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-Part 1


* What are debris avalanches and debris flows? Debris avalanches and debris flows (both popularly called "mudslides") are shallow landslides, saturated with water, that travel rapidly down slope as muddy slurries. The flowing mud carries rocks, bushes, and other debris as it pours down the slope. A debris avalanche is a fast moving debris flow that travels faster than about 10 mph or approximately 25 yards in about 5 seconds. Speeds in excess of 20 mph are not uncommon and speeds in excess of 100 mph, although rare, do occur locally.

* What are dangers posed by debris avalanches? Debris avalanches pose hazards that are often overlooked. Houses in the path of debris avalanches can be severely damaged or totally demolished. Persons in these structures can be killed or severely injured. Most rainstorms are of such low intensity that they do not trigger debris avalanches. However, when the ground is already saturated from previous rain, even relatively short high-intensity rainstorms may trigger debris avalanches.

* What causes debris avalanches and debris flows? The most common cause of debris avalanches and debris flows is the combination of heavy rainfall, steep slopes and loose soil. Most fairly steep slopes have enough soil and loose rock for potential landslides. Although "stable" when dry, such slopes can produce local debris flows without warning. Normally the source of the excess water is intense rainfall, although broken water pipes or misdirected runoff concentrated by roads, roofs, or large paved areas may trigger, or help to trigger, debris avalanches and flows. In California most debris flows occur during wet winters.

* Where do debris flows and debris avalanches occur? Slopes burned by range and forest fires are especially vulnerable because of the absence of vegetation and roots to bind the soil. The areas directly down slope are especially subject to damage from debris flows. Debris flows are known to start on slopes as low as 15 degrees, but the more dangerous, faster moving flows are more likely to develop on steeper slopes. About two thirds of all debris avalanches start in hollows or troughs at the heads of small hillside drainage courses. Typically, a debris avalanche bursts out of a hillside and flows quickly downs lope, inundating anything in its path. Because the path of debris flow is controlled by the local topography, just like flowing water, debris avalanches and flows generally follow stream courses.

This info was provided by Calif. Dept. of Conservation, Division of Mines/ Geology and was edited by Carol Felixson. 03/96

Part 2


To be safe, assume that all drainages in steep, hilly, or mountainous areas are capable of carrying debris flows, especially if relatively loose, sandy soils are present in the watershed. Areas that have been burned over by regional fires are especially vulnerable.

Avoid building sites at the bottoms and mouths of steep ravines and drainage courses. These areas are the most likely to be inundated by debris flows. The outer 'banks" of bends along such ravines also should be avoided because swiftly flowing debris avalanches can "ride up" out of the bottom of the stream channel where it bends.

Avoid building on or below steep slopes. In general, the steeper the slope, the greater the risk. If these areas must be used, consult with a soils engineer and an engineering geologist. These specialists will be able to evaluate the potential for mudslide problems and give advice on the best way to minimize the risk to life and property.

The hazard from debris flows that occur in man-modified slope cuts can be decreased by 1) limiting the height and slope of cuts and fills, 2) properly compacting fills and keying them into bedrock, and 3) properly controlling the flow of water onto slopes. If steep cuts or fills occur below the discharge points of runoff water from streets, downspouts, or similar drainage facilities onto a slope, it may be wise to obtain advice from an engineering geologist or erosion control specialist.

In some cases, walls can be built to deflect potential mudflows away from or around structures. To be effective, diversion walls must be properly designed and maintained regularly.

Stay alert to the amount of rain falling locally during intense rainstorms. Buy a rain gauge (an inexpensive plastic one will suffice) and install it where it can be checked frequently. Whenever rainfall has exceeded 3 or 4 inches per day or 1/4 inch per hour, the soil may be waterlogged, and more rain can trigger mudflows. Any sudden increase in runoff is cause for concern. Watch for new springs or seeps on slopes; cracks in snow, ice, soil, or rock; bulges at the base of slopes, the appearance of holes or bare spots on hillsides; tilting trees, or increased muddiness of streams. Also listen for unusual rumbling sounds or noises that may indicate shifting bedrock or breaking vegetation.

This info was provided by Calif. Dept. of Conservation, Division of Mines/ Geology and was edited by Carol Felixson. 03/96


Contact T-CEP:    310-455-3000   email:
P.O. Box 1708    Topanga, CA 90290