Santa Cruz County Debris Flow Presentation, Sept. 29, 2020
9:42PM Sep 29, 2020
santa barbara county
santa cruz county
Good morning chair, cabinet and members of the board. My name is Carolyn Burke. I'm the senior civil engineer and environmental planning. I'm here today with Ken edler, Assistant Director of Public Works to speak with you about a deadly post fire threat that many have not heard of before debris flows. The photo that you see on the title slide was taken in the upper watershed of molesky Creek near Boulder Creek during the multi agency postburn reconnaissance to evaluate San Lorenzo Valley Water District infrastructure. You will learn during this presentation. The steep burns slopes and debris field channel shown in this photo illustrates a set of conditions that is primed to produce devastating debris flows during even short duration rain events. Unfortunately, this scenario is repeated on steep slopes throughout the burned area. And the resulting debris flow threat affects not only homes on hillsides or within the burn area, but also homes on flat ground a mile or more from charred slopes. They're unpredictable nature and powerful force make it imperative that we act swiftly to save lives by educating our community on the risks of debris flows, and the only effective means of protection, early warning and evacuation. As noted earlier, our main objective today is to educate constituents about a likely unfamiliar geologic phenomena referred to as debris flows, and spread awareness of the absolute destruction of debris flow can leave in its wake. We will first learn a little about the physical processes that lead to debris flows, and how the chances of a debris flow occurring are heightened after a fire event. Next, we'll explain the risks to life and property associated with debris flows. and highlight the reasons why a debris flow cannot be written out or fought or escaped by those in its path. The only way to avoid this hazard is to evacuate and evacuate early. This will be brought home with the deadly debris flow scenario that unfolded in Montecito in Santa Barbara County in 2018. We will wrap up with an overview of the county's response including risk assessment, evacuation mapping and communication plan in the coming months. A debris flow occurs when high intensity rainfall saturates loose soil, rock and other debris such as fallen trees high up in the mountains creating a moving mass that travels down slope. The left image is a diagram of what this looks like from a bird's eye view. This mass gets larger as it travels down the slope with mud rock with the mud rock slurry picking up cars houses, people and anything else in its path. As the mass gets larger, it also gets faster and can reach speeds upwards of 30 miles an hour. The huge mass does not stop when it reaches the bottom of the slope but instead flattens out spreading and depositing debris and mud in all directions. The area covered by this debris is often referred to as the alluvial fan. Due to the speed of the mass this area of deposition can be miles from the starting point of the debris flow. the right image on this slide is of the town of Montecito and Santa Barbara County and shows the alluvial fan left by a debris flow that occurred after the Thomas fire in 2018. As you can see the burned hills in the background that produced the debris flow with mud, rocks and debris still carving channels over one mile away. Unfortunately, debris flows are not new to Santa Cruz County and occurred during all types of rain years with devastating results including last lives. The two lower photos show the aftermath of debris flows that occurred during rain events. In more mountainous areas of the San Lorenzo Valley and North Coast. Not only did debris and golf the homes but they were knocked off their foundations and you can see in the photo on the left. A tree has also pierced the rear wall of the home. The upper photos show a debris flow that occurred in Davenport in 2010 above Newtown. As you can see from the relatively gentle slopes it is difficult to predict when and where the conditions will be just right to create a debris flow. They can occur where they never have in the past and can revisit previous sites.
