**2. Data and Approach**

This study reviews the unit cost information of measures divided over six main flood adaptation categories (flood-proofing buildings, protection, beach nourishment, coastal nature-based solutions, riverine adaptation, and urban drainage) and adds new information. Flood protection measures are provided for both urban and rural areas.

The most relevant and recent overview studies that pertain to these different categories of flood adaptation form the basis of the analysis. In addition to these existing review studies, the following new research aspects are provided: (1) cost estimates for adaptation measures related to urban drainage (e.g., green roofs and pumping capacity), and (2) coastal-defense measures (sluices, groins, breakwaters, and rip-rap). Sources of uncertainty in cost estimates are discussed, as are the major gaps in our knowledge of adaptation costs. Recommendations for future research are also offered.

Research shows that, for the following reasons, it is hard to estimate the unit cost of flood-management measures in terms of finding reliable data:


In view of these challenges, and considering the very diverse set of flood-management measures, the focus in this study is on just two types of unit cost estimates, which are often appraised in detailed engineering studies: (1) construction costs and (2) operation and maintenance costs (O&M). Construction costs are fixed, one-time expenses that include costs for planning, purchasing materials and machinery, land acquisition, construction labor, permits, etc. Operation and maintenance costs include yearly costs needed to operate (e.g., storm surge barriers and sluices), maintain (e.g., cleaning sewer systems), monitor (e.g., restored mangroves), and replace equipment [23].

The approach followed in this paper to select unit cost prices comprises several steps:


A peer-reviewed paper is a first step in a quality check of data, and some studies have conducted quite advanced quality checks and statistical analyses to indicate the uncertainty margins of the cost estimates. For example, a review by Bayraktarov et al. [19] on the cost of nature-based solutions considers a number of studies that were used to perform statistical estimates. However, such review papers are rare, and most reviewed papers describe single case studies, for which cost estimates are based on reports from engineering companies, expert knowledge provided during workshops, or estimates that were communicated via personal communication.

Another issue that pertains to quality and comparability is that it is not always clear what is included in the cost estimates and what sub-categories have been neglected [14]. This problem has been addressed by providing upper and lower limits of cost estimations. However, due to the above-mentioned limitations, the cost estimates that are listed in this paper have high uncertainty parameters; they are probably conservative estimates, as not all cost categories are included in the estimation.
