Study site description
The study site was near the Emerald Bay on the southwest shore of Lake Tahoe in California, USA (Fig. 1); Latitude 38.94°N, Longitude 120.08°W. Vegetation is comprised of mixed conifer forest with significant areas covered by meadows, riparian areas, or bare granite outcrops (Coats, Larsen, Heyvaert, Thomas, Luck & Reuter, 2008; Nevada Division of Environmental Protection, 2011). Lake Tahoe is the largest alpine lake in North America and has long been renowned for its exceptional clarity. However, erosion resulting from forest clearing and urban development over the past century and a half has degraded that clarity, leading to the establishment of a Total Maximum Daily Load for fine sediment particles, nitrogen, and phosphorus that are allowed to enter the lake (Nevada Division of Environmental Protection, 2011). The climate is comprised of wet winters and dry summers (Csb according to the Köppen classification; Ackerman, 1941). Average annual precipitation at the Echo Peak SnoTel station located 10 km south of the site is 1496 mm (NRCS, 2020). Site hydrology is dominated by rain-on-snow events (Brooks et al., 2016). Soil textures within the study area are dominated by a “gravelly loam coarse sand” derived from decomposed granite (Brooks, Dobre, Elliot, W, & Boll, 2010; Lew, Dobre, Elliot, Robichaud, Brooks, Srivastava & Frankenberger, 2020). For WEPP Modeling purposes, it was assumed to be a “sandy loam” in the WEPP soil database, for either forest, low severity, or high severity erodibility (Elliot, 2004), with relatively high hydraulic conductivity and rill erodibility for a given disturbance condition (unburned, low, moderate or high severity).
The Emerald Fire started early in the morning of October 14, 2016. Fueled by high winds (gusting up to 89 km h-1), burning 71 ha (red boundary in Figure 1) by the following day, before heavy rainfall extinguished the fire (http://wildfiretoday.com/2016/10/14/200-acre-emerald-fire-at-lake-tahoe-slowed-by-rain). Precipitation depths of 81, 56, and 114 mm were recorded at the nearby Echo Peak SnoTel Site 10 km south of the fire on October 14, 15 and 16, respectively. (NRCS, 2020). Highway 89 (Road 1 in Figure 1) and another paved road (Cascade Road; Road 2 in Figure 1) pass through the northeast part of the burn site. According to our field survey, road 1 is flat without drains with the gradient of about 4 percent (2.3º). Road 2 is crowned with drains on both sides, and a gradient of about 7 percent (4.0 º). The burned area above and between the roads is generally uniform with an average gradient of about 40 percent (21.8 º). The average gradient of hillslope below road 2 is about 17 percent (9.6 º). There are 4 sediment basins installed to trap road sediment below road 2 (Figure 1).
In the days following the fire, the California Department of Transport removed a large amount of sediment that was deposited on Highway 89 (Road 1) in order to keep the road open. Eldorado County had to empty four sediment basins that had overtopped on cross drains below Cascade Road (Road 2; Vollmer, 2016).
Following the fire, a USDA Forest Service Burned Area Response (BAER) specialist evaluated the erosion risks using the Erosion Risk Management Tool (ERMiT; Robichaud, Elliot, Pierson, Hall, & Moffet; Young, 2016). Stakeholders within the Tahoe Basin then asked the authors two questions: 1) What was the impact of the two roads on post fire erosion and sediment delivery? and 2) How accurate were the ERMiT erosion predictions? In order to answer these questions, it was necessary to develop some novel GIS and erosion modeling methods. A report was submitted to the stakeholders with our findings (Elliot et al., 2018). This paper presents the methodologies and findings related to surface erosion in that report.