ENVI 485 02/20/07

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ENVI 485 02/20/07

• STEAMS AND FLOODING (cont.)
• MASS WASTING

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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.

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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.

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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.

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Rainiest years in San Diego history

• 1. 1883-84    25.97
• 2. 1940-41   24.74
• 3. 1977-78   18.71
• 4. 1921-22   18.65
• 5. 2004-05    22.81

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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
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Meandering River, showing forms and processes
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Meander on the Colorado River
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Erosion
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Koyakuk River, Alaska, showing meander bends,
point bar, and cut bank
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Show animation
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Braided channels in Granada, southern Spain with
multiple channels, steep gradient, and coarse
gravel
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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 rivers velocity Leading the change in river dynamics

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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
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Floods In The US
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Flooding

Flooding Overbank flow condition, discharge greater than channels 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

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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

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Example of a discharge-frequency curve for
Patrick River
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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

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Smaller floods are more affected by urbanization
than larger floods
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Mean annual flood RI 2.23
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Effect of dam on erosion
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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

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Adjustments to Flood Hazards

The structural approach Engineering barriers Levee augmentation Channelization River-channel restoration Flood insurance Flood-proofing
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Floodplain without and with levees
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07_28b Placing riprap to defend the bank
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Natural vs. channelized stream
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Concrete channel in LA
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07_28a Urban stream restoration by controlling
erosion and deposition
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Landslide/Mass wasting
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Factors that influence slope stability (all
related to shear stress)

• 1) Slope
• 2) Fluid
• 3) Vegetation
• 4) Earthquakes
• 5) material type (clays) geologic structure
• 6) human activities

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General classification of landslides

• 1) Slides
• rock and/or sediment slides along Earth's surface
• 2) Falls/Topples
• rocks or soils fall or bounce through the air
• 3) Flows
• sediment flows across Earth's surface
• Slow flow is creep
• Fast flow is an avalanche
• 4) Complex (combination of the above)

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• Slides
• Distinct basal surface
• Rotational slide (Slump)
• Curved basal surface
• Translational slide
• Flat basal surface

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Mass movements occur when the downward pull of
gravity overcomes the forces (usually frictional)
resisting it.
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Problematic Formations in California

• Capay Formation
• Capistrano Formation
• Catalina Schist
• Chico-Martinez Formation
• Clarmont Shale
• Contra Costa Group
• Coyote Formation
• Fernando Formation
• Franciscan Formation
• La Habra Formatino
• Ladd Formation (Holz shale member)
• Meganos Formation
• Mehrten Formation
• Merced Formation
• Modelo Formation

• Monterey Formation
• Moreno formation
• Orinda Formation
• Otay Formation
• Pelona Schist
• Pico Formation
• Placerita Series
• Puente Formation
• Purisima Formation
• San Pedro Formation
• Santa Monica Slate
• Sespe Formation
• Siesta Formation
• Topanga Formation
• Trabuco Formation
• Valley Springs Formation
• Vaqueros Formation

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Slope Stability Analysis

• Requires an accurate characterization of
• 1. Surface topography,
• 2. Subsurface stratigraphy,
• 3. Subsurface water levels and possible
  subsurface flow patterns,
• 4. Shear strength of materials through which the
  failure surface may pass, and
• 5. Unit weight of the materials overlying
  potential failure planes.

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Dont Forget Earthquakes!

• A seismic slope stability analysis requires
  consideration of each of the above factors for
  static stability, as well as characterization of
• 1. Design-basis earthquake ground motions at the
  site, and
• 2. Earthquake shaking effects on the strength and
  stress-deformation behavior of the soil,
  including pore pressure generation and rate
  effects (which can decrease or increase the shear
  strengths relative to the static case).

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Recognition and avoidance of landslide hazards

• Detailed analysis of hillslope stability requires
  the expertise of engineers. Planners need to
  decide
• (1) whether a site is in a stable area
• (2) whether there is enough uncertainty to
  warrant a detailed site investigation by an
  expert
• (3) whether the site is so obviously unstable
  that it should be avoided.

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California Landslide Laws and Regulations

• The State of California requires analysis of the
  stability of slopes for certain projects.
• The authority to require analysis of slope
  stability is provided by the Seismic Hazards
  Mapping Act of 1990 (Chapter 7.8, Sections 2690
  et. seq., California Public Resources Code).
• The Act protects public safety from the effects
  of strong ground shaking, liquefaction,
  landslides, or other ground failure caused by
  earthquakes. The Act is a companion and
  complement to the Alquist-Priolo Earthquake Fault
  Zoning Act.
• Chapters 18 and 33 (formerly 70) of the
  Uniform/California Building Code provide the
  authority for local Building Departments to
  require geotechnical reports for various
  projects.

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Techniques for evaluating landslide hazards

• (1) Past hillslope failures
• Direct evidence in airphoto, indirect evidence of
  altered vegetation, subtle topographic features,
  deposits formed by hillslope failures.
• (2) Conditions that are conductive to hillslope
  failures
• Steep slope gradients, mechanically weak
  geological material, poor permeability, high
  water table, seepage in the vicinity of steep
  slope.
• (3) De-stabilizing effects of planned development
  Undercutting, overloading, changing hydrologic
  conditions.