Fire increases both the likelihood and severity of debris flows in several ways. First, wildfires burn trees, leaf litter and other organic matter, which leaves a waxy residue on soils and makes them effectively water repellent. This means rainfall does not percolate into the ground and instead all the water is delivered directly downslope. This results in a larger amount of water. Reaching mountain drainage is all at one time. What is collected in those mountain drainages is also influenced by wildfire burned slopes devoid of roots and organic matter whose structure and moisture would typically hold the surface soils together, begin to erode even under dry conditions. This eroded material, along with fallen trees collects in small drainages and swells throughout the mountains. As you can see in these photos, then the rain hits when it comes to debris flows. The most dangerous type of rain event is one that is short but intense. This leads to large volumes of water being delivered downslope almost instantaneously. When such a rain event hits postfire larger volumes of water coming off water repellent slopes quickly saturate the larger amounts of loose soil rock and debris and drainage isn't swales, creating a semi liquid mass that travels down slope as a debris flow. Efforts to determine the probability and locations of potential debris flows began weeks ago, with dispatch of the CAL FIRE watershed emergency response team referred to as the wort comprised of personnel from California Department of Forestry and Fire Protection in California Geological Survey, the work conducted a rapid evaluation of post fire debris flow hazards and their potential impacts to life safety and property. Their assessment included both field reconnaissance and computer modeling to arrive at a preliminary estimate of where debris flow likelihood has been increased, as well as identification of residential and infrastructure targets at risk of being impacted by debris flows. The left image is a map developed by the work that shows their estimates of debris flow probability for each drainage segment in the burn area. As you can see, the highest probabilities are in the western burn areas which correlates with higher burn severities. They should not be confused with the areas of highest life safety risk, which are defined by both the increased probability of debris flow coupled with their potential to hit existing homes. It should also be noted that the word mapping is preliminary and it's currently being refined by local staff as will be explained later, when an issue identified early on by both word and county staff was that word burns severity estimates were likely lower than field conditions due to the fact that their estimates were based on aerial surveys that showed green tree canopies where the understory had actually been severely burned. This means that the probabilities shown on these maps should be assumed to be higher than listed. The illustration on the right shows watershed basins in the Boulder Creek area included in the work report, due to historic evidence of debris flow activity in the form of existing alluvial fans are areas where debris has been deposited at the base of the drainage in the past. This is only one example of a scenario that is repeated throughout the north coast, Davenport and other areas of the San Lorenzo Valley, located within or downslope of the burn area. As noted in our last slide, people living down slope of the burn area have an increased risk when their homes are located in or near drainage channels, on or below a burn slope and at the mouth of the drainage channel. Here we have an illustration of a high risk debris flow scenario that is repeated throughout the brand area. The arrows are showing drainage and debris being deposited into existing drainage ditches. The slurry flows down slope striking homes within the drainage channel. Before fanning out to strike homes in the neighborhood on flat ground at the mouth of the channel. It may be difficult to picture how a home may be positioned literally in a drainage channel, but is more common than one might think. As these are typically seasonal drainages and not raging rivers. The lower photo is of a burned home site and just as scenario, a small seasonal drainage channel runs to the left of the home site and through a culvert in the foreground, both of which are currently buried by debris. You'll note that there are larger rock strewn throughout the photo. These are actually evidence of past debris flows that have deposited rocks from the upper watershed throughout the drainage. Due to the life safety risks described earlier in this presentation. as it sits the burn home site cannot be reoccupied even on a temporary basis. In the background to the right you'll see an existing home that will need to be on alert this winter and heed early evacuation notices.
It is important to educate residents in our affected areas on the unique risks posed by debris flow and why unlike fires or floods, there's no way to fight or write out a debris flow in place. Early evacuation is the only sure way to survive a debris flow event. debris flows are triggered by high intensity short duration bursts of rain. By short we mean on the order of 15 minutes. Typical forecasts predict rain totals per day or per hour but more sophisticated modeling is required. To predict systems that can produce these quick bursts. The high intensity rainfall that can trigger debris flows occurs in the upper burn area, which may not be the case at lower elevations where many debris flow targets exist. That means a resident on flatland at the mouth of the drainage may look out their window and see sprinkles totally unaware that a burst of rain high in the mountains above is generating a debris flow that will reach them within minutes. Even if a resident somehow knew a debris flow was starting in the upper watershed, they still would not have enough time to evacuate to the speed with which debris flows can travel more than 30 miles per hour. Once you can see or hear a debris flow, it's too late. Finally, due to the unpredictable nature of debris flows, residents cannot predict their risk of being impacted based on their history with the property. As we have shown the risk of debris flow already exists in the Santa Cruz Mountains. And even if your home has been safe for the past 30 or 50 years, we are now grappling with a post fire burned scenario that we have not seen in decades longer. The threat is real and it is worse now than ever. The only sure way to keep yourself safe from debris flows is to evacuate and evacuate early. The importance of early evacuation by residents was made clear during the devastating debris flow that hit the community of Montecito and Santa Barbara County on January 9 2018. The photo here is of the recovery effort in the aftermath of the event. And while it shows the physical devastation left in the wake of the debris flow, it does not begin to describe the horrific scenes encountered as a tally of lost lives mounted over the coming days. like Santa Cruz County Santa Barbara County was in the final days of the fight against the Thomas fire with which ravaged 280,000 acres including the hills above Montecito. Were report was also prepared for the areas down the slope of the burn zone which identified the increased debris flow risk for the community of Montecito. The county worked quickly to establish evacuation maps and coordinate with federal and state personnel to monitor weather forecasts. And for the first time in Santa Barbara County history and evacuation warning was issued for a storm event. The evacuation warning was issued more than 24 hours ahead of the anticipated storm, giving residents ample time to get out of harm's way. Unfortunately, only an estimated 28% of residents in the evacuation area heeded the warning before the debris flow hit on January 9 2018 at 3:45am. While most residents were sleeping in their beds, the debris flow began in the mountains above Montecito and ran all the way to the Pacific Ocean. 425 structures were destroyed totaling 400 and 50 million in damages. Highway one was buried under feet of mud slurry and debris that rescuing personnel described as quicksand. 21 lives were lost and two are still missing and presumed dead. In the face of such devastating loss, one can only ask how this could have been avoided. County staff have been in contact with Santa Barbara County officials to learn how we can educate our constituents to be prepared and escaped such a disaster.
