Flooding

In July of 1976, a summer storm grew over the Colorado Rockies in an area stretching between Loveland and Estes Park, Colorado. The Big Thompson Canyon River collected the rainfall from the storm, which had little chance of absorption along its steep rocky walls. Rainfall amounts were so heavy that transpiration, groundwater, and runoff couldn't keep up and went into overload. The river became a torrent, quickly doubling and tripling in size, as everything in its path gave way to the floodwaters.

Within a few hours' time, a flash flood was moving boulders 3 meters in diameter down the canyon, trapping people in their cars and destroying homes and businesses. The National Weather Service had measured normal flow of the river at 137 ft3/sec of water, but during the flood surge, the river's flow rate was expanded to 31,200 ft3/sec. In all, over 20 centimeters of water fell in less than two hours. Following the devastation, 145 people were dead or missing, 418 houses were destroyed, and 138 others were damaged. Fifty-two businesses were destroyed and over $35 million in damages was chalked up to one bad storm.

During the fall of 1996, an area along the Red River in the Midwest also received record amounts of rain. That winter, extremely cold air froze the excess water before it could run off, and the record rainfall was followed by record snowfall. For several winter months, snow levels grew. Then, when the temperatures finally began to rise, melting became a big problem. Not only did the winter's snow melt, but the frozen rainwater from the previous fall melted, too. The Red River and surrounding drainage basin streams couldn't handle this record excess. So, in the spring of 1997, a large area of the Northern Plains region suffered massive flooding. River levels rose to over 9 meters above flood stage and towns along the Red River like Grand Forks, North Dakota, were shut down because of flooding. Cities were completely incapacitated, with entire downtown sections under meters of water. Large chunks of floating ice blocked the river's normal flow and forced it out of its banks into nearby homes and businesses as well. Table 5-3 lists record amounts of rainfall.

Table 5-3 Rain record amounts can fall in very short time periods.

(cm)

Date

Location

1 minute

3.8

Nov. 26, 1970

Barot, Guadeloupe, West Indies

42 minutes

30.5

June 22, 1947

Holt, Missouri, USA

2 hours, 10 minutes

42.3

July 18, 1889

Rockport, W. Virginia, USA

2 hours, 45 minutes

55.9

May 31, 1935

D'Hanis, Texas, USA

9 hours

108.7

Feb. 28, 1964

Belouve, La Réunion, Indian Ocean

10 hours

140 ■

Aug. 1, 1977

Muduocaidang, Nei Monggol, China

24 hours

109 1

July 25-26, 1979

Alvin, Texas, USA

Soil and Rock

As we've seen, soil is made up of tightly packed particles of different shapes and sizes. As water hits the ground, it sinks down into pores and between soil particles. A high-porosity soil can hold large amounts of water because of its many pore spaces. If the pores are interconnected and permit water to flow easily, the soil is considered permeable. Sands, gravels, and other rocky soils allow rapid infiltration due to high permeability. However, clay particles are tiny and the pore arrangement between clay particles causes clay soils to be fairly rainproof and resistant to infiltration.

Soil's water retention is a big factor in whether an area will flood. Frequently, soil layers do a good job for a while, but with continued rain, soil reaches saturation and then it's all over—runoff begins.

A soil's initial water content is also important. Commonly, water infiltrates dry soils faster than wet soils. A huge storm that lasts for a week, like a tropical storm, with high amounts of rainfall also affects infiltration. When rain or snow melt reaches the soil surface faster than it can seep through the pores, the water collects at the surface or may travel downhill to the nearest stream channel in a drainage basin. This aspect of soil's absorbent ability is one of the reasons why brief, high-intensity storms, like the Big Thompson Canyon storm, often create more flooding than light rains over a longer period of time.

Whether water is able to move through the ground depends on a region's glacial and bedrock geology. In the northern United States, during the last Ice Age, glaciers covered much of the land surface and left behind till, outwash, and lake deposits. Till, a rock-and-soil mixture of all sizes, has low permeability when clay is present. Outwash is made up of highly permeable sand and gravel open to groundwater flow. Lake deposits can be clay, silt, or sand, with permeability dependent on sediment type.

The type of bedrock structures under glacial deposits also plays a part in groundwater transport. Sandstone channels water when the spaces between grains are connected, creating high permeability. Limestone fractures with many interconnecting splits can also conduct water easily. Finely grained rocks like shale and slate, however, usually have low permeability.

Survival Basics

Survival Basics

This is common knowledge that disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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