geography: contemporary urban environments - urban drainage

why is precipitation typically higher in urban areas?

due to increased presence of hygroscopic nuclei, and urban areas also experience more thunderstorms

what does the increase in precipitation change?

the inputs to the urban drainage system

how does urban form influence the hydrological cycle?

• less permeable surfaces → increases surface runoff which increases likelihood of flooding and higher levels of evaporation

  • reduces infiltration which prevents groundwater recharge

• river discharge increases as possible pollutants/waste water may be disposed of in water systems

• reduced evapotranspiration - can influence local microclimates, potentially making urban areas warmer and drier.

• may alter natural natural waterways - streams may be straightened, deepened, or lined with concrete, which changes flow patterns and increases flow velocities.

• water is artificially imported into the system - increasing inputs

• artificial drainage system added

what are the two issues associated with urban river catchment management?

h2o quantity and h2o quality

what is urban catchment management?

the management of urban water and surface water runoff within a given urban drainage basin.

how does urban catchment alter river discharge?

increase in the volume of water in some areas whereas areas further down the stream might dry up because the lack of infiltration reduces the groundwater flow

how can river discharge be calculated?

cross sectional area (wxh) x speed

what is a flood hydrograph?

graph that shows how a drainage basin responds to a period of rainfall

what do urban hydrographs look like compared to normal hydrographs?

tend to be flashy, where rivers are more likely to flood due to lower interception and infiltration rates.

what do urban river’s base flows look like compared to normal rivers?

lower in urban rivers compared to rural rivers as less is fed into the system via soil and rock stores, these rivers fill quickly during rain events and are more likely to flood.

what do urban river’s base lag time’s look like compared to normal rivers?

urban river is shorter due to drainage systems which speed the flow of water through urban areas and a higher peak discharge is evident in urban rivers.

what do urban river’s rising/falling limbs look like compared to normal rivers?

steep rising and falling limbs as water reaches urban rivers quicker via drainage networks.

what are the overall differences in an urban river’s hydrograph compared to a normal rivers?

• urban

  • shorter basin lag time - impermeable surfaces

  • steeper rising limb - water management systems = quicker flow to river

  • steeper falling limb - is less underground storage of water so the vast majority of the water will have entered the river

• rural

  • longer basin lag time - stem flow and interception

  • gentler rising limb - moisture absorbed and transpired by trees

  • gentler falling limb - water will take longer to return to the river because of groundwater flow

what is a SUD?

• sustainable urban drainage system

aims to mimic natural drainage regimes within the built environment, thereby reducing flooding, improving water quality and the value of the environment to people and biodiversity

what is urban catchment management?

management of urban water and surface water runoff within a given urban drainage basin

aim is to minimise flooding, drought, water pollution, channel erosion and improve ecosystems that rely on the river

what are the issues of urban catchment management?

• Decisions made by local authorities and private engineers (usually without consultation with local residents) → generates conflict and resentment

• management schemes are disruptive and expensive

• Ecological balance is disturbed through increased water temperatures and waste water discharge

• Concentrated water pollution and spread of viruses

• Decreased water flow in dry periods impact aquatic ecosystems

what is soft engineering?

using knowledge of river basin processes to work with nature

what is hard engineering?

building structures to 'push back' against nature

what are some examples of soft engineering approaches to urban drainage management?

• afforestation: increases interception and reduces throughflow and surface runoff + reduces flood risk and water pollution

• floodplain zoning: restriction on building on certain areas of a floodplain 

• riverbank conservation: reduce lateral bank erosion and collapse through planting to stabilise banks

• riverbank restoration: aims to restore the river channel to its original course by removing/reversing past management strategies

what are some examples of hard engineering approaches to urban drainage management?

