EFFECTS OF CHANNEL GEOMETRY ON FLOODPLAIN INUNDATION FREQUENCY AND SEDIMENT ACCRETION, WALNUT CREEK, IOWA, USA

Floodplain storage commonly represents one of the largest sediment fluxes within sediment budgets. In catchments responding to large scale disturbance, floodplain inundation frequency may change over time with progression of channel evolution and associated changes in channel geometry. The primary objective of this study is to investigate the effects of channel geometry on floodplain inundation frequency and sediment accretion within Walnut Creek, a third-order alluvial channel stream located in central Iowa, USA. Over the past ~150 years, the Walnut Creek catchment has undergone intensive land cover (e.g., conversion of tallgrass prairie to agriculture) and hydrologic (e.g., tile drainage, channelization) alterations. In response, the main stem of Walnut continues to exhibit channel incision and widening, with a 17% increase in mean cross sectional area over the past ~20 years.

To accomplish study objectives, individual Hydrologic Engineering Center – River Analysis System (HEC-RAS) models were created for the years 1998 and 2014. HEC-RAS uses a basic energy equation to calculate water surface elevations at a series of channel cross section locations, based on a given discharge and resistance (e.g., channel shape, roughness). In both years, a set of 25 georeferenced channel cross sections were measured in-field by investigators. During HEC-RAS model creation, in-field cross sectional data were inserted into a 3 m digital elevation model (DEM) in order to more accurately represent channel geometry. Models were run for a set of discharges that ranged from 10 to 0.04% exceedance probability, which allowed threshold floodplain inundation discharges to be identified for both years. Suspended sediment duration curves were utilized in conjunction with model-calculated flood extent and flood depth to estimate volume of floodplain-deposited sediment for both years. Changes in flow-specific sediment accretion volume will be used by investigators to relate floodplain sediment accretion rates and sediment storage dynamics to changes in channel geometry and channel evolution stage. This work is part of a larger effort to create detailed sediment and phosphorus budgets for the Walnut Creek watershed.