Debris flows and El Niño in southern California landscape

Debris-flows occur in predictable places

Locations where roads and buildings have greater potential for debris-flow hazard are:

• At and next to the foot of a steep slope or at sites benched into steep slopes (especially slopes of 26 degrees and steeper),
• At or near the junctions of ravines with canyons,
• Near the upper points of alluvial fans,
• In and near existing channels across alluvial fans.
  

Locations where relatively flat terrain, such as a narrow canyon flood plain or an alluvial fan, adjoins a steep slope, such as a canyon wall or a steep mountain front, are most likely to be exposed to small debris flows from small, steep drainage channels, referred to as first and second order drainages (Click here or on the image to the left to learn about stream order and drainage networks). First and second order drainages are gentle depressions and small gullies in steep hillsides that are dry except when serving as channels for surface runoff during rainstorms. The size of debris flow increases with a longer slope, and the speed of a debris flow increases with steeper slopes.

Examples of debris flows: Photographs of debris flow paths and the houses they destroyed.

Locations in and near the mouths of relatively steep, larger ravines, which are generally second and third order drainage channels, can be vulnerable to unexpectedly large flow surges if surface runoff is bulked by debris flows in the drainage basin upstream.

Still larger drainages, such as canyons with relatively narrow flood plains (for example, fourth and fifth order drainages), generally have gentler gradients, and larger volumes of slurry are required to maintain flow as channels widen. During intense rainstorms these larger streams can receive both increased surface runoff and increased frequency of debris flows in tributary sub-basins, which may cause large surges of debris-laden flood water. Floods in canyons and at alluvial fan heads can surge many times, as individual debris flows discharge into flooding channels upstream.

Debris flow hazards and rainfall: what does heavy rainfall do?

Debris flow hazards and wildfires: effects of removing vegetation

How to prepare for debris flows

Geologic mapping in southern California

For further information, contact David Miller

http://geology.wr.usgs.gov/wgmt/elnino/scampen/landscape.html, 29 December 1997, Contact: El Niño Web Team