ENVI 485 02/20/07 - PowerPoint PPT Presentation

1 / 88
About This Presentation
Title:

ENVI 485 02/20/07

Description:

envi 485 02/20/07 steams and flooding (cont.) mass wasting – PowerPoint PPT presentation

Number of Views:71
Avg rating:3.0/5.0
Slides: 89
Provided by: SarahC170
Learn more at: http://home.sandiego.edu
Category:

less

Transcript and Presenter's Notes

Title: ENVI 485 02/20/07


1
ENVI 485 02/20/07
  • STEAMS AND FLOODING (cont.)
  • MASS WASTING

2
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.

3
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.

4
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.

5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
(No Transcript)
11
(No Transcript)
12
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

13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
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
19
Meandering River, showing forms and processes
20
Meander on the Colorado River
21
Erosion
22
Koyakuk River, Alaska, showing meander bends,
point bar, and cut bank
23
Show animation
24
(No Transcript)
25
Braided channels in Granada, southern Spain with
multiple channels, steep gradient, and coarse
gravel
26
(No Transcript)
27
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

28
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
29
Floods In The US
30
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

31
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

32
(No Transcript)
33
(No Transcript)
34
Example of a discharge-frequency curve for
Patrick River
35
(No Transcript)
36
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

37
(No Transcript)
38
(No Transcript)
39
Smaller floods are more affected by urbanization
than larger floods
40
Mean annual flood RI 2.23
41
(No Transcript)
42
(No Transcript)
43
Effect of dam on erosion
44
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

45
Adjustments to Flood Hazards

The structural approach Engineering barriers Levee augmentation Channelization River-channel restoration Flood insurance Flood-proofing
46
Floodplain without and with levees
47
07_28b Placing riprap to defend the bank
48
Natural vs. channelized stream
49
Concrete channel in LA
50
07_28a Urban stream restoration by controlling
erosion and deposition
51
Landslide/Mass wasting
52
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

53
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)

54
  • Slides
  • Distinct basal surface
  • Rotational slide (Slump)
  • Curved basal surface
  • Translational slide
  • Flat basal surface

55
(No Transcript)
56
(No Transcript)
57
Mass movements occur when the downward pull of
gravity overcomes the forces (usually frictional)
resisting it.
58
(No Transcript)
59
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

60
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.

61
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).

62
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.

63
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.

64
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.

65
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

66
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.

67
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.

68
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.

69
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.

70
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.

71
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.

72
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.

73
concrete
74
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.

75
3) fluid removal
76
3) fluid removal
77
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.

78
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.

79
2) retention structures
80
Diversion walls, Taiwan
81
Close-up of previous slide note tires for
cushioning
82
Flumes for diverting debris flows, Lamosano, Italy
83
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.

84
Chicken wire
85
(No Transcript)
86
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.

87
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.

88
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)
Write a Comment
User Comments (0)
About PowerShow.com