Characterizing Duration and Frequency of Flood Events Across Geomorphic Settings

Design floods are used in the development of structural and non-structural measures to reduce flood damages, restore ecosystem functions, and manage environmental contaminants in riparian zones. While parameters of design flood hydrographs such as peak discharge, event duration, and event volume are significant for ecosystem processes and engineering applications, most Flood Frequency Analyses (FFA) focus solely on annual maximum discharges. We investigated event-specific discharge-duration dynamics at 33 unregulated USGS stream gages within the US state of Vermont. Building on the method of Feng et al. 2017, flood events from 15-minute discharge timeseries were extracted using a simple threshold method, and statistical models were developed for both frequency of discharge exceedance and conditional duration of discharge exceedance. These relationships were further compared to basin, river network, and channel parameters at each gage, and it was found that basin slope and drainage area accurately define the conditional distribution of flood duration on peak discharge. These results enable design event duration estimation and Monte-Carlo simulation in ungaged catchments, which may be used to improve the predictive capacity of hydraulic and ecosystem models or to inform emergency management plans and flood insurance rates. Furthermore, the approach to modeling event frequency used in this analysis demonstrates a potential improvement on Over-Threshold FFA methods.