Floods & Hyrdographs

IMPERMEABLE GEOLOGY

Eg: Boscastle 2004 floods - slate.

Rocks such as old red sandstone, slate, clay, coal measures etc are impermeable and do not allow water to percolate through them.

Limited percolation in the drainage basin around a river on in the river channel itself leads to the soil becoming saturated quickly, creating lots of overland flow (OLF) / surface runoff.

OLF is the fastest flow of water so it will be transported to the river channel faster, making the river fill up swiftly and forcing it to burst it's banks quickly.

IMPERMEABLE SOIL

Eg: Boscastle 2004 - lots of clay

In the winter (especially in tundra biomes) the soil can be frozen hard, which doesn't allow water to percolate through. This increases OLF and the same thing happens again.

During summer months, the soil can become baked hard. This also doesn't allow water to percolate through. Again, this increases OLF...

In Boscastle, the soil is full of clay so water often cannot percolate thoroughly enough.

STEEP-SIDED VALLEYS

On steep valley sides, there is a lot less soil for water to percolate through, creating more OLF.

Also, the force of gravity will make water travel faster to the river channel.

PRECIPITATION AMOUNT / TIME OF YEAR

In spring months, flooding can be more common as snow melt, glacial melt is at it's key time.

Rapid snow melt can result in flooding. As well as releasing large amount of water, the underlying ground remains frozen, thus preventing infiltration.

Monsoon rains (or even just heavy rains) can result in ground saturation at a faster level, eventually leading to a flood as the river cannot hold the water in. This is common in countries such as Korea and other Asian countries.

URBANISATION

High residential areas are prime factors that increase flooding in rivers. As OLF is the fastest transport of water, impermeable surfaces such as concrete and tarmac speed up the transporting of water from the ground to a river channel.

This increased runoff, leads to increased risk of flooding.

Also, the lack of green areas within cities makes rivers near an area of high population density are more likely to flood than ones in open land.

Lack of forests, grass / pasture areas lead to decreased interception, thus making it easier for water to get to the river channel.

AGRICULTURE

Animals & machines trample the soil, compacting it so less infiltration leads to more OLF.

Animals also eat vegetation so less interception leads to less natural stores of water and no roots to take up water so more OLF.

On top of this, there is increased erosion so lots of sediment in the river channel which reduces capacity and makes it more likey for the river to flood.

SOIL TYPE

Sandy soils are porous which means that there are lots of air spaces so lots of infiltration which means there is very little run off.

Peaty soils - near the source of the River Tawe - absorb lots of water and slowly realease it slowly, decreaseing flood risk.

GEOLOGY

Rocks such as limestone, sandstone, slate, granite etc are all porous rocks - meaning they have air spaces within them to allow water in.

This means water can percolate through them, decreasing the risk of flooding as water can either be stored in them or travel slowly to the river channel.

AGRICULTURE

Ploughing aerates the soil, this allows water to infiltrate inwards and travel slowly down to the river channel, decreasing the risk of flooding.

Afforestation creates a large amount of 'stores' of water within plants and trees. It also creates a large area of interception, which overall stops rain water from reaching a river channel too quickly, if at all.

On top of this, the EU pays farmers to let their land go to fallow, plant trees on their land (afforestation) and to create new habitats within these areas.

DAMS (+ case study)

These are the perfect way of controlling flooding.

For example, the Aswan Dam on the River Nile in Egypt was built in order to control the seasonal floods of the river. If the river flooded just right, farmers would have fertilized land thanks to the silt in the water. If it flooded too much, villages could be washed away and lives lost. If it didn't flood at all then the country would starve and famine would run rife. The building of the Aswan Dam allowed the government to control the flood waters, ensuring that they would be the correct height every year. Unfortunately, this lead to siltation - a build up of silt behind the dam walls - meaning that farmers now have to use chemical fertilizers, which leak into the river and have been the cause of a decrease in fish throughout the Nile.  The Egyptian government had to borrow a lot of money in order to build the dam. They have managed to pay it all off with the creation of HEP in the dam and the increase in tourism on Lake Nasser - the reservoir behind the dam.

OVERVIEW

A flashy hydrograph consists of several characteristics:

• Steep rising limb
• Steep falling limb
• Shorter lag-time
• Higher peak discharge

There are several things that can make a hydrograph flashy. Please read on for more.

IMPERMEABLE GEOLOGY (PHYSICAL)

This can make a hydrography FLASHY as much of the water will transfer to the river via OLF as the soil will become saturated quickly.

Impermeable geology can also create a LOWER BASEFLOW as very little water can enter the rocks on the bed of the river channel. 

AGRICULTURE (HUMAN)

Animals & machines trampling and compacting soil makes it very difficult for water to infiltrate - creating an increase in OLF. This can lead to a flashy hydrograph as water will transfer to the river much faster, making it fill up quicker.

Animals also remove vegetation which lessens interception. This leads to a flashy hydrograph for the same reasons as above.

PHYSICAL GEOLOGY (PHYSICAL)

Steep valleys create a FLASHY hydrograph as there is very little soil on steep-sided valleys, creating limited infiltration.

Also, under the pressure of gravity water will transfer downhill significantly faster than if it were to travel along a gently-rising valley side.

CLIMATE (PHYSICAL)

This is an obvious one.

In climates that experience a monsoon season or glacial / snow melt in spring, FLASHY hydrographs are much more likely at certain times of year. 

For example, when a monsoon rain hits the soil, bedrock etc becomes saturated very quickly. This leads to a lot of OLF - which leads to a FLASHY hydrograph. 

In spring / summer, increased glacial and snow melt often occurs on top of mountainous areas, meaning they are steep sided. The increase in water mixed with the steep-sided valleys means that  a lot of water will reach the river channel faster.  

VEGETATION

Grassland means less interception and less root systems to take up water. This means most the water will eventually get to the river, leading to a flashy hydrograph.

URBANISATION (HUMAN)

An increase in impermeable surfaces, drains and limited vegetation creates a flashy hydrograph as water is transferred to a river channel directly.

There is no interception, infiltration and percolation possibilities in urbanised areas, making flooding much more likey and therefore a FLASHY hydrograph.

OVERVIEW

Gentle hydrographs differ enormously to flashy ones:

• Longer peak discharge
• Longer lag-time
• Gentle rising limb
• Gentle falling limb
• Very little alteration to baseflow

PERMEABLE GEOLOGY (PHYSICAL)

This allows water to percolate and can create a store of water aswell as a gentle flow.

Rocks such as limestone, sandstone and slate etc are all porous rocks - meaning that allow water into airspaces.This means water can percolate through them, decreasing the risk of flooding as water can either be stored in them or travel slowly to the river channel.

This can also lead to an increased baseflow - which you should add into your exam.