envi 485 02/15/07 intro. to natural hazards & disasters steams and flooding case study 1

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. occurrenceinfluenced byhumans?

Catastrophicpotential

flood 86 yes H

earthquake 50+ yes H

landslide 25 yes M

volcano 1 no H

coastalerosion

0 yes L

expansivesoils

0 no L

Effects of Selected Hazards in the U.S.

Hazard Deaths/yr. occurrenceinfluenced byhumans?

Catastrophicpotential

hurricane 55 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 GageVelocity X Area = Q

Rating curve

USGS National Streamflow Program

http://water.usgs.govhttp://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 right-of-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