• dams+floodwalls+reserviors: built to prevent flooding and ensure a regular supply of water

• river straightening: increases the flow of water by increasing the gradient of a river's channel and removing natural meanders

• levees/embankments: can be built or increased using concrete or dredged material from channels

• channelisation: adds a liner to a straightened river channel to reduce friction which aims to improve flow rate

what are the arguments for and against soft engineering?

for - overall a cheaper method of management compared to hard engineering and can improve the local environment

against - planning restrictions limit building of new homes and businesses and difficult to implement in already urbanised areas

what are the arguments for and against hard engineering?

for - prevents flooding of homes and businesses - reducing costs of repairs and loss of revenue.

against - very expensive to build and maintain - they alter wildlife habitats and prevent aquatic migration.

• they affect river flow downstream - increased silting or flooding. 

how do SUD’s improve water quality and biodiversity?

• storing runoff and releasing it slowly

• collecting and re-using surface water at source

• allowing water to infiltrate into the ground 

• filtering out pollutants

• allowing sediments to settle by controlling the flow of the water

how are SUD’s different to traditional ways of managing flood risk urban areas?

• mimics nature and typically uses rainfall close to where it falls

• transports surface water, slows runoff down before it enters watercourses - provides areas to store water in natural contours and can be used to allow water to soak into the ground or used for transpiration

• considered to be environmentally beneficial

• a sequence of management practices and strategies designed to efficiently and sustainably drain surface water →minimises pollution +manages the impact of water quality on local water bodies

why are SUD’s more sustainable than traditional methods?

• manages runoff volumes from hard surfaces + provides opportunities for using runoff where it falls

• protects water quality and natural flow regimes in water courses

• helps the environment - provides an attractive habitat for wildlife and provides opportunities for evapotranspiration from vegetation and surface water

• helps the community - encourages groundwater/aquifer recharge

why do SUD’s benefit the water cycle?

replicates as closely as possible the natural drainage from a site before development

• delivers the most benefits for water quality, quantity and biodiversity

what are the four pillars of SUD design?

• water quantity

• water quality

• amenity

• biodiversity

what is the water quantity pillar of SUD’s?

controls the quantity of run off to support flood risk management and maintain/protect the natural water cycle

what is the water quality pillar of SUD’s?

manages the quality of the runoff to prevent pollution

what is the amenity pillar of SUD’s?

create + sustain better places for people to inhabit

what is the biodiversity pillar of SUD’s?

create and sustain better places for nature

what factors should be considered in decisions about drainage?

long-term environmental and social factors

what are some examples of SUD techniques?

• swales- wide shallow drainage channels

• permeable road and pavement surfaces - use of porous block paving and concrete

• infiltration trenches - gravel filled drains and filter strips

• rain gardens - shallow landscape depressions planted with flowers and shrubs

what are some of the benefits of SUD’s?

• slowing down surface water runoff and reducing the risk of flooding

• reducing the risk of sewer flooding during heavy rain

• preventing water pollution

• recharging groundwater to help prevent drought

• providing valuable habitats for wildlife in urban areas

• creating green spaces for people in urban areas

what are some of the components of SUD’s?

source control - green roofs, rainwater harvesting and permeable surfaces

swales and convergence channels

filtration - filter strips

infiltration - basin

retention + detention - detention basin

inlets, outlets and control structures

what is a method of filtration?

filter strips

what are SUD filter strips?

gently sloping vegetated strips of land that provide opportunities for slow infiltration

• designed to intercept runoff as an overland street flow from upstream development and often lie between a hard-surfaced area and a receiving stream/surface water collection

treat runoff by vegetative filtering promote settlement of particulate pollutants and infiltration

what are the advantages of filter strips?

• well suited to implementation adjacent to large impervious areas

• encourages evaporation and can promote infiltration

• easy to implement and low construction costs

• easily integrated into landscaping and can be designed to provide aesthetic benefits

• water quality improvement - removes pollutant removal

• flood control - runoff reduction and erosion (prevents loss of topsoil)

• low maintenance compared to other solutions

what are the disadvantages of filter strips?

• not suitable for draining hotspot runoff or for location where there is risk of groundwater contamination

• space requirements - needs significant land area

• sediment accumulation - sediment can accumulate over time which reduces the effectiveness of the filter strips

• performance variability - effectiveness of filter strips can vary depending on factors such as slope, soil type, vegetation type, and rainfall intensity.

