Author(s): Rohan Benjankar; Elowyn Yager; Klaus Jorde; Gregory Egger
Linked Author(s): Klaus Jorde, Rohan Benjankar
Keywords: Kootenai River; Levee; Reservoir operation; Hydrological alteration; Hydrodynamic model; Inundation area; Shear stress; Floodplain restoration
Abstract: The Kootenai River floodplain in Northern Idaho, USA, is almost completely disconnected from its main channel over an 80 km reach because of levees built in the1930's and an altered hydrological regime from the operation of Libby Dam since1972. River modification, tributary regulation and the decrease in flood frequencies and magnitudes have completely changed the floodplain physical processes, which are the key driving forces for floodplain ecology. In this research, we present preliminary results to reconnect the abandoned historic floodplain by intentionally breaching levees in a 2D hydrodynamic model. We considered an area of about 800 ha that was historically wetland but is currently protected with 3 to 5 m high levees and is used as agricultural land. Under the current flow regime with intact levees, this area is never inundated even during a flood with a historic recurrence interval (RI) of 25 years. After the levee breach, the area reconnects with the channel during a flood with a historic RI of 1.1 year. The 2D hydrodynamic model is also used to simulate the spatially distributed flood inundation and shear stresses for four different hydrologic conditions. The shear stresses and inundation areas are compared between all events. Flooding and shear stress are crucial physical processes that lead to floodplain morphodyanmics, which can create species diversity in vegetation. We used a sediment transport model to study erosion, deposition, and topography changes on the floodplain after each event. The levee breach caused significant sediment deposition and erosion on the floodplain and could improve habitat for riparian vegetation.