Erosion Rates from Badlands National Park

Abstract

Erosion rates from fossil-bearing strata at Badlands National Park in South Dakota (Figure 1) will be used to prepare an erosion model for paleontologists to utilize for management of fossil resources throughout the Park. Strata being monitored has been regarded as highly erodible, estimated at ~2.5 cm/yr, although no documented measurements have been identified. Stratigraphic units, and their contained fossil beds, are low-angle (2 to 3 degree dip) and thick (up to ~100 m) sequences that consist primarily of fine-grained silt and clay units with intercalated thin sandstone beds representing mostly fluvial deposits and paleosols. The clay provides cohesion and swells when wet resulting in typical badlands weathering profiles that are steep and deeply gullied. The rugged topography and often remote fossil sites have historically challenged Park paleontologists in their efforts to catalog and protect these resources. Current measurements of erosion rates are made using erosion pins, erosion trays at the toe of slopes, and photogrammetry techniques. In addition, a tipping-bucket rain gauge is used at each instrumented site. Erosion pins consist of 0.47 cm diameter stainless steel rods 45.7 cm long, three of which are driven perpendicularly into the slope leaving approximately 8 cm exposed. A flat stainless steel plate (10 cm x 15 cm) is inserted over the exposed pin through a hole and the measurement is acquired using a 0.025 mm resolution digital caliper. The flat plate provides measurements averaged over an area. Six to eight measurements are collected from each pin and averaged to attain a value. Erosion trays consist of hard plastic trays that are 1 m long, 10 cm deep, and 13 cm wide that are dug into the bedrock so they are flush with the surface and butt against the toe of the near vertical slope. The height of the slope above the tray multiplied by the length provides an area from which the eroded material were derived. The co-located rain gauge provides the precipitation amount utilized in determination of erosion rates. At specific locations, a high-resolution photogrammetry system is utilized to image a slope and render 3-D images. Subsequent images from the same location will be subtracted to determine an average slope retreat and will be compared to the physical measurements from the pins and trays. Preliminary measurements from fall 2010 have indicated that erosion is linked to precipitation. A rare 4.4 cm precipitation event occurred at Badlands over the course of 1.5 days in August 2010. The slope wash overfilled the erosion trays rendering invalid data. However, the erosion pins at two sites (a north-facing and a south-facing slope) recorded up to 10.6 mm of slope retreat. Installation of additional sites in 2011 and additional photogrammetry images will be utilized to determine Park-wide as well as site-specific erosion rates.