Floods: A detailed discussion of cause and effect
Introduction
People have long been attracted to floodplains. Here rivers deposit the topsoil picked up elsewhere, so the land is fertile. Floodplains are both flat and near water, so irrigation, ploughing and transport (usually aided by the river) are all made easier. The heavy settlement along the lower reaches of Egypt's Nile, India's Ganges, Bangladesh's Brahmaputra-Padma, the Yellow River and Yangtze of China, and the Tigris and Euphrates of Iraq are all examples of floodplain civilizations.
Floodplains are desirable places to live, not only in agricultural societies, but also in industrial countries where the floodplains often host large capitals that use the river water for industry and its mouth as a harbor for shipping.
The floodplain of a river is a clearly definable physical feature of its valley. It is the almost flat area that borders the river. A floodplain is built up of layers of sediment deposited by the river when it periodically overflows its normal banks. Steep narrow valleys in mountain regions have no floodplains at all, but a large complex system of converging rivers in a lowland region may have a floodplain over a hundred kilometers wide. There is a natural tendency for a river to deposit sediment in its channel during times of low flow, so that an equilibrium is arrived at where the river comfortably fills its main channel under normal conditions. Therefore the river will spread out automatically onto its floodplain during periods of high flow-- after all, floodplains are for floods.
But expansion of towns soon forces them to spread out--all too commonly onto the floodplains, where they were immediately in danger. In the United States alone there are currently an estimated 10 million people living in areas subject to flooding.
A flood is too much water in the wrong place, whether it be an inundated city or a single street or a field flooded due to a blocked drain. Among the trigger mechanisms are dam or levee failures; more rain than the landscape can dispose of; the torrential rains of hurricanes; tsunamis; ocean storm surges; rapid snow melts; ice floes blocking a river; and burst water mains.
Flooding is generally defined as any abnormally high streamflow that overtops the natural or artificial banks of a stream. Flooding is a natural characteristic of rivers. The floodplains are normally dryland areas. They are an integral part of a river system that acts as a natural reservoir and temporary channel for flood waters. If more runoff is generated than the banks of a stream channel can accommodate, the water will overtop the stream banks and spread over the floodplain. The ultimate factor of damage, however, is not the quantity of water being discharged but how high the water goes above normal restraints or embankments. Furthermore, floods can form where there is no stream, as for example when abnormally heavy precipitation falls on flat terrain at such a rate that the soil cannot absorb the water or the water cannot run off as fast as it falls.
Of all the disasters except droughts, flood disasters affect the most people. But there are many more flood disasters than droughts, and the number affected by floods is increasing much more rapidly than those suffering droughts. In fact, flooding is one natural hazard that is becoming a greater threat rather than a constant or declining one. Floods are caused not only by rain but also by human changes to the surface of the earth. Farming, deforestation, and urbanization increase the runoff from rains; thus storms that previously would have caused no flooding today inundate vast areas.
Not only do we contribute to the causes of floods, but reckless building in vulnerable areas, poor watershed management, and failure to control the flooding also help create the disaster condition. Ecologists have recently found evidence that human endeavors may directly be affecting the weather conditions that produce extensive and heavy rains. Irrigation of dry lands creates moisture conditions that contribute to increased humidity and evaporation, which in turn lead to increased rainfall. This is particularly heightened in desert areas where large lakes are built to provide water either for irrigation or for nearby settlements.1
Historical Examples
Bangladesh, 1974
Bangladesh is a riverine country where recurrent flooding is both common and necessary. Every year large areas are submerged during the monsoon season and fertilized by deposits of fresh alluvium, i.e., the soil deposited by moving water. However, if the waters remain stagnant for too long, these beneficial floods become major disasters. Such was the case in the summer and fall of 1974 when flooding extended over nearly one-half of the country and stagnated for more than a month. At least 1,200 people died in the floods and another 27,500 died from subsequent disease and starvation. Approximately 425,000 houses were destroyed or severely damaged and the losses to agriculture were estimated at U.S. $325.9 million. A total of 36 million people suffered severe hardship and losses due to the disaster.
The devastation of the floods can be attributed to more than just a malicious act of nature. Neglect and lack of administrative control were also contributing factors.2 Under colonial rule regular dredging had helped to maintain adequate river depth. After independence, however, protective measures were lax and silting of rivers and deforestation resulted in gradually increasing flood levels. Slow environmental degradation left Bangladesh virtually defenseless against destructive flooding.
