Flash Flooding from the Waldo Canyon Burn Scar

In June 2012, the Waldo Canyon wildfire occurred in mountainous terrain immediately to the west of Colorado Springs, Colorado. The fire burned with a moderate to severe intensity resulting in hydrophobic soils over much of the burn scar. The hydrophobic soils cause rapid runoff resulting in flash flooding, even with modest rainfall amounts. This presentation will discuss the experience of the National Weather Service office in Pueblo with several flash floods which occurred in summer 2013. Flash flooding from the Waldo Canyon burn scar can occur very quickly, result from modest rainfall totals, and be catastrophic. Flash flooding can start within 15 minutes of the commencement of heavy rain. Significant flash flooding can occur with rainfall totals as small as 15mm (0.6 inches), but rainfall rate is critical. The flash floods typically occur with rain rates larger than 50 mm/hour (2 inches/hour). Past flash floods have resulted in cars floating down Highway 24 (a major four-lane corridor in the mountains) resulting in one fatality. Significant flooding occurred in Manitou Springs, Colorado, and there was substantial damage to the water supply infrastructure for Colorado Springs.

One of the main factors which increase the false alarm potential is the vastly different runoff characteristics between locations within and outside of the burn scar. Most of the summer thunderstorms in the vicinity of the burn scar can have rather small (but intense) precipitation cores. If a thunderstorm just misses the burn scar, the runoff could be minimal, while significant flooding would occur if the heavy rain fell on the burn scar.

The burn scar is in an area with steep terrain (up to 70 percent grade), ranging from the Pikes Peaks massif to the high plains immediately to the east. The complex terrain in the area affects the strength and movement of thunderstorms in the vicinity of the burn scar. A subtle change of the storm path towards or away from the burn scar can determine whether or not locally heavy rainfall will fall within the burn scar (producing significant flash flooding) or just outside of the burn scar (with only modest runoff). In addition, storms have rapidly developed over or just upstream of the burn scar (resulting in flash flooding) and conversely strong storms have rapidly weakened as they approached the burn scar.

The small size of most of the basins affects the location of the flash flooding. Most basins in the burn scar have an area around 5 square kilometers (2 square miles) and each basin will produce flash flooding at different locations. Pinpointing which basins will be under the heavy precipitation core can be challenging ahead of time as subtle movements in the storm will result in different basins being affected.

Decision support ahead of the event is also very important for the warnings to be effective. The decision support associated with Waldo Canyon burn scar flash flooding will also be discussed.