The River Quaggy Case Study

Location: 51° 27' 21", 0° 1' 41"

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Project summary

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The Environment Agency implemented a scheme to increase the level of flood risk protection to homes and properties along the Quaggy River from Sutcliffe Park downstream to Clarendon Rise in Lewisham, a length of some 4km. The Quaggy is a tributary of the River Ravensbourne, it rises as the Kyd Brook in Bromley and flows over a length of 13km through Bromley, Greenwich and Lewisham to join the River Ravensbourne at Loampit Vale. The River Ravensbourne joins the River Thames at Deptford.

In 1968 extensive flooding affected over 400 residential and commercial properties, and flooded Sutcliffe Park. Serious flooding occurred in June 1992 when about 100 residential and commercial properties were flooded. The solution originally proposed to address this flooding called for large scale channel works to accommodate the predicted flood flows. These works would have required considerable widening and deepening of the river and the construction of significant lengths of retaining wall, leading to the loss of a large number of trees, and other natural features of the river.

The flood alleviation scheme for the Quaggy was the key driver to restoring this river.

Sutcliffe Park & Weigall Road Flood Storage Areas: When in use as a flood retention area, the park will hold up to 85,000m3 of flood water. The flood storage area located downstream at Weigall Road, provides a flood storage volume of 65,000m3 in an area currently used as a sports field, allowing for dual use of an open space. The overall Quaggy FAS includes two upstream flood storage areas in urban open spaces at Sutcliffe Park and Weigall Road Sports Ground, which significantly reduced environmental impacts by minimising the need for works (wall raising) downstream. The flood storage reservoirs hold the peak flows, thereby reducing the flow downstream and the levels required for flood protection. Through this scheme the London Borough of Greenwich has allowed the Environment Agency to ‘make space for water’ in Greenwich to reduce the risk of flooding to the residents of Lewisham. See Figure 2 for locations.

Sutcliffe park operates as part of the flood alleviation scheme providing flood storage when required. The original concrete box culvert is still utilised as part of the flood defence and, with additional control structures within it and attached to it, the culvert directs floodwater into and out of the park, see Figure 4. Downstream of the low flow inlet a small weir within the culvert directs the low flow continuously through the park. Downstream of the high flow inlet a flume constructed within the culvert constricts high flows and forces water into the lake. During flooding, the park is designed to fill up from the centre to prevent the creation of islands within it. If the storage area were to reach capacity a spillway directs the overflow back into the culvert.

The low level footpaths are above the 1 in 1 year event flood level, hence minimising regular disturbance to users.

Sections 2,4 and Manor Park

The Quaggy River between the Weigall Road flood storage area and the confluence with the Ravensbourne is dominated by narrow concrete and brick-lined channels running primarily between rear gardens, with occasional small sections of naturalised banks along rear gardens and open spaces, see Figure 6. It is a typical urban river with a very restricted floodplain, numerous properties and riparian owners and a wide uniform river bed.

The overall design aim for Sections 2 and 4 was to achieve a 1 in 70 year level of flood protection by incorporating flood defences set back from the river in all possible areas including in rear gardens, enhancing retained hard structures through the careful incorporation of environmental features, retaining as many trees in the river corridor as possible through sensitive design and improving the geomorphology of the channel upon completion of the works.

A reduced scope of works at these downstream locations was possible due to the implemented flood storage areas upstream. The reduced flows downstream also give the scheme the characterisics of an integrated urban drainage solution through Lewisham as there is capacity in the river for storm water overflows.

Two stretches of natural riverbank along this section of works were lower than the necessary level for a 1:70 flood protection, thereby affecting properties, mostly terraced homes, to a high risk of flooding. Historically, this problem was addressed on the Quaggy by providing concrete or brick river walls at the bottom of gardens to contain flood waters. This solution was deemed unacceptable on this project for environmental, social and flood defence reasons. Instead an innovative solution was proposed whereby flood defences were to be incorporated into garden structures through careful design. This provided exciting garden designs for residents and improved levels of flood protection for these properties, flood storage areas within gardens and preservation and enhancement opportunities along natural riverbanks. HDA Landscape Architects have been responsible for the revised garden designs.

In one location consisting of ten properties, these set back defences were incorporated into existing decks and included features such as steps or planting beds, giving them the appearance of new and interesting garden features, see Figure 7. In the other location of set back defences, consisting of twelve properties, many of which have been sub-divided into flats with very deep and often subdivided gardens, the set back defence took the form of a low wall within the garden. In some properties, this wall acts to provide divisions between gardens of differing ownership, and in others it divides formal garden areas from wilderness areas associated with the river. In all instances, the walls incorporate planting beds, steps and other features as requested by residents. The riverbank in these two sections of properties will be secured by coir rolls pre-planted with locally sourced flowering plants and grasses to enhance the residents’ enjoyment of their river access and hopefully increase awareness of the river in general. In both of these areas, residents were asked to contribute as much as they were willing to the design of walls, garden features and planting plans to ensure long term designs which met multiple objectives of flood defence, community involvement and environmental benefits.

Other factors which increased the risk of flooding along this section of river were crumbling river walls falling into the river and creating blockages, pipes and low footbridges crossing the channel above mean water levels causing blockages during flood events, and steep earth banks subject to scour, thus undermining existing trees and causing blockages. To address these concerns, existing poor quality walls have been repaired or replaced with landowner input into reinstatement; pipes have been buried; a footbridge lifted and related ramps and steps constructed. Banks were stabilised with coir rolls to reduce the risk of scour and to enhance the channel by restricting the width in localised sections.

Prior to construction, eleven baseline surveys were carried out including surveys of riverine flora, trees, bats, fish, birds and mammals to inform designs in progress and enable the process of environmental impact assessment. As a result of these surveys, working methods on site have sought to identify key environmental features and to retain them where possible. This has meant educating the contractor’s staff through toolbox talks and site supervision, leading to site staff who recognise some of the locally rare plants and adopt sensitive methods when working near them.

Monitoring surveys and results

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Sutcliffe Park

  • The number of park visits increased by 73%
  • People stay longer, on average 47 minutes per visit compared to 34 minutes
  • Total time spent in the park per person per month increased by more than 3.5 hours
  • 28% of people surveyed started visiting only recently, due to the improvements
  • More people visited more often, stayed longer and were more likely to visit to exercise and for health
  • Analysis showed that visitors’ self esteem increased the longer they had spent exercising in the natural environment.
  • Local green spaces are an important health resource for surrounding communities.


83% of visitors feel differently