Global Investment Costs for Coastal Defense through the 21st Century Report to the World Bank 10 This results in a new range of costs represented by low and high estimates of dike unit costs as documented in Appendix 3. The low costs are similar to the earlier unit costs in DIVA and the high costs are about three times higher. As the stock of dikes grows, so the maintenance costs also grow and can become significant, as shown in Section 4. Hence it is important to track these maintenance costs and make sure they are considered, rather than forgotten or ignored.

The capital costs of existing sea dikes up to 2015 (the base year) are not considered in the analysis. It is simply assumed that defenses corresponding to the standards in Table 4 have been provided. However, the cost of post-2015 maintenance of these pre-existing dikes is considered.

## **3.4.2 River dikes**

River dikes are built on both sides of the lower coastal-influenced reaches of selected rivers (see Figure 2). To calculate the length of river to protect with dikes, it is estimated how far the 1-in-1,000-year extreme sea level event could penetrate inland along the river. This is done by dividing the water level of the 1-in-1,000-year event by the river slope. Hence, the impact length of the 1-in-1,000-year event increases with sea-level rise. For simplicity, we use the same unit costs as sea dikes. Due to geometric considerations, river dikes are on average half the height of sea dikes. This also implies that river dikes are built at sea-level elevation in the river mouth and their base elevation increases with distance from the river mouth. Due to the absence of wave action, the river dike maintenance costs are assumed to be half the maintenance cost of sea dikes (0.5% of the capital cost per year).

As with sea dikes, existing river dikes’ capital and maintenance costs in 2015 (the base year) are not considered in the analysis, but post-2015 maintenance is considered.

# **3.5 Surge barrier costs**

Surge barriers require a location. This was developed using expert judgement to define a single potential surge barrier position for each river distributary. The selected position was based on a trade-off between minimizing the barrier length and maximizing the length of avoided river dikes, while considering local conditions (Figure 4). The few existing barriers are not considered, as for most of them there are already plans for substantial upgrade of even the construction of new replacement barriers (Lavery and Donovan, 2005; Tarrant and Sayers, 2012). Hence, all barrier capital (construction) and maintenance costs are considered in the analysis. As with river dikes, the same protection levels are assumed as for the open coast requirement. If surge barriers are built, existing river dikes are kept (maintained) but no longer raised with sea level rise. Surge barriers stay constant in width, but are incrementally raised with rising sea level.

The unit costs for surge barriers are taken from Table 3 of Jonkman et al., (2013). We use the minimal (US$97,000 per meter height and meter width) and maximal (US$374,000 per meter height and meter width) unit costs as the low and high cost scenarios in this study, respectively. Maintenance costs are assumed to be 1%/year following the maintenance costs for sea dikes.