Reducing uncertainty in estimating rare flood events using paleoflood analyses: Insights from an investigation near Stillhouse Hollow Dam, Texas

A reconnaissance-level paleoflood investigation was completed to support characterization and reduce uncertainties in large hydrologic loadings near Stillhouse Hollow Dam, Texas. Desktop analysis identified several remnant flights of Holocene terrace surfaces along the Lampasas River near Stillhouse Hollow Reservoir that were used as physical evidence of past large floods. Field geomorphic mapping, soil exposures, and analysis of aerial imagery demonstrated that lower terraces (Qt1, Qt2) were inundated during historic and modern peak flows. A higher terrace (Qt3), formed about 3,600 years ago, had soil profile characteristics suggestive of a non-exceedance boundary for large fluvial discharges. Two-dimensional hydraulic modeling, coupled with interdisciplinary collaboration, estimated that a minimum discharge of about 300,000 cfs would be needed to just inundate the Qt3 terrace, in an unregulated …