Parent Layer: Lake County

Name: Lake Alluvial Fans Points

Display Field: fan_type

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Description: With the continued development of new technologies and in response to developing wildfire-ready watersheds as well as to overall preparedness for post-wildfire hazards, there is an urgent need to map areas that are at risk of geologic hazards including debris flows, mudflows, and flooding. The use of LiDAR technology allow us to map alluvial fans with more precision to alert cities and counties, land owners, etc. to concerning areas following a fire event.The CGS is providing LiDAR-based mapping to identify alluvial fans and debris flow-prone areas in Lake County. Debris flows or mudflows are significant hazards associated with alluvial fans (discussed solely as “debris flow” in the remainder of this text). According to the American Geosciences Institute, Alluvial fans are “A low, outspread, relatively flat to gently sloping mass of loose rock material, shaped like an open fan or a segment of a cone, deposited by a stream (esp. in a semiarid region) at the place where it issues from a narrow mountain valley upon a plain or broad valley, or where a tributary stream is near or at its junction with the main stream, or wherever a constriction in a valley abruptly ceases or the gradient of the stream suddenly decreases; it is steepest near the mouth of the valley where its apex points upstream, and it slopes gently and convexly outward with gradually decreasing gradient.” LiDAR allows more extensive mapping of alluvial fan features where debris-flow hazards could occur. The magnitude of debris-flow hazards will likely increase following wildfire events. The CGS is using readily available LiDAR technologies to more completely define alluvial fan areas at risk for debris flows, especially after wildfire events. This study seeks to compile, map, and define known and previously unrecognized alluvial fans with the aid of high-resolution LiDAR imagery. MethodsAlluvial fans were digitized from 1-m (3.3 ft) resolution LiDAR, high-resolution aerial imagery. The intent of this study is to identify and map the extents of alluvial fans. This includes interpretation where the fans have been modified by human activities. The methods of identifying alluvial fans from the available LiDAR imagery included the following steps:Produce hillshaded imagery, 2-ft, 5-ft, and 10-ft contour lines, and slope-derived maps from available LiDAR-based digital elevation models (DEMs).The hillshaded imagery was examined at different scales, in conjunction with aerial imagery (ESRI World Imagery, Google Earth imagery).Identify areas with the traditional fan shapes that are downslope from steep stream channels.Alluvial fans are mapped using LiDAR-generated 2- to 5-ft contours on mean slopes generally 5–20° and are located downslope of the fan's apex, the feeder channel and its source area. Feature has a traditional fan shape. Alluvial fans are mapped at scales ranging from 1:24,000 to 1:5,000. Using the polygon tool in ArcGIS, digitize polygon features of alluvial fans beginning at the author-interpreted apex of the fan. The apex was generally identified as the break in the contour or where it slopes gently and convexly outward at a decreasing gradient, predominantly where the deposition of sediments begins. These alluvial fans are in the feature class “Lake_alluvial_fans.”Boundaries are approximate. The extent of the alluvial fans was limited to the most recent depositional features/landforms and contours and may not encompass the entire fan feature. Changes in the edges of the mapped alluvial fans should be expected with detailed site-specific work and other’s interpretation The accuracy of the mapped alluvial fan is based on the mapper’s interpretation, current LiDAR imagery at the time of mapping, observations of alluvial fans during field checks (if performed), and our general understanding of the scope of the project. Talus slopes, comingled aprons of colluvium, alluvium and debris, and other geomorphic features were distinguished from alluvial fans deposited by floodwaters or debris-flow events by the steepness of adjacent slopes, lack of a stream channel, and the mean slope across the mapped polygon. These features are labeled as “Lake_high_angle_fans.” These features generally have mean slopes greater than 20° and may have a traditional fan shape or an irregular shape. However, it should be noted that the mapper/author, in an attempt to capture alluvial fans, mapped what other professionals may classify as another type of geomorphic feature. These features will be further evaluated during field checking in the Summer/Fall of 2024. Revisions to these mapped alluvial fans should be expected to vary.Other geologic hazards such as landslides, debris flows, hydrocompaction, subsidence, sinkholes, and rockfall were not mapped, as those features are out of the scope of this study.

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