IGCSE Geography: Flooding: Why it happens, and how to prevent it

IGCSE Geography: Flooding – Why it happens, and how to prevent it

Flooding:

Causes of Flooding:

Precipitation – Rain, Snowmelt: High levels of precipitation can cause a river to burst its banks and flood.

Steep Slopes – Gradient of Land: If land is particularly steep, normal storm events may escalate into small floods. This is due to a short lag time. When there is rain, the steepness of the land means that not long after, there will be discharge or a flood.

Permeability of Surface – Soil, Vegetation, Rock type ability to absorb water: A surface with good permeability can absorb water during rainfall, reducing the chance of a flood.

If a surface is impermeable, water cannot be absorbed, and instead flows off the surface, leading to a flood.

Human Activities – Developments, Settlements: If an area is heavily built up (e.g. tarmac roads which are impermeable), water has no way of being absorbed naturally (no vegetation, rocks, and soil). Thus, the lack of absorbent material in urban landscapes means rainfall could lead to flooding.

How Flooding is Controlled:

Classifications:

NIMBY (acronym: Not in my backyard) – This refers to strategies aimed at preventing floods from affecting locals ONLY.

Catch and Hold – Strategies that aim to prevent the flood or slow it down.

Run Away – Accepting that floods occur, and ensuring to be in a safe zone, e.g., stilted buildings when floods occur.

Settlements can be impacted by flooding on both the higher and lower courses of rivers, and stopping a flood at one location means that the flood may still occur downstream.

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Preventions:

Here are a few of the main ones you should remember!

Floodwalls – A NIMBY technique. This is a reinforced, retractable wall which blocks off rising water levels to protect a certain location.

Channelization – This involves straightening the river stream using certain equipment. The river flood is diverted away downstream at a high velocity.

Control Dams – These are reservoirs, large areas of space which take in excess water rises for control. Similar to wells and catchments. The water trapped can be purified and used by the population.

Floodways – Prohibiting development in certain areas, such as the floodplain, with no human settlements, etc. Floodways allow for protection against 100-year floods. A 100-year flood refers to large scale floods which occur rarely, “once in a hundred years”.

Afforestation – Planting trees improves the surface area for infiltration, and water being absorbed reduces the risk of flooding.

Dredging – A costly method to remove silt in streams. Removing silt increases water capacity. However, this method has the disadvantage of damaging the ecosystem, requiring heavy machinery, and causing changes in the water habitat.

Levees – These can be natural or artificially made.

Natural Levees – When river speed decreases due to friction with the floodplain, the load it is carrying is deposited. The coarser, heavier material is deposited first onto the floodplain. Over time, the material layers together and forms a barrier on the river edge, preventing small floods when a river bursts its banks.

Artificial Levees – Humans often use local resources, like wood, bark, etc., to create small levees on the river edge and floodplain. This acts as local protection, protecting their houses, animals, crops.

It's also important to understand that there are Hard flood management techniques and Soft Flood management techniques.

Hard Techniques: Long-lasting and slightly expensive (e.g., Dredging)

Soft Techniques: Operate on a local scale, done by locals and not expensive, done with local material, on a very small scale (e.g., Levees).