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Road erosion in dry and wet tropical settings of the Northeastern Caribbean
  • Carlos Ramos-Scharron
Carlos Ramos-Scharron
University of Texas at Austin

Corresponding Author:cramos@austin.utexas.edu

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Unpaved roads are ubiquitous features of anthropogenic landscapes that facilitate the use and extraction of coveted resources. Until recently, unpaved roads had been overlooked as significant drivers of erosion and sediment yields in the Northeastern Caribbean. This paper summarizes findings of two decades of work documenting the role human disturbance on surface erosion and sediment yields. Project objectives have been to: (1) quantify the role of roads on surface erosion and watershed sediment yields; (2) identify controlling factors; (3) evaluate the effectiveness of mitigating measures; and (4) assess road hydrological and sediment connectivity. In the small, dry-tropical coastal watersheds of the U.S. Virgin Islands and Puerto Rico, land use change is driven by low-density urban development associated to tourism. In this setting, ephemeral streams deliver runoff to coral-bearing waters only ~4–5 times every year. However, unpaved roads covering only ~0.1–3% of the land surface can increase runoff delivery up to ~40 times per year. In this setting, road erosion may be up to four orders of magnitude above background and they contribute 80–99% of sediment yields. Mitigation strategies have included road drainage improvements, road paving, and sediment detention ponds. In the sun-grown coffee growing region of the wet tropical highlands of Puerto Rico, roads are a key source of the sediment affecting downstream water resources. A high erosion potential exists due to its steep relief, copious rainfall (1.6-2.1 m y-1), and considerable soil exposure. Here, watershed-scale sediment yields are 3–30 Mg ha-1 y-1, yet surface erosion rates under its natural forested cover are only ~0.25 Mg ha-1 y-1. Unpaved road erosion on these settings is 14–780 times faster than on forested hillslopes. Gravel reduces road erosion rates by 66 – 90%, but the effect seems to fade after one to two years. Cultivated lands contribute 5–63% of farm-scale surface erosion, and roads are responsible for the remainder—even though they cover 8–15% of the total farm area. In the 45-km2 Lucchetti Watershed, surface erosion from coffee farms equals 1–18% of its long-term sediment yield. The residual sediment is believed to originate from sources being quantified presently; these include road cutslopes, gullies, and landslides.