WINTER STORM MUDSLIDE AWARENESS
Mudslides or debris avalanches, combined with trees felled by high winds,
and saturated ground have resulted in power outages, closed roads, damaged
property, and loss of life. 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 cause debris avalanches in areas of steep terrain and lose
soil. Normally, the source of excess water is intense rainfall, although
broken water pipes or misdirected runoff concentrated on roads, roofs, or
large paved areas can contribute to debris avalanches. Debris flows are
known to start on slopes as low as 15 degrees, but more dangerous, faster
moving flows are more likely to develop on steeper slopes and follow
natural stream courses.
Assume all drainage in hilly or mountainous areas is capable of
carrying debris flows, especially if relatively loose, sandy soils are
present in the watershed when soil is saturated. Areas that have been
burned over by large fires are especially vulnerable. The following
indications of potential damaging debris avalanches include:
 |
New cracks or
widening of cracks in foundations, concrete or tile floors, retaining
walls, driveways, roads, or soil parallel to the contours of a hill
above and below a structure. |
|
Excessive water
seepage from the toe of a cut, such as behind a structure. |
|
Doors or windows
stick or jam for the first time, and new cracks appear in plaster or
brickwork. |
|
Stairs, fences,
walls, utility pools, or trees that start to lean, or trees that have
been previously leaning and are now straight. |
|
Any drainage that is
blocked by small soil slumps. Ponding can occur behind the slumps, and
when saturation occurs, the entire system can move and create a debris
avalanche. |
|
Bare ground that
shows signs of erosion, movement or bulging. |
If
these conditions are observed, homeowners are advised to contact a civil,
soils, or geo-technical engineer to ascertain the debris (mud) avalanche
potential, in order to take appropriate action. Homeowners whose property
is protected by retaining walls and drainage systems are advised to:
|
Ensure that
interceptor drains are clear and free of blockage. |
|
Check retaining
walls drainage outlets, and ensure that they are flowing freely. |
|
Blocked drains
should be annually flushed from the "clear-out" or rodded out. |
|
Check retaining
walls to ensure that they are not sagging or leaning away from the
hillside. |
For additional
information, contact your local Planning and Building Inspection
Department, or the Office of Emergency Services at 796-1900.
Return to
top of page
|