• limited capacity - during heavy rainfall events, filter strips may become saturated and less effective at filtering pollutants and reducing runoff.

what are rain gardens?

are small depressions in the ground that can act as infiltration points for roof water and other surface water

• most likely to be implemented on private property close to buildings

• in order for roof water to reach a rain garden, property drainpipes are often disconnected from the drainage system

what are the advantages of rain gardens?

• easy to retrofit

• small and attractive features that may improve open spaces

• can reduce run off

• easy maintenance

• slowing down the flow of water- rain gardens help in reducing peak discharge rates during heavy rainfall events, minimizing the risk of flooding.

• carbon sequestration: plants in rain gardens can absorb CO2, contributing to the reduction of greenhouse gases.

what are the disadvantages of rain gardens?

• limited impact on volume reduction

• requires landscaping and management

• unsuitable for areas with steep slopes

• Proper design often requires expertise to ensure the garden functions correctly and integrates well with the surrounding landscape.

• Seasonal Performance: During dry periods, rain gardens may require supplemental watering to maintain plant health, while in extreme rainfall events, they may become overwhelmed.

• Standing Water: If not properly designed/ maintained, rain gardens can create standing water, which may become breeding grounds for mosquitoes.

what are detention basins?

surface storage basins/facilities that provide flow control through storing stormwater runoff - also facilitates some settling of particulate pollutants

• normally dry and the land can function as a recreation facility

• however some basins provide a habitat area for wildlife

what are the advantages of detention basins?

• can cater for a wider range of weather events

• simple to design and construction

• potential for dual land use

• Peak Flow Reduction: Detention basins temporarily store stormwater runoff, reducing the peak flow rates and mitigating the risk of downstream flooding.

• Delayed Discharge: release water at a controlled rate, reducing the immediate impact on the drainage system.

• Infiltration: designs incorporate features that promote infiltration, allowing stormwater to recharge local groundwater supplies.

what are the disadvantages of detention basins?

• Drowning Risks: Open water in detention basins can pose a drowning risk, particularly to children.

• Climatic Sensitivity: The effectiveness of detention basins can be influenced by local climatic conditions, with varying performance during prolonged droughts or excessive rainfall events.

• Sediment Overload: Heavy sediment loads can reduce storage capacity and affect the basin's efficiency over time.

what is the SUD’s case study?

Marylebone

what SUD’s have been implemented into Marylebone?

bioretention areas retrofitted into a busy public highway in Marylebone

what are the physical constraints for SUD’s in Marylebone?

the site is a live high street with significant pedestrian and vehicle traffic

→ rain gardens had to be smaller than intended and lined with an impermeable membrane to protect services

what were the benefits of the SUD’s in Marylebone?

• no flooding during storm events in late 2019 and early 2020

• silt trap and plants improve the water quality entering the sewer and capture pollutant particles to improve air quality

• community education on urban flooding and water management

• enhanced amenity value and pedestrian safety through replacement of dual carriageway used for temporary parking replaced with large rain gardens

what is river restoration project case study?

cheonggyecheon river restoration project

what is the background of the cheonggyecheon stream?

had been covered by a highway for decades due to the rapid industrialization and urban development in post-war South Korea.

By the early 2000s, the area had become heavily congested and polluted, prompting the Seoul Metropolitan Government to undertake the project

what were the objectives of the cheonggyecheon river project?

• Environmental Restoration: Revive the natural ecosystem and improve water quality.

• Urban Renewal: Enhance the urban landscape and provide green public space.

• Cultural and Historical Preservation: Restore historical sites along the stream.

• Economic Revitalization: Boost local economy through tourism and increased property values.

• Flood Control and Water Management: Improve flood resilience and stormwater management.

how was the restoration implemented?

• Demolition and Excavation: The highway covering the stream was dismantled, and the streambed was excavated and cleaned.

• Water Supply System: A water circulation system was installed to ensure a continuous flow of clean water, using water from the Han River.