The most devastating effect of the floods was on the agricultural sector. Although agriculture accounts for 60 percent of the Gross Domestic Product (GDP) and employs 80 percent of the population, Bangladesh has not been able to feed itself. Ninety percent of the flooded 1.6 million hectare (four million acres) was rice lands, the country's major crop. Little of this could be recuperated since the replanting season had already passed.
Supply problems were compounded by the lack of a buffer stock, absence of foreign exchange to purchase food, and failure of food aid shipments to arrive in time. The longstanding foodgrain gap increased from 1973-74 to 1974-75. As a result, many people who escaped drowning died of starvation.
The country was already in the midst of a serious payments crisis when the floods increased the problem. A reduction in export revenue from the Bangladesh jute crop coincided with massive import requirements for food assistance after the disaster. The result was a severe trade imbalance and an increase in the current account deficit by U.S. $250 million.
Per capita income and income distribution also suffered during this period. The countryside became increasingly impoverished as many small landholders lost both their crops and land. The decline in agricultural output led to higher levels of unemployment and underemployment. Masses of destitute rural people migrated to urban areas where job opportunities proved equally scarce. Added to this was a 50 percent inflation rate fueled by escalating prices of essential commodities now in short supply. In sum, the flood directly caused deterioration in levels of output and combined with rising unemployment and inflation to disturb the once moderately equitable nature of Bangladesh society.3
Yellow River, China
China's Yellow River has the dubious distinction of being responsible for more human deaths than any other individual feature of the world's surface. The cause for this is the river's unique form and configuration. For nearly 4,000 kilometers (2,500 miles) it flows through the mountains and plateaus of northern China, and on its route through the easily eroded loose soils it picks up an enormous quantity of silt. The flow of the river may be 40 percent yellow silt (which gives it its name) when it arrives at Kaifeng. From there it travels another 800 kilometers (500 miles) to the sea across the great Yellow Plain--essentially a massive alluvial fan, sloping more steeply than a true delta--which is also 800 kilometers wide and spreads around both sides of the mountains of Shantung. The river gradient across the plain is far higher than in a normal delta, but the Hwang Ho, as the Yellow River is known in Chinese, is still unable to carry its sediment load, and the plain is made of redeposited silt.
From Kaifeng, 15 channels radiate across the plain. Each time the Yellow River tops one of these, it causes enormous floods before resuming a single channel. The floods have drowned unbelievable numbers of people on the crowded plain, and the destruction of crops results in famine and yet more deaths. In only three floods since 1887 the Yellow River has killed over 6,000,000 people.
The levees, which were started over 2,500 years ago, have had to be constantly raised by the labour-intensive methods for which the Chinese are famous. There is nothing with which to build them except the silt. The constant raising means that the Yellow River now crosses its plain about 7.5 meters (25 feet) above the surrounding countryside, between inner and outer levees that form a belt 19 kilometers (12 miles) wide. The silt is the cause of the problem, for it is constantly deposited in the river channel. The river rises to yet higher levels and the Chinese are left with a literally never-ending task of building the levees higher still. Because of this the Yellow River now has no tributaries for over 640 kilometers (400 miles), and millions of people live below river level with the constant threat of flooding. There are no hills in the plain, no escape routes in the event of a flood. And the average area flooded each year is 8,200 square kilometers (3,000 square miles). Because the plain is below river level, it cannot drain. Regions stay flooded to the horizon for a year at a time. Once a major levee break lets the river completely escape, it occupies a braided course perhaps 24 kilometers (15 miles) wide for up to 10 years before it settles itself into a new channel.4
Geographical Distribution
Flooding is the most universal of natural hazards. It occurs on each continent and is a potential threat wherever there is rainfall or coastal hazards. With the exception where rainfall is never more than very light, every watershed is a potential site for flooding. Every coastline that is vulnerable to tropical cyclones or tsunamis is also at risk to flooding.
The most noted floods are associated with the world's great rivers. However, the lesser floods on smaller rivers or upstream tributaries may cause cumulatively more damage, even though receiving less public attention.
Natural Preconditions for Disasters
All of the earth's water (including the atmosphere) is part of a system referred to as the hydrological cycle (Figure 6.1). Beginning with the moisture in the air, water vapor enters the atmosphere by evaporation from bodies of water and by transpiration (the giving off of water vapor) from plants and trees. Once aloft, the moisture cools and collects into clouds as it rises higher into the atmosphere. When temperature and moisture content reach the proper stage, the vapor in the clouds condenses, and the water in the clouds falls to the earth as rain or snow.
Once returned to the surface, the water may evaporate again rapidly, or it may soak down into the earth and remain as groundwater for thousands of years until at last it again finds its way to an outlet. But regardless of where the precipitation falls, or how long it remains, eventually it is recycled.
