Post Fire Debris Flow Threat #2

[Speaking: Jim Bowers, USGS California Hydrologic Monitoring Program Chief. Standing  on the canyon hillside.]

Were in the upper reaches of the Santiago Creek drainage in Modjeska Canyon in Central Orange County.  It's located pretty much in the center part of the Santiago fire burn area. One of our biggest concerns of what science we can bring to the table to help with response to the fires is the mapping and the assessing the risk of debris flow hazards. Where we are right here, I can look around where I'm standing, has the appearance of being a relic of a previous debris flow, there's a lobe of material here it's kind of domed in the center, I can see at the base of the bushes there's rocks and sand and mud pushed up against them.  You can see over here on the bank there's poorly sorted material, larger boulders, all jumbled up. The debris flow comes down almost like one solid unit with a very small proportion of water in it. With the steep slopes, the recently burned hillsides, there's pretty much an unlimited supply of material with a very minor amount of brief duration and intense rain can get these triggered and moving down this canyon. Just downstream here there are a lot of homes that are real close to this peaceful little stream but in these conditions with the denuded hillsides there could be a lot of material coming down that they wouldn't be able to stop with normal flood control measures.

After the 2003 fires, it was pretty much an El Niņo neutral year, and we had one storm come in on Christmas day that produced the majority of our rainfall for the year over a two day period.  The post fire conditions up in the San Bernardino Mountains and San Gabriel Mountains produced some massive debris flow up there so it doesn't take a really wet year to produce these things.  If one heavy rainfall cell happens to come up and hit over this basin that could do it. Just a few inches of rain over a 24-hour period or a brief downpour can trigger one of these events.