Watershed Emergency Response Team Report

Following selected wildfires, California State Watershed Emergency Recovery Teams (WERTs) are deployed to conduct post-fire assessments. WERTs identify types and locations of threats to life-safety and property (i.e., collectively known as “Values-at-risk" or VARs) from debris flows, flooding, rockfall, and surface erosion that are elevated due to wildfire. The WERT report also evaluates the burn intensity, which is helpful for assessing the damage to the ecology and the likelihood of recovery for vegetative communities.

WERTs develop preliminary emergency protection measures for the identified locations, and communicate the findings to responsible local emergency management agencies. Napa County is evaluating this data and working with property owners to evaluate and monitor VARs and other potential hazards throughout the winter. The Engineering division will be coordinating with owners who are concerned about threats to their life and property, or who's properties are associated with VARs, to conduct storm patrols and provide early warnings for storm events which may cause debris flow, flooding, or other hazards. 

The WERT found that twenty-two (22) percent of the fire burned at moderate to high soil burn severity, whereas the remaining portion of the fire (78%) burned at low or very low/unburned soil burn severity. Past discussions with Soil Scientists from the United States Forest Service have indicated that soils with higher clay content tend to have less heat penetration than sandier soils (personal communication, Brad Rust and Dave Young, USFS Soil Scientists). This may play a part in the lower soil burn severities observed in the Hennessey Fire, as soils in these areas generally have a relatively high clay content.

Ninety-six (96) Values-at-Risk (VARs) were identified within and downslope/downstream of the fire. Fifty (50) of the VARs are polygons that encompass several individual structures or extended alignments of road. In general, the majority of VARs are likely to be subject to localized flooding and/or localized potential for debris flows.

Twenty-one (21) sub-watersheds (i.e., pour points) were specifically analyzed for increased post-fire sediment-laden flood hazards. Various rapid analytical methods were used to predict runoff increases utilizing 2- and 10-year recurrence interval 30-minute rainstorms. Results are variable depending upon modeling assumptions (i.e., assuming flood versus debris flows). The range of increase is predicted at 10% to 170%, with the lowest increase at the pour point on Putah Creek, and the highest increase at the pour point on Cold Canyon. The Cold Canyon and Ulatis Creek pour points have the largest potential post-fire flood response, with the 10-year rainfall event modeled as capable of producing a bulked peak flow with a 500-year recurrence interval.

Based on review of the USGS debris flow model for the Hennessey Fire, a storm scenario of approximately 0.35 inches in 15-minutes (i.e., 1.54 in hr-1 or 36 mm hr-1 for the 15-minute duration) represents a 50-percent chance that a debris flow may initiate within 50-percent of the modeled basins. Basins that are modeled as high in the USGS debris flow model may be susceptible to debris flow initiation at lower thresholds.

The ERMiT post-fire surface erosion model predicts mostly a minimal to moderate increase in the rate of surface erosion across the burned area due to the preponderance of low and very low soil burn severity.

For up-to-date information and early warnings, monitor Napa One Rain for high-intensity storm events. Additionally, monitor your property throughout the winter and especially through the first few rain events. Tension cracks, scarps and slumps, and rills of any sign are common early signs of erosion hazards. 

View the County of Napa's GIS WERT Report Post-Fire Map here.

Review the WERT report for the LNU Complex Fire (Hennessey Fire) here.

Review the WERT report for the Glass Fire here.