HomeMy WebLinkAboutMulti-Hazard Mitigation Plan: 06. Landslides And Rock Falls CIVIL DEFENSE AGENCY
COUNTY OF HAWAII
920 ULULANI STREET HILO,HAWAII 96720
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6. Landslides and Rock Falls
Chapter 6:Hazard Analysis—Landslides and Rock Falls
CHAPTER 6 - LANDSLIDES AND ROCK FALLS
6.1 Description of Hazard
Landslides are one of the natural processes of erosion resulting from mass transfer, the
movement of material down the side of slopes under the influence of gravity. Mass transfer
may also occur as falling, sliding or rolling of individual rocks or rock fragments,washing of
loose materials by rain runoff and soil creep. Although gravity is the primary reason for
landslides, other naturally occurring factors include:
• Erosion by rivers or ocean waves creating over steepened slopes;
• Rock and soil slopes that are weakened through saturation by heavy rain and runoff,
• Stress created by earthquakes;
• High winds uprooting trees along steep slopes.
Human made causes of landslides may include grading, terrain cutting and filling and devel-
opment on hillsides or excessive slopes.
Landslides can occur on any sloping terrain given the right conditions of soil, moisture and
the angle of slope. This ongoing process is generally slow and rarely noticeable.
Occasionally landslides will occur in a spectacular rush or an abrupt collapse, often triggered
by another natural hazard such as an earthquake or flooding.
6.1.1 Hazard Areas
Areas that may be considered prone to landslides may include the following:
• On existing old landslides
• On or at the base of slopes
• In or at the base of minor drainage hollows
• At the base or top of an old fill slope
• At the base or top of a steep cut slope
• Developed hillsides where leach field septic systems are used.2t
Roadcuts and other altered or excavated areas of slopes are particularly susceptible to debris
flows and abrupt collapse. Several areas along the Hamakua Coast are chronic problem areas
particularly during periods of heavy rainfall. In addition, the three major gulches Maulua,
Laupahoehoe and Kaawalii, which are known for the "horseshoe" turns on State Highway
19, present rock fall problems. The rock fall problems arise during times of heavy rain as
well as strong winds which sway the trees along the walls of the gulch back and forth and
loosen the dirt and rocks underneath it.
Homes that are built along the edge of the Hamakua coast cliffs are also susceptible to abrupt
collapse particularly during times of heavy rainfall. These cliffs along the northeast coast of
Mauna Kea range in height from 50 to 350 feet. The sea cliffs are eroded through a continu-
ous process of wave action at the base of the cliff which cuts a notch and undermines the
21 USGS Fact Sheet FS-071-00
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Chapter 6:Hazard Analysis—Landslides and Rock Falls
higher section of the cliff which eventually collapses and drops off. A photograph taken
following a landslide caused by the Kiholo Bay earthquake is shown in Figure 6-1.
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Figure 6-1. Massive coastal escarpment landslides into the ocean,Hamakua Coast
(Photograph courtesy of Hawaii Civil Defense Agency)
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Chapter 6:Hazard Analysis—Landslides and Rock Falls
Figure 6-2. Examples or earthquake-induced rock falls causing road hazard or closure
6.2 Significant Catastrophic Events
Giant catastrophic slides have occurred around the major Hawaiian Islands thousands of
years ago. At least 15 giant landslides have been identified by United States Geological
Survey (USGS)marine scientist with the most recent occurring approximately 100,000 years
ago off the Kona coast. Each of these slides has resulted in huge land losses to the islands
and resulted in large waves that have carried rocks and sediments as high as 1000 feet above
sea level. Although these giant landslides have the potential for enormous loss of life,
property and resources, these events are infrequent in human terms, occurring perhaps once
every few tens of thousands of years. (USGS,Volcano Watch, 1997)
The U.S. Geological Survey (USGS) suggests that we should not be overly concerned about
the giant landslides because they are so infrequent (once every few tens of thousands of
years) and because"we can do virtually nothing to mitigate such gigantic slides and tsunami-
except to follow evacuation routes predetermined by Civil Defense and hope that the tsunami
is not too high." (USGS Volcano Watch, 1997) "Worrying realistically means directing our
limited resources to the hazardous events most likely to occur-the ones whose effects we now
have the ability to mitigate through education and foresight." (USGS Volcano Watch, 1997)
A significant landslide mentioned in historical times is a mudflow triggered by the largest
earthquake in recorded history in April, 1868. The mudflow killed 31 people in Wood
Valley in the Ka'u district. No other landslide event has been mentioned as resulting in any
loss of life.
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Chapter 6:Hazard Analysis—Landslides and Rock Falls
6.3 Probability of Occurrence
Landslides and rock falls typically occur as a result of another hazard or force of nature, and
due to the multitude of forces that may cause landslides and rock falls assigning or
calculating the return period of events of a destructive magnitude would be extremely
difficult. However, it is possible to determine areas that have a higher potential of risk for
such events as was done in a study was conducted by URS. In this study a conceptual level
slope risk map was prepared, using an adaptation of the slope hazard methodology given in
the FEMA 2007 HAZUS-MH MR3 Technical Manual. The approach involved the
interactions of three primary slope hazard input criteria simplified to low, medium and high
hazard susceptibility, described below:
Topography: A high resolution slope map of the entire island is composed of recent aerial,
lidar and satellite 10m DEM topographic survey data from USGS and FEMA sources. Slope
criteria are categorized as:
1. <20degreees (low hazard susceptibility)
2. <20-40 degrees (medium hazard susceptibility)
3. >40 degrees (high hazard susceptibility)
Geologic groups: Soil and rock categories assigned based upon strength and susceptibility to
landslide was derived from existing UDSA NCRS surface maps and USGS geologic maps.
