PATTERNS AND MECHANICS OF GULLY EROSION, LOESS HILLS, WESTERN IOWA

The thick loess deposits of western Iowa are susceptible to gully erosion, with individual gullies sometimes exceeding 25 meters in width and 7 meters in depth. Gully erosion destroys farmland and engineering structures and reduces water quality by increasing the sediment load delivered to streams. One gully near Treynor, Iowa, has been mapped and studied at irregular intervals since the 1960's. Our hypothesis is that seepage forces, associated with groundwater flow to the gully, trigger bank failure and gully growth at this site. Our goal is to document changes in gully growth rates and morphology and to monitor groundwater flow and bank collapse using an array of continuously recording sensors. Old and new surveys were digitized to study changes in the geometry of the gully. These surveys indicate substantial gully growth since the 1970's but at rates less than those measured in the 1960's. From 1967 to 1999, 9,000 m³ of sediment have been eroded from the gully by surface runoff. Pressure transducers and extensometers have been installed adjacent to the actively eroding headcut of the gully to monitor hydraulic head and bank deformation. Air temperature, rainfall, and barometric pressure are logged at the nearby Deep Loess Research Station. Data indicate that collapse of the gully walls is driven by rainfall and snowmelt events that increase gradients in hydraulic head normal to the gully walls. Freezing and thawing also cause bank deformation, however, and may increase the susceptibility of the banks to groundwater-driven collapse.