"Translation" and summary of WERT report on post-CZU-Fire debris flow hazards.
Santa Cruz County recently released a study https://www.smcgov.org/.../CZU%20Lightning%20Complex...
completed October 1, 2020, by the Watershed Emergency Response Team (WERT) Evaluation: CZU Lighting Fire that assessed post-fire rockfall, debris flow, and flooding hazards. It's pretty technical, but it contains critical information, so I think it is important to "translate" it for a general reader.
The WERT report contains an elaboration on the debris flow hazard models produced earlier by the USGS, which I explained how to use in previous posts. This report shows on maps in Appendix C the locations of more than 100 areas that have "Values at Risk", i.e., homes and infrastructure that are at risk from debris flows. These maps also show USGS "watchstreams" (streams that will be subject to debris flows or sediment- and debris-laden floods if the basins above them generate a debris flow), and show areas of creek and river floodplains considered by FEMA/DWR to be subject to winter flooding. After the fire, these areas will be more prone to flooding, even in the absence of debris flows, because of the elevated run-off expected from the burn zone. You can determine if your home or business is in a flood zone by looking at the maps or entering an address into the FEMA Flood Map Service Center at https://msc.fema.gov/portal/home.
HAZARDS FROM DEBRIS FLOWS AND DEBRIS-LADEN FLOODS BY AREA
This is my attempt to summarize the results of the WERT report by area. See the report for specific recommendations for each area. If you have any questions about your own property, consider consulting a California-registered Professional Geologist or Professional Engineer (which I am not), because when it comes to floods and debris flows, the determinations are very site-specific.
West side of Ben Lomond Mountain
The worst debris flow hazards are on the west side of Ben Lomond Mountain, where a combination of intense burning, steep slopes, and easily eroded sedimentary bedrock combine to make for basin probabilities of 60-100% of a debris flow forming in a single high-intensity rain event in West and East Waddell Creek, Scott Creek, Big Creek, and Molino Creek. Based on the hazards map, it wouldn't be surprising if Highway 1 were hit by debris flows this coming winter north of Davenport Landing and north and south of Ano Nuevo State Park.
The area around Bonny Doon has a low probability of debris flows, largely because the area is relatively flat, but also because the burning intensity was low to moderate. Homes along Empire Grade are unlikely to be subject to large debris flows because they are on a ridge. Davenport lies adjacent to the floodplain of San Vicente Creek, which along with its major tributary Mill Creek, head in the burn zone around Bonny Doon. Some of the tributary basins, by virtue of their large size, are capable of generating moderate-sized debris flows. So, although the probability and hazard are considered Low, there is a possibility of inundation of the lower reaches of San Vicente Creek, which is subject to occasional winter flooding in any case.
Highway 9 and 236 corridors
The Basin Hazards for the Hwy 9 corridor are low to moderate. This is because the east side of Ben Lomond Mountain is so steep, the drainage areas are small, so the amount of sediment and debris that can be swept away is relatively small. All the basins but one draining eastward into the San Lorenzo River are considered Low hazard, due to the low probability (0-10%) of debris flows being generated. The exception is the Clear Creek drainage, which has a moderate (20-40%) probability of generating a large debris flow in an intense rainfall event. Much of Brookdale is built on an old alluvial fan emanating from where Clear Creek emerges from the mountains near the Brookdale Lodge. Were a large debris flow to come down Clear Creek, homes on that alluvial fan well above the level of the San Lorenzo River potentially could be in its path.
Several creeks that drain into Boulder Creek have Moderate hazards, including Jamison Creek, Peavine Creek, and Foreman Creek. All of these creeks have alluvial fans where they exit the mountains, and these alluvial fans have homes on them, e.g., parts of Jamison Creek Road and Fallen Leaf Drive, Acorn Drive and Brook Lane, Boulder Brook Drive and adjacent Hwy 236, respectively. Given the number of creeks feeding into Boulder Creek that head in the burn zone, low-lying homes along Hwy 236 from the Boulder Creek Golf and Country Club to the town of Boulder Creek are at risk of debris flows and muddy floods even though they are outside the burn zone.
The town of Boulder Creek was built at the confluence of Boulder Creek and the San Lorenzo River, and parts of it are subject to winter flooding. Given that the section of the San Lorenzo River from Boulder Creek to just north of Riverside Grove would receive debris flows from creek tributaries, as would Boulder Creek, the low-lying parts of the town of Boulder Creek are at risk of being inundated by sediment- and debris-laden floods. In addition, the western part of town, including the cemetery and the elementary school, are built on a alluvial fan at the mouth of Harmon Creek. Its small basin is assessed as having a Low Probability of generating a Moderate-sized debris flow.
South of the town of Boulder Creek, the San Lorenzo River has numerous creek tributaries that head in the burn zone, each increasing the probability that the river would be affected by a debris flow during an intense rainfall event. Areas particularly at risk of inundation by a San Lorenzo Rivers swollen with debris are low-lying areas in southern Ben Lomond, Highlands County Park, parts of Felton, Paradise Park and the floodplain of the river south to the town of Santa Cruz, where it forms spreads over a area near the outlet of the river to the sea.
The probability of debris flows being generated in the CZU burn area adjacent to Felton is low (0-20%) due to low to very low burn intensity, and the predicted volumes are small. People whose property is only a few feet above Fall Creek, Bull Creek, Shingle Mill Creek, and Gold Gulch Creek should probably keep in mind that there is a possibility of debris flows affecting their property, though the hazard is considered low. More likely is flooding due to elevated run-off from the burned area. The new Felton Library and the adjacent fire department are only a few feet in elevation higher than Bull Creek, potentially placing them at risk. The much greater risk is to downtown Felton, where a San Lorenzo River swollen with debris flows that enter from creeks upstream could overflow its banks. Areas such as Covered Bridge Park and Felton Grove could potentially see effects similar to worse than the great flood of 1982.
SANTA CRUZ COUNTY RESPONSE
Because the routes of debris flows are unpredictable in detail, and they are fast-moving, temporary evacuation from areas at risk in advance of predicted heavy rainfall events is the best way to save lives. As of this writing, October 25, the evacuation plans are under development by CalFire, local fire chiefs, and the Sheriffs Office in collaboration with County staff. In developing its plans, the County is taking into account the recommendations of the WERT report plus more detailed mapping by County staff of structures at risk than could be completed in the rapid time frame of the WERT evaluation.
For information about Santa Cruz County planning for debris flow hazards, developing early warning systems and evacuation plans, see the special presentation to the Board of Supervisors on September 29, 2020 and the Fire Recovery Town Hall on October 7, 2020, co-sponsored by Supervisors McPherson and Coonerly, posted on Supervisor McPherson's Facebook page. Check there for updates.