Three Decades of Road and Trail Runoff and Erosion Work in the Northeastern Caribbean – A Research Program Perspective

Abstract

<b>Abstract.</b> Erosion is a key environmental concern for the Northeastern Caribbean because it can diminish soil productivity, damage infrastructure, and threaten human life. Sediment released by erosion can be delivered to streams where it can degrade water quality and aquatic habitat, reduce reservoir storage capacity, and threaten coral reefs. Road erosion has been recognized as a key concern since the 1990s (MacDonald et al., 1997; Larsen and Parks, 1998), and a considerable body of research has been conducted over the past 30 years. In some tropical dry coastal watersheds of the US Virgin Islands and Puerto Rico, unpaved roads, as well as foot or off-road vehicle trails may be the primary sediment sources. In wetter settings, like the forested and actively-cultivated landscapes of highland Puerto Rico, sediment contributions include those from unpaved farm roads, but also from cropland erosion and landslides. Infiltration capacities of unpaved roads and trails are ~6 – 20 mm hr^-1 and those are only 10 – 40% of adjacent undisturbed areas (Ramos-Scharrón and LaFevor, 2016). Rainfall intensities in the region frequently exceed the infiltration capacity of unpaved roads and trails. The frequent generation of overland flow is particularly important in tropical dry coastal watersheds as the ephemeral streams might carry streamflow and deliver sediment to coastal waters a handful of times per year under undisturbed conditions, but when dissected by roads the same streams will deliver runoff and sediment to coastal waters up to a few dozens of times per year. Surface erosion from road travelways and cutslopes are 15 – 220 Mg ha^-1 yr^-1 or one to four orders of magnitude higher than undisturbed hillslopes or even steep croplands in both dry and wet settings (Ramos-Scharrón and MacDonald, 2007; Ramos-Scharrón and Thomaz, 2017). Road and trail erosion rates are highly dependent on slope to the 1.5^th and 1.6^th power, contributing road surface area, time since grading, and vegetation cover. Recognition of these factors provides useful guidance for efforts to reduce runoff and erosion. Watershed scale sediment production rates in tropical dry settings are 0.3 to 3.7 Mg ha^-1 yr^-1 depending on unpaved road density and these rates are 3 – 40 times greater than background. On actively cultivated tropical wet settings, watershed scale sediment production rates are 15 – 60 Mg ha^-1 yr^-1 with road induced surface erosion and landsliding responsible for 50 – 95% of the total (Ramos-Scharrón et al., 2021). Past studies have tested the effectiveness of different Best Management Practices such as surface graveling, road drainage improvements, and sediment detention ponds. Additional work is needed to evaluate the amount of and controls on gullying below road drains, particularly in actively cultivated lands, along with the site-specific causes of road-induced slope failures.