The surface drainage system should remove all flow of rainwater from the road’s surface, and from the highway slopes as well as the runoff from adjacent land. Surface drainage systems are also important in the proper management of polluted runoff and in minimizing environmental impacts. The surface drainage can be divided into transverse drainage and longitudinal drainage.
Transverse Drainage — Typically used to allow existing water courses to pass under/over the road which would, otherwise, form a physical barrier. These are normally constructed as aqueducts or culverts (see Chapter 12, Fig. 12.3).
Longitudinal Drainage — The main objective is the fast collection and removal of the rainwater that falls upon the road’s immediate surroundings, and of the water from the adjacent areas, edges, excavation slopes and central reserve. This is fundamental for maintaining the safety of traffic by eliminating water films and puddles from the road surface (which can result in aquaplaning) at the same time reducing the possibility of water infiltration into the pavement’s layers or foundation, which may reduce its load carrying capability.
Longitudinal surface drainage systems include gutters, channels, ditches, swales, galleries and collectors, complemented by their respective manholes, catchpits and sumps. Surface drainage is not the main topic of this book, so readers should look elsewhere for detailed information on this topic (e. g. Kasibati & Kolkman, 2006).