Watershed Scale Riparian Flood Inundation Frequency Analysis

Abstract

Minnesota is home to an abundance of water resources and consequent risk of frequent flooding, high water levels, and saturation of surface soils. While widely known for its lakes, it also has thousands of miles of rivers and streams containing diverse geomorphic characteristics throughout watersheds with varying land use. This study analyzes flood inundation frequency (FIF) to help determine the context for contaminant transport from riverine riparian areas. Riparian flooding is a likely candidate for contaminant transport, however, our current understanding of the relationship is limited. FIF is defined as a quantity encompassing both the time and area over which a river’s surrounding riparian zone is inundated. Incorporating both duration and inundated area into this analysis gives weight to events that occur more frequently but inundate smaller areas in comparison to the infrequent, high flow events. FIF was analyzed in five watersheds throughout Minnesota, primarily comparing agricultural vs. forested land use, with some consideration of wetland presence. Sites within a watershed were selected to characterize a lower flow, upstream area, typically near the headwaters, and a higher flow, downstream area. Watersheds are modeled in HEC-RAS, using flow data obtained from continuous hydrologic simulations or regionally-calibrated regression equations. Results indicate that the predictability of flood inundation frequency increases with drainage area, particularly in agriculturally dominated areas. Downstream areas in the forested watersheds also exhibited a predictable flood inundation frequency, but were slightly more variable than the downstream areas in the agricultural watersheds. The flood inundation parameters for upstream sites were relatively indiscernible from one another due to the small drainage area.