Forensic Hydrological Investigation of the Blanco River Flood May 2015, Wimberley, TX

Abstract:

A forensic hydrological investigation of a major flash flood was conducted for the Blanco River in south-central Texas. The unprecedented flood occurred during the early morning hours of May 24^th leaving 12 dead in the towns of Wimberley and San Marcos. Hundreds of homes were damaged or destroyed, two reinforced concrete bridges were washed off their piers, and nearly 100 high water rescues were made the following day.

The present work characterizes the meteorological setup leading to the event, describes the flood hydrology using the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model, and reports on an extensive field campaign seeking to document high water marks throughout the 1200 km² basin.

Results indicate high precipitable water values, large CAPE, and strong mid and upper level winds aided in impressive divergence over the region. This allowed for storms to continually produce heavy rainfall over the same areas. Large regions of the catchment received greater than 200 mm across the upper portion of the basin with 24 hr maximums around 330 mm.

GSSHA simulations indicate good performance when compared to a stage hydrograph recorded mid-catchment. The remaining USGS gauges failed early on during the rising limb of the hydrograph. Model estimates indicate peak streamflow was approximately 5500 cms with stage values nearing 13 m as the flood wave moved through the town of Wimberley. Approximately 125 locations were examined for high water marks along the mainstem of the river using RTK GPS. Stage values ranged from 12 – 18 m.