CHANNEL DYNAMICS AND FLOODPLAIN ALTERATION: BLANCO RIVER FLOOD MAY 23-24

The critical zone where humans interact with the processes and forces of Earth results in disasters of all flavors. Floods are a common natural disaster claiming lives, displacing individuals and families, destroying properties, and degrading land on and adjacent to the floodplain. Floods are the most frequent and devastating natural disasters in the world. The Texas Hill Country is no exception to this. Its geographic location, physical characteristics and landuse make this a region highly susceptible to flash-flooding; historically receiving world-record rainfall amounts. Local physiographic, the Balcones Escarpment, meteorological factors, drainage density, geology, confined valleys coupled with landuse create an area of extreme potential. This area has experienced major flooding in 1929,1998 and again in 2015.

Above average rainfall resulting from a persistent weather pattern resulted in May 2015 becoming one of the wettest months on record. Increased precipitation saturated soils across the region, resulting in excess runoff as precipitation continued. During May 23-24, a series of convective thunderstorms produced record rainfall and resulting flash floods throughout the 1,607 km² Blanco River watershed. This event produced record stage at both USGS gauges located along the Blanco River, with a recorded stage at Wimberley rising to 13.68 m.; 3.35 m above the previous highest recorded level in 1929. The river rose 31 ft. between 2230 hrs. and 0100 hrs. (May 23-24) leaving the 100-year floodplain and inundating the 500-year floodplain in numerous locations, which resulted in considerable damage along several miles of the river. Based on the length of record, this flood had a recurrence 55 of 91.

We documented damaged structures and morphologic changes along two sections of the Blanco River. The river exposed bedrock in several locations outside of the current channel as a result of stream power a large volume of sediment was transported. Undercut banks, large root balls, and hundreds of down trees are visual indicators to the force of the discharge in the river. As a result of the rapid rise in discharge, numerous structures were destroyed and carried downstream, bridges became impassible, buildings suffered severe water damage, and several people lost their lives.