Description: The CGS is providing lidar-based mapping to refine the currently mapped alluvial fans and debris flow-prone areas in Pitkin County. This new mapping is based on Pitkin County’s existing geologic hazard maps but also includes new areas defined by lidar. Debris flows or mudflows are significant hazards associated with alluvial fans (discussed solely as debris flow in the remainder of this article). 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. Pitkin County has expressed concern about their preparedness for post-wildfire natural hazards and contracted CGS to use 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. Methods: Alluvial fans were digitized from 1-m (3.3 ft) resolution lidar, high-resolution aerial imagery obtained from the Colorado Water Conservation Board and Department of Natural Resources (https://coloradohazardmapping.com/lidarDownload) and collected by Merrick-Surdex Joint Venture, LLP and Merrick & Co. from 5/28/2020 to 10/19/2020. 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 alluvium fans from the available lidar imagery included the following steps:Produce hillshaded imagery, 5-ft contour lines, and slope-derived maps from available lidar.The hillshaded imagery was examined at different scales, and alluvial fans were mapped at scales ranging from 1:24,000 to 1:5,000. Identify areas with the traditional fan shapes that are downslope from steep stream channels.Alluvial fans mapped using lidar generated 5-ft contours on slopes generally <15-20° and is located downslope of the fan's apex, the feeder channel and its source area and has a traditional fan shape.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. 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. 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 or lack of a stream channel. These features are labeled debris fans_aprons_other. 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 2023. 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 a part of this study.
Service Item Id: a2331de0ec974cd7a81f13ada4e3df3e
Copyright Text: Colorado Geological Survey, Pitkin County