A Duration‐Over‐Threshold Model for Flood Frequency and Flow Regime Characterization

Abstract Proper characterization of river flow is essential for the development of structural and non‐structural measures to reduce flood damages, restore ecosystem functions, and manage environmental contaminants in riparian zones. The duration of flood events is an important feature that drives riverine processes and functions such as erosion, geomorphic adjustment, habitat suitability, and nutrient and water quality dynamics. Despite this, most flood characterization methods focus on relating the magnitude of annual‐maximum discharges to frequency, without addressing the duration of flood events. We investigated event‐specific discharge‐duration dynamics at 33 USGS stream gages within the US state of Vermont. Building on the method of Feng et al., 2017, https://doi.org/10.1061/(asce)he.1943‐5584.0001573 , flood events from 15‐min discharge timeseries were extracted using an automated threshold method. A statistical model was fit at each gage for both frequency of discharge exceedance and conditional duration of discharge exceedance. This Duration‐Over‐Threshold model estimates the arrival rate of a discharge threshold, q , being exceeded for a given duration, d . Fitted model parameters were compared to basin and channel physiographical characteristics to develop regional regression equations and examine potential watershed processes underlying the duration dynamics. Model parameters summarizing event duration were best predicted by drainage area, mainstem slope, and soil depth/type. The regional regression equations enable design event estimation in ungaged catchments of the study region, which may be used to improve the predictive capacity of hydraulic and ecosystem models, outline a range of potential geomorphic trajectories, or inform emergency management plans and flood damage rating curves.