With a storm forecast this weekend, the County of San Diego is prepared to alert residents living near burned hillsides to threats of flooding and mudflows. Homes that receive warning calls from the County’s AlertSanDiego system are selected from a GIS-based map of burn areas that considers the flooding and debris flow potential, parcel by parcel.
“We’re breaking new ground with this,” said Rand Allan, associate meteorologist with the County Flood Control District. “When the Reverse911 system worked so well for the 2007 fires, we decided to try the same thing for flooding,” Allan said.
The County first used its AlertSanDiego system, an emergency notification system similar to Reverse911, to warn residents of flooding during a storm on Nov. 30, 2007. It has activated the system five times this year.
Hillsides burned by wildfire are susceptible to mudflows because there is no vegetation left to anchor the soil, and the rain runs down the hillside instead of soaking into it.
After the fires, the County used maps created by the U.S. Geological Survey and FEMA that showed the potential for flooding and debris flow. The County then transferred the information into a GIS-based map, and divided the burn areas into geologically-related zones. A pre-existing network of automated rain gauges within and near the zones provides real-time data.
Once an alert threshold is met, the County decides whether to activate the AlertSanDiego system. The County may send advanced warnings if predicted rain amounts are significantly high.
In some zones, smaller subsets were created because those homes faced a higher risk. In a storm, one home may receive a warning call while a home down the street does not.
For more information, visit the County’s Flood Control District Web site.