SURFACE GEOLOGIC AND GEOMORPHIC CHARACTERIZATION FOR PALEOFLOOD ANALYSES: APPLICATIONS TO USACE DAM SAFETY RISK ASSESSMENTS

Geologic and geomorphic characterizations provide critical input to paleoflood analyses (PFA) and are improving future approaches in USACE dam-safety risk assessments nationwide. Geologic data provide the primary basis for interpreting pre-historic and early historic paleoflood chronologies, and for extending records of extreme floods beyond limited systematic records. Using field- and office-based methods, geologic data characterize fluvial processes and document the occurrence (or non-occurrence), magnitude, and timing of large, late Holocene floods. The approach involves progressive reductions of uncertainties through tiered analyses with progressively greater detail. Tier 1 efforts address watershed PFA viability via office-based analyses and limited field observations, yielding recommendations for minimizing uncertainties in initial hydrologic loading estimates. Tier 2 PFA are completed where results are likely to improve confidence and reduce uncertainties in hydrologic loadings and benefit the risk assessment. Tier 2 PFA involve an integrated program of geologic and hydraulic analyses to identify and characterize paleostage indicators (PSI) and non-exceedance bounds (NEB) that constrain late Holocene paleoflood chronologies. Tier 2 efforts often include geologic and geomorphic characterizations of riverine flood-terrace and slackwater deposits to identify and date specific floods in the historic and pre-historic record, coupled with detailed hydraulic modeling to characterize peak flood magnitudes. These efforts involve state-of-art deposit and soil characterization, radiometric and OSL dating, and use of 1D and/or 2D hydraulic modeling to define flood stages. The best-estimates and ranges in peak discharge and age for all PSI/NEB are incorporated into flow-frequency analyses through use of perception thresholds or flow intervals, and sensitivity runs provide guidance on the value of PFA results in reservoir-stage frequency analyses. Tier 3 efforts are then used to resolve specific technical issues or further reduce uncertainties, as needed. Since 2015, USACE PFA in varied geographic and meteorologic domains in 17 states nationwide have reduced uncertainty in hydrologic loadings, providing insight into future geologic inputs to USACE dam-safety risk assessments.