ENVI 485 022007

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ENVI 485 02/20/07

• STEAMS AND FLOODING (cont.)
• MASS WASTING

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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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Regulation of the Floodplain

• Flood hazard mapping
• Floodway floodway fringe district
• Area of the floodplain covered by a 100 year
  flood
• Issues associated with calculating
• O.k. for some uses
• Many communities dont have the maps
• Flood Insurance Act (1968) Flood Disaster
  Protection Act
• Federally subsidized insurance programs
• Flood insurance required for federally insured
  mortgages
• Communities must enforse floodplain zone
  regulations to qualify
• Unintended consequences of these laws?

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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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Concrete channel in LA
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Natural vs. channelized stream
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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)