Floodplain Analysis for the Garcia Trading Post Area Canyon de Chelly National Monument, Chinle, Arizona

Rosenlieb, Gary; Smillie, Gary

Floodplain analysis - 100-year flood, 500-year flood, worst case flood. Includes management recommendations. This report presents the results of a study that quantified flood flows and associated hydraulic characteristics for the 100- and 500-year and worst case floodplains for Chinle Wash at the Garcia Trading Post near Canyon de Chelly National Monument (CACH). Additionally, the effectiveness and possible impacts of a levee, one alternative to protect the site from flooding, were analyzed. The study was conducted by the Water Resources Division to provide Southwest Region planners with information necessary for development planning and to permit compliance with the National Park Service's Floodplain Guidelines. The study consisted of estimation of flood magnitudes, survey of channel and floodplain geometry, determination of hydraulic roughness, and computer simulation of flood flows through the area. The 100- and 500-year flood peaks were estimated using regional regression equations developed by the U.S. Geological Survey. The worst case flood peak was extracted from a previous study by the U.S. Army Corps of Engineers (COE). The COE backwater computer model, HEC-2, was then used to predict water-surface elevations and expected depths and velocities for the floods of interest. Water surface elevations were superimposed on a topographic map and depicted on channel profiles to show the extent of inundation in the study area and the flood depths relative to the floor elevations of key structures at the site. The results of the study reveal that all structures on the Garcia Trading Post site are within the 100- (base floodplain) and 500-(critical floodplain) year floodplains. The main floors of buildings 5, 11, and 12, as referenced from a Southwest Region planning map, and the lower floor of the Garcia Trading Post would be inundated by about 1 to 2.5 feet of water during a 100-year flood event. The upper floor of the Garcia Trading Post would be 2 to 3 feet above the expected 100-year flood level. A 500-year flood event would inundate all floors of all structures with flood depths ranging from 1 to 5 feet. The combination of flood depth and velocity during a 100-year flood may topple able-bodied adults that may be trapped in the flood. The Garcia Trading Post site could be protected from the 100- and 500-year floods by a levee that could be constructed on the south overbank of Chinle Wash. However, such a structure would increase flow velocities through Chinle Wash by about 70 percent during the 100- and 500-year flood events. Increased velocities may alter the existing sediment transport characteristics through Chinle Wash, which in turn could increase the risk of failure of a levee if not properly designed. A levee may also exacerbate flood situations at the Thunderbird Lodge and the CACH campground. Construction activities at the site may adversely impact wetlands. Consultation with the U.S. Fish and Wildlife Service or the COE is recommended to determine if mitigation is required.