Many disasters originate as naturally occurring events, often starting as a part of the environment's normal functions. Their smaller, less destructive counterparts are things the world experiences all the time without concern. It is when they become excessive that they become a disaster that impinges upon the safety of others. Even when they are at that weaker, normal stage, it isn't difficult to be aware of the potential threat that they can produce.

This article will look at another natural disaster classification, hydrological disasters. Topics will include an explanation of what is involved with a hydrological disaster, the risks associated with them, and some of the more common events. The causes behind some of these disasters and what can contribute to their severity will also be discussed.

Experts deem any violent or abnormal movement of water on land as a hydrological disaster.1 This can include bodies of water that overflow, heavy precipitation, changes in water quality, or atypical behavior of water sources. Flooding and events that cause flooding are often identified as hydrological disasters, although this also can include disasters of other classification such as meteorological disasters like hurricanes and storms.

There isn't any kind of strict time frame needed for hydrological disasters to develop. Insufficient drainage often impacts the build-up of water or other liquids when things like heavy precipitation occurs. A large storm can cause flash flooding when it causes excessive rainfall in a time frame of six hours or less.2 Several days of heavy rainfall which bring enough water on normally dry land or overload bodies of water (e.g. rivers) can also cause a flood to develop, just not as fast as a flash flood.

As with geophysical disasters, the risk of a hydrological disaster developing can depend on factors of location. Communities that are close to major bodies of water-oceans, seas, large lakes and rivers, etc.-are going to have a higher risk of experiencing a hydrological disaster than those that not located near water sources. Proximity to the source of any kind of disaster is always going to be a risk, as there is simply a higher change of vulnerabilities from physical exposure.3

With hydrological disasters, risk can involve geologic factors. Low-lying ground often is prone to flooding because gravity will pull the flood waters down. That is why, safety-wise, most experts recommend that those who are experiencing hydrological disasters like tsunamis get to higher ground. Even the type of ground can impact the risk of a hydrological disaster happening, as certain materials do not allow for easy drainage. Man-made surfaces in urban areas, such as roads and parking lots, need sewers and storm drains because concrete and asphalt are not water absorbent. Thus such environments are easily prone to flooding caused by hydrological disasters as there often isn't enough drainage available, and debris can easily block drainage access on underpasses, basements, parking lots and garages, and densely populate areas like a city's downtown.4

Flooding

Floods are, by far, the most common disaster on Earth. They can happen anywhere and with any level of severity, from a depth equal to a puddle to a raging destructive force. Many other disasters, hydrological or otherwise, can produce flooding in addition to the damage specific to the primary event.7 In some cases, flooding can occur by accident-e.g. broken piping, spills, etc. Some common causes of flooding include:

Heavy Rainfall-As mentioned, high levels of precipitation in a short amount of time can produce flood conditions. In those instances, the water literally has nowhere else to go and gathers where gravity and momentum directs it. This can be exacerbated by poor or inadequate drainage.

Ice/Snow Melt-Areas that had high levels of precipitation in the winter months often end up with a potential risk of flooding from snow and ice melt. Most winter melt produces a runoff that the excess water travels along to an appropriate place (e.g. natural bodies of water, storm drains), but it can build up if it melts too quickly. As with heavy rainfall, this can cause flooding when the melt has nowhere to go and is hindered by drainage issues.

Structural Failure-Most cases of accidental flooding is a result of some kind of structural failure. This can include piping, dams, embankments/levees, and dykes-anything that is designed to contain liquid. Structural failures can occur because of poor maintenance or construction. Flooding can also occur if such structures exceed their capacity and begin to overflow. It is possible for an instance of structural failure that leads to flooding could be a result of another disaster, including a previous flood.

Most hydrological disasters like floods can be predicted thanks to warning signs. The presence of any of the above causes, for instance, can be used to herald an approaching flood. The warning system used for floods alters the public to two things: if the conditions for flooding are present, and if a flood or flash flood is happening or is about to happen.8 When the conditions are right and a flood is possible, but has not yet occurred, a flood watch is issued. When a flood is expected to happen soon or is already happening, a flood warning is issued. With flash floods, a warning is usually issued instead of a watch, as the potential is often too high for it to not happen.

