FLOODPLAIN SEDIMENTATION TRENDS AND IMPLICATIONS FOR WATERSHED SEDIMENT AND PHOSPHORUS EXPORT, WALNUT CREEK, IOWA

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 was to investigate changes in watershed-scale floodplain sedimentation over an approximate 20 year period and the resulting impacts to watershed sediment and phosphorus export. Walnut Creek is 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 the study objective, individual Hydrologic Engineering Center – River Analysis System (HEC-RAS) models were created for the years 1998 and 2014. 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 covered a range of stream gauge-informed exceedance probabilities, which allowed threshold floodplain inundation discharges to be identified 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.