(limitations)= # Disclaimer and Limitations ## Disclaimer This hazard assessment depicts locations that may be inundated by a postfire debris flow. The results are based on a combination of multiple simulations selected to reflect the uncertainty in hazard due to the size and composition of debris flows. Rainfall is simplified into hypothetical rainstorms called 'design storms' in which a constant rainfall intensity falls for 15 minutes. Should no high-intensity rainstorms occur within the burned area, debris flows may not occur. Similarly, storms with greater rainfall accumulation or intensities, or longer durations, than those evaluated may present more severe hazards than those estimated by hazard assessments. The impact of multiple, sequential storms is not assessed. Additionally, intense rainfall may occur in one portion of the burned area but not another. The spatial pattern of hazard within the drainage basins upstream of the runout zones is not directly assessed by this method; consequently, the entire extent of upstream drainage basis are considered areas of high hazard. Finally, assessments generated by this method are specific to debris flows; significant hazards from flash flooding may exist and would require a separate hazard assessment. See the “limitations section” for additional limitations. Hazard assessments generated by this method are provided due to the need for timely "best science" information. Assessments are provided on the condition that neither the U.S. Geological Survey nor the United States Government may be held liable for any damages resulting from the authorized or unauthorized use of an assessment. ## Limitations The methods used to generate postfire debris-flow runout hazard assessments involve simplification of multiple aspects of physical processes that may occur during a debris-flow runout event. Here, we list some of the key simplifications made. **Topography:** Hazard assessments are made on 10-m topography sourced from the USGS National Map. Topography is used as provided and may not represent the topographic surface a debris flow interacts with during a debris-flow runout event. For example: (1) the topography may have changed between data collection and the present; (2) debris retainment structures may be more or less full than at present; (3) three-dimensional structures, such as bridges and culverts, may not be fully represented; and (4) deposition and entrainment during a debris-flow runout event may change topography enough to redirect flow. **Uncertainty in runout path:** Hazard assessments are based on a discrete number of simulations selected to reflect the most likely range of debris-flow size and composition. Thus, hazard assessments document uncertainty within the considered scenarios. Because other sources of uncertainty may exist, including the difference between observed and hypothetical storms, the extent of runout paths should not be considered as precise or interpreted as a hard boundary. **Spatial scale of use:** We suggest not using hazard assessments at a scale larger than 1:10,000. **Building-level interpretation:** Hazard assessments are not expected to be reliable at the spatial scale of an individual building or individual parcel ([Barnhart and others, 2024](https://doi.org/10.5194/nhess-24-1459-2024)). Consequently, hazard assessments are not appropriate for site-specific evaluations or for use setting site-specific insurance rates. **Rainfall:** Hazard assessments are generated based on a series of idealized rainstorms in which a uniform intensity of rain falls for 15 minutes. Results are depicted as if the design storm occurred over the entire burned area. The likely spatial extent of high-intensity rain may vary in different regions based on the meteorological conditions of that region. Additionally, this method only considers a single storm. Multiple, sequential storms are not considered. **Deposition and entrainment:** Hazard assessments are based on combining the maximum flow depth at any individual location across multiple simulations. As a result, the hazard assessments do not indicate the expected debris deposit depth. Additionally, the model used for runout simulations was not configured to entrain material; thus, the generated results do not reflect debris-flow growth through entrainment. **Watershed recovery:** The likelihood of these postfire debris-flow scenarios may diminish through time as upstream watersheds recover from the fire. This product is intended for use for the first year after a fire. **Geographic applicability:** Generation of debris-flow volumes is based on the empirical relation between rainfall intensity and volume in the transverse ranges of southern California ([Gartner and others, 2014](https://doi.org/10.1016/j.enggeo.2014.04.008)). Debris-flow likelihood is based on the empirical relation for the western United Stated developed by [Staley and others (2017)](https://doi.org/10.1016/j.geomorph.2016.10.019). ## References Barnhart, K.R., Miller, C.R., Rengers, F.K., and Kean, J.W., 2024, Evaluation of debris-flow building damage forecasts: Natural Hazards and Earth System Sciences, v. 24, no. 4, p. 1459–1483, [https://doi.org/10.5194/nhess-24-1459-2024](https://doi.org/10.5194/nhess-24-1459-2024). Gartner, J.E., Cannon, S.H., and Santi, P.M., 2014, Empirical models for predicting volumes of sediment deposited by debris flows and sediment-laden floods in the transverse ranges of southern California: Engineering Geology, v. 176, p. 45–56, [https://doi.org/10.1016/j.enggeo.2014.04.008](https://doi.org/10.1016/j.enggeo.2014.04.008). Staley, D.M., Negri, J.A., Kean, J.W., Laber, J.L., Tillery, A.C., and Youberg, A.M., 2017, Prediction of spatially explicit rainfall intensity–duration thresholds for post-fire debris-flow generation in the western United States: Geomorphology, v. 278, p. 149–162, [https://doi.org/10.1016/j.geomorph.2016.10.019](https://doi.org/10.1016/j.geomorph.2016.10.019).