• Ecological Restoration: Native plants and trees were introduced, and the stream was designed to mimic natural river conditions.

• Infrastructure and Amenities: Walkways, bridges, public art installations, and recreational areas were constructed along the stream.

how much did the project cost?

$281 million and was funded by the Seoul Metropolitan Government

why did the river restoration scheme begin?

• accelerated industrialisation and modernisation

• the freeway had safety risks and divided north and south areas of the city

• overpass had a negative impact on businesses nearby - crime

what were the 3 key aims of the project?

• create ecological and recreational opportunities

• emphasis from city authorities on health, sustainability and social responsibility

• mirrors South Korea’s planning promises of improving QOL and the importance of functioning ecosystems

what were some of the key features of the river restoration scheme?

• car use discouraged and rapid bus lanes added

• 22 bridges added in total including 12 pedestrian bridges and 10 motorist bridges to improve north and south comms

• the Hanang river used as a source of water pumped into the river to deal with variable flow rates - maintained a regular flow of 40cm depth

• middle zone of river has fountains and waterfalls to increase biological oxygen supply

• final zone of the river - stream is widened and designed to look overgrown - marks the end of the river where it flows into a wetland conservation area

what were the environmental impacts on Seoul’s river catchment?

• flow rates within the river were maintained at a constant rate - 120,000 tons of water pumped in daily from the Han river

• provides flood protection for an up to 200 year flood event - embankments, vegetation enables infiltration

• river provides cooling effect + plants reduce the UHIE and filter water

• surrounding temp reduction by 2.5c as a result of reducing cars and introducing the ecosystem

• recued small-particle air pollution by 35%

• evapotranspiration rates increased because of the introduction of plants to the system

• a 2.2 - 2.8% increase in wind speeds in the corridor

what were the social impacts on Seoul’s river catchment?

• 18.1 million visitors by 2008

• contributed to 15% increase in bus and 3.3% subway usage between 2003 and 2008

• before the restoration, residents who lived next to the river were twice as likely to suffer from respitory diseases compared to residents in other areas of the city

• stream became a popular recreational space for residents and tourists, offering walking paths, seating areas, and cultural events.

• project revived historical landmarks and increased public awareness of Seoul’s heritage.

• attractive and green urban space improved the quality of life for local residents.

what were the economic impacts on Seoul’s river catchment?

• tourists who visit the scheme contribute up to 2.1 billion to the economy per year

• property values in the vicinity of the stream saw significant appreciation, reflecting the area’s improved desirability.

  • increased the price of land by 30-50% for properties within 50 meters of the project

• increased number of businesses by 3.5% in the area of the project from 2002-2003, which was double the rate in downtown seoul

  • increased number of people working in the area by 0.8% compared to the downtown Seoul area which had a reduction of 2.5%

what were the positive impacts of the river restoration project on the hydrological cycle?

• improved water quality: project involved removal of pollutants and debris from the river - improved quality + positively impacted the cycle as provided downstream habitats

• increased water infiltration: restoration efforts often include measures to promote natural infiltration into the ground which replenishes groundwater sources

• enhanced natural flood control: the project aimed to restore the natural floodplain functions of the river which can regulate waterflow during periods of rainfall

• promotion of biodiversity: restoring natural habitat/ecosystems can enhance rivers resilience by supporting species that contribute to nutrient cycling and water filtration

what were some negative impacts of the river restoration project?

• technical difficulties: managing water quality and ensuring a continuous flow posed significant challenges, requiring innovative engineering solutions.

• ensuring ongoing maintenance and sustainability of the restored stream necessitated continuous investment

• business and residents: removal of the elevated highway and the restoration process led to the displacement of many small businesses who operated in the area + residents living there

• area under the former highway had become a hub for informal economic activities, including street markets and small-scale commerce. The restoration project disrupted these activities, impacting the livelihoods of those involved.

• gentrification: improvement of the area led to increased property values and rents, which contributed to the gentrification of surrounding neighborhoods - made it more difficult for lower-income residents and small businesses to remain in the area.