In Part 1 of this chapter on Floods , we reviewed the main types of floods, and recognized river system floods as the most common form. Year by year, theyare the most dangerous, disruptive events affectingmany people's lives, and are the most expensive interms of property destruction and repair costs.
The other common form of flooding -
-is discussed elsewhere. They are usually rapidly developing but brief events, often unexpected, andusually result from very heavy local rainfallin the headwaters of a stream.
But back to the river system floods. Let's look at what's behind them.
Causes of Floods
I don't think I'll surprise you by stating that the most importantcause of flooding is widespread, heavy rain. Heavy rain is hard to ignore, particularly when it continues for days. Its effects are magnified when the rain bringing system extends over most of a river catchment area, so that all tributaries are soon at bank full stage or are in flood. When they meet the main channel, itscapacity for removing the water is rapidly exceeded, andextensive flooding is inevitable.
Heavy rain itself can be difficult to define, except after the event. In some areas where rainfall is normally light and spread out, falls of 3-4 inches may be enough to cause flooding.In other areas, falls of this size may be common, but the stream channels have adapted to handle the amount of water,and no flooding occurs.
And while it is true that a major rainfall event can overcomeall natural conditions and man made structures that may minimiseflooding, even prolonged drought, there are many other factorsthat can contribute to major flooding.
Saturated or Frozen Ground. In normal times, soil has a great capacity to soak up a large amount of water,preventing it from running off. But when the ground is already saturatedor frozen, all rain or melt water moves over the top and intothe streams.
Full streams. Quite often, if the ground is saturatedwater is already running off and streams and rivers areclose to bank full stage. Any extra rain will rapidly causeflooding.
Full dams. Partly full dams can be used as a floodcontrol measure by intercepting and holding back the earlyrun off. In some cases, if a flood is expected to occur, waterauthorities will begin releasing water before the expected peak to create some capacity in the dam to reduce flooding downstream. Although this may cause early nuisance flooding, itwill help reduce the effects of the main flood.
But if the streams and the dams are already full, once againalmost all the rain will end up in the river.
Despite everyone's best efforts, these three conditions tendto occur together and reinforce each other. This situation contributed to the Mississippi Flood of 1993, when a seriesof heavy thunderstorms over several weeks eventually overwhelmed natural and artificial checks on flooding.
Snowcover. As a general rule, ten inches of freshlyfallen snow is equivalent to one inch of rain. But overwinter, it compacts, and a ten inch snowpack spread overa large area can add a lot of runoff to a river systemif it melts quickly.
Ice Jams. Two sorts of ice jams can form andcause flooding.
Local freezing can dam up unfrozen water behind the ice, generally only causing local flooding. The same thingcan happen when ice breaks up, is transported downstreamas blocks and sheets, and forms a dam at a natural orman made obstruction, such as a bridge.
Apart from its potential to damage or destroy bridges andother structures, and its contribution to upstream flooding,the main threat of such an ice dam is if it suddenly breaks.Dangerous downstream flash floods can result, and usuallyevery effort is made to remove the ice jam as quickly as possible. Log jams pose similar threats.
Frozen ground, a heavy snowpack, and ice jams all contributed to devastating flooding on the Red River at Grand Forks, North Dakota during Spring, 1997. This combination wasprobably enough for a major flood, but the situation wasmade far worse by events months earlier. Record Fall rains were followed by a quick and sustained freeze.
So not only did record amounts of melting snow have to get away, but so too did the frozen Fall rainwater. Ice jamsforming during the thaw also contributed to the record floods.
Out of control steamship breaks levee, Junior, Louisiana, 1927
Floods and Home Weather Stations
River floods, by their nature, are best studied after theevent. Flash floods, covered elsewhere,are most commonly associated with individual thunderstorms.
Unless a flood is caused by a single episode of widespreadand persistent and heavy rain, it can be very difficult toput your finger on the initial cause of a flood while it is happening.
As we have seen, the start of the 1997 Red River ND floodbegan almost half a year before.
Similarly, the 1993 Mississippi Flood developed over sometime. Was the cause in the first of the train of thunderstorms that moved through the region, or was it in the first thunderstorm after the ground was saturated and the streams were running full, or was it at some other time. It can be hard to tell.
But if you have experienced a flood in your area, it isan interesting exercise to go back through records andthe archives of the NWS to try and pick the point atwhich the flood first became likely, as well as anysubsequent events which made the flood inevitable, such as further heavy rain.
Reports on river heights, estimates of flood peaks,and predicted extent of flooding can also be illuminating, and at worst will give you an idea of thevast amount of information available to the officialflood forecasters at the time when it's all happening.
All the photos used in this article are reproduced by courtesyof the NOOA Photo Library, and were taken during floods inthe first few decades of last century. Historic and recent photos in this collection are well worth a look
Like to Know More?
Apart from the links in the text above, and the excellentbooks, DVDs, videos and other material mentioned in the More Flood Resources pages, you can find outmore about the weather conditions that cause floods onthe Severe Weather page.
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