Why do physical factors cause flooding? (AS-level standard)

What is flooding? Flooding is where the input to a river exceeds the output. In other words, flooding occurs when a river is so full that it is unable to hold any more water, and therefore any excess water will flow out of the river and onto land. An example of a flooding event often used at AS level is that of the Boscastle 2004 flood.

But what physical factors cause the water level to become so high that it causes flooding? Well, firstly we need to understand what a ‘physical factor’ is. Physical factors cause flooding as a result of a natural change such as impermeable ground, steep slopes, a high drainage density and sparse vegetation. Impermeable surfaces can cause flooding, as if water is unable to infiltrate and move by base flow through it, then this water will instead run along the grounds surface, known as overland flow. By running along the grounds surface, this will then cause the water to run more quickly towards the river, reducing the lag time. This means that with less water infiltrating and more water running straight to the river, during periods of heavy rainfall, the river will be holding more water and have a greater discharge. The river may then flood if the cross-sectional area of the river is too small to hold all the water which is moving via overland flow into the river.

Steep slopes are another physical factor which can cause flooding. Imagine you roll a ball down a hill. If the hill is steep, then the ball will roll down the hill faster, while if you put the ball on a flatter section of land, the ball will roll at a slower rate. This principle works with rain too, as on steep slopes, water will run via overland flow quicker into the river. Also because of the steep angles of the slopes, rain is less likely to infiltrate into the ground in these areas. With more overland flow and a quicker overland flow rate, this can also cause flooding.

High drainage densities can also cause flooding. This physical factor is mainly seen in very large river systems such as the Ganges. This is because the drainage basin contains lots of tributaries which feed into the main river. Therefore in periods of heavy rainfall, water will flow into these tributaries. The water will then flow through these tributaries until the tributary meets the main river channel. However, with a large number of tributaries meeting the main river at the same time, a large volume of water will be moving into the main river at one point. With so much water reaching the river at one point, then this will cause the volume of water in the main river to increase rapidly, potentially causing the volume of water to be too much, which will then lead to flooding.

An area where there is sparse vegetation is the final main physical factor which may cause the river to flood. For example, where there is little vegetation, this means that less rainfall is intercepted and stored by trees and grasses. As well as less interception, sparse vegetation means less water can infiltrate into the ground through stem flow and percolation. Therefore with more water on the grounds surface and less infiltrating into the ground or being stored in vegetation, then overland flow will occur causing water to reach the river at a quicker rate, in turn causing an increased risk of flooding. These four factors are the main reasons why flooding occurs, and often, several of these physical factors may interact with each other to cause the river to flood. 

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