*3.6. Measures Against Local Inundations*

Traditionally, measures against local inundations in urban environments consist of engineered drainage systems with concrete channels, pipes, and culverts to quickly drain water under gravity towards larger water bodies such as lakes, rivers, or seas (Table 8). In locations that lie below sea level or in low-lying areas where drainage under gravity is difficult, pumping systems may assist in draining the water towards larger water bodies. The capital costs of engineered urban-drainage systems are very high, so nature-based "sustainable urban drainage solutions" (SUDS) have recently been developed to retain rainwater (see also Appendix E Table A6). Examples of SUDS are green roofs and wetlands (For an extended review of SUDS see [23,92]). Most cost estimates are made for individual measures such as pipelines or ditches. For such measures, unit costs are based on the diameter of length of the proposed storm-water drainage infrastructure [93]. The U.K. Environment Agency (EA) [94] sugges<sup>t</sup> that maintenance costs for SUDS in the United Kingdom range from 0.5% to 10% of the construction cost, with the exception of an infiltration trench (20%).

*Sewer pipes:* Construction costs in the United Kingdom and United States for implementing sewer pipes vary between \$215–453/m and \$61–861/m for concrete and metal pipes, respectively. The diameter varies between ~0.15–0.45 m and 0.25–2 m for concrete and metal pipes, respectively. Maintenance (cleaning and inspection) of sewer pipes in South Africa is estimated at ~\$10/m [23].

*Pumping stations*: Appendix D Table A5 shows cost figures for pumping systems in several countries. Factors that determine costs (length of pipes, distance to source area, type of motor, drainage area, etc.) are described by Marzouk and Ahmed [95]. Costs vary between \$0.4– and \$1.7 million/m3/s. The numbers show that cost are not largely linked to local labor costs, as costs are sometimes higher in developing countries than in developed countries. This may indicate that these systems are highly sophisticated and require labor from external, specialized businesses.

*Retention and retention ponds* are designed only for flood control and are also known as dry ponds. In a city environment, a pond is intended to retain storm water for a period of time, releasing the water after the storm. An outlet pipe (or control device) is mostly placed at the bottom elevation of the detention volume to allow the pond to drain dry. Unlike dry retention ponds, wet retention ponds hold a permanent pool of water. The bottom of a wet retention pond is often excavated below the water table, thereby allowing fauna and vegetation in the water to consume nutrients and suspended pollutants to settle. Costs vary between \$15 and \$50/m<sup>3</sup> for cases in the United Kingdom and United States [94,96]; the higher cost is for wet retention ponds [96]. The costs of legal fees, land costs, and other unexpected or additional costs are not included in these estimates. Further reading and detailed cost estimates can be found in a report by the Environmental Protection Agency (EPA) [97].

*Green roofs* are mostly developed upon the flat roofs of new or existing buildings (Figure 4). They are designed to store and evaporate rainwater to reduce the run-off peak to the sewer system [98]. A green-roof consists of a vegetation layer, a substrate layer (which retains water and anchors the vegetation), and a drainage layer (to discharge water). The construction cost of a green roof is dependent on the thickness (typically 0.15 m or more) and variety of vegetation (grasses, herbs, and shrubs). Costs vary from \$32–39/m<sup>2</sup> in South Africa to \$114–225/m<sup>2</sup> in the United States.

*Parks and green zones* are designed to meet the recreational needs of urban populations, but also have an important role in the hydrological regulation of cities, thereby mitigating local inundations [99]. Development costs of a park in the United States are \$1521/m<sup>2</sup> [100]. Maintenance costs of parks in the United Kingdom vary between \$0.4 and \$2/m<sup>2</sup> [99].

**Figure 4.** Example of a green roof and its main components (source: [101]).


