DEBRIS FLOW
MITIGATION
TECHNIQUES
Check Dams
Check dams are commonly employed to arrest channel downcutting and store
sediment.
These can also be employed to create flatter surfaces
and retard development of destructive snouts on
Drop structures and debris fences
•
Debris fences and
drop structures are
intended to impede
flow, and thereby
retard development
of destructive snouts,
which can easily
overflow natural
channels
Debris Fences
• Debris fences can be designed to absorb the kinetic impacts of
either rockfalls or debris flows.
• This shows a debris fence designed by Geobrugg Protection
Systems of Switzerland. These fences employ flexible anchors with spiral cable ties, a coiled ring brake (fuse) on the restraining cable tieback, which releases after a threshold load impacts the system.
• Ring net barriers were
originally developed for use as underwater
antisubmarine nets during the Second World War, by Allied and Axis powers
Debris Basins
•
Debris basins provide
the most fail-safe
method to mitigate
damage to developed
areas from debris flows.
•
They need to be sized
to retain 3X the 100 yr
flow of the channel
Fail-Safe Debris Basins
• Debris Basins need to be
designed-in-depth with sufficient redundancy to
survive extreme events, with large volumes of clastic
sediment, trees, shrubs, etc.
• Bollards needed on overflow
spillways
Duty to Maintain Basins
• Debris basins must be mucked out periodically to maintain
storage capacity, as shown here.
• Los Angeles County mucks between 25 and 50 million cubic
A-wall diversion structures are intended to deflect fluid debris and route it around high-value structures
There must be some
accommodation for storage of the deflected debris,
either in streets, a basin, or channel reach below the structure
A-Walls
•
This shows a
combination A-wall and
retaining wall
constructed above a
multi-story home in
east Los Angeles
•
Note debris flow scars
in zero order watershed
to left of residence
AVOID Flow
Constrictions
Hwy 138 box culvert Heath Creek serves to constrict flow
Natural bedrock narrows at head of Heath Creek near Wrightwood, CA constricts flows, which periodically surge downstream, enveloping everything in their path
One of the easiest ways to avoid problems with debris flows is to prevent surge flows, which occur because of channel constrictions. These
Provide
unobstructed flow
path
• Whenever possible,
maintain unobstructed flow paths across
channels prone to debris flowage, as shown here.
• Use clear span bridges
over the 100-year
recurrence frequency flow channel
• Armored channels can be sized for debris flows using accepted
principles of fluid mechanics in the debris flow literature, emanating from CA, CO, Or, WA, British Columbia, Japan and Switzerland. This example is from Colorado (from P. Santi).
Provide Benches or basins for debris
storage
• Don’t allow debris to
flow into developed parcels
• Provide storage area
above or below developed parcels
• Construct debris
benches and low head basins, shown at left.
• Jersey Barriers and concrete K-rails, like those shown here,
can easily be employed to train debris along benches or roads. Several design charts exist for sizing these (from P. Santi)
Reinforced concrete shelters have been employed along highways and railroads around the world to safely convey debris flows over these corridors. Such structures still require maintenance. These examples are from Taiwan and British Columbia.
NEW HIGH-TECH TOOLS
FOR IDENTIFYING
•
Digital map image of the West Lost Trail Creek
Landslide created by overlaying USGS DOQQ aerial
photo on 10 m Digital Elevation Model, then rotating in
space, using ArcGIS 9.1
Synthetic Aperture Radar
(SAR)
•
SAR has ability to see through cloudsand woody vegetation, up to about 4 inches thick.
•
It provides acute verticalexaggeration of geomorphic
features, crucial to landslide hazard mapping
•
Image resolution and densitycontrols scale of mappable features, such as these debris lobes
•
LiDAR image
of Paine
Run, Virginia revealed
an active fault scarp
never previously seen,
mapped, or even
imagined!
•
“Bare Earth”
assessment has
incredible potential for
landslide mapping
LiDAR
Seattle/Toe Jam Hill Fault Zone
•
Another USGS
LiDAR imaging
project that
revealed
hitherto
unmapped
Holocene fault
scarps
•
Common
problem in
wooded terrain
•
Algorithms allow for rapid screening of large land areas, whencombined with neural networks to create an artificial intelligence (AI) program to enable discriminate mapping.
•
Accuracy depends on quality and scale of digital elevation models•
Will supplant stereopair aerial photographic methods forreconnaissance mapping of potential landslide hazards
•
Limitations would be spelled out on map products, recommendingsite-specific investigations be employed to determine presence, depth and character of past landslippage.
•
Property would not be “condemned” unfit for development solely oninterpretation of the regional hazard map.
Recent USGS Landslide Maps
The USGS has used San Mateo County, CA as their prototype hazard mapping area since 1969. These plots show debris flow hazard map (at left) and landslide incidence map (at right) for Montara Mountain area of northern San Mateo County, CA.
•
End Users will demand increasingly detailed assessments. Landslide hazardmaps generated using GIS can be overlain on tax assessor parcel maps will be used like FEMA Flood Insurance Rate Maps, spreading risk to lenders and
•
Future map products will appear like this, overlaying mapinformation on DEMs and digital color images. These products relate well to the general public, planners, and decision makers. ArcGIS 9.1 product.
MONITORING
SYSTEM
Installation of twelve items in the field
Debris detection sensor
Line 3 Line 1 Line 2 GeoPhone Tiltmeter Master logger Water contents sensor Water contents sensor
*Master logger : 1 set (rain gauge included) *Water contents sensor : 4 sets (RF incl.) *Debris flow detection sensor : 3 sets (RF logger included) *Geophone : 1 set
1st check dam RF logger
Water contents Slope displ.
Debris flow Detection logger Geophone logger
Wire sensor
Detailed photographs of the system
2nd check dam
Wire sensors
Detailed photographs of the system
Wire sensor Tensioning of the wire sensor Wire sensor fixation device
GeoPhone and wire sensor logger GeoPhone sensor
Excavation of the ground Sensors and RF logger
Master logger
Installation of web camera
Detailed photographs of the system
Window of the site image by the web camera
Sensor mapping
Tables of the measured data
Graphs of the data