Anaylsis of Flood and Debris Flow Hazards Following Short Duration, High Intensity Rainfall Events on New Mexico Burn Scar Areas During Summer 2011 and 2012

Wildfires burned nearly 870,000 acres of forested lands in the State of New Mexico during the 2011 and 2012 fire seasons. In northern New Mexico the Las Conchas wildfire (2011) was the the second largest in the state's history at 156,593 acres, impacting portions of the Santa Clara, Cochiti, San Ildefonso, and Santa Domingo Pueblos, and also portions of Bandelier National Monument and the Valles Caldera National Preserve. Some of the Las Conchas wildfire burn area had been burned by the Cerro Grande Fire in 2000. The Track Fire (2011) burned much of the Chicorica Creek watershed, a major source of water for the City of Raton. In southern New Mexico the Little Bear Fire (2012) near Ruidoso burned much of the Rio Bonito watershed, a major source of water for the City of Alamogordo and Holloman Air Force Base. Freshly burned landscapes are at risk of damage from erosion hazards such as those caused by flash flooding and debris flows. The risk of hydrologic hazards may persist for years after a fire and can impact water resources, ecology, businesses, homes, reservoirs, roads, and utility infrastructure in the wildland/urban interface. Following the Las Conchas, Track, and Little Bear fires, several high volume (low frequency) floods occurred in and downstream of the burn scars as the result of otherwise typical summer monsoon thunderstorms. Personnel from the U. S. Geological Survey and the National Weather Service visited parts of the significantly impacted burned areas, conducted debris-flow assessments and collected data to support numerical modeling (slope-area computations) and documentation of high volume floods downstream of the burn scars. For basins burned by the Las Conchas fire, in response to a 10-year recurrence design storm of 28 mm of rain in 30 min, the probabilities of debris flows exceeded 80% for two-thirds of the modeled basins. Similarly, for basins burned by the Track Fire, the probabilities of debris flows exceeded 80% for the majority of the Raton Creek tributary basins in Railroad Canyon and also for six basins that flow into Lake Maloya, the main water supply for the City of Raton. This presentation will show examples of the damage in and downstream from the burn scars, review the calculations of the debris flow probabilities, and emphasize why debris-flow hazard assessments and flood-frequency predictions conducted in wildfire threatened areas before fires occur would help land and resource managers plan for, and mitigate the effects of, post-wildfire hazards.