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 landscapes
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: http://www.Weather.Gov/mtr
Local river forecast center: http://www.Cnrfc.NOAA.Gov
NWS mobile: http://Mobile.Weather.Gov
California flood preparedness: http://www.Water.CA.Gov/floodsafe/CA-flood-preparedness/fpw_home.Cfm
Map service center: https://MSC.Fema.Gov
US Army corps of engineers: http://www.Usace.Army.Mil/