Atkins Report & Debris Flow Rebuilding FAQs


For parcels where rebuilding sites are subject to debris flow hazards, pre-application Geologic Hazard Clearances (GHCs) have resulted in a requirement for further geologic evaluation before a clearance can be approved. In some cases, debris flows are the only potential geologic hazard on the building site, in other cases there may be other geologic hazards such as slope instability or landsliding present in addition to debris flows. In both cases the further geologic evaluation that was required would produce recommendations on how to mitigate the hazards on site.

The requirement for further geologic evaluation and mitigation comes from Santa Cruz County Code (SCCC) Chapter 16.10, which requires that staff confirm through peer review of technical reports and designs that geologic hazards can be adequately mitigated prior to approving a development. Pursuant to the Board of Supervisors “CZU Rebuild Directive” issued 9/14/21, for eligible, in-kind rebuild projects, property owners may choose to “opt out” of the requirement for geologic reports, County peer review of geologic and geotechnical reports, and County confirmation of the adequacy of proposed mitigation measures .

While the CZU Rebuild Directive allows those rebuilding in-kind to “opt out” of County geologic evaluation and mitigation requirements, the California Building Code still requires submission of a geotechnical report that includes an evaluation of slope instability. Building Plan Check review includes verifying that the geologic report recommendations are incorporated into the building permit plans and calculations. (The term “geotechnical report” is used interchangeably with “soil report”).

Geotechnical engineers typically work with a geologist to define the various parameters necessary to design debris flow mitigation measures. Due to the time and cost to produce site-specific geologic reports, the Community Foundation of Santa Cruz County funded a flood and debris flow hazard study (“Atkins Study”) that, among many broader beneficial uses, assists rebuilders by providing debris flow depth and velocity data for use by geotechnical engineers in lieu of the results of a full geologic evaluation. This depth and velocity data is available for the areas identified in the Atkins report as being within a ”Primary Debris Flow Path”.

The following information should clarify how to interpret the results of the Atkins Study, and how the data provided in the study may streamline the permit process for those rebuilding in-kind in debris flow hazard areas. It is important to note that the first step in the permit process is applying for a Geologic Hazard Clearance. This pre-application review of geologic constraints on a building site will identify whether your site is subject to geologic hazards, and if so, whether the only hazard is debris flow , or if other conditions must be addressed as well.

(Please also note that two other pre-clearances to confirm adequate septic and fire access, are also required prior to making an application for a building permit. This ensures that the technical challenges in these three areas are addressed before investing in final building plans, and that your various professionals are working together as a team.)



The Atkins Study consists of a hydraulic (water mechanics) model that shows where muddy water will flow in an intense storm. Debris flows may include higher amounts of soil, trees, rocks and debris, and will generally inundate the same locations as muddy water based on the topography of the affected drainage basins. While debris flow material varies from water in the weight of the material and flow mechanics, the inundation areas, velocities and flow depths yielded by the hydraulic model are reasonable estimates for those of a debris flow.


The Atkins Study debris flow map (in addition to other pertinent maps) is used by geologic staff to determine whether CZU rebuild home sites are subject to potential debris flow hazards during the Geologic Hazard Clearance in-office review. Building sites that fall within the areas labeled “Primary Debris Flow Path” and/or “Uncertain Debris Flow Path” are considered subject to debris flow hazards. Please note that if your building site falls OUTSIDE the Primary and/or Uncertain Debris Flow paths the building site may still be prone to debris flow hazards (see FAQ 10 below).

Areas shown within the “Primary Debris Flow Path” boundary may experience higher velocities and more destructive forces during a debris flow event. Depth and velocity model data is available within the Primary Debris Flow Path areas.

"Uncertain Debris Flow Path" boundaries delineate areas where the debris flow could jump out of the “Primary Debris Flow Path” due to a change in grade, channel blockage or other factors. These areas are at elevated risk of inundation by debris flow, but due to uncertainties in the direction flow will take, they are at relatively lower risk than those in the “Primary Debris Flow Path” areas. Due to the many variables that influence debris flow path, the Atkins Study model output does not include depth or velocity values for the "Uncertain Debris Flow Path" areas.


