Impacts of unpaved roads on runoff and erosion in a dry tropical setting: Isla De Culebra, Puerto Rico

Purpose Unpaved road erosion represents a key source of terrigenous sediment threatening Caribbean coral reefs, yet no empirical data existed to document this in Culebra, Puerto Rico. Here, we compared unpaved road erosion to that from undisturbed hillslopes and evaluated the effects of road grading frequency and slope on erosion. Methods A total of 46 plot-scale rainfall simulation experiments were used to measure runoff response and erosion rates from undisturbed hillslopes and unpaved roads with varying slopes and time since grading. Rainfall rates recorded over a one-year period combined with empirically derived infiltration capacity curves and erosion rates allowed for annualizing runoff and sediment production for natural hillslopes and four road types representing different grading frequencies and slopes. Results Infiltration rates from roads were between a tenth and a quarter of those from natural hillslopes and that lead to roads generating runoff five times more frequently than natural hillslopes annually. Road erosion rates were between 330 and 760 times greater than those from undisturbed hillslopes, depending on slope and grading. Roads represent a dominant source of sediments responsible for increasing watershed-scale erosion rates from 1.1 to 25 times above background rates. Conclusion Unpaved roads represent a major source of the sediment that threatens the coral reefs of Culebra. Therefore, future new road building must be kept to a minimum and unpaved roads must be the focus of coral reef protection efforts.