ANALYSIS OF LONG-TERM FLOOD CHRONOLOGIES TO IMPROVE HYDROLOGIC LOADING FOR USACE RISK ASSESSMENTS IN NEW ENGLAND

Recent paleoflood analyses (PFA) have improved hydrologic-loading data sets for risk-based decision making for watershed assessments in New England and throughout the Nation, using a tiered approach that reduces uncertainty through progressively more-detailed efforts. Tier 1 efforts focus on watershed viability to provide PFA data and recommendations for minimizing uncertainties in hydrologic loading estimates. With confirmation of the site’s viability in Tier 1, the project progresses to a Tier 2 PFA to improve confidence and reduce uncertainties in hydrologic loadings through reach-specific data collection and interpretation. Tier 2 PFA constrain flood chronologies via integrated geologic, hydrologic, and hydraulic analyses, including detailed characterizations of riverine flood-terrace and slackwater deposits to identify and estimate ages of specific flood events in the historic and pre-historic record, coupled with detailed 1D/2D hydraulic modeling using HEC-RAS software to characterize peak flood magnitudes. These efforts involve characterization of fluvial deposits and relative soil development, state-of-art laboratory analyses to estimate flood ages, and hydraulic modeling to define flood water-surface elevations. The best-estimates and ranges in peak discharge and age for each flood datum are included into flow-frequency statistics through use of perception thresholds and flow intervals. Tier 3 PFA efforts may be conducted to resolve specific technical issues as needed to further reduce uncertainty. Using PFA results in flow-frequency analyses has shown that frequencies of rare peak discharges can be either over- or under-estimated compared to analyses using only historical and systematic hydrologic data.

In New England, PFA completed for the West River (VT) and Naugatuck River (CT) provide geomorphic data on stages and magnitudes of rare floods over watershed-scale spatial extents, therefore improving existing data sets usually containing only a few high-water-mark observations, discharge guesstimates, and/or anecdotal notions. In addition, these PFA provide physical evidence and defensible discharge calculations of multiple pre-historic floods, which extends hydrologic record lengths by hundreds of years and improves confidence in extreme hydrologic loadings.