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California, North Bay Mountains

Public Information Statement

Statement as of 6:30 AM PDT on October 22, 2014

October 20th through 24th is California flood preparedness week!
The National Weather Service forecast office for the San Francisco
and Monterey Bay areas will feature a different educational topic
each day during the preparedness week.

Today's topic: debris flows and the hydrology of burned

Debris flows recall that a flood is defined as any high flow,
overflow, or inundation by water which causes or threatens
damage. Alternatively, debris flows are not floods, but are
fluidized masses of sediment and organic materials (trees, logs,
and any other vegetation that gets dislodged and transported
downslope) that flow downslope. They are typically (although not
always) caused by heavy rainfall which saturates soils and
weakens the structural integrity of soil layers. Materials
dislodged in this way then flow downslope and follow valley
networks down to creeks and rivers. Debris flows may occur during
a storm or following a storm. Debris flows can occur during small
storms when creek levels are relatively low, which means that the
debris flow itself may be the only hazard produced by the storm.
Debris flows can also occur during heavy rainfall events, where
the addition of debris flow materials to already swollen creeks
and rivers can exacerbate flooding hazard. Predicting the exact
timing and location of debris flows will always be impossible,
because it would require knowing all the intricacies of water and
soil throughout the whole Bay area. However, the Weather Service
and United States geological survey are currently working together
to develop techniques that will help determine the likelihood of
debris flows during a storm event. Pay attention to flash flood
warnings issued by the Weather Service... information on debris flow
likelihood will appear in these statements.

Debris flows can be associated with wildfire burn scars because
fire weakens root networks that hold soils together and because
fire removes vegetation and plant litter (decaying leaves,
branches, etc) that protects soil from the energy of rainfall
impact. Additionally, fire can change the efficiency with which
water flows into soils, accelerating the weakening and
lubrication of soils on hill slopes. The Southern California adage
of fire + rain = debris flow does not always hold true, but
wildfires do accentuate debris flow risk. Be extra cautious and
vigilant of debris flow potential in burned terrain, and know how
to prepare and respond quickly.

Hydrology of wildfires wildfires and the burned landscapes left
behind can alter the way water behaves during a storm in
significant ways. A landscape without plants, trees, Leaf litter,
and other organic debris can Route rainfall and runoff more
efficiently to creeks and rivers. Living plants are no longer
there to soak up water (called evapotranspiration), and root
networks which hold soils together can be damaged or completely
burned away, leaving conduits beneath the surface that funnel
water downslope more quickly. Fire can also alter the chemistry
of subsurface layers in the soil, which may then repel water
(like the saying GOES, off a ducks back!). this effect is known as
hydrophobicity, or hydrophobic soils (literally, fear of water).
These changes can mean that more water gets to creeks and rivers
faster, which can cause flash flooding during storms that would
not normally create flood risk.

Wildfires can also increase the likelihood of debris flows
occurring. If more runoff is flowing more quickly into channels,
that water is both more likely to erode more sediment off of
hill slopes and is more likely to transport higher volumes of
sediment down channels in the valley bottoms. The increase in
sediment and debris delivery and transport in valleys and channels
can result in highly debris-laden debris flows! Wildfire can
also increase the likelihood of debris detachment and soil
instability on hill slopes, which could lead to a debris flow. This
is because fire destroys roots that hold soil together and because
hydrophobic soils create a layer of soil above that becomes
waterlogged and prone to detachment (remember, the hydrophobic
layer is underneath the surface). Voila debris flow.

Again, in burned terrain, be cautious and vigilant. The movement
of water and debris in burned terrain may be different than what
you would normally expect and the risk from flooding and debris
flows may be increased. Stay safe, and listen for flash flood
watches and warnings from the Weather Service and your local
emergency personnel.

Join US tomorrow for information on coastal flooding hazards.


Important flood websites
local NWS office:

Local river forecast center:


NWS mobile:

California flood preparedness:


Map service center:

US Army corps of engineers:

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