If your project meets eligibility requirements to “opt out” of County peer review of technical reports and adequacy of mitigations, your geotechnical engineer will use their professional skills and judgement as a state-licensed engineer to determine how to consider the hazard in the structure and/or site design. Once plans are completed, your geotechnical engineer will need to produce a “plan check letter” stating that the geotechnical recommendations have been properly incorporated into the plans. Recovery Permit Center (RPC) plan check staff will collect that letter and help to ensure the geotechnical report is reflected in the home design and calculations.

Atkins Study data is just one source of information that will be used by the project geotechnical engineer to determine how the rebuilt home design will address the potential for debris flow. In some cases the geotechnical engineer may request additional information from a geologist to assist with their recommendations; if the property owner has “opted out” of SCCC 16.10, this additional geologic information will not be peer reviewed by County staff.

County staff have met with local geotechnical engineers on both an individual and group basis to discuss how they may utilize the Atkins Study data to satisfy the requirements of the California Building Code, and will continue to assist however possible to facilitate use of this valuable data by industry professionals.

If you plan to rebuild in conformance with 16.10 and not opt into the rebuilding directive, your geotechnical engineer may provide a more expanded analysis, and will make specific recommendations for mitigation measures that can lower the risk to the rebuilt home.

Technical professionals can download the Atkins Study shapefiles and raster data via the County Geographic Information Services (GIS) open data website:


Atkins Study debris flow zone maps are available as an electronic .PDF file on the Recovery Debris Flow web page and are also hosted on the County GIS web application as the “Atkins Study” layer under the "Hazards and Geophysical" tab.

The Atkins Study .PDF maps are at a 1:500 scale and provide building footprints that are based on optical tracing of aerial photographs with a +/- 10 foot accuracy. Not all buildings shown are dwellings, and only buildings visible from the air are shown. It should be noted that parcel layers in the CZU burn area are highly inaccurate (up to +/- 300 feet in some locations), so the most reliable way to determine the location of your home site relative to the reported debris flow hazard areas is by zooming to the site’s geographic coordinates via the “Measure Lat/Long” tab at the top of the County GIS web screen; your design professional may be able to assist with this.


If your building site touches the debris flow hazard area, your project is considered to be within an area subject to potential debris flow hazards and requires further evaluation.


No, if your site was shown within the Atkins primary or uncertain debris flow hazard areas and is moved outside the Atkins Study debris flow hazard zones, no further mitigation for debris flow hazards is required to comply with County Code Chapter 16.10. The location of your building site relative to the Atkins Study debris flow hazard area will be confirmed during the Geologic Hazard Clearance in-office review of your build site. Please note that if your building site falls OUTSIDE the Atkins study "primary" and/or "uncertain" Debris Flow paths the building site may still be prone to debris flow hazards (see FAQ 8 below).


NO! As a tool for fire survivors who wish to rebuild, the Atkins Study focused on larger drainage basins that have affected parcels with destroyed structures. This means that basins with all standing structures were not included in the Atkins Study detailed mapping areas. The best sources of data for determining whether you are at heightened risk for winter debris flows and/or evacuations are the debris flow hazard maps on the Recovery Debris Flow web page, which are also available as layers on the County GIS web application titled "CZU Potential Debris Flow Hazard Areas" layer under the "Hazards and Geophysical" tab.9. Are there other Important things to know?

Is it key that your geotechnical engineer, civil/structural engineer and architect or design professionals all work together to ensure the project geotechnical report recommendations are properly and consistently reflected in the plans and calculations. Your geotechnical engineer will be asked to provide a plan review letter stating that this is the case. You should ensure that the geotechnical engineer is prepared to provide this letter.

After building permit issuance, the geotechnical engineer must stay involved with the project to ensure it is constructed in conformance with the geotechnical report recommendations. Communication among your team throughout construction is key to success!


Maybe. Some drainages within the Atkins detailed study area were not assigned "primary" or “uncertain” debris flow hazard areas; in these locations, the County-generated debris flow hazard polygons will be utilized to determine whether additional geologic evaluation is required. In areas within the Atkins detailed study area that were assigned "primary" and/or "uncertain" debris flow hazard areas, these debris flow hazard areas are used for County geologic hazard screening purposes. The location of the building site relative to designated debris flow hazard areas will be confirmed by County geologic staff during the Geologic Hazards Clearance review.