THREE-DIMENSIONAL MODELING OF THE DEGRADATION OF CIVIL WAR EARTHWORKS

Three-dimensional modeling of the degradation of Civil War earthworks near Charleston, South Carolina, finds that modeling the downslope debris flux as linearly proportional to gradient closely fits degradation. The constant of proportionality (frequently referred to as “diffusivity”) is substantially lower than found in studies of scarps in cooler climates, suggesting that soil creep resulting from freezing and thawing may be less active.

Engineering sketches of the earthworks made shortly after capture by Union forces provide a reasonable estimate of initial morphology. The modeling is performed using an explicit finite differences approach, easily implemented with a spreadsheet program. Three-dimensional modeling has the advantage over two-dimensional modeling in that it is not limited to scarps with ridgelines that are straight in plan view, thus making it applicable to a much broader range of geomorphic features.

Modeling based on the debris flux being a non-linear function of gradient did not improve the fit over a linear function. Although this contradicts other studies, it may result from the fact that the initial height of the earthworks did not vary in the surveyed area. As with all models based on the debris flux being solely a function (any function) of gradient alone (i.e., debris flux is independent of scale), the model in this study is probably only appropriate for transport-limited hillslopes not subject to substantial erosion by wash processes.