INITIAL PALEOFLOOD VIABILITY FOR IMPROVING LONG-TERM HYDROLOGIC LOADING ALONG THE CONTOOCOOK, PISCATAQUOG, AND MERRIMACK RIVERS, NEW HAMPSHIRE

Paleoflood analyses (PFA) lengthen the effective flood record, reduce hydrologic uncertainties, and thus improve confidence in long-term hydrologic loading for risk assessments. USACE is assessing the viability of a PFA on the Contoocook, Piscataquog, and Merrimack Rivers in New Hampshire to provide a better understanding of hydrologic loading. The goals of the viability assessment are to (1) improve information on the systematic floods of record (in 1936 and 1938), (2) develop information on poorly known historical floods, and (3) assess the viability of the Merrimack, Piscataquog, and Contoocook Rivers for yielding timing and discharge data on comparable or larger pre-historic floods. Historical research indicates that the largest discharge events in the region occurred in March 1936 and September 1938, which are the largest floods since the early to mid-1700s on the Contoocook, Piscataquog, and Merrimack Rivers. These floods are well documented and flood crest stages and photographic evidence of highwater marks will be used to calibrate the hydraulic model and identify flood-related deposits from pre-historic floods. Geomorphic analyses of aerial imagery and high-resolution topographic data identified historic and pre-historic fluvial terraces at several sites that likely contain flood-related stratigraphy. Field reconnaissance will test whether these sites contain undisturbed flood deposits to characterize pre-historic floods. Field data will be collected to interpret paleoflood chronologies from the flood stratigraphy, relative soil development, geomorphic position, and numerical-age estimates. Paleodischarges will be estimated from detailed hydraulic modeling via HEC-RAS software. The PFA results will increase the effective record length beyond the historic and systematic flood record and improve confidence in the timing and magnitude of rare or extreme flood loading.