Managing the flood

Devastating floods in parched land; few who witnessed the Mozambique floods, either at first hand or through the media, will not have wondered at the perverse cruelty that drowned and destroyed people and property and swept away the means of survival. But floods, like most things, are good in moderation. For river basins and their populations, they are an essential part of the natural, and human cycle, revitalising land, restoring groundwater, nurturing fish and plants. In dry areas, for example in sub-Saharan Africa, the retreat of floodwaters are the signal for 'recession agriculture' to begin. Farming families living near the Senegal River traditionally harvest two crops a year, one during the rainy season, on the sandy uplands (millet and beans), and a second on the floodplain once the annual floods have retreated (maize, watermelons and potatoes).

Seasonal floods bring not only water, but also much needed sediment, a rich source of nutrients to maintain fertility. Many communities depend for food and income on fish which spawn and feed on the flooded land. And the floodplain grasses and trees that have evolved to cope with the high water levels provide nutritious grazing for livestock, and timber, fuel and an incredible variety of other products for people. No wonder that the whole social and cultural framework of floodplain communities is linked to the annual floods.

Damming of large rivers, whether for hydropower or irrigation projects, has major impacts on downstream floodplains. Specialist ecosystems which have evolved to depend on the dynamics of high and low water are not suited to the constant, average water levels that that may be released from hydropower reservoirs. In the absence of floods both natural and agricultural systems adapt; dryland plants and practices frequently take over. In north-western Nigeria, the damming of the Sokoto River reduced the area of annual flooding by half, and as a result a quarter of families have been forced to stop recession agriculture entirely. Similar effects have been seen on rivers in Niger, Chad, Sudan and Mali. After the building of the Manantali dam on the Senegal River, up to 800,000 people lost access to floodplains that had provided most or part of their means of survival.

In addressing the problems faced by downstream communities, some dam authorities have attempted to simulate the effects of the annual rains, by deliberately releasing large amounts of water during the normal flooding period. In the majority of cases, managed releases are quite a recent strategy, but in Zambia, flood releases have been made from the Kafue river's Itezhi-tezhi dam for over twenty years. Exceptionally, even the design of the dam incorporated the concept; 15% of the water held by the dam was designated for flood releases. In 2000 a meeting of stakeholders was held in Lusaka to review the flood release programme, and to discuss the lessons learned from the World Commission on Dams.

The main message to come from the Lusaka meeting was that an effective programme of flood releases is far from simple; indeed it was difficult for the participants to establish whether the twenty year programme had really made very much difference to lives of the one million people who live on the Kafue Flats. The area actually falls between two dams, upstream to the west the Itzhi-tezhi, and downstream the Kafue Gorge dam. Over the course of the year higher water levels at the eastern end of the floodplain had significantly reduced grazing areas, while at the western end the floodplain was permanently dry, which some suggest has led to higher rates of animal disease from tick bites. People living on the floodplain were notified when flood releases were due, but were not able to anticipate levels of inundation that would occur. As a result they actually lost crops to the floods and were sometimes forced to move their cattle very quickly to avoid the rising water. All were agreed that a much greater degree of co-ordination was needed, particularly between the energy supply company that controlled the dams, and the downstream communities. Effective flood management demanded an integrated control programme, for example using embankments and levees to protect some areas, while enhancing flooding in areas where more water was required. Regulation could be best achieved by an independent body, and the meeting supported the formation of a Kafue River Basin Authority.

Other river basins have shown clearer benefits from flood releases. In Cameroon, the Waza-Logone floodplain ecosystem and farming systems have been substantially restored following a re-inundation programme. Attempts to release floodwater from the Pongolapoort reservoir on South Africa's Phongolo river initially failed because of poor timing. However, a successful programme is now underway, with farmers, fisher-folk and others participating in the management process through a number of stakeholder water committees. In Kenya managed releases are being considered in the design of the Grand Falls dam on the Tana river.

There certainly remain questions about using flood releases. One difficulty facing the engineers is how to allow sediment as well as water to be released. And for existing dams, a belated starting of a flood release programme may not be a helpful option. Once dams have been in place for a few years, floodplain ecosystems change, and so do human activities. Infrastructure develops in areas that are, for the most part, reliably dry. But the experience of Mozambique again offers a warning. Huge, devastating floods do happen, and not even the very biggest dams are capable of stopping them. Yet where natural floodplain ecosystems and farming systems can be preserved, they are much better adapted to surviving the giant floods that sweep away dryland lives and livelihoods. Future dam building projects should, agrees the World Commission on Dams, pay more attention to the needs not just of displaced communities on the upstream side, but also to those on the downstream side whose lives depend on the ebb and flow of the flood.

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