Hydrological effects of urbanisation on flood risk

Urbanisation refers to the development of towns and cities, which significantly alters natural landscapes and hydrological processes. This transformation often increases the likelihood of flooding, especially in the lower parts of drainage basins where urban areas are commonly located.

Key hydrological responses to urbanisation

• Removal of vegetation - Clearing trees and plants reduces interception (where rainfall is captured by leaves) and evapotranspiration (water returning to the atmosphere). This leads to more water reaching the ground, increasing surface run-off and sedimentation in streams.
• Construction activities - Building houses, roads, and drainage systems decreases the ground's ability to absorb water, lowering infiltration rates. This results in higher storm flows during rain and reduced baseflows (sustained river flow) in dry periods.
• Full urban development - As residential, commercial, and industrial areas expand, surfaces become less porous (e.g., concrete and asphalt). This shortens the time for run-off to reach rivers, increasing peak discharges, compressing flow timing, and significantly raising run-off volume and flood damage potential.
• Storm drains and sewers - These systems provide local flood relief by quickly channelling water away. However, they can concentrate floodwaters downstream, potentially worsening flooding in other areas.

Visualising urbanisation impacts

Urbanisation changes how water moves through a landscape. Initially, a forested area intercepts rainfall, allowing slow infiltration into soil. After vegetation is cleared, surface run-off increases, and soil compaction heightens landslide risks due to weakened root structures. Once urbanised, with buildings and storm sewers, run-off surges dramatically as water is rapidly directed into water bodies via impermeable surfaces and pipes.

Impacts of deforestation on flood hydrographs and risk

Deforestation, the large-scale removal of trees, mirrors urbanisation in its effect on water movement but often occurs over wider areas due to reasons like agriculture, industrial growth, tourism, or making space for urban expansion. This alteration heightens flood risk across broader regions.

Effects of deforestation on hydrological processes

• Reduced interception and evapotranspiration - Without trees, less rainfall is captured or returned to the atmosphere as vapour, leading to more water flowing over the surface.
• Increased overland flow - With vegetation gone, water moves quickly across the land rather than being absorbed, shortening the time lag (time between rainfall peak and river peak flow) on flood hydrographs.
• Higher peak flows - The rapid run-off results in sharper, higher peaks on hydrographs, indicating a greater volume of water reaching rivers faster.
• Broader flood risk - Deforestation increases the chance of floods with higher magnitudes (larger volumes), greater frequencies (more often), and shorter recurrence intervals (less time between events).

Effects of channel modifications on water flow and flooding

Channel modifications involve altering river paths through methods like channelisation, enlargement, and straightening. These changes are often made to manage water flow and reduce local flood threats, but they can have varied impacts on hydrological systems.

Types of channel modifications and their impacts

• Channelisation and straightening - Creating or altering channels to make them straighter speeds up water movement. This reduces the time lag between rainfall and peak river flow, as water travels faster through the modified path.
• Enlargement with levees - Raising riverbanks (levees) or widening channels allows rivers to hold more water, potentially leading to higher peak flows. However, the main goal is to divert water away from an area to lessen local flood risk.
• Downstream considerations - While these modifications can protect specific locations, they may increase flood risks downstream by accelerating and concentrating water flow to other areas.

Variations in flood magnitude and frequency

Not all floods are identical in their impact or occurrence.

Differences in flood events

• Magnitude variation - Floods range from minor events, such as annual floods that may only be a few centimetres deep, to rare, high-magnitude floods that can reach a metre or more in depth.
• Frequency and impact - High-frequency, low-magnitude floods happen more often but cause less damage. In contrast, low-frequency, high-magnitude floods are rarer but far more destructive due to their power and the extensive areas they affect.
• Planning implications - Recognising these variations helps in designing infrastructure and emergency plans that account for both common, smaller floods and the devastating potential of larger, less frequent events.