At any given moment, only about .005 percent of the earth's estimated 1,360 million cubic kilometers of water is actively involved in the hydrological cycle. But because of fluctuations in the cycle, the actual amount of water available to various regions of the world can vary dramatically, often bringing searing drought or devastating flood.5
Disaster Event
Flood Types
Flash floods are local floods of great volume and short duration. A flash flood generally results from a torrential rain or "cloudburst" on relatively small and widely-dispersed streams. Runoff from the intense rainfall results in high flood waves. Discharges quickly reach a maximum and diminish almost as rapidly. Flood flows frequently contain large concentrations of sediment and debris. Flash floods also result from the failure of a dam or from the sudden breakup of an ice jam. Flash floods are particularly common in mountainous areas and desert regions but are a potential threat in any area where the terrain is steep, surface runoff rates are high, streams flow in narrow canyons, and severe thunderstorms prevail.
Riverine floods are caused by precipitation over large areas or by melting of the winter's accumulation of snow, or by both. These floods differ from flash floods in their extent and duration. Whereas flash floods are of short duration in small streams, riverine floods take place in river systems whose tributaries may drain large geographic areas and encompass many independent river basins (see Figure 6.2). Floods on large river systems may continue for periods ranging from a few hours to many days. Flood flows in large river systems are the distribution of precipitation. The condition of the ground (amount of soil moisture, seasonal variations in vegetation, depth of snow cover, imperviousness due to urbanization, etc.) directly affects runoff.
In most cases the most devastating flooding from rainfall is that associated with tropical cyclones. Catastrophic flooding from rainfall is often aggravated by wind-induced surcharge along the coastline. Rainfall intensities are high and the area of the storm is broad-based; these two factors together are capable of producing extreme flood discharges in both small and large river basins.
The size of catchment area usually governs the character of flooding. On very large rivers, such as the Nile and the Mekong, river flow is relatively slow to change in the downstream reaches. Flood waters are, therefore, mostly a combination of numerous and widespread rainfall events possibly with considerable snow-melt contribution. In large river basins, flooding is usually seasonal and of major significance. Peak discharges are maintained over a relatively long period of days or even weeks.
Flood-producing rainfall, with or without snow-melt, can also be of extratropical or weather frontal character. It may alternatively be the result of a large atmospheric depression with moisture-laden winds, moving from a marine environment onto and over a land mass. Rainfall in these events is generally widespread and can be heavy. Intensity can be high and is generally influenced by topographic relief.6
Coastal salt water flooding is usually caused by a combination of circumstances that may include astronomical tides, storm surges, or tsunamis. The latter two events are covered respectively in the lessons on tropical cyclones and tsunamis.
Flood Characteristics
The dangers of flood waters are associated with a number of different criteria, not necessarily independent of each other but creating different types of clearly recognizable hazards. A summary of the criteria and related hazards is given below.
Depth of water--Building stability against flotation and foundation failures, flood proofing, and vegetation survival have different degrees of tolerance to inundation. In each case these can usually be identified and the depth hazard established.
Duration--Time of inundation is of utmost importance since damage or degree of damage is often related to it. This applies to structural safety, the effect of interruption in communications, industrial activity and public services, and the life of plants.
Velocity--High velocities of flow create high erosive forces and hydrodynamic pressures. These features often result in complete or partial failure of structures by creating instability or destroying foundation support. Dangerously high velocities can occur on the floodplains as well as in the main river channel.
Rate of rise --The rate of rise of river level and discharge is important in its relation to the time available for giving flood warnings or making arrangements for evacuation and flood fighting arrangements. Rate of rise can therefore influence planning permission for floodplain occupation and its zoning.
Frequency of occurrence--Total potential damage in a floodplain relates to the cumulative effect of depth, duration and velocity hazards measured over a long period of time. This will very often, but not exclusively, influence decisions on planning permission, especially if the hazard can be measured in quantitative terms. Cumulative frequency of occurrence of the various hazards is a consideration that farming communities throughout the world have always taken into account, usually on the basis of experience and intuitive reasoning, as they decide the type and intensity of agricultural or livestock farming to employ in regions susceptible to floods.
Seasonality --Inundation of land during a growing season can have a completely destructive effect on agricultural production, as severe in fact as a prolonged drought. If flood waters occur during cold weather and if they derive predominantly from snow- melt with possible ice flows, general discomfort and subsistency levels of affected communities are also considerably influenced. Seasonality in large floods is therefore an important influence on severity of flood hazard.7