Introduction
Water is the single largest factor in road damage; as a consequence, it is generally considered the prime enemy of road infrastructure. Water is responsible for 80 percent of damage to unpaved roads (Chinowsky and Arndt 2012) and 30 percent of damage to paved roads. Water management is a critical area of study in the development of roads, and the focus is on managing water to minimize the damage to road infrastructure. Therefore, the impact of roads on their surrounding landscape and surface hydrology is often considered secondary. This lopsided focus translates into significant direct impacts on natural hydrology and landscapes and into missed opportunities to harvest water for communities along the road. Conventional road design often blocks and changes runoff, interferes with subsurface flows, and changes water patterns. Roads cause erosion, local flooding, and sedimentation. Failure to anticipate how roads will affect natural water flows results in road bodies that act as dams and become primary causes of drainage congestion and waterlogging, disturb wetland hydrology, and prevent fish movement. For instance, transect surveys undertaken along roads in upland Ethiopia and Uganda show that there may be 8–25 flash points—such as local erosion, flooding, sedimentation, or waterlogging—for every 10 kilometers of road. Ibisch et al. (2016) estimate that, at present, 20 percent of the global land surface is within one kilometer of a road, which is where most people live and where economic activities are concentrated. Given the vast coverage of the road network and its proximity to the human population, ensuring that roads contribute to water management may have the potential to improve water security globally. An estimated one in four people currently depends on groundwater as the primary water source. Yet too few regions maximize the potential for roads to play a positive role in management of water resources, for instance, by directing road water to recharge groundwater or through short-term water storage in roadside ponds. Many examples emerge from a variety of countries; for example, research in Tigray, Ethiopia, finds that road runoff affected 70 percent of roadside farmers, but only 20 percent were making productive use of that runoff (Teweldebrihan 2014). Furthermore, even though roads can play many positive roles managing water in low-lying coastal areas, data from two coastal polders in 3
