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3.

Guaranteed Floods?

In nature, river flooding and moderate soil erosion are normal. The silt forms alluvial flood-plains, ideal for farming: fertile soil, high water-table, easy irrigation, often a temperate climate. Flood-plains were the cradles of most early civilizations: the Nile, Tigris-Euphrates, Indus, Ganges and Brahmaputra, Yangtze and Yellow River (Huang He). Even today, nearly all great cities lie on flood-plains. But, as US Vice President Al Gore put it recently, 'the reason they call them flood-plains is that it is plain that they flood'. For millennia, floods have been welcomed for the fertility they bring from the hills - and feared for the devastation that can sometimes come with the nutrients. 'Would that a lion had ravaged mankind, rather than the flood', lamented the Epic of Gilgamesh in Babylon 5,000 years ago. So engineers have worked to pacify floods with canals, dikes and dams, trying to minimize erosion and other damage while maximizing irrigation and fertilization of the fields. Agriculture, architecture and culture evolved to survive annual inundation as well as occasional destructive floods. Sometimes the price was high. ChinaÕs Yellow River, first diked over 4,000 years ago, drowned or starved a million people in its 1887 flood, and at least another million people in 1931. So when modern technology allowed bigger, better, longer, higher dams and dikes civil engineers went on a building spree. The Aswan High Dam is the classic flood control mega-scheme. Its aim was to hold back the annual Nile floods, and release the waters throughout the year. It does this very well. It also holds back 98% of the sediment, and does not release it later in the year. The silt is now forming a new delta in Lake Nasser. The old delta, 1000 km and more downstream, is the site of four fifths of Egyptian farmland. No longer enriched by sediment from the Ethiopian highlands, it is salinizing fast, has one of the highest levels of pesticide and fertilizer usage in the world, and is eroding and sinking into the Mediterranean. On the other side of the globe, the United States boasts 100,000 dams, 5,500 over 15 meters high. Of non-Alaskan US rivers over 600 miles (960 km) long, only the Yellowstone is still undammed. From 1960-1987, the US federal government alone spent $35-40 billion on storage dams, levees and other flood control measures. But flood damage still grew from $1 billion in 1950 to $8 billion a year in the mid-1990s (constant dollar adjusted for population growth). Much anti-flood engineering has a simple motive; send the water downriver to flood someone else. (Sandbagging the top of a levee is often distinctly un-neighborly.) Many ecologists and engineers now charge that river regulation has gone too far. Paradoxically, by adopting the unrealisable goal of zero tolerance of floods, we may have guaranteed major flooding. Storage dams can flatten flood peaks, and dikes can protect towns. But preventing a river from spreading over its natural flood plain, draining its wetlands, and confining it to a narrow stream-bed between high levees, is likely to raise flood crests downstream. Combine that with faster runoff from deforested watersheds, flood-plain agriculture which cannot tolerate floods, and unwater-proofed domestic and commercial buildings, and you may get a recipe to replace many small floods with a few devastatingly big ones. Many now think that more feasible SD goals are to moderate rather than prevent floods, to allow some flood-plains to flood, and focus more on flood-coping strategies (see DI#5).
[flood control can harm ecosystems and livelihoods]

Japan: a dike too far?

Mountainous Japan, with limited cultivable land, has traditionally treasured its river valleys. During the Edo era (1603-1868), dikes backed by forest belts weakened flood flow, trapped coarser sediments, and allowed slow-moving water rich in finer sediment to flood the fields about once a decade. Houses were built on mounds with elevated floors, and boats were kept for emergencies. But from the Meiji era (1868-1912), more modern civil engineering led to a new wave of dike-building, and a population explosion as wetlands were drained for cultivation. Social attitudes changed, and became intolerant of even the smallest flood. Today, high concrete levees line most rivers. Land development policy assumes there will never be floods, and many flood-plains are densely covered with homes, factories and offices. Natural riverine ecosystems have all but disappeared. But the mega-floods which come once every few centuries could prove uncontrollable in canalized rivers lacking designed over-spills. 'We may be certain that such a flood would prove disastrous,' warns Dr. Takashi Okuma, civil engineering professor at Niigata University (World Rivers Review, Feb 1997).

Word Watch extreme event n. the earthquakes, tsunami, volcanic eruptions etc. which trigger human disasters, and especially extremes of weather (droughts, hurricanes, floods etc) linked to climate change.

hubris n. arrogant ambition, ultimately leading to downfall (the revenge of the gods, the ancient Greeks said, when mortals got above themselves.) Maybe synonymous with the concept of flood prevention.

In Depth Philippi, Nancy. Floodplain Management. London, UK: Academic Press, 1996.


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