It should be noted that a flood doesn't necessarily have to be made of water, but any viscous liquefied substance. There are actually two historical floods that occurred that did not involve water, but were just as devastating. The 1919 Great Molasses Flood occurred when a ruptured rum-production tank at the Purity Distilling Company in Boston's North End unleashed 2.3 million gallons (or 26 million pounds) of molasses in a 15-foot, 35 mph wave that killed 21 people and injured over 150 people.9 It smashed houses, buildings, and an elevated train track's supports and left a sticky waist-high mess that took several days to clean up. The other historical non-water flood occurred about 100 years previously in 1814, this time from a brewery in London. About 610,000 liters (or 161,145 gallons) of beer flooded the Tottenham Court Road area when a vat ruptured and knocked over several other vats, producing a 15-foot wave that destroyed two houses and killed seven people.10

Tsunamis

Tsunamis are hydrological disasters that are actually caused by geophysical disasters, like earthquakes, volcanic eruptions, and landslides. For them to occur, the geophysical disaster needs to trigger significant hydrological movement down to the ocean floor. Often, this can be accomplished when an earthquake happens underneath the ocean and the tremors from the quake cause the tsunami to develop. This produces a massive tidal wave that surges towards land and can reach up to 10 miles inland in just a few minutes.11 The wave of a tsunami increases in height as it travels and is often moving around 20 mph once it reaches land, but they can move up to 500 mph as they travel across the ocean.

Most tsunamis can form in just a few minutes after the geophysical even that triggers them, which might not be enough time for an official warning to be issued.12 They do, however, come with warning signs that precede their arrival. Tsunamis cause water to suddenly recede from the shore, exposing the ocean floor underneath as the tidal wave builds up further out from land. When this happens, it is best to move as far inland as possible and get to higher ground before the water comes back. Geophysical events that can trigger a tsunami, if the conditions are right (e.g. occurred under the ocean's surface), can prompt a tsunami watch.

Storm surges are rises in water levels caused by major oceanic storms like hurricanes. The storms pick up and push large quantities of water as they move across the ocean, depositing it on land once they reach shore.14 The are noted as tidal anomalies, where levels exceed the regular sea level and the predicted level during normal tides. Keep in mind that this isn't a result of poor math when predicting changes in sea levels during a storm, a noticeable difference of several feet of water that is brought inland.

These events can often cause significant flooding and damage once they reach land, and in some cases may be just as bad as the storm that brought it ashore. Experts state that storm surges are often the greatest threat with hurricanes due to the threat they pose.15 Unfortunately, the dangers that a storm surge are not always taken into consideration with events like hurricanes. Storm surges are basically like mini-tsunamis, as they can push water far inland and the water along the shore may recede out into the ocean before returning in 10-20 foot waves.

Avalanches

Like tsunamis, avalanches are disasters that combine hydrological and geophysical aspects into one event. They occur in mountainous areas where snow, ice, and heavy rains build-up on the sides of mountains and slopes. If they become destabilized by their own weight, geophysical or geological events (e.g. earthquakes), or the effects of human actions (e.g. snow sports) then the accumulated snow will be dislodged and literally come crashing down.16 Essentially, if the (frozen) water on the side of a mountain is disturbed, then you've got an avalanche. Keep in mind that the intensity of an avalanche can vary from the massive crashes of snow depicted in movies to harmless patches of powder that look closer to a snow drift.

Avalanches are often made up of more than just snow or ice, and can include other debris that it collects on its journey downwards. This is similar to landslides and mudflows, and can be extremely dangerous for anything or anyone in its path. Buildings, flora, infrastructure, transportation vehicles, and utility lines can be easily damaged or destroyed in an avalanche. The mountainous regions where avalanches are possible often can be cut off from the outside world when the disaster strikes, as major roadways and means of travel can be blocked off by the debris. Communities that are not located in those regions but rely on those routes or resource lines-electrical, cable, telecommunication, etc.-that go through them can be impacted.

Of course, avalanches do pose a direct threat to people. Getting caught in an avalanche is nothing like what the movies show, where a person falls in a pocket in the snow and is rescued later with the help of a St. Bernard carrying a little barrel of whiskey. Avalanches that are on the stronger end of the spectrum can travel at 80 mph or more in just a few seconds; these usually are fatal to anyone in their path.17 Slower avalanches can still result in crush injuries and broken bones from the sheer force of the impact of the snow and ice, not to mention any equipment (e.g. skis or snowmobiles) the person has with them. If possible, it is recommended that anyone who is caught in an avalanche try to outrun it-which may be possible if you're skiing or snowmobiling in the area-reach for a tree, or swim through the snow. If you can't, then clear as much space around you to allow for breathing and/or make an opening in the snow above you; this may be difficult as settled avalanches are very solid and hard to move in. Wait for rescue or carefully pull yourself out if you're close enough to the surface-only if you're close to the surface, as you might sink if you're deep in the debris.