We pass it on to Ken Adler.
The map shows on this slide how far debris flows can extend beyond the fire area. This is the the Montecito debris flow in Santa Barbara County. On the map. The area above the red line is the burned area and the light blue areas along the pink hatched areas are the locations of the debris flows. The dots represent affected structures with red being destroyed and orange having major damage. The debris flows extended over two miles below the burn area. The picture on the bottom left is of highway 101 which was closed for 12 days. And the picture on the right shows typical debris that hit a house and some of the drainage is in Santa Cruz County have debris that up and up in the swells that looks just like this. County staff recently met with Santa Barbara officials and they stress the importance of the following debris flows can extend out many miles. Early consistent public messaging is a must. evacuation preparation must be done early and people are less likely to evacuate for potential debris flow especially if they previously evacuated for a fire. Santa Barbara County did a survey after the debris flows and found that 75% of the people evacuated for a fire but only 28% evacuated for debris flow. So they strongly, strongly stress we need to get the message out that debris flows need to be taken as seriously as a wildfire. We included this slide of rainfall totals from October 13 2009. to stress the urgency of the issue center. Santa Cruz County does sometimes get significant rainfall early in mid October. There was a similarly sized event in October of 2004 as well. But again, we want to stress that it's not about the amount of rain in 24 hours, it's more about the intensity over a short period of time.
The county is taking many steps to protect lives.
The county geologist is leading a team of geologists in the field right now to map the debris flow hazards and expand on the word report. They're also identifying structures at risk and the hazard areas. We're also getting help mapping from the California geologic survey as well as the USGS. On the evacuation planning side of things we've already been having local multi agency coordination which includes CAL FIRE, local fire protection districts, the sheriff's office, Caltrans, the city of Santa Cruz, the San Lorenzo Valley Water District, state parks and county departments including public works planning, environmental health, j s staff, health services, and the Office of Emergency Services. The National Weather Service has also been brought into the conversation because determining when to evacuate is just as important as where to evacuate. field work on the mapping effort should be done this week and we'll be meeting with Cal Fire and local fire districts next week about mapping and development of evacuation zones and evacuation planning. Part of evacuation planning is also thinking about where to stage emergency response vehicles, which includes heavy equipment, fire trucks, ambulances, etc. as we get into the planning for evacuations, we'll be talking about how to make the maps available to the public. We want to get those released as soon as possible. We're already having conversations with the National Weather Service and we'll be having frequent conversations with them throughout the rainy season regarding the events that could trigger debris flows. Public Works flood control staff are also working to fine tune our rain gauges and we're also working with the Santa Rosa Valley Water District and the city and the City of Santa Cruz to tie their gauges into our monitoring systems. We're also going to be having discussions about how to get the message out when a call is made. Better evacuations are needed.
when communicating to the public, it's important that we do it in both English and Spanish and that the risks and evacuation plans are clear. For communication will want to use social media townhall meetings board meetings like this, press briefings and other avenues. We're already working with the county's Pio and will continue to do so on the most effective ways to get the message out. When we get shelter locations ready, those will need to be publicized as well. So people will know those options are available. Whenever possible, we need to issue warnings and evacuation orders as far in advance as possible. And obviously we need to do our messaging and clear terminology.
It's also important to message what
what it's also important to message to the public what they what they can do. First evacuation is the best thing to do. As Carolyn mentioned, the repos are fast and occur without warning. We haven't seen a fire of this size and many decades and the hillsides are going to behave in waves in ways that people have not experienced before. Also people reoccupying damaged homes and other people with existing homes downslope of the burn area should contact a licensed professional geologists to get an assessment of their property. People should sign up for emergency alerts, such as the app called Code Red, and pay attention to weather forecasts. Some people may be evacuated multiple times. So it's good to have a plan. Residents should be prepared with what they want to take with them and know where they're going to go ahead of time. Next is to stay at work, stay alert and heed evacuation warnings. Finally, we'll do our best to warn people but don't wait for someone to tell you to evacuate if you don't feel safe where you live leave. This also applies to areas that are outside of evacuation zones. So as far as the next steps, we're going to continue to communicate our mapping efforts and we'll get those out to the public as soon as possible, hopefully within the next couple of weeks. Oops. evacuation planning is an ongoing effort. Public messaging has started but we're really going to ramp up that when we have the maps. We're going to ramp that up when we have to have the maps completed and work with people counties Pio on the most effective way to get the message to the public. And finally, we'll continue to update the board on these planning efforts. That's the end of our presentation and we're available for any questions.