Unit 1 - Essays - Storm Hydrograph UPDATED Flashcards
‘Urbanisation always results in an increase in channel flow.’ With the aid of examples, how far do you agree?
Paragraph 1: Increased Channel Flow Due to Impermeable Surfaces
Point: Urbanisation creates more impermeable surfaces like concrete and asphalt, leading to greater surface runoff.
Explanation: Hard surfaces prevent water from soaking into the ground, causing rainwater to flow rapidly into drainage systems and rivers.
Examples and Evidence:
London: Between 1990 and 2010, increased urbanisation led to a 15% rise in surface runoff into the River Thames.
Mumbai: Over 75% of the city is covered by impermeable surfaces, contributing to frequent urban flooding.
Spatial Variation: The impact is more significant in densely built cities like Mumbai compared to smaller towns with more green spaces.
Temporal Variation: As cities expand over time, the extent of impermeable surfaces increases, amplifying the effect on channel flow.
Development: Explain how increased surface runoff reduces the lag time between rainfall and peak discharge, heightening flood risks, especially during storms.
Paragraph 2: Increased Channel Flow from Channelisation Projects
Point: Channelisation involves straightening and concreting river channels to manage water flow and reduce flood risk.
Explanation: While this method helps control floods locally, it increases the velocity of water, leading to higher channel flow downstream.
Examples and Evidence:
Los Angeles River: Channelisation has led to faster water flow, increasing the risk of downstream flooding.
Sheffield, UK: Channelisation of the River Don resulted in a 20% increase in peak flow velocity during storm events.
Spatial Variation: The effects are particularly noticeable in older urban areas with extensive channelisation.
Temporal Variation: The impacts of channelisation are long-term, as concrete channels are not easily altered.
Development: Discuss how increased velocity can carry more sediment downstream, potentially causing siltation and altering river dynamics.
Paragraph 3: Decreased Channel Flow Due to Water Extraction and Diversion
Point: Urbanisation can also reduce channel flow through water extraction and the diversion of water for urban supply and industrial use.
Explanation: High water demand in cities leads to large-scale water extraction from rivers and the construction of infrastructure that diverts water away from natural channels.
Examples and Evidence:
Las Vegas: Extracts 500 million gallons daily from the Colorado River, significantly reducing channel flow downstream.
Hoover Dam: Has caused a 20% reduction in downstream flow, impacting the Colorado River’s ecosystem.
Thames Tideway Tunnel (London): Diverts stormwater directly to the sea, bypassing natural river channels.
Spatial Variation: These effects are more prominent in arid regions or areas with significant infrastructure for water supply.
Temporal Variation: Seasonal water demand and drought conditions can further influence channel flow reduction.
Development: Explain how reduced channel flow can lead to ecological impacts, such as habitat loss for aquatic species and reduced water quality.
Paragraph 4: Mitigating Channel Flow Through Sustainable Urban Drainage Systems (SUDS)
Point: Modern urban planning uses SUDS to reduce surface runoff and manage water flow sustainably.
Explanation: SUDS include features like permeable pavements, green roofs, and retention basins that allow water to infiltrate the ground rather than rushing into rivers.
Examples and Evidence:
Copenhagen: Implementation of SUDS reduced peak flows by 25% during heavy rainfall.
Green Roofs: Absorb rainwater, delaying runoff and easing the pressure on drainage systems.
Spatial Variation: The effectiveness of SUDS is more apparent in newly developed areas with strict planning regulations.
Temporal Variation: The benefits of SUDS are maintained as long as they are properly maintained, offering a long-term solution to managing urban water flow.
Development: Discuss how SUDS not only manage water flow but also improve water quality by filtering pollutants.
Conclusion
Summary: Urbanisation often increases channel flow due to impermeable surfaces and channelisation, but water extraction and SUDS can reduce this effect.
Judgement: While increased channel flow is a common outcome of urbanisation, the overall impact depends on local management strategies and the adoption of sustainable practices.
Final Thought: Effective urban planning that includes green infrastructure can significantly mitigate the negative impacts of urbanisation on channel flow.
