9 - Urban Drainage

Question 1

Using the type of flood hydrograph and reasoning.

Describe post-urban precipitation and surfaces.

Answer 1

Flashy hydrograph response…discharge increases soon after the start of the storm, steep rising limb and short lagtime, high peak discharge, steep falling limb.

More precipitation is due to more convectional rainfall (due to urban heat island effect) and more condensation nuclei for raindrop formation.
Less infiltration - groundater sources are low and depleted due to human water use.
Less evaporation as water runs off the surface fast.
More surface run-off as impermeable surfaces stop water being absorbed.
Run-off is channeled through man-made pipes which transport water to rivers faster.

Question 2

What has urban drainage traditionally consisted of and why were these systems an issue?

Answer 2

Underground pipe systems.

They didn’t take into account amenity aspects such as: water resource management, community facilities, landscaping potential and wildlife provision.

Question 3

What is catchment management?

Answer 3

A way of managing rivers and improving drainage systems by looking at the whole river catchment and the interactions between water and land.

Question 4

Name and describe the 4 issues associated with river catchment (basin) management in urban areas.

Answer 4

1. River flow - increased flow = flooding and erosion of river banks during wet periods, decreased flow = harms aquatic wildlife and source becomes unreliable during dry periods.
2. Sewage - very high flows can overload the water system, resulting in raw sewage on the surface.
3. Pollution - oil, grease, toxic chemicals from cars, pesticides and nutrients from parks and gardens, road salt, viruses from pest-waste.
4. Temperature - higher water temp = ecosystems are disturbed.

Question 5

Name the 3 main approaches of catchment management

Answer 5

1. SUDS
2. River restoration and conservation
3. Hard engineering

Question 6

What are SUDS as a catchment management approach?

Answer 6

Sustainable Urban Drainage Systems that control pluvial flooding (flooding from rain) and take pressure off of rivers.
They aim to imitate natural drainage systems.

Question 7

Describe and evaluate green roofs as a SUD.

Answer 7

Multi-layered system, cover roof with vegetation, designed to intercept and retain precipitation and reduces volume of runoff and peak flow.

Strengths: applied in high density developments, ecological, aesthetic, no additional land take, manages UHI effect, insulates building.

Limitations: cost, not appropriate for steep roofs, maintenance.

Question 8

Describe and evaluate rainwater harvesting as a SUD.

Answer 8

Stores and reuses rainwater, reduces rate and volume of runoff, water butts are common, small-scale e.g. gardens.

Strengths: effective, reduces demand on water mains, low relative cost and maintenance.

Limitations: complex to install, limited aesthetic, perceived public health risk.

Question 9

Describe and evaluate pervious surfaces as a SUD.

Answer 9

Porous = water infiltrates across entire surface.
Permeable = water infiltrates through a pattern of voids. Can temporarily store water in ground or reuse water.

Strengths: suitable surface for pedestrians and vehicles = dual use of space; reduces peak flow, flood risk, effect of pollution, need of drainage deep in ground and runoff; good community acceptability.

Limitations: cannot be used where large sediment may get washed, only used on highways where there is low traffic, long-term clogging if its poorly maintained.

Question 10

What is river restoration and conservation as a catchment management approach?

Answer 10

Involves soft-engineering.
More aesthetic, cost varies and sustainable.
e.g. Cheonggyecheon

Question 11

What is hard-engineering as a catchment management approach?

Answer 11

The most traditional approach, involving structures like dams, embankments and drainpipes.
Often unaesthetic, expensive and unsustainable

Question 12

What are the 6 key principles that influence the planning and design process of SUDS? (CHAIFA)

Answer 12

1. Conveying (slowly transport water on the surface)
2. Harvesting and using the rain close to where it falls
3. Attenuation (store run-off and release it slowly)
4. Infiltration (allow water to soak into the ground)
5. Filtering out pollutants
6. Allow sediment to settle by controlling water flow

All of these enable SUDS to mimic natural drainage.

Question 13

What 6 benefits should SUDS achieve to be considered successful? (FRIEDA)

Answer 13

Flood risk management
Recreation opportunities
Improved water quality
Enhanced biodiversity
Drought management
Air quality improvements

Question 14

What catchment management was used in Olympic Park, London?

Answer 14

SUDS - wetlands, swales, rainwater harvesting.

Question 15

Fill in the blanks: The benefits of the catchment management used in the Olympic Park in London are that it:
- accounted for potential impacts of ___ ___,
- appropriate methods of ___ and treatment,
- protect both ___water and local water ___,
- ___ biodiversity and amenity of the ___,
- safe and ___able,
- life ___ managed.

Answer 15

climate change, interception, ground, courses, enhance, public, maintain, costs

Question 16

Fill in the blanks: The challenges of the catchment management used in the ___ Park in London are that:
- there was limited ___ and coordination of discharge,
- there were differing ___ ___ for each element.

Answer 16

olympic, integration, design criteria

Question 17

What catchment management was used in the Cheonggyecheon River Project, Seoul, China?

Answer 17

River restoration and conservation - restored a river that had been built over in the 1960s into a 5.8km ecologically sensitive green pedestrian corridor, hugely popular for residents and tourists.

Question 18

What were the benefits of the river catchment managment used in the Cheonggyecheon River Project, Seoul, China?

Answer 18

Economic: 3.5% in new businesses, more tourism and more entertainment

Social: 64000 visitors a day, more pedestrian activity

Environmental: UHI reduced (temp 3-4.C lower), air flow along stream = cool wind corridor, air quality, water quality, noise and smell improved, lost habitats and wildlife reestablished, increased interception from vegetation reduces flood risk.

Question 19

What were the challenges of the river catchment managment used in the Cheonggyecheon River Project, Seoul, China?

Answer 19

Water flow could not be completely restored and needs to be pumped from nearby Hanang River. It’s an “artificial stream” and natural and historical value is yet to be restored.