Title: ENVI 485 02/20/07
1ENVI 485 02/20/07
- STEAMS AND FLOODING (cont.)
- MASS WASTING
2San 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.
3San 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.
41927 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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12Rainiest 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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18River 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
19Meandering River, showing forms and processes
20Meander on the Colorado River
21Erosion
22Koyakuk River, Alaska, showing meander bends,
point bar, and cut bank
23Show animation
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25Braided channels in Granada, southern Spain with
multiple channels, steep gradient, and coarse
gravel
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27Effects 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
28Effects 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
29Floods In The US
30Flooding
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
31Flood 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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34Example of a discharge-frequency curve for
Patrick River
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36Urban 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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39Smaller floods are more affected by urbanization
than larger floods
40Mean annual flood RI 2.23
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43Effect of dam on erosion
44Regulation 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
45Adjustments to Flood Hazards
The structural approach Engineering barriers Levee augmentation Channelization River-channel restoration Flood insurance Flood-proofing
46Floodplain without and with levees
4707_28b Placing riprap to defend the bank
48Natural vs. channelized stream
49Concrete channel in LA
5007_28a Urban stream restoration by controlling
erosion and deposition
51Landslide/Mass wasting
52Factors 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
53General 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
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57Mass movements occur when the downward pull of
gravity overcomes the forces (usually frictional)
resisting it.
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59Problematic 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
60Slope 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.
61Dont 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).
62Recognition 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.
63California 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.
64Techniques 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.
65Planning
- Avoid building on steep slopes
- Avoid building in hazardous areas
- Restrict or eliminate human activities in these
zones - Make wise use of hazards maps
66Site 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.
67Landslide 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.
68Landslide 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.
69Avoidance
- 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.
70Grading
- 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.
71Engineered 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.
72Mechanical 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.
73concrete
74Dewatering
- 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.
753) fluid removal
763) fluid removal
77Containment
- 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.
78Deflection
- 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.
792) retention structures
80Diversion walls, Taiwan
81Close-up of previous slide note tires for
cushioning
82Flumes for diverting debris flows, Lamosano, Italy
83Slope 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.
84Chicken wire
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86Resistant 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.
87Resistant 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.
88Summary - 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)