Development Aspects Impressum Editors

Using natural infrastructure to manage water quantity

Natural infrastructure offers significant opportunity to meet the challenge of man- aging water availability, including sustaining mean availability (over the seasonal cy- cle) and regulating availability in the extremes of drought and flood (Boxes 3 and 4).

Box 3: Natural infrastructure solutions to flood risk reduction

Catastrophic flooding is emerging as one of the most significant sources of increas- ing vulnerability due to three main factors: increasing human populations and physical infrastructure development in high flood-risk areas (particularly megaci- ties in deltas or floodplains in developing countries); loss of natural infrastructure, particularly wetlands services, that regulate water flows; and most probably the increasing frequency and severity of extreme weather events under climate change.

Most modern flood management plans now consider the use of natural infrastruc- ture such as floodplains and wetlands, and increasingly are also using better man- agement of soils and land cover. Key services of this natural infrastructure include their ability to absorb water rapidly and release it slowly (regulate), and to increase ecosystems resilience by regulating erosion, and therefore landslides, and sediment transfer. These services alone account for some of the highest land/nature values thus far calculated; for example, US$33,000 per hectare per annum for wetlands for hurricane risk reduction in the USA (Costanza and others 2008).
For example, in Viet Nam planting and protecting nearly 12,000 hectares of man- groves cost US$1.1 million, but saved annual expenditures on dyke maintenance of US$7.3 million (Tallis and others, 2008). Similarly, according to Emerton and Keku- landala (2003), the Muthurajawela Marsh, a coastal wetland in a densely populated area in North Sri Lanka, provides several more-visible ecosystem services (agricul- ture, fishing and firewood), which directly contribute to local incomes (total value: US$150 per hectare and per year), but the most substantial benefits, which accrue to a wider population, are related to flood attenuation (US$1,907 per hectare) and industrial and domestic wastewater treatment (US$654 per hectare).
China runs one of the largest payments for ecosystem services schemes worldwide: the Grain-to-Greens Programme to tackle soil erosion. Soil erosion is believed to be a principal cause of the extreme flooding that took place in 1998. Planting trees or maintaining pasture has restored 9 million ha of cropland on steep slopes. In addition to flood-risk reduction, co-benefits include wildlife conservation, such as positive impacts on Giant Panda habitats (Chen and others, 2009).
However, the economic arguments for natural infrastructure are not always clear- cut. For example, in the case of the Maple River Watershed, USA, Shultz and Leitch (2001) stated that ecosystem restoration delivered insufficient risk reduction.
In many highly managed systems some previous wetlands are now occupied by high-value assets such as buildings which are costly to remove , and there is of- ten the perception that restoring natural infrastructure is an expensive option. But this is not always the case. Managed risk transfer at the catchment scale can be a solution to overall risk management. For example, parts of London (UK) are very

3.