Engineering Geology

The US Gulf of Mexico coastal region has repeatedly been subjected to major flood events. Local geotechnical site characteristics and geomorphology can change due to sediment transport processes during such events. However, field measurements during extreme conditions are challenging. This paper discusses initial attempts at a combined geotechnical and geophysical site investigation of the uppermost layers of riverbeds following severe flooding events at three different rivers in Texas: the Guadalupe, Brazos, and Colorado Rivers in terms of sediment strength derived from a portable free fall penetrometer, backscatter intensity recorded by a chirp sonar, and soil sample characterization. Results show low strength sediments (50 kPa) and larger grain sizes (d(50) similar to 0.3 mm), sediment strength of the Guadalupe and Colorado Rivers displayed more variations around bridge piers. The spatial variations likely resulted from sediment remobilization processes and local scour under severe hydrodynamic conditions. Both, geotechnical and geophysical results, reflected the observed variations in the riverbed sediments; nonetheless, a quantitative correlation among the rivers was impeded by challenges primarily related to limitations of spatial accuracy and the significant riverbed heterogeneity, as well as shallow water limitations of the chirp sonar. National Science Foundation National Science Foundation (NSF) [CMMI-1822307] Published version The authors would like to acknowledge the National Science Foundation for funding the research and work presented through grant CMMI-1822307. The authors express gratitude to Julie Paprocki, Dennis Kiptoo, Matthew Florence, and Brian Harris for their data collection efforts. The authors would also like to thank Jean-Louis Briaud and Iman Shafii for their assistance with the EFA testing. The authors would also like to thank the reviewers of this article for constructive feedback that contributed to this article.