Engineering Geologic Assessment of Risk to Visitors: Canyon Lake Gorge, Texas

Presented here are the results of a study of geological hazards conducted in Canyon Lake Gorge of Central Texas. Canyon Lake Gorge formed in 2002 when the emergency spillway of Canyon Lake was overtopped. Since that time, the gorge has been opened to public tours, and the organization governing the gorge has expressed concern regarding visitor safety. The surveys in this study gathered data through field observations and supplemented those data with non-destructive tests from an impact test hammer. The goal of this study was to gather original field data on potential hazards of the gorge with the hope that insight from these data could be used to enhance visitor safety in the gorge. The field observations made in this study identified the presence of undercut rock ledges that could present varying degrees of risk to visitors. Easily eroded clayey wackestone facilitated formation of these potential hazards. Lithologies such as packstone and grainstone serve to form ledges atop the wackestone. Preexisting fractures and joints in the ledge forming rock, which compound the danger of the unstable masses of undercut ledges, provide failure planes. This study identified current areas of unstable masses by location and differentiates the degree of risk present at each location, using simplified classes of low, medium, and high risk. Level of risk was determined primarily by the potential injuries incurred. Often, the height was dependent upon the thickness of an easily eroded wackestone bed that undercuts ledge forming rock. Canyon Lake Gorge is a young and dynamic geomorphological environment seeking equilibrium through gravity facilitated erosional events. In time, natural formation of riser beds will mitigate the potential hazards of some undercut ledges. Based on the potential hazards identified in Canyon Lake Gorge, four safety recommendations are proposed:

• Visitors should always be guided by trained personnel. This practice is in place.
• Visitors should be educated on the dangers of Canyon Lake Gorge before entering.
• Unavoidable hazards should be evaluated for ways to mitigate risk.
• The gorge should be continually monitored to insure safety of the visiting public.