Hydrologic Modeling and Flood Frequency Analysis for Ordinary High Water Mark Delineation

Abstract : This document explores hydrologic modeling and flood frequency analysis for ordinary high water mark (OHWM) delineation performed for Clean Water Act implementation and other applications. OHWM delineation in streams and rivers is primarily based on field indicators, and the streamflow recurrence interval corresponding with the OHWM varies between different sites. However, recurrence intervals are the primary metric that hydrologists use to characterize the variability of streamflow, and recurrence intervals provide context for understanding the OHWM. This document tests modeling techniques for estimating flow frequency and assesses their utility for OHWM delineation. The principal conclusions are that (1) there are multiple ways to estimate streamflow recurrence intervals, each having benefits, limitations, and slightly different results; (2) the uncertainty in flow frequency analysis is one reason why the OHWM typically should not be delineated based on the inundation extent of a specific flow recurrence interval; and (3) despite this uncertainty, quantitative flow frequency analysis can assist in OHWM delineation by helping to eliminate or support potential OHWM locations observed in the field. Two separate companion documents focus on (a) hydraulic modeling for OHWM delineation and (b) the combined use of hydraulic modeling, flow frequency analysis, and field evidence for OHWM delineation.