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Deciduous Tree

Option A - Freshwater – Drainage basins

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Magnifying Glass Tilted Left

Option B - Oceans & coastal margins

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ringed-planet

Option C - Extreme Environments

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Rocket

Option D - Geophysical Hazards

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Performing Arts

Option E - Leisure, Tourism And Sport

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Flexed Biceps

Option F - The Geography Of Food & Health

Option G - Urban Environments

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Office Building

Unit 1 - Changing Population

Unit 2 - Global Climate - Vulnerability & Resilience

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Globe Showing Europe Africa

Unit 3 - Global Resource Consumption & Security

<ul>
	<li>Flood: When a river cannot contain all the water it is transporting in its channel, the water flows onto a flood plain.</li>
	<li>Small floods: Typically contained on the flood plain close to the river.</li>
	<li>Large floods: Cover a larger part of the flood plain and occur less often.</li>
</ul>

The recurrence interval is the frequency with which events of a certain size occur. High-magnitude, low-frequency events might occur once every 50 years, whereas low-magnitude, high-frequency events may occur yearly.

<ul>
	<li>Most flows are contained within normal operating levels.</li>
	<li>Disasters are caused by extremes of floods (or drought) and catastrophes are caused under very extreme conditions.</li>
	<li>A catastrophe has a higher magnitude but a lower frequency than a disaster.</li>
</ul>

<ul>
	<li>
	Precipitation type and intensity: Highly intensive rainfall can produce a flood, while low-intensity rainfall likely infiltrates into the soil and percolates slowly into the rock, thereby reducing the flood peak. For example, imagine a heavy downpour vs a drizzle during a garden party. The former would flood your backyard quickly, while the latter would slowly soak into the ground.
	</li>
	<li>
	Temperature and evapotranspiration: Temperature affects both the type of precipitation and the evaporation rate. High temperatures lead to more evaporation, thus less water getting into rivers. However, warm air can hold more water, increasing the potential for high peak flows in hot areas.
	</li>
	<li>
	Antecedent moisture: If it has been raining previously and the ground is saturated, additional rainfall will quickly produce a flood. Think of a sponge - once it&#39;s fully soaked, additional water will just spill over.
	</li>
	<li>
	Drainage basin size and shape: Smaller basins respond more quickly to rainfall conditions. Picture a small pond versus a large lake during a rainstorm: the small pond fills up and overflows much quicker.
	</li>
	<li>
	Drainage density: High drainage density, such as urban basins with a network of sewers and drains, flood very quickly.
	</li>
	<li>
	Porosity and impermeability of rocks and soils: Impermeable surfaces cause more water to flow overland, creating greater peak flows. In contrast, permeable rocks and soils, like chalk and gravel, allow water to infiltrate and percolate, reducing peak flow.
	</li>
	<li>
	Slopes: Steeper slopes create more overland flow and higher peak flows.
	</li>
	<li>
	Vegetation type: Broad-leafed vegetation intercepts more rainfall, reducing the amount of overland flow, peak flow, and increases time lag.
	</li>
	<li>
	Land use: Impermeable surfaces or reduced vegetation cover increase overland flow. Urbanization increases the magnitude and frequency of floods by creating highly impermeable surfaces, increasing drainage density, and constricting natural river channels.
	</li>
</ul>

Urban hydrographs differ from rural ones as there are more impermeable surfaces in urban areas (roofs, pavements, roads, buildings) and more drainage channels (gutters, drains, sewers). This results in:

<ul>
	<li>A shorter lag time</li>
	<li>A steeper rising limb</li>
	<li>A higher peak flow (discharge)</li>
	<li>A steeper recessional limb.</li>
</ul>

Urbanization also impacts hydrological processes by increasing erosion due to more water getting into rivers, increasing flow and transport of materials due to enlarged channels, and decreasing erosion due to riverbank protection schemes.

Deforestation impacts flood hydrographs similarly to urbanization. It reduces interception, increases overland flow, and reduces evapotranspiration, causing flood hydrographs to have shorter time lags and higher peak flows. In the Himalayas, for instance, changes in flooding and increased deposition of silt in parts of the lower Ganges-Brahmaputra are due to a combination of high monsoon rains, steep slopes, and seismically unstable terrain, ensuring rapid runoff and high sedimentation, irrespective of vegetation cover.

Understanding The Impact Of Urbanization On Flood Risks

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Standard Level

Geography SL

Option A - Freshwater – Drainage basins

Understanding The Impact Of Urbanization On Flood Risks

Table of content

Flood basics

Recurrence interval

Relationship between flood magnitude, frequency, & economic loss

Unlock the Full Content!

Option A - Freshwater – Drainage basins

Understanding The Impact Of Urbanization On Flood Risks

Table of content

Flood basics

Recurrence interval

Relationship between flood magnitude, frequency, & economic loss

Unlock the Full Content!