Earthquake: The Long Road Back : Fire, Quake and Rain : The Mudslide Cycle

Southern California’s hillsides were built for speed-a result of seismic drama. Steeped and fractured by eons of earthquakes, then denuded by wildfire, they routinely melt into mudslides during the rainy season. Here is a look at the latest danger in a season of disaster.

Rivers of Rock

The San Gabriel and Santa Monica mountains are young and restless. Earthquake by earthquake, they are growing as rapidly as any range on Earth-and constantly shedding their outer layers. Many slopes in the San Gabriels are beyond the so-called angle of repose-the steepest angle on which loose material will remain. So even in dry weather, the hillsides surrender a continuous stream of material, damming dry stream beds below. These rivers of rock swell significantly after two unnervingly common local events: brush fire and earthquake.

Built to Burn

Chaparral-the distinctive combination of woody shrubs and seasonal grasses that grips the hillsides-is built to burn. Some species of shrubs in a chaparral community cannot reproduce without a scorching wildlife to incubate and germinate their seeds. To survive the region’s hot summers and long droughts, many of these plants have a waxy or oily coating on their leaves to reduce water loss. In a firestorm puffed up by Santa Ana winds, these coatings are a potent fuel.

Natural Waterproofing

Dead leaves retain the waxy and oily coatings plants use to reduce evaporation. Fire can burn these coatings or melt them; when they melt, they soak into the hillside soil and re-solidify into a water-resistant layer just below the surface. Rain will tend to collect in the soil above this layer, oversaturating it.

Starting Small

In the first few rains after a fire, water quickly saturates soil above the water-resistant layer of plant oils and waxes that have condensed below the surface. Waterlogged surface soil drips like runny paint, creating thousands of tiny stream beds called rills. More rain deepens these rills, vastly increasing the speed of the storm runoff racing down them and depositing slippery mud among the rocks accumulated in stream beds below.

Attracting Trouble

Smaller storms set the stage for big trouble by sending more debris into the canyons. Catastrophic mudslides usually require one of Southern California’s notorious high-intensity weather systems-those made up of two, three, four or more storms in a row off the Pacific. Often, the biggest storms, capable of dumping 7 million tons of water per square mile, seem to be most intense over burned hillsides. Meteorologists suspect this may have something to do with moisture condensing on ash particles, or with the extra heat reflected by burned ground, but no one knows for sure.

Slip and Slide

Heavy storms can quickly and unpredictably oversaturate the uppermost soil layers, causing the hillsides to slip. This mud acts as a lubricant, and rocks, ash and other fire debris flow freely downhill. Slick, fast-moving mud also lubricates and propels rocks and boulders that have accumulated in stream beds, compounding the mudslide into a disastrous debris flow.

Defense Tactics

Potential for flood damage is intensified in steep foothill areas where vegetation has been destroyed by fire and the slope weakened by quakes. In such cases, storm water comes off the hills carrying soil, rocks, trees and brush. In Los Angeles County, the Department of Public Works maintains a network of debris basins that offer at least some protection for communities downslope. After the fires in October and November, nearby residents were alerted about what to expect and how to prepare.

Capacity Debris Basin In cubic yards 1. West ravine 44,900 2. Fern 30,600 3. Fair Oaks 23,800 4. Devonwood 5,700 5. Las Flores 55,600 6. Rublo 127,200 7. Kinneloa 14,100 8. Kinneloa West 14,200 9. Sierra Madre Villa 402,700 10. Carriage House 6,100 11. Sunnyside 3,400 12. Bailey 128,800 13. Auburn 31,600 14. Carter 14,600 15. Sierra Madre Dam 136,400

Surface Saturation

Heavy rains can cause street flooding and minor mudslides, but a longer-term problem occurs when the soil gets oversaturated. Here is a look at what can happen during the rain season: 1. When rainfall is less than 6 inches, there tend to be few problems. With more than 6 inches of rain, soil begins to saturate and can absorb less water. Small mudslides with a few feet of soil washing away can occur. 2. With more than 10 inches, real problems begin. These include large mudslides during the storms and, later in the year, the chance of massive landslides as water undermines weak rock layers below the soil.

The Worst Case

A major long-term risk of saturated soil is that it will be weakened and later give way.

In some areas of the Southland-especially in coastal places-the underlying bedrock is extremely weak. The Northridge quake has further destabilized the ground in many areas.

Heavy rainfall during the winter can seep into this weaker layer.

The seepage can loosen the soil.

In the fall, minor rains can make the weakened area vulnerable to a big slide.

Insurance coverage: Homeowner’s insurance does not cover earth movement. Federal flood insurance-available nationwide in flood-prone areas, including Southland hillsides-does cover damage from debris flows in certain cases. Generally, federal claims adjusters decide after the fact whether a disaster is covered, based on the composition of the displaced earth.

Next step: The danger from this year’s fires won’t subside when the rainy season passes. History shows that burned areas are likely to burn again within a few years because particularly flammable plants-grasses and flowering annual plants-usually are the first to reappear. These plants make for a colorful spring, but will burn readily when dried by summer heat and autumn winds. The good news is that once the woody chaparral plants re-establish themselves, these grasses and flowering annuals are suppressed by the vigorously growing shrubs. This usually reduces fire danger for about a decade.

People Versus Nature

In Southern California, hillsides slip even when there hasn’t been a brush fire or quake. Cautious homeowners can plan a multitiered defense against mudslides and debris flows.

A. Debris basins: Built and maintained by the county and some homeowner associations, these dams of concrete or earth are designed to catch mud and debris at the bottom of mountains or hills, before homes are threatened. Ranging in size from a wading pool to a small lake, they are effective only when regularly cleaned out.

B. Deflection barriers: A temporary defense erected to protect individual homes when rushing streams of muddy water indicate a slide may be imminent. Often built of wood with earthen reinforcement, these barriers cannot stop a mudslide or debris flow. They are used to divert them away from homes.

C. Slope drain system: Any hillside drainage system should be well-maintained, and always kept clear of debris and undergrowth.

D. Sandbags or hay bales: Used primarily for “low-flow” protection, or only a few feet of water. Sandbags will not seal out water, but will help divert the flow. In general, sandbags should be about half-full; bags should be placed with the folded side uphill.

E. Jute mesh or plastic sheeting: When properly installed and maintained, such covering can protect bare soil from storm erosion. Experts emphasize that drainage is critical; make sure water runs off directly to a street or storm drain.

For a free booklet, “Homeowner’s Guide for Flood, Debris, and Erosion Control,” write to: Los Angeles County Department of Public Works, Box 1460, Alhambra, Calif. 91802-1460.

Sources: L.A. County Department of Public Works; L.A. County Fire Department; U.S. Forest Service; Caltech; “Roadside Plants of Southern California” by Thomas J. Belzer; “Native Shrubs of Southern California” by Peter H. Raven, “The Control of Nature” by John McPhee; “ A Homeowners Guide to Fire and Watershed Management at the Chaparral / Urban interface” by Klaus W. H. Redtke; Los Angeles Times files.

Graphics by MICHAEL HALL

Researched by MARK STEIN, MICHAEL HALL, TOM MULLIGAN and VICKY MCCARGAR / Los Angeles Times