Climate adaptation measures for local flooding of the Dutch national highway network

Abstract

This master thesis provides an approach to develop Adaptation Pathways to prevent local flooding of the Dutch national highway network, using the A2 between the Vught and Sint-Michielsgestel junctions as a case study. Local flooding as result of extreme precipitation has the highest urgency on the Dutch national highway network, due to its increasing chance of occurrence as a result of climate change and increasing consequences on the traffic flow. Hydrological modelling through a bucket approach, complemented by the groundwater dynamics of time series analysis, was done to quantify a range of tipping points between 6 and 60 mm/hr for the Adaptation Pathways, i.e. the hourly precipitation intensity at which water exceeds the edge line of the road for a precipitation event with a maximum return period of 10 years. This was done for three scenarios that embed uncertainties in interception and infiltration parameters and variations in the boundary conditions of groundwater levels. The bucket approach revealed that processes of interception by porous asphalt and sewer discharge are most influential to delay the tipping point, whereas infiltration is the least influential to delay the tipping point. The modelling results showed that adaptation measures that result in (1) an increase in the storage capacity of the catchment of the road (either on the road itself or in the verge) or (2) an increase in the discharge capacity of the catchment of the road are possible to control flooding of the case highway. Possible adaptation measures to increase the storage capacity on the road that were identified are cleaning porous asphalt, increasing the thickness of the porous asphalt and installing a Plastic Road. Possible adaptation measures to increase the storage capacity of the verge that were gathered are removing the top layer of the verge, installing an infiltration trench, raising the road and creating a descending verge. An increase in the discharge capacity of the catchment of the road can be established through a new sewer layout, increasing the sewer capacity and installing manholes with holes. A focus group revealed the interests of stakeholders with respect to flooding. (1) Traffic flow, (2) water buffering capacity, (3) water quality, (4) ecological value and (5) costs and maintenance reflect the criteria relevant to the stakeholders of the focus group and therefore provide information on the societal robustness of an adaptation measure. A scorecard ranked the potential adaptation measures according to the defined criteria which indicate the societal robustness of adaptation measures. Overall, the design of Adaptation Pathways for measures to prevent local flooding on the Dutch national highway network revealed a wide range of possible futures for the defined measures, in which the fulfillment of the T=10 year design storm that is associated with 2050 requirement is either already met or possible through the implementation of adaptation measures. The combination of the Adaptation Pathways map together with the scorecard provided an assessment of the climate resilience of the A2 as an integral part of the surrounding environment. It was found that the designed Adaptation Pathways promote incremental changes, such as cleaning porous asphalt and enlarging the sewer capacity, which enhance path-dependency. Moreover, it was discovered that using a combination of Adaptation Pathways with a scorecard provides insight on transformational adaptation measures that focus on achieving hydrologic neutrality. Hydrologic neutrality can be either be achieved by storing precipitation in the catchment of the road or by discharging precipitation to a nearby location out-side the catchment of the road, where it is used. The findings of the research can be generalized to others places of the Dutch national highway network, to the design of Adaptation Pathways elsewhere and to a variety of climate effects. Finally, as the transformational change towards hydrologically neutral highways complies with the principles of systematic water management of the 21st century and low impact development, one could argue for generalization towards hydrologically neutral highways.