Spatial Distribution of Collapsed Pipes: a Case Study From Goodwin Creek Watershed, Mississippi.

The role of internal erosion of soil pipes in ephemeral gully formation is largely overlooked in the USA yet some of the earliest research on gully erosion did acknowledge its role. Pipe collapses are occasionally seen on landscapes thereby providing evidence, after the fact, that internal erosion had been occurring below ground due to pipe flow. These collapsed pipes are not only a visible indicator of soil loss, but also change the spatial pattern of runoff and water erosion. The objective of this study was to determine the factors controlling the spatial distribution of collapsed pipes in agricultural fields in Goodwin Creek Watershed. Ground reconnaissance of a 2.5 km² area was carried out and the location and size of pipe collapses were measured with differential GPS. The surface elevation of cropland was surveyed with differential GPS. The interval between each measurement points was about 5 m and the interval between tracks of the vehicle was about 10 m. The mean slope gradient was 2.61 degrees. The landuse where pipe collapses occurred was dominated by cropland (cotton and soybean). In the cotton parcels, the soil was tilled with furrow intervals of about 0.9 m and furrow depths of 10 cm. According to field observations, runoff will dictated by tillage roughness until runoff was sufficient to overtop furrows. For the collapsed pipes in the parcel without furrows, the surface drainage area was calculated by running the DEM. As the diameter of collapsed pipe was only about 10% of the grid size of 10m, it was assumed that 10% of the runoff flowing into a grid will flow into the collapsed pipe. For the collapsed pipes in the parcel with furrows, the calculation of surface drainage area was based on both topography and tillage roughness. The length of each furrow segment was measured on the DEM. For each collapsed pipe, the surface drainage area was calculated by multiplying the length of the affected furrow segments by the furrow width. A total of 145 collapsed pipes were identified. The mean maximum depth, area and volume was 0.12m, 0.34m² and 0.02m³. Supposing the shape of the collapsed pipe’s upper edge is circular, the mean diameter was 0.66m.