Part 7Part 7

DEBRIS FLOW DEBRIS FLOW MITIGATION MITIGATION

TECHNIQUESTECHNIQUES

Check DamsCheck Dams

Check dams are Check dams are commonly employed to commonly employed to arrest channel arrest channel downcutting and store downcutting and store sediment.sediment.

These can also be These can also be employed to create employed to create flatter surfaces and flatter surfaces and retard development of retard development of destructive snouts on destructive snouts on rapidly moving debris rapidly moving debris flows.flows.

Drop structures and debris fencesDrop structures and debris fences• Debris fences and Debris fences and

drop structures drop structures are intended to are intended to impede flow, and impede flow, and thereby retard thereby retard development of development of destructive destructive snouts, which can snouts, which can easily overflow easily overflow natural channels natural channels

Debris FencesDebris Fences

• Debris fences can be designed to absorb the kinetic Debris fences can be designed to absorb the kinetic impacts of either rockfalls or debris flows.impacts of either rockfalls or debris flows.

• This shows a debris fence designed by This shows a debris fence designed by Geobrugg Geobrugg Protection SystemsProtection Systems of Switzerland. These fences of Switzerland. These fences employ flexible anchors with spiral cable ties, a coiled employ flexible anchors with spiral cable ties, a coiled ring brake (fuse) on the restraining cable tieback, which ring brake (fuse) on the restraining cable tieback, which releases after a threshold load impacts the system.releases after a threshold load impacts the system.

• Ring net barriersRing net barriers were were originally developed for originally developed for use as underwater use as underwater antisubmarine nets antisubmarine nets during the Second during the Second World War, by Allied World War, by Allied and Axis powersand Axis powers

Contour GradingContour Grading

Contour grading removes most of the colluvium from bedrock hollows so it cannot mobilize into destructive debris flows.

Debris BasinsDebris Basins

• Debris basins Debris basins provide the most provide the most fail-safe method to fail-safe method to mitigate damage to mitigate damage to developed areas developed areas from debris flows.from debris flows.

• They need to be They need to be sized to retain 3X sized to retain 3X the 100 yr flow of the 100 yr flow of the channel the channel

Fail-Safe Debris BasinsFail-Safe Debris Basins

• Debris Basins need to be Debris Basins need to be designed-in-depth with designed-in-depth with sufficient redundancy to sufficient redundancy to survive extreme events, survive extreme events, with large volumes of with large volumes of clastic sediment, trees, clastic sediment, trees, shrubs, etc.shrubs, etc.

• BollardsBollards needed on needed on overflow spillways overflow spillways

Bollards on overflow crest to catch boulders

Duty to Maintain BasinsDuty to Maintain Basins

• Debris basins must be mucked out periodically to Debris basins must be mucked out periodically to maintain storage capacity, as shown here.maintain storage capacity, as shown here.

• Los Angeles County mucks between 25 and 50 million Los Angeles County mucks between 25 and 50 million cubic yards of debris per year.cubic yards of debris per year.

A-WallsA-Walls

A-wall diversion A-wall diversion structures are intended structures are intended to deflect fluid debris to deflect fluid debris and route it around high-and route it around high-value structuresvalue structures

There must be some There must be some accommodation for accommodation for storage of the deflected storage of the deflected debris, either in streets, debris, either in streets, a basin, or channel a basin, or channel reach below the reach below the structure structure

A-WallsA-Walls

• This shows a This shows a combination A-wall combination A-wall and retaining wall and retaining wall constructed above constructed above a multi-story home a multi-story home in east Los Angelesin east Los Angeles

• Note debris flow Note debris flow scars in zero order scars in zero order watershed to left of watershed to left of residenceresidence

Perforated Riser IntakesPerforated Riser Intakes

Experience has shown that perforated riser intake towers are a wise precaution to employ in debris basins, along with overflow spillways. This allows debris to accumulate in the basin and less turbid standing water to decant into the riser and pass on downstream, during extreme flow events.

AVOID Flow AVOID Flow ConstrictionsConstrictions

Hwy 138 box culvert Heath Creek Hwy 138 box culvert Heath Creek serves to constrict flowserves to constrict flow

Natural bedrock narrows at head of Heath Creek near Natural bedrock narrows at head of Heath Creek near Wrightwood, CA constricts flows, which periodically Wrightwood, CA constricts flows, which periodically surge downstream, enveloping everything in their pathsurge downstream, enveloping everything in their path

One of the easiest ways to avoid problems with debris flows is to One of the easiest ways to avoid problems with debris flows is to prevent surge flowsprevent surge flows, which occur because of channel , which occur because of channel constrictions. These constrictions can be natural or man-made, as constrictions. These constrictions can be natural or man-made, as shown here. shown here.

Provide Provide unobstructed unobstructed

flow pathflow path• Whenever possible, Whenever possible,

maintain maintain unobstructed flow unobstructed flow pathspaths across across channels prone to channels prone to debris flowage, as debris flowage, as shown here.shown here.

• Use clear span Use clear span bridges over the 100-bridges over the 100-year recurrence year recurrence frequency flow frequency flow channelchannel

• Armored channelsArmored channels can be sized for debris flows using can be sized for debris flows using accepted principles of fluid mechanics in the debris flow accepted principles of fluid mechanics in the debris flow literature, emanating from CA, CO, Or, WA, British literature, emanating from CA, CO, Or, WA, British Columbia, Japan and Switzerland. This example is from Columbia, Japan and Switzerland. This example is from Colorado (from P. Santi).Colorado (from P. Santi).

Provide Benches or basins Provide Benches or basins for debris storagefor debris storage

• Don’t allow debris Don’t allow debris to flow into to flow into developed parcelsdeveloped parcels

• Provide storage Provide storage area above or below area above or below developed parcels developed parcels

• Construct debris Construct debris benches and low benches and low head basins, shown head basins, shown at left.at left.

• Jersey Barriers and concrete K-rails, like those Jersey Barriers and concrete K-rails, like those shown here, can easily be employed to train debris shown here, can easily be employed to train debris along benches or roads. Several design charts exist along benches or roads. Several design charts exist for sizing these (from P. Santi)for sizing these (from P. Santi)

Reinforced concrete sheltersReinforced concrete shelters have been employed along have been employed along highways and railroads highways and railroads around the world to safely around the world to safely convey debris flows over convey debris flows over these corridors. Such these corridors. Such structures still require structures still require maintenance. These maintenance. These examples are from Taiwan examples are from Taiwan and British Columbia. and British Columbia.