A WATERSHED SCALE FLOOD INUNDATION FREQUENCY ANALYSIS

Minnesota is home to an abundance of water resources. Its unique surplus of freshwater differs from arid, western states that cope with droughts, aquifer depletion, and water restrictions. Instead, there is risk of frequent flooding, high water levels, and saturation of surface soils. While Minnesota is 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 to help determine the context for contaminant transport from riverine riparian areas. Flood inundation frequency was studied in five characteristically different watersheds throughout Minnesota. The watershed selection primarily compares agricultural vs. forested land use, as well as 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 StreamStats. Flood inundation frequency is used as a quantity encompassing both the time and area over which a river corridor’s surrounding riparian zone is inundated, with units of acre-days. Incorporating both duration and inundated area into this analysis gives weight to events that occur more frequently, but do not inundate large areas to the extent of high flow events.

Preliminary results indicate the agricultural, flat watersheds have higher cumulative acre-day inundation than the forested watersheds. In the agriculturally dominated watersheds, higher flows (between the 2% and 5% flow exceedance) were responsible for the greatest riparian inundation susceptibility, whereas in the forest dominated watersheds, lower flows (e.g. 10% flow exceedance) were responsible for the greatest riparian inundation susceptibility. The implications for contaminant transport depend on the nature of contaminant properties. The riparian areas in forested watersheds are more frequently flushed, potentially causing contaminants and particles to enter the adjacent waterway. On the contrary, riparian areas in agricultural watersheds have longer periods of dry land surfaces, potentially creating an environment conducive to increasing the mobility of contaminants.