Flooding Factors

For several months now the UK has been subjected to a series of floods.  Some areas (such as Somerset) have been flooded for weeks, others have had it come and go several times.  For this post I want to have a quick look at some of the factors that cause flooding.

Coast or River?  It might sound like a daft question, but whether the flood is coming from the sea or a river will determine its characteristics, its causes, the solutions and the aftereffects.  A coastal flood will usually result from a storm surge or a high tide, though other events such as a lowering of the land due to an earthquake can also cause flooding.  A river based flood is usually the result of excessive rainfall, melting snow/ice or the breaching of a dam (natural or artificial).

1. Tides

Moon Orbit & Tides 1

I hate to state the obvious but no the Earth, Moon & Sun in the above diagram are not to scale, neither is the ocean blue for the tides or the distance between the various objects.  The diagram is of my own making, but the images of the Sun, Earth & Moon are from Wikipedia.

It has been know since the days of Issac Newton that the Moon’s gravity forms the tides.  As the Moon moves around the Earth it’s gravity pulls on the water in the ocean (see diagram above).  In addition to the twice daily cycle of high/low tides there is the approximately 29 day Lunar Cycle.  Twice a Lunar Cycle (roughly twice a month) the gravitational forces of the Moon and the Sun line up and create an even larger tide (Spring Tide).  Twice a cycle the Sun’s gravity helps to counteract the Moon’s and we get a lower tide (Neap Tide).  Both of these are offset slightly by other factors such as the rotation of the Earth.  These spring tides can cause serious flooding to coastal areas, especially when it coincides with an extreme weather event such as a storm surge.  This is what happened to the UK in December 2013 as well as the great North Sea flood of 1953.

2. The Weather

OK so this is the obvious one…the more it rains the more likely you are to get a flood.  It’s not just the volume of water that falls from the sky but also the time over which it rains.  Very heavy rain over a few hours can create the quick but devastating flash floods.  More prolonged rainfall creates the slower creeping floods such as those currently covering large areas of the UK.  With the current situation there has been several large storms over the winter, each one adding extra water to the system.  The ground has become saturated and can no longer absorb any more water, resulting in more runoff and thus flooding.  Melting snow and ice can also contribute to this, especially when there is a sudden rise in temperature causing rapid melting.

In addition to rainfall strong winds can create large waves and a storm surge.  A storm surge is formed by the lowering of air pressure under the storm (causing the local sea level to rise slightly) and the constant strength and same direction of the wind blowing over the water surface and generating a much bigger wave than normal.  As this wave-like action moves towards the coast the reducing water depth forces the wave to be higher (basically the same amount of water is trying to fit into a smaller space of sea) and with the strong wind behind it, the water has only one place to go; up and over, flooding the adjacent land.  When combined with high/spring tides this can overwhelm coastal defenses.

3.  Vegetation

Vegetation coverage and the type of vegetation will help control the amount of water that actually hits the ground.  As a general rule the greater the vegetation (and particularly leaf) coverage the greater the chance of the rainfall being intercepted and absorbed by the plants, and thus less will reach the ground.  The water gets caught in dips & depressions in the leaves whilst the roots absorb some of the water at ground level.  A lack of vegetation, or leaf coverage during winter will increase the chances and severity of flooding.  There is an additional danger of mud/landslides from areas that have been deforested as the roots are no longer there to hold the soil together, and any fractures that the roots once occupied will be filled with water instead, destabilising the slope.

4. Soil & Rock Types

When it rains some of the water is absorbed by the soil/rock of the area  (a process called infiltration).  The type of soil and the spaces (fissure & pores) in between the various soil particles will determine how much water can be absorbed by the soil.  Surfaces such as compacted clay and tarmac will absorb less water than something like loose sand and will thus create more run-off.  Linking into vegetation roots open up spaces in the soil (and rock) increasing the amount of water that can flow through it.

As for rock the same principle is true.  Rocks that have large spaces between particles such as sandstones and limestones absorb more water than rocks which have more tightly packed grains.  Some rocks such as granite will absorb almost nothing.  A special note also needs to be made of limestone.  Rain water is naturally slightly acidic (usually around pH 5) and will dissolve limestone & chalk given enough time.  creating underwater channels that will eventually for underwater rivers, lakes & caves.


Smoo Cave, Durness, NW Scotland

5. Topography & Geomorphology

The shape of the land will ultimately determine where a flood will be.  Rivers will drain a catchment area, with the water flowing down the river channel to the exit point (usually the sea or a lake).  It doesn’t have to rain in lower down the channel for it to flood.  Take a look at the Nile.  Most of the water that feeds the Nile comes from the Ethiopia Highlands, and yet it would flood in the delta in Egypt.  The size of the catchment area is determined by the topography.  Hills & mountain ranges will often provide an edge to this area.  The larger the catchment area, the more rain you will likely gain, but at the same time the larger the river will normally be.  High rain in one part of the catchment area will cause flooding in most areas further down stream.