Adapted geologic group assignments for the Island of Hawaii include:
1. shallow rock, fresh volcanics (low hazard susceptibility)
2. clay surficial soils,weathered rock (medium hazard susceptibility)
3. weak soft soils, ash deposits, mapped historic slide talus
(high hazard susceptibility)
In addition, URS interpreted historic Hawaii DOT landslide locations, as medium hazard for
locations with preliminary FHWA Rockfall Hazard Rating System (RHRS) of less than 200
and high hazard for locations with preliminary RHRS values greater than 200.
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Chapter 6:Hazard Analysis—Landslides and Rock Falls
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LOW MEDIUM HIGH
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Figure 6-3. Slope hazards and soil types on Hawaii Island
Soil moisture: The HAZUS MH4 slope hazard methodology considers soil moisture as either
wet or dry, with wet soils posing increased slope hazard. Soil moisture assignments are
derived from recent NOAA rainfall mapping of the island since regional groundwater and
soil moisture data is unavailable island wide. Areas receiving greater than 2000 mm annual
precipitation are considered wet soil, corresponding largely to the windward side of the
island. In addition, coastal areas below elevation 200' are considered wet due to potential
groundwater seepage gradients from higher elevations, except in the and Kona coast areas.
6-5 Hawaii County Multi-Hazard Mitigation Plan
Chapter 6:Hazard Analysis—Landslides and Rock Falls
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Figure 6-4. Precipitation and groundwater condition on Hawaii Island
• HAZUS-MH MR4 provides susceptibility categories combining slope angle, soil type
and soil moisture in ten categories (I to X), with an assigned yield acceleration to
each category. The combined susceptibility categories when mapped thus represent
zones of potential landslide triggering under different levels of ground shaking.
• The ten HAZUS risk zones were converted to polygon shape files and mapped
to census blocks/tracts, with an interface to Geodatabase (NAD83) so that
HAZUS SHAKEMAP runs will then interpret triggering criteria and assign a
triggered or non triggered assessment for a given earthquake scenario, and
then incorporate loss estimates with other HAZUS estimate modules.
6-6 Hawaii County Multi-Hazard Mitigation Plan
Chapter 6:Hazard Analysis—Landslides and Rock Falls
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Figure 6-5. Landslide susceptibility map of Hawaii Island
HAMS assigned landslide sucscepibility categories for the Island of Hawaii.This figures is a conglomerate of
spatially assigned topography,geology and soilo mostures relationships with superimposed mapped historical
landslides
6.4 Risk Assessment
The random nature of landslide and rock fall events presents a difficult challenge in assessing
the risk in relation to other hazards that occur at more probabilistic intervals. As discussed in
the previous section the HAZUS Earthquake model incorporates potential losses due to
landslides into its economic loss predictions. Currently there is no way of quantifying the
total risk of landslides and rock falls due to other hazards such as heavy rain and coastal
erosion, other than history.
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Chapter 6:Hazard Analysis—Landslides and Rock Falls
6.5 Mitigation Strategies
6.5.1 Previous/Current Efforts
The State Department of Transportation tries to address the landslide and rock fall problems
through their maintenance budget. The more chronic problem areas will require additional
CIP (Capital Improvement Project) funding that has not been provided to date; although
landslide events are a concern for our roadways, utilities and structures, and data is available
regarding the frequency and severity of these events. Discussions with State and County
highway maintenance agencies have yielded only anecdotal information regarding landslide
problem areas. Rockfall protection projects have been conducted along stretches of Hawaii
Belt Road. Figure 6-6 illustrates locations where historic landslides have impacted major
roadways in Hawaii County.
Legend
Slope Hazards
_ - Historic Landslides
Other Roads
Major Roads
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OUTH KONA
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Figure 6-6. Map of historic landslide locations impacting major roads
Individual homeowners are attempting to address the problem of the collapse of the
Hamakua Coast sea cliffs by reinforcing the cliff sides and anchoring their structures.
Additional information is needed to assess these efforts and to determine adequate setbacks
for future construction.
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Chapter 6:Hazard Analysis—Landslides and Rock Falls
6.5.2 Future Plans
Improving the landslide susceptibility map:
Project Description Status
New UH study is to generate GIS maps are included,which To start in 2010,completion
potential slope stability hazard could be used for prioritization of subsequent to this hazard
maps which could be used to investigations as well as assessment mitigation plan update
identify zones that may warrant tools for earthquake preparedness.
further detailed evaluation during UH will take the URS'information
project planning. from the landslide-potential map to
develop 2nd generation maps that
will identify priority criteria and
critical areas for more detailed
study(e.g.,road cuts and lifelines).
• Subdivide the island into subtracts for slope hazards to allow practical use of more
slope susceptibility categories giving greater resolution. Digital file sizes limited the
island wide mapping to 3 slope angle categories. Resolution of the final mapping
varies from layer to layer due to disparate data sources. The scale of the 17" x 22"
map is 1:333,732.
• Refine key slope areas, colluvium, debris flows and poor soils intersect w/boring
data and shear strike measurements (field and from maps).
• Refine soil wetness map with well and groundwater model data.
• Incorporate final HDOT RHRS data.
• Validate estimated generalized allowable slope angle relationship for local
fractured/weathered basalts, soils, and ash with lab testing.
• Incorporate more detailed offshore submarine slope modeling.
• Incorporate more detailed USGS slope modeling with:
• Simple rigid block deformation modeling(Newmark)using shallow friction
behavior of basalts and andesites, ignoring soil strength.
• Assigning spatial probability& susceptibility by event
• Mapping of critical accelerations using Factor of Safety contours.
• Consider inaccessible damages on very steep (>60degrees)high(>1000ft)
cliffs at the North Kohala Ditch and general trends of landslide damage on
leeward(west) slopes.
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