Cost-Effectiveness Analysis for Coastal Protection of the North Coast of Zealand, Denmark

NIRAS has developed a shoreline management plan and EIA for improving the coastal protection along the 60 km North Coast of Zealand, Denmark. The project was made for Halsnæs Municipality, Gribskov Municipality and Elsinore Municipality in collaboration with DHI and Hasløv & Kjærsgaard Architects. The shoreline management plan concerns large scale regionally coordinated beach nourishment along stretches with threatened properties. The nourishment scheme includes 35 km beach distributed at 8 locations within all three municipalities. The nourishment strategy includes initial beach nourishment and maintenance beach nourishment at 5 years interval. The beach nourishment scheme is designed and optimised in combination with existing revetments to provide optimal protection of the hinterland with a 50 years perspective. The proposed beach nourishment will raise the level of the beach up to +2.0 m above mean sea level (MSL) in front of the revetments to protect the structures and the hinterland. More than 700 shore and coastal protection structures have been registered along the coast. The conditions of each structure have been assessed for scenarios with and without beach nourishment. The existing protection structures have been compared to calculated designs prepared for a 50-year return period storm and with a design lifetime of 50 years (2070), taking sea level rise and potential erosion into account. The structural site assessment survey was developed further into a cost evaluation tool to assess the total cost of strengthening and extending the existing revetments. Cost estimates have been made for the baseline scenario where existing revetments are rebuild and strengthened in cases they do not fulfil the defined design criteria. In parallel cost estimates have been prepared for rebuilding and strengthening the revetments combined with beach nourishment scenarios. The result of the cost-effectiveness analysis concluded that the most cost-effective coastal protection scheme at the North Coast for the next 50 years is obtained by combing strengthening the revetments with large scale beach nourishment. The main reason for this is, that existing revetments need less upgrading when the protecting beach is made higher and maintained with beach nourishment. Groynes and beach breakwaters were generally not included in the shoreline management as this would increase construction cost significantly compared to maintaining the beaches with beach nourishment alone. In the future some additional beach breakwaters or similar may be considered at exposed locations where the nourished sediments erode too fast.