Flooding Revision Notes

Flooding often begins with intense rainfall or prolonged periods of rain. When rain falls heavily over a short time, the ground may not absorb water quickly enough, leading to surface runoff. Similarly, extended rainfall saturates the soil, filling all pore spaces with water and preventing further infiltration. This causes additional rainwater to flow over the surface into rivers and streams, increasing their volume rapidly.

Melting snow is another significant contributor, especially in spring. Snowpack accumulated over winter melts as temperatures rise, releasing large volumes of water into rivers and streams. This sudden input can overwhelm river channels, causing them to overflow their banks. For example, in mountainous regions, rapid snowmelt combined with spring rains can trigger floods downstream.

• Intense rainfall exceeds soil absorption capacity, increasing runoff.
• Prolonged rain saturates soil, preventing further infiltration.
• Melting snow adds substantial water volume to rivers, especially in spring.

The shape and slope of the land, known as topography, strongly affect how water moves across the landscape. Steep slopes cause water to flow quickly downhill, reducing the time for infiltration and increasing the speed and volume of runoff entering rivers. Conversely, flat areas allow more water to soak into the ground and slow runoff.

For example, in hilly or mountainous areas, rainwater rapidly collects in valleys and river channels, which can lead to flash floods. In contrast, floodplains with gentle slopes may experience slower, more prolonged flooding as water spreads out over a wider area.

• Steep slopes increase runoff speed and volume, raising flood risk.
• Flat terrain allows more infiltration and slower runoff.
• Topography determines flood type: flash floods in steep areas, slow floods in flat regions.

Soil characteristics influence how much water can be absorbed. Saturated soil has no capacity to take in more water, so additional rainfall becomes surface runoff. Soil compaction, often caused by heavy machinery or livestock trampling, reduces pore space, limiting infiltration even if the soil is not fully saturated.

For instance, farmland with compacted soil after repeated ploughing or grazing may generate more runoff during storms compared to natural woodland soil. This increases the volume of water reaching rivers quickly, contributing to flooding downstream.

• Saturated soil cannot absorb additional water, increasing runoff.
• Compacted soil reduces infiltration capacity, even if not saturated.
• Agricultural practices can compact soil, raising flood risk.

Human actions significantly affect flooding by altering the natural water cycle. Deforestation removes trees that intercept rainfall and absorb water through roots, reducing infiltration and increasing runoff. Farming can compact soil and remove vegetation cover, further limiting absorption.

Urban development replaces permeable surfaces with impermeable ones like concrete and asphalt. This prevents water from soaking into the ground, causing rapid runoff into drainage systems and rivers. For example, a city with extensive paved areas will experience faster and higher peak river flows during storms than a forested area of the same size.

These changes increase the volume and speed of water entering rivers, overwhelming their capacity and causing floods.

• Deforestation reduces interception and absorption, increasing runoff.
• Farming compacts soil and removes vegetation, limiting infiltration.
• Urban areas have impermeable surfaces, causing rapid runoff and higher flood peaks.

Coastal flooding can result from natural events such as storm surges and tsunamis. Storm surges occur when strong winds and low atmospheric pressure push seawater onto the land, raising sea levels temporarily. Tsunamis, caused by underwater earthquakes or landslides, bring massive waves that flood coastal areas.

Additionally, rising sea levels due to climate change increase the baseline water level, making coastal areas more vulnerable to flooding from storms and high tides. This gradual increase means that events which previously caused minor flooding can now lead to more severe and frequent inundation.

For example, low-lying coastal cities face higher flood risks as sea levels rise, requiring improved flood defences and planning.

• Storm surges push seawater inland, causing coastal floods.
• Tsunamis bring large waves that flood coastal zones.
• Rising sea levels from climate change increase coastal flood frequency and severity.