Impacts from the 2015 Memorial Day Weekend Blanco River, Texas, Flood & Lessons Learned in Communicating the Message

Flash flooding and river flooding, along with two tornado outbreaks, occurred across South-Central Texas during the Memorial Day Weekend of 2015, during which time several rivers across the Hill Country of Texas were packed with tourists for the weekend. Close to a foot of rain fell on May 23rd in the headwaters of the Blanco River, resulting in a historic flood wave that impacted the communities of Wimberley and San Marcos. The Blanco River at Wimberley rose close to 40 feet in just four hours. A Flash Flood Emergency was issued, the first time this was ever done for the Blanco River. Numerous high water rescues occurred throughout the late evening and morning hours. 11 fatalities occurred along the Blanco River, with over 1000 homes damaged or destroyed. Interstate 35 was closed as the Blanco River overtopped it. In some places the river exceeded the 500 year flood inundation. Search and rescue continued for days after the flood, during which time additional storms impacted the Blanco River area.

Days leading up to the flood Decision Support tools, such as strongly worded emails and graphical briefings, alerted core emergency management and first responder partners of a potentially dangerous flooding possibility across South-Central during the upcoming Memorial Holiday Weekend. During the flood, phone briefings and NWSChat played crucial roles, in both maintaining a consistent message and alerting media and emergency management partners of the rapidly increasing magnitude of the flood. Strong relationships and trust, built between the National Weather Service in Austin / San Antonio and emergency management officials in Hay County and the City of San Marcos, aided in the confidence for decision makers to make calls for evacuations when presented with the urgent nature of the developing flood situation. Rapid fire social media posts were also used successfully to spread the word with the general public on the unfolding event.

Many lessons have already been learned from this event. While the Reverse 911 System worked in some cases for those that had registered their cell phones, especially downstream towards the City of San Marcos, in other cases tourists staying in rentals along the Blanco River weren't registered and did not get notification. Also potentially playing a role was the influx of tourists over the holiday weekend, unfamiliar with the rapid nature of river rises in this area during flash floods. There are also currently no river gauges along roughly the first 40 miles of the Blanco River, upstream of Wimberley. Combined with limitations in current hydrologic modeling, this likely played a role in assessing the magnitude of the flood wave before reaching Wimberley. Plans for additional upstream gauges, better inundation mapping, and building upon proven Decision Support tools are currently underway to mitigate threats to life and property along the Blanco and other rivers in South-Central Texas.