Raging Waters

Raging Water

Raging waters causes lots of damages that cannot be avoided.

The Inevitable Catastrophe 

Up to now, in recent posts we have focused on the process of drainage formation and evolution and on the variety of landscape features formed by streams. Now we turn our attention to the havoc that a stream can cause when flooding takes place. Floods can be catastrophic they can strip land of forests and buildings, they can bury land in clay and silt, and they can submerge cities. A flood occurs when the volume of water flowing down a stream exceeds the volume of the stream channel, so water rises out of the channel and spreads out over a floodplain or delta plain, or fills a canyon to a greater depth than normal. 

Floods happen 

1. during abrupt, heavy rains, when water falls on the ground faster than it can infiltrate and thus becomes surface runoff; 
2. after a long period of continuous rain, when the ground has become saturated with water and can hold no more; 
3. when heavy snows from the previous winter melt rapidly in response to a sudden hot spell; or 
4. when a dam holding back a lake or reservoir, or a levee or retaining wall holding back a river or canal, suddenly collapses and releases the water that it held back. 

Examples of seasonal floodplain flooding.

Geologists find it convenient to divide floods into two general categories. Floods that occur during a “wet season,” when rainfall is heavy or when winter snows start to melt, are called seasonal floods. Floods of this type typically take place in tropical regions during monsoons, and in temperate regions during the spring when storms drench the land frequently and or a heavy winter snow pack melts. When seasonal floods submerge floodplains, they produce floodplain floods, and when they submerge delta plains they produce delta-plain floods (figure above a–c). 

Flash floods can occur after torrential rains.

Events during which the flood waters rise so fast that it may be impossible to escape from the path of the water are called flash floods (figure above a, b). These happen during unusually intense rainfall or as a result of a dam collapse (as in the 1889 Johnstown flood) or levee failure. During a flash flood, a canyon or valley may fill to a level many meters above normal. In some cases, a wall of water may slam downstream with great force, leaving devastation in its wake, but the flood waters subside after a short time. Flash floods can be particularly unexpected in arid or semiarid climates, where isolated thundershowers may suddenly fill the channel of an otherwise dry wash, whose unvegetated ground allows runoff to reach the channel faster. Such a flood may even affect areas downstream that had not received a drop of rain.

Case Study: A Seasonal Flood 

In the spring of 1993, the jet stream, the high-altitude (10–15 km high) wind current that strongly affects weather systems, drifted southward. For weeks, the jet stream’s cool, dry air formed an invisible wall that trapped warm, moist air from the Gulf of Mexico over the central United States. When this air rose to higher elevations, it cooled, and the water it held condensed and fell as rain, rain, and more rain. In fact, almost a whole year’s supply of rain fell in just that spring some regions received 400% more than usual. Because the rain fell over such a short period, the ground became saturated and could no longer absorb additional water, so the excess entered the region’s streams, which carried it into the Missouri and Mississippi rivers. Eventually, the water in these rivers rose above the height of levees or broke through levees, and spread out over the floodplain. By July, parts of nine states were underwater (see a in first figure). 

The roiling, muddy flood uprooted trees, cars, and even coffins (which floated up from inundated graveyards). All barge traffic along the Mississippi came to a halt, bridges and roads were undermined and washed away, and towns along the river were submerged. For example, in Davenport, Iowa, the river front district and baseball stadium were covered with 4 m (14 ft) of water. In Des Moines, Iowa, 250,000 residents lost their supply of drinking water when flood waters contaminated the municipal water supply with raw sewage and chemical fertilizers. Row boats replaced cars as the favoured mode of transportation in towns where only the rooftops remained visible. In St. Louis, Missouri, the river crested 14 m (47 ft) above flood stage. 

For 79 days, the flooding continued. When the water finally subsided, it left behind a thick layer of sediment, filling living rooms and kitchens in floodplain towns and burying crops in floodplain fields. In the end, more than 40,000 square km of the floodplain had been submerged, 50 people died, at least 55,000 homes were destroyed, and countless acres of crops were buried. Officials estimated that the flood caused over $12 billion in damage. Comparable flooding happened again in the spring of 2011, in the Mississippi and Missouri drainage basins. 

Case Study: A Flash Flood 

On a typical sunny day in the Front Range of the Rocky Mountains, north of Denver, Colorado, the Big Thompson River seems quite harmless. Clear water, dripping from melting ice and snow higher in the mountains, flows down its course through a narrow canyon, frothing over and around boulders. In places, vacation cabins, camp grounds, and motels line the river. The landscape seems immutable, but as is the case with  so many geologic features, permanence is an illusion.

