Potential hazards from flood in part of the Chalone Creek and Bear Valley drainage basins, Pinnacles National Monument, California

Abstract

Areas of Chalone Creek and Bear Valley drainage basins in Pinnacles National Monument, California, are subject to frontal storms that can cause major flooding from November to April in areas designated for public use. To enhance visitor safety and to protect cultural and natural resources, the U.S. Geological Survey in cooperation with the National Park Service studied flood-hazard potentials within the boundaries of the Pinnacles National Monument. This study area extends from about a quarter of a mile north of Chalone Creek Campground to the mouth of Bear Valley and from the east monument entrance to Chalone Creek. Historical data of precipitation and floodflow within the monument area are sparse to nonexistent, therefore, U.S. Soil Conservation Service unit-hydrograph procedures were used to determine the magnitude of a 100-year flood. Because of a lack of specific storm-rainfall data, a simulated storm was applied to the basins using a digital-computer model developed by the Soil Conservation Service. A graphical relation was used to define the regionally based maximum flood for Chalone Creek and Bear Valley. Water-surface elevations and inundation areas were determined using a conventional step-backwater program. Flood-zone boundaries were derived from the computed water-surface elevations. The 100-year flood plain for both streams would be inundated at all points by the regional maximum flood. Most of the buildings and proposed building sites in the monument area are above the elevation of the 100-year flood, except the proposed building sites near the horse corral and the east monument entrance. The 100-year flood may cause reverse flow through a 12-inch culvert embedded in the embankment of Old Pinnacles Campground Road in the center of Chalone Creek Campground. The likelihood of this occurring is dependant upon the amount of aggradation that occurs upstream; therefore, the campground area also is considered to be within the 100-year flood zone.