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Instructions on how to determine if your property is in a debris flow hazard zone

Go to the WERT report:

Zoom in on the hazard map at the beginning of Appendix C (page 70 in the .pdf) to find your home.


Your home may be in a hazard zone if it is in, on, or near:

• a red, orange, or yellow watershed (high, medium, and low hazard, respectively),

• purple streams that are likely to become debris-laden if a debris flow forms upstream,

• FEMA flood zones shown with blue diagonal lines, or

• one of the colored dots.

If not, you aren't in a significant debris flow hazard zone. Still, if your home is located in the burned zone, you could experience small local debris flows on slopes above your home, and anywhere near streams coming off Ben Lomond Mountain could see local flooding due to greater run-off in the burned area and bulking up of creeks by sediment eroded from burned slopes. 

If your home looks like it may be in a debris flow hazard zone, then find your home on one of the 26 more-detailed maps that follow p.70 in the .pdf of the WERT report. These maps have enough detail to locate your home precisely with respect to hazard zones. If you are near one of the numbered dots or areas, you can look up the site number on the “Value at Risk” table (starting on page 56 of the .pdf) to see the description of the hazard and the recommended action.


West of San Lorenzo River and Boulder Creek from Brookdale to Boulder Creek to Jamison Creek

As the most difficult areas to predict the paths of debris flows are alluvial fans, more detailed mapping was done by geologists and geomorphologists employed by the County and the California Geologic Survey of areas from Brookdale to Jamison Creek where alluvial fans are common and are favored sites for homes and businesses. If you live in this corridor, get more detail on the spatial distribution of hazards using the following directions.

Go to the California Geological Survey Boulder Creek Post-WERT Study at

The hazards are shown on page 88 in Plate 1b, Runoff Inundation Hazard Map. Note that north is to the left on this map.

In order of decreasing hazard:

Orange: High Energy: Regions likely to be occupied by large, channelized debris flows capable of sweeping away everything in their path.

Mustard: Moderate Energy: Regions likely to be occupied by moderate-energy, thinner debris flows where larger debris flows spill out of channels onto the flatter ground of alluvial fans or where the watershed can generate only small debris flows.

Yellow: Active Fan: Areas of alluvial fan with possible inundation if debris flows jump out of their channels due to blockage, typically at culverts where creeks cross Hwy 9 or Hwy 236 (green dots), or where the creek takes a sharp bend and the debris flow overflows the channel and keeps going straight downhill.

Tan: Inactive Fan: Areas of alluvial fan unlikely to be inundated unless debris flows take a dramatically different path from current channel due to exceptional blockage or takes a different path starting at the top of the alluvial fan.


The map also shows area that might be flooded by the San Lorenzo River due to increased run-off or bulking by debris.


Paid for by Gail Mahood for SLVWD Director
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