Abstract
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.