Runoff and road erosion at the plot and road segment scales, St John, US Virgin Islands

Previous studies have identified unpaved roads as the primary source of ero sion on St John in the US Virgin Islands, but these studies estimated road erosion rates only as annual averages based primarily on road rill measurem ents. The goal of this project was to quantify the effect of unpaved roads on runoff and sediment production on St John, and to better understand the key controlling factors. To this end runoff and sediment yields were measur ed from July 1996 to March 1997 from three plots on naturally vegetated hil lslopes, four plots on unpaved road surfaces and two cutslope plots. Sedime nt yields were also measured from seven road segments with contributing are as ranging from 90 to 700 m(2). With respect to the vegetated plots, only the two largest storm events gene rated runoff and there was no measurable sediment yield. Runoff from the ro ad surface plots generally occurred when storm precipitation exceeded 6 mm, Sediment yields from the four road surface plots ranged from 0.9 to 15 kg m(-2) a(-1), and sediment concentrations were typically 20-80 kg m(-3). Dif ferences in runoff between the two cutslope plots were consistent with the difference in upslope contributing area. A sprinkler experiment confirmed t hat cross-slope roads intercept shallow subsurface stormflow and convert th is into surface runoff. At the road segment scale the estimated sediment yi elds were 0.1 to 7.4 kg m(-2) a(-1). Road surface runoff was best predicted by storm precipitation, while sedime nt yields for at least three of the four road surface plots were significan tly correlated with storm rainfall, storm intensity and storm runoff. Sedim ent yields at the road segment: scale were best predicted by road surface a rea, and sediment yields per unit area were most strongly correlated with r oad segment slope. The one road segment subjected to heavy traffic and more frequent regrading produced more than twice as much sediment per unit area than comparable segments with no truck traffic. Particle-size analyses ind icate a preferential erosion of fine particles from the road surface and a rapid surface coarsening of new roads. Published in 2001 by John Wiley & So ns, Ltd.