*3.2. Flood Protection*

Different types of engineered flood-protection measures exist, from simple earth-filled dikes to concrete sea walls and sophisticated storm surge barriers. Table 4 shows different flood-protection methods with their unit costs. Costs are provided both for developing a new structure and for strengthening existing structures.

*Storm surge barriers:* Storm surge barriers are engineering structures in rivers or estuaries which are designed to protect the high value of economic assets and urban areas from coastal flooding. Storm surge barriers can have movable gates to allow shipping and tidal flows, which are closed during an extreme flooding event. Non-navigable barriers allow only the inflow and outflow of water. Costs vary greatly and are largely determined by the share of the movable parts of the design: Extra gates or a shipping lock can sharply increase costs. Furthermore, geographical and hydrodynamic requirements, such as "span" or "hydraulic head," determine, respectively, the size and the required strength of the design; they may also increase costs [14]. Aerts et al. [16] estimate the cost of movable parts to be between \$0.45 billion/km and \$3.6 billion/km, depending on the type of gates and head of the barrier (Appendix C Table A4). General costs, including both closure dams and movable parts vary between \$0.27 and \$3.6 billion/km, where the lower cost range includes barriers with relatively long closure-dam parts. Operation and maintenance costs vary between \$0.6 and \$22 million/year, depending on the length and number of moveable parts of the barrier system [16].

*Sea and river dikes* are designed to resist the forces of large coastal storm surges in areas with urban populations and valuable economic assets. Dikes are made of various fill materials such as concrete, clay, and sand, and they are covered with a layer of resistant vegetation or armoring material such as asphalt or boulders (Figure 2). Armoring for sea dikes is more robust than river dikes because of wave impacts, and costs are consequently higher. The cost of a new sea dike in the United States is estimated by Aerts et al. [16] at \$28.8 million/km, and for Vietnam at \$2.3 million/ [36]. For The Netherlands, Jonkman et al. [14] estimate costs at \$19.3–27.2 million/km per meter of dike raised, and for Vietnam at \$0.9–1.6 million/km per meter of dike raised. These case-study results are in the same range as those determined by a large study by Prahl et al. [22], who estimate the cost of raising sea dikes in European cities at \$21.8–31.2 million/km per meter of dike raised. River dikes are generally cheaper than sea dikes at \$12.1–18.2 million/km for a new dike in the United States [37,38] and \$5 million/km per meter of dike raised in Canada [13]. Maintenance is estimated at \$0.15 million/km and \$0.03 million/m for sea dikes in The Netherlands and Vietnam, respectively [14], or between 0.01% and 1% of the initial investment cost [17].

*Rural earthen dikes*: Rural earthen dikes are peat- or clay-filled dikes applied in rural areas, with design standards <1/100. Jonkman et al. [14] estimate the cost of a rural dike in The Netherlands at \$5.1–14 million/km per meter of dike raised, while for Canada this is \$2.1 million/km per meter of dike raised [13]. According to studies in developing countries, for new earthen dikes (1–3 m in height), costs range from \$0.1–0.2 million/km (Mozambique) [39] to \$0.9–1.5 million/km (Vietnam) [14].

*Floodwalls*: Other types of levees can be made from steel piles or concrete and are often designed as T-walls [40] (Figure 2). The foundations of these structures are also made from concrete or steel to provide stability and prevent seepage and piping [14]. Costs for a new 7-m T-wall are estimated at \$31 million/km [40]. The cost of a deployable floodwall is \$6.6 million/km [29]. Operation and maintenance costs of dikes and floodwalls vary between 0.01% and 1% [17]. Another study provides numbers detailed estimates for the operation and maintenance of two flood-protection alternatives in Texas at 0.5% of the total investment costs [41].

Other protection measures:

*Breakwater*: Offshore breakwaters are above-water structures parallel to the shore which reduce wave heights, provide shelter to a harbor and prevent sediment deposition in the entrance channel of a port. There are three main types of breakwaters: (1) rubble-mound breakwaters, which consist of a core of small rocks covered with large rocks or concrete elements; (2) vertical-wall breakwaters, which are filled with concrete blocks or sand; and (3) vertical-composite breakwaters, which are concrete structures founded on rubble substructures where the *caissons* (or concrete blocks) are placed on a high rubble foundation [42]. Costs range from \$1.4–6 million/km in developed countries to \$63 million/km for complex structures (e.g., the breakwater of the ports of Los Angeles and Long Beach [17]. For developing countries, costs vary between \$0.13 and \$0.5 million/km [21]

*Rip-rap:* Rip-rap, also known "rock armor" or "rubble," is rock or other material used to armor shorelines, streambeds, bridge abutments, pilings, and other shoreline structures against erosion (Figure 2). The unit cost for riprap used for protecting coastal zones is estimated at \$292–780/m [43,44] or \$80/ton [41], with maintenance costs at 2–4% [45].

**Figure 2.** Types of flood protection: (**a**) a cross-section of a typical Dutch sea-dike, filled with sand and clay [14] (**b**) Two types of steel and concrete floodwalls applied [46] (**c**) Rip-rap [17].





1 Values calculated using the 2016 consumer price index (CPI); 2 P = peer-reviewed; n.a. = not available.

A *bulkhead* is a retaining wall that is generally made of steel or wood which stretches ~3–10 m below the water surface and at least 1–3 m above. They are often used to protect pier walls in ports and harbors and are built to prevent soil erosion, flooding, and maintain sufficient navigation width. Recent bulkheads are made of vinyl or concrete, and wooden bulkhead pilings are usually the least expensive. Aerts et al. [16] apply a unit cost range between \$12.7 and \$51.9 million/km.

A *sandbag wall* is composed of individual bag that are filled with sand, often during the flood event. Though the method is considered effective, it is time- and labor-consuming. Sandbag walls have trapezoidal or triangular cross-sections, which means that, the higher wall, the more bags are needed. Estimates for a wall of +1 m in the United States are between ~\$200,000 and \$400,000 per km. One sandbag costs \$3–6, and to protect a door opening with a sandbag wall of +1.2 m requires 72 bags at ~\$210–420 (Appendix F Table A7).

Note that some flood protection related to river-bank maintenance has a link with "nature-based solutions" (see Section 3.4).

### *3.3. Coastal Protection by Beaches and Dunes*
