DIRECT MEASUREMENT OF EROSION RATES USING REPEAT SURVEY TECHNIQUES

Direct measurements of erosion are difficult to obtain because they typically require long time periods in order to observe measurable change. We used erosion pins to measure erosion rates on two abandoned lots in areas with exposed bare soil where a system of rills had developed. We also measured channel bank erosion using repeat surveys of channel cross-sections. Each erosion pin site had an approximate slope of 30 degrees, but site 1 consists of silty clay loam soils of the Waynesboro series with few chert fragments and site 2 consists of silt loam soils of the Christian series with numerous chert fragments. In 2005, 20 erosion pins were installed a site 1 and 32 pins were installed at site 2 and repeat measurements were made intermittently until 2013. Erosion pins were 35 cm-long, 1.5 cm-diameter rebar rods driven into the soil. The amount of erosion was measured as the change in height between the land surface and the top of erosion pins. At site 1, erosion occurred at all pins and measurements made between 2005 and 2013 indicate an average erosion rate of 19.1 cm per pin (2.3 cm/yr or 350 Mg/ha/yr). Compared to values reported in the literature, this erosion rate is similar to rates measured from constructions sites using silt fences, but much lower than rates from agricultural plots including row crops. At site 2 between 2005 and 2009, erosion occurred at 21 of the 32 pins at an average rate of 1.0 cm/yr (152 Mg/ha/yr). However, 11 pins at site 2 had become buried with sediment indicating deposition. The lower erosion rate and the deposition that occurred at site 2 can be attributed to the growth of vegetation at this site during the study period. In contrast to site 2, site 1 has remained barren of vegetation throughout the study period. Repeat surveys of stream cross-sections between 2004 and 2009 indicated lateral bank erosion rates of 0.03 to 0.04 m/yr, as well as fluctuations in stream bed elevation, the likely result of bedload movement. Erosion rates measured in this study represent only a fraction of the downstream sediment yield of (1000 Mg/ha/yr) measured from suspended sediment sampling in an earlier study in this watershed. This finding indicates that sources other than bare soil areas must be contributing sediment to the basin outlet. One possible source is the large amount of sediment stored in caves throughout the study area.