If many of the smaller streams all converge at the same point and are of a similar length (say if the catchment area/drainage basin is roughly circular in shape), then any excess water will reach that point at the same time, similar to rush hour traffic in the morning & evening.  But instead of congestion you get a higher flood level then if the streams were of different lengths, which would have the excess water reach the meeting point at different times.

The cross-section of the river valley will effect the speed of the flood.  A steeper slope will see the flooding take place sooner after the rain than a gentle slope.  A wider, flatter valley is probably indicative of a wide and active floodplain…even if it hasn’t flooded in decades, geology works on a different timescale to the human memory.  It may sound a little obvious but a floodplain is how a river manages excessive rainfall and it is suppose to flood, so expect flooding.

Another topographical factor is the bottle-neck principle.

River bottle neck 1This is a bit of a simplification, but the above diagram demonstrates it nicely.  If a river encounters an area of resistance (in this case harder rock – but it can also be something like an area of flood defenses with an undefended area upstream.  As the water flows downstream it encounters a narrower stretch with resist material either side.  The water can’t flow through the gap quickly enough, nor can it erode the tougher rock quickly enough.  As a result the water backs up and floods the area upstream of the narrow gap.  This can cause a problem in areas which have some form of solid flood defence, say in the town, which acts as a bottle-neck, the flooding then takes place in the farmland upstream of the town and because of the bottle-neck it can make the flooding worse.

6. Urbanization

As said in the soil section, covering the area in tarmac and concrete will reduce the infiltration & absorption rate.  In addition to this modern drainage systems mean that there is a very efficient removal of rain water from the buildings/road surfaces, resulting in a rapid influx of water from a substantial urban area into a river.  Almost none of it will be absorbed and there will be little to slow it down.  This will increase the chance of flooding.  Combine this with the trend of building on floodplains and you can have a real problem.

Building on a floodplain is just asking for an area of the town to be flooded.  Humans tend to have short memories and people saying things like “well it’s never flooded before, and I’ve lived here all of my life” isn’t going to help.  Most people have only vague memories of their childhood, and just because you don’t remember it happening doesn’t mean it didn’t.  Also the Earth works on a longer time frame and there are certain events that will only happen every century or two, well beyond living memory.  Basically it is a floodplain, it will flood at some point, and so if you build on the land because there hasn’t been a flood in 50 years…you might just get caught out.  The problem is that the population is growing and people need places to live.  Old floodplains are an attractive site because they tend to be flat and require less work before you can build a house on it.  This not only puts people, homes & businesses in harms way, but also changes the ground surface to a less porous one, reduces vegetation and increases runoff from drains etc. all increasing the risk of flooding.

As part of urbanization we try to control rivers with dams, levees and weirs.  These often help to control the river & flooding, but also can lead to a false sense of security.  People will often build close to the levee and if there is a breach the flooding will be rapid and without warning.  There are also dams and reservoirs for non-natural substances and a failure of these will result in a toxic flood, such as the Ajka Alumina plant accident in Hungary on 4th October 2010.

7. Unique & unusual phenomena

Occasionally there are unusual events that cause flooding.  An example would be an earthquake or a volcanic eruption.  Such events will cause phenomena such as tsunamis as the Japanese tsunami of 2011 demonstrated.

800px-SH-60B_helicopter_flies_over_SendaiImage taken from the Wikipedia entry on the Japanese Tsunami 2011.

Earthquakes can also lower the land surface respective to the coast, causing flooding.  Examples of this would be the 2011 Japanese earthquake and the 1964 Alaskan earthquake.

A warming climate and an increased melting of polar/mountain ice will increase the amount of water in neighbouring streams and ultimately the sea.  Any rise in sea level will increase the risk of flooding for coastal areas, combine that with storm surges and tides and you have a lethal outcome.

A special note needs to go to the jökulhlaup, a rare phenomenon that is the result of geothermic heating or a volcanic eruption underneath a glacier.  Iceland tends to be the main sufferer of such events, but the volcano-glacier combination can be found elsewhere.  Basically the eruption melts the base of the glacier very rapidly and the water then escapes underneath the ice to emerge at high speed flooding an wide area downstream.

I hope that this has helped explain some of the major causes of flooding, and the factors that can influence it.  With more rain & wind on the way I think that flooding will be in the public image for a while.  I plan on writing more about flooding and some of the strategies that can be used to manage/control such events.  Until then stay dry.

References: Fundamentals of the Physical Environment (3rd Edition) by P. Smithson, K. Addison & K. Atkinson (2002).  Geology (2nd Edition) by S. Chernicoff (1999).  Environmental Resources by A. S. Mather & K. Chapman (1995).  Global Casino (3rd Edition) by N. Middleton (2003).  As already noted the tsunami picture and the images of Sun, Moon & Earth I got from Wikipedia, as was the checking of a couple of the dates for specific events.

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