On July 31, 1976, easterly winds blew warm, moist air from the Great Plains toward the Rocky Mountain front. As this air rose over the mountains, towering thunder heads built up, and at 7:00 P.M. rain began to fall. It poured, in quantities that even old-timers couldn't recall. In a little over an hour, 19 cm (7.5 inches) of rain drenched the watershed of the Big Thompson River. The river’s discharge grew to more than four times the maximum recorded at any time during the previous century. The river rose quickly, in places reaching depths several meters above normal. Turbulent water swirled down the canyon at up to 8 m per second and churned up so much sand and mud that it became a viscous slurry. Slides of rock and soil tumbled down the steep slopes bordering the river and fed the torrent with even more sediment. The water undercut house foundations and washed the houses away, along with their inhabitants (see above figure b). Roads and bridges disappeared, and boulders that had stood like landmarks for generations bounced along in the torrent like beach balls, striking and shattering other rocks along the way. Cars drifted downstream until they finally wrapped like foil around obstacles. When the flood subsided, the canyon had changed forever, and 144 people had lost their lives.

Living with Floods 

Flood Control  

Holding back rivers to prevent floods.

Mark Twain once wrote of the Mississippi that we “cannot tame that lawless stream, cannot curb it or confine it, cannot say to it, ‘go here or go there,’ and make it obey.” Was Twain right? Since ancient times, people have attempted to control courses of rivers so as  to prevent undesired flooding. In the 20th century, flood-control efforts intensified as the population living along rivers increased. For example, since the passage of the 1927 Mississippi River Flood Control Act (drafted after a disastrous flood took place that year), the U.S. Army Corps of Engineers has laboured to control the Mississippi. First, engineers built about 300 dams along the river’s tributaries so that excess run-off could be stored in the reservoirs and later be released slowly. Second, they built artificial levees of sand and mud, and built concrete flood walls to increase the channel’s volume. Artificial levees and flood walls isolate a discrete area of the floodplain (figure above a–c). 

Although the Corps’ strategy worked for floods up to a certain size, it was insufficient to handle the 1993 and 2011 floods when reservoirs filled to capacity, and additional run-off headed downstream. The river rose until it spilled over the tops of some levees and undermined others. “Undermining” occurs when rising water levels increase the water pressure on the river side of the levee, forcing water through sand under the levee. In susceptible areas, water begins to spurt out of the ground on the dry side of the levee, thereby washing away the levee’s support. The levee finally becomes so weak that it collapses, and water fills in the area behind it. In some cases, the Corps of Engineers intentionally dynamites levees along a relatively unpopulated reach of the river upstream of a vulnerable town. This diverts water out onto a portion of the floodplain where the water will do less damage, and prevents the flood waters from over topping levees close to the town.

Another solution to flooding in some localities may involve restoration of wetland areas along rivers, for wetlands can absorb significant quantities of flood water. Also, where appropriate, planners may prohibit construction within designated land areas adjacent to the channel, so that flood water can fill these areas without causing expensive damage. The existence of such areas, which are known as flood ways, effectively increases the volume of water that the river can carry and thus helps prevent the water level from rising too high.

Evaluating Flooding Hazard  

When making decisions about investing in flood-control measures, mortgages, or insurance, planners need a basis for defining the hazard or risk posed by flooding. If flood waters submerge a locality every year, a bank officer would be ill advised to approve a loan that would promote building there. But if flood waters submerge the locality very rarely, then the loan may be worth the risk. Geologists characterize the risk of flooding in two ways. The annual probability of flooding indicates the likelihood that a flood of a given size or larger will happen at a specified locality during any given year. For example, if we say that a flood of a given size has an annual probability of 1%, then we mean there is a 1 in 100 chance that a flood of at least this size will happen in any given year. The recurrence interval of a flood of a given size is defined as the average number of years between successive floods of at least this size. For example, if a flood of a given  size happens once in 100 years, on average, then it is assigned a recurrence interval of 100 years and is called a “100-year-flood.” Note that annual probability and recurrence interval are related:

    annual probability = 1/recurrence interval 

For example, the annual probability of a 50-year flood is 1/50, which can also be written as 0.02 or 2%. 

Unfortunately, some people are misled by the meaning of recurrence interval, and think that they do not face future flooding hazard if they buy a home within an area just after a 100-year flood has occurred. Their confidence comes from making the incorrect assumption that because such flooding just happened, it can’t happen again until “long after I'm gone.” They may regret their decision because two 100-year floods can occur in consecutive years or even in the same year (alternatively, the interval between such floods could be, say, 210 years).

The conceptual relationship between flood size and probability.

The recurrence interval for a flood along a particular river reflects the size of a flood. For example, the discharge of a 100-year flood is larger than that of a 2-year flood, because the 100-year event happens less frequently (figure above a). To define this relationship, geologists construct graphs that plot flood discharge on the vertical axis against recurrence interval on the horizontal axis (figure above b).

Knowing the discharge during a flood of a specified annual probability, and knowing the shape of the river channel and the elevation of the land bordering the river, hydrologists can predict the extent of land that will be submerged by such a flood. Such data, in turn, permit hydrologists to produce flood-hazard maps. In the United States, the Federal Emergency Management Agency (FEMA) produces maps that show the 1% annual probability (100-year) flood area and the 0.2% annual probability (500-year) flood risk zones (figure above c).

Figures credited to Stephen Marshak.

>