Part 7 DEBRIS FLOW MITIGATION TECHNIQUES Check Dams

Part 7 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 rapidly moving debris flows.

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

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

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 3 X the 100 yr flow of the channel

Fail-Safe Debris Basins Bollards on overflow crest to catch boulders • 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 yards of debris per year.

A-Walls A-wall diversion structures are intended to deflect fluid debris and route it around highvalue 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

Perforated 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 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 constrictions can be natural or man-made, as shown here.

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.