Opportunities

cross-drainage structures, and flood embankments is a powerful strategy for enhancing climate resilience, reducing flood-disaster risks, increasing agricultural production, and ensuring the durability and reliability of road infrastructure in coastal areas.

Coastal areas account for 20 percent of the world’s land area, but they are home to more than half of the global population and an equal portion of economic activities (World Bank 2008). They offer a rich variety of ecosystems with a range of services, such as storm protection, water purification, nutrient recycling, fish spawning, and recreation (tourism). They also sustain food production through crops, fisheries, and aquaculture.

Because of their location, coastal systems are among the most productive but are also one of the most threatened (dayton et al. 2005). These regions are at the forefront of climate change and face major risks from sea-level rise, storm surges, floods, changing rainfall patterns, coastal erosion, and tsunamis. furthermore, coastal lowlands also face pressure from changing river regimes, evolving sedimentation patterns in coastal deltas, and land subsidence (World Bank 2008). In some coastal lowland areas, these changes may have greater impacts than those caused by climate change. Land subsidence in the Ganges delta, for instance, is estimated to be 18 millimeters per year (Brammer 2014), which is much more than expected global sea-level rise, projected to be 1–2 millimeters per year (Church et al. 2001).

Roads have a significant influence on the development of coastal areas. Because they are often combined with permanent embankments, roads may influence the duration and extent of inundations and the dynamics of flooding in coastal deltas (photo 5.1). Roads also fragment the landscape and interrupt the natural flow of water and the movement of sediments and nutrients important for biological diversity, fertile agriculture, and fisheries (douven, Goichot, and Verheij 2009). douven et al. (2012) argue that a resilience approach rather than a resistance approach often works better in coastal areas. a resilience approach consists of managing the road infrastructure along with the surrounding landscape, adapting to the broad opportunities of the area rather than reclaiming and protecting as much land as possible, and accepting risks while building in mechanisms to deal with these risks. The resilience strategy aims to minimize the consequences of floods while maintaining natural floodplain dynamics as much as possible. In contrast, the resistance strategy aims to prevent and regulate floods and has a major impact on natural floodplain dynamics (douven, Goichot, and Verheij 2009).

OPPORTUNITIES

Roads, water management, and flood protection are strongly connected in low-lying coastal areas, but this connection is usually not systematically operationalized, which results in a major missed opportunity and, in many cases, creates substantial drainage-related problems.

There is considerable scope for an integrated approach in which roads can become instruments for water management and flood resilience in coastal areas. The three main opportunities are (a) roads that contribute to improved agricultural water management, (b) roads combined with flood embankments, and (c) roads that serve more systematically as temporary

flood shelters and evacuation routes. These opportunities are discussed below.

Roads for improved water management within low-lying coastal areas

Roads, bridges, culverts, and gates strongly influence water flows, water distribution, and water levels in low-lying areas. networks of internal roads, including small village roads and pathways, divide areas into compartments with relatively higher and lower lands. In this way, road infrastructure may impede drainage and create waterlogging, affecting land use and the soil’s capacity to absorb rain during high rainfall events. Cross-drainage structures (bridges, culverts or gated culverts, and pipes) are often insufficient or too narrow, and obstruct water flows. Likewise, bridge sills may be too high, impede drainage, and cause waterlogging. although the impact of roads on area hydrology is often a neglected aspect of road design, roads can be powerful instruments for better regulating water levels in fields, contributing to improved agricultural production. If properly finetuned, roads in low-lying coastal regions can create areas with both low and high water levels and thus allow more varied, multiple-cropping agricultural practices. at present, road alignments are generally not selected with adequate consideration for catchment hydrology. as mentioned, water-crossing structures may have inadequate dimensions, be incorrectly located or absent, and are rarely provided with systems of gates that enable active management of water levels, such as storing and releasing water between different sections of low-lying coastal areas. at the same time, new roads designed without attention to required drainage are often quickly damaged by erosion and subsidence. In summary, combining road development with water management brings multiple benefits, including less waterlogging, less road damage, improved agricultural production, and improved overall livelihoods of local communities.

Roads combined with flood embankments

Roads and flood embankments are strongly linked. Many flood embankments are also used as roads, with the top of the embankment serving as a subgrade for the road pavement. In turn, some roads function as embankments of rivers, channels, and canals. Sometimes these transport and flood protection functions are mismatched, such as when a paved road is developed on an embankment that has not yet reached its safe and climate-proof level. In these cases, the embankment height cannot easily be increased because of the road. In some instances, the height of an existing embankment is reduced to enable construction of a wider road that better serves the structure’s transport functions. In addition, roads tend to compact embankment bodies. This compacting force often makes the embankment stronger but also may cause subsidence of the embankment body; where roads increase the risk of subsidence, they also threaten the essential flood protection function of an embankment. The construction of bridges in the flood embankment may also either weaken or strengthen the flood protection functions. These problems are best addressed by merging road and embankment development and reconceiving roads and embankments as an integrated concept, which involves