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산사태 강의자료-3

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(1)

DEBRIS FLOW

MITIGATION

TECHNIQUES

(2)

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

(3)
(4)

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

(5)

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

(6)
(7)

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

(8)

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

(9)

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

(10)

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

(11)

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

(12)

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

(13)

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

(14)

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).

(15)

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.

(16)

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)

(17)

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.

(18)

NEW HIGH-TECH TOOLS

FOR IDENTIFYING

(19)

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

(20)

Synthetic Aperture Radar

(SAR)

SAR has ability to see through clouds

and woody vegetation, up to about 4 inches thick.

It provides acute vertical

exaggeration of geomorphic

features, crucial to landslide hazard mapping

Image resolution and density

controls scale of mappable features, such as these debris lobes

(21)

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

(22)

Seattle/Toe Jam Hill Fault Zone

Another USGS

LiDAR imaging

project that

revealed

hitherto

unmapped

Holocene fault

scarps

Common

problem in

wooded terrain

(23)

Algorithms allow for rapid screening of large land areas, when

combined 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 for

reconnaissance mapping of potential landslide hazards

Limitations would be spelled out on map products, recommending

site-specific investigations be employed to determine presence, depth and character of past landslippage.

Property would not be “condemned” unfit for development solely on

interpretation of the regional hazard map.

(24)

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.

(25)

End Users will demand increasingly detailed assessments. Landslide hazard

maps 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

(26)

Future map products will appear like this, overlaying map

information on DEMs and digital color images. These products relate well to the general public, planners, and decision makers. ArcGIS 9.1 product.

(27)

MONITORING

SYSTEM

(28)

 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

(29)

1st check dam RF logger

Water contents Slope displ.

Debris flow Detection logger Geophone logger

Wire sensor

 Detailed photographs of the system

(30)

2nd check dam

Wire sensors

 Detailed photographs of the system

(31)

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

(32)

Window of the site image by the web camera

Sensor mapping

Tables of the measured data

Graphs of the data

참조

관련 문서

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