3.5.2. Operation and Maintenance

Operational activities include inspections and periodic evaluation studies, and they are estimated at \$4049/km [78]. Maintenance measures include weed control (vegetation clearance), obstruction/dirt removal, de-silting, and small bank repairs. Apart from the generic factors that determine costs (e.g., labor rates), the maintenance costs depend on the "target condition" of the channels and whether channel-clearance measures are implemented manually or mechanically. For well-maintained channels ("Grade 2"; [78]), costs for manual maintenance are higher than for mechanical methods; they vary between \$5730–51,311 km/year and \$1680–17,096 km/year for manual and mechanical maintenance, respectively [78].

### 3.5.3. Nature-Based "Soft Bank" Protection and Water Buffering

In Section 3.2, a number of hard-engineered bank-protection measures (e.g., Riprap) were described. However, in more rural areas and in areas with lower protection standards, nature-based methods may often both reduce flood risk and improve environmental values such as biodiversity. Methods include implementing brush mattresses, revegetation, using biodegradable geotextiles to stabilize grade, and the application of logs or other natural materials resistant to erosive flows. Costs vary between \$54,000 and \$978,000/km for smaller rural river branches [49,78]. Most of these measures only have a design life of 3–15 years [89].

Other channel-related measures:

*River detention areas*: River detention areas are larger bath-tub-like systems (>3 million m3) surrounded by a dike and designed to temporarily retain peak river discharges. Detention areas are located along river channels and capture floodwater above a pre-defined water-level. At this point, a control device (a pipe or a spillway) is overtopped with flood-water into the detention/retention area. Spillways are also applied to activating side channels in case of extreme water levels [90]. Along the river Rhine in Germany, multiple detention areas between 3.6–32 million m<sup>3</sup> are being developed to reduce flood peaks [91]. The cost of the largest "*detention polder Hordt*" (870 ha, volume: 32 million m3) is estimated at \$98 million [83]. A similar flood-detention project has been proposed in the Rio Piura in Peru, including a detention area of 2600 ha, a 20-km new dike with construction costs estimated at \$429 million, and operation and maintenance estimated at \$5 million/year [84]. With a dike 3 m in height, the volume would be +/−373 million m<sup>3</sup> with unit construction costs of \$1.9/m3.

*Inland wetland and water buffering*: Creating wetlands in upstream areas enhances the buffer-capacity of ecosystems to absorb—at least temporarily—peak rainwater before it drains into the main river channels. Wetlands also have the ability to reduce nitrogen, phosphorous, pesticides, and sediment loading in open-water systems. Restoration of inland wetlands is less expensive than restoration of coastal wetlands. The cost for the former is estimated at \$45,752/ha [19]. Tyndall and Bowman [85] estimate restoration costs for inland wetlands in Iowa (United States). For this case study, costs vary depending on site planning and design, excavation activities, installation of control structures (e.g., levees), and the opportunity cost of land made unsuitable for agricultural production. Design and construction costs are \$10,022/ha, and yearly maintenance costs over a time span of 40 years are \$785/ha [85].