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Planning

• Avoid building on steep slopes
• Avoid building in hazardous areas
• Restrict or eliminate human activities in these
  zones
• Make wise use of hazards maps

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Site Investigation and Geologic Studies of Slope
Stability

• 1. Study and review of published and unpublished
  geologic information (both regional and site
  specific), and of available stereoscopic and
  oblique aerial photographs.
• 2. Field mapping and subsurface exploration.
• 3. Analysis of the geologic failure mechanisms
  that could occur at the site during the life span
  of the project.
• 4. Presentation and analysis of the data,
  including an evaluation of the potential impact
  of geologic conditions on the project.

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Landslide Mitigation

• Slopes that possess factors of safety less than
  required by the governing agency, or with
  unacceptably large seismic slope displacements,
  require avoidance or mitigation to improve their
  stability.

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Landslide Mitigation

• (1) hazard avoidance,
• (2) grading to improve slope stability,
• (3) reinforcement of the slope or improvement of
  the soil within the slope, and
• (4) reinforcement of the structure built on the
  slope to tolerate the anticipated displacement.

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Avoidance

• The simplest method of mitigation may be to avoid
  construction on or adjacent to a potentially
  unstable slope.
• The setback distance is based on the slope
  configuration, probable mode of slope failure,
  factor of safety, and potential consequences of
  failure.
• The required setback cannot generally be
  accurately calculated, therefore a large degree
  of engineering/geologic judgment is required.

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Grading

• Grading can often be performed to entirely or
  partially remove potentially unstable soil
• The available grading methods range from
• Reconfiguration of the slope surface to a stable
  gradient (flattening)
• Removal and recompaction of a soil that is
  preferentially weak in an unfavorable direction
  and its replacement with a more homogeneous soil
  with a higher strength.

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Engineered Stabilization

• A grading solution to a slope stability problem
  is not always feasible due to physical
  constraints such as property-line location,
  location of existing structures, the presence of
  steep slopes, and/or the presence of very
  low-strength soil.
• In such cases, it may be feasible to mechanically
  stabilize the slide mass or to improve the soil
  with admixture stabilization.

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Mechanical Stabilization

• Retaining walls
• Deep foundations (i.e., piles or drilled shafts)
• Soil reinforcement with geosynthetics, tieback
  anchors, and soil nails.
• Common admixture stabilization measures include
  cement and lime treatment as well as Geofibers.

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concrete
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Dewatering

• Water can reduce shear strength of the soil,
  reduce the shear resistance through buoyancy
  effects, and impose seepage forces causing slope
  failure.
• Both passive and active dewatering/subsurface-wate
  r-control systems can be used.
• A slope can be "passively" dewatered by
  installing slightly inclined gravity dewatering
  wells into the slope.
• Vertical pumped-wells also can be utilized to
  lower subsurface water levels.
• The effectiveness of dewatering wells is
  dependent on the permeability of the soil.

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3) fluid removal
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3) fluid removal
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Containment

• Loose materials, such as colluvium, slopewash,
  slide debris, and broken rock, can be collected
  by a containment structure capable of holding the
  volume of material that is expected to fail.
• The containment structure type, size, and
  configuration will depend on the anticipated
  volume to be retained.
• Debris basins, graded berms, graded ditches,
  debris walls, and slough walls can be used. In
  some cases, debris fences may be permitted,
  although those structures often fail upon
  high-velocity impact.

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Deflection

• Walls or berms that are constructed at an angle
  to the expected path of a debris flow can be used
  to deflect and transport debris around a
  structure.
• Required channel gradients may range from 10 to
  40 percent depending on the expected viscosity of
  the debris and whether the channel is earthen or
  paved.

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2) retention structures
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Diversion walls, Taiwan
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Close-up of previous slide note tires for
cushioning
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Flumes for diverting debris flows, Lamosano, Italy
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Slope Protection for Rock Slopes

• Woven wire mesh is hung from anchors drilled into
  stable rock and is placed over the slope face to
  help keep dislodged rocks from bouncing as they
  fall.
• Wire mesh systems can contain large rocks (3 feet
  in diameter) traveling at fast speeds.
• It is also possible to hold rocks in position
  with cables, rock bolts, or gunite slope
  covering.

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Chicken wire
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Resistant Structures

• Examples of structural systems that can resist
  damage include mat foundations and very stiff,
  widely spaced piles.
• Mat foundations are designed to resist or
  minimize deflection or distortion of the
  structure resting on the mat as a result of
  permanent displacement of the underlying ground.
  The mat foundation itself may move or settle
  differentially, but the mat is sufficiently stiff
  to reduce bending in the structure to a tolerable
  level.

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Resistant Structures

• Another instance where a building can be designed
  to resist damage to earth movement involves
  structures built over landslides experiencing
  plastic flow.
• Flows that do not move as a rigid block can be
  penetrated with a series of widely spaced stiff
  piles.
• These piles are designed to resist loading
  imposed by moving material.

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Summary - Reduction of landslide hazards

• 1) slope reduction
• Reduce the slope angle
• Supporting material at base of slope
• Reduce the load by removing rock or soil
• 2) retaining structures
• 3) fluid removal
• 4) vegetation
• 5) Others (soil hardening, piles, rock bolts)