carried in chemical solutions
Show animation
Factors in Stream flow
• Several basic factors control the way a stream
behaves:
– Gradient [h/x] (expressed in meters per kilometers)
– Stream-cross-sectional area [A]
• (width * average depth, expressed in square meters)
– Average velocity of water flow [v]
• (expressed in meters per second)
– Discharge [Q] (expressed in cubic meters per second)
– Load (expressed in kilograms per cubic meter)
• Dissolved matter generally does not affect stream behavior
Discharge, Velocity, & Channel
Shape
• The relationship of discharge, velocity, and
channel shape for a stream can be expressed
by the equation:
Q
=
A
*
V
Discharge
Cross-sectional
Average
(m3/s)
area of stream
velocity
(width x average
(m/s)
depth) (m2)
Flood stage
• The elevation (in meters, feet, etc.) of the
water above normal at the highest point in
the flood
Stream Gage
•
•
•
•
•
USGS: 7000 nationwide
Measures water level (gage height) every 15 minutes
Sends data via satellite every 4 hours
“rating curve” converts gage height to discharge
Rating curve modifed after technition measures streamflow
on site every 6 weeks
Measuring
Stream
Velocity
Calculating Discharge from a Stream Gage
Velocity X Area = Q
Rating curve
USGS National Streamflow
Program
http://water.usgs.gov
http://waterdata.usgs.gov/ca/nwis/uv?11023000
San Diego River Watershed
Two USGS stream
gages
•Fashion Valley:
429 sq. mi
Santee Stream Gage
482 Stream gages in CA
1927 Flood
• Photo taken on
February 2, 1927 shows
the Old Town railroad
bridge washed out by
the flood. This rail rightof-way still exists - you
can see it looking east
from I-5; Friars Rd. runs
underneath it.
San Diego River
• 1852 - Since San Diego Bay was a deeper
harbor, and the San Diego River carried heavy
silt deposits, it was decided to deflect the San
Diego River into False Bay (Mission Bay)
• The project was completed in two years by
Indian laborers who reportedly hauled building
materials in baskets. The Darby dike washed out
one year after its completion and the San Diego
River returned to its old course.
San Diego River
• 1862 – Possibly the largest flood in the
history of the San Diego River occurred
(almost 100,000 cfs).
• 1875- New dike constructed (cobblestone
face two to three feet thick). A small
channel was constructed on the north side
of the dike that the river was diverted into
the eastern part of Mission Bay.
River Erosion
• Erosion types
 Abrasion by sediments transported by river
 Hydraulic action of moving water
 Chemical corrosion
• Erosion location
 Down cutting
 Lateral: Concentrating on the outer bends
 Headward erosion
Meandering River, showing forms and processes
Meander on the Colorado River
Erosion
Koyakuk River, Alaska, showing meander bends, point bar, and
cut bank
Show animation
Braided channels in Granada, southern Spain with multiple
channels, steep gradient, and coarse gravel
Effects of Land-Use Changes
• Changes in infiltration rate: Change of the
amount of water flowing into a river
• Soil erosion: Change in the amount of
sediments in a river
• Amount of water and sediments in river:
Changes in the velocity of water flow
• Changes in river’s velocity: Leading the
change in river dynamics
Effects of Land-Use Changes
• Forest to farmland
 Increases soil erosion, stream deposition
 Increases gradient and velocity
 Increases river-channel erosion
• Urban build-up
 Increases impervious cover
 Increases certain flood frequency
 Reduces the lag time of flood
Floods
In
The US
Flooding
• Flooding: Overbank flow condition, discharge
greater than channel’s holding capacity
• Stage: The height of the water level in a river
at a given location at a given time
• Hydrograph: a graph that plots stream
discharge (Q) against time (t)
• Lag time: The amount of time between the
occurrence of peak rainfall and the onset of
flooding
Flood magnitude
• Recurrence interval
– Discharge (Q) on a stream is measured over a period of
time (N)
– Each flood is ranked (highest discharge = 1) (M)
– Recurrence interval: (N + 1)/M
• Probability of a flood of a given magnitude in a
year is 1/recurrence interval
Example of a discharge-frequency curve for Patrick River
Urban development and flooding
• Flooding usually increased by urban development
– Affected by impervious cover
– Storm sewers
• More water reaches stream
• Water reaches stream faster
• Affects the relationship between rainfall-runoff
– Reduced lag time = “flashy discharge”
Smaller floods are more affected by
urbanization than larger floods
Mean annual flood: RI = 2.23
Effect of dam on erosion
Regulation of the Floodplain
• Floodplain belongs to the river system and
the river WILL reoccupy it.
• Flood hazard mapping
– Floodway & floodway fringe district
• Area of the floodplain covered by a 100 year flood
• O.k. for some uses
Adjustments to Flood Hazards
• The structural approach
 Engineering barriers: Levee augmentation
 Channelization
 River-channel restoration
• Flood insurance
• Flood-proofing
Floodplain without and with levees
07_28b Placing riprap to defend the bank
Natural vs.
channelized
stream
Concrete channel in LA
07_28a Urban
stream restoration
by controlling
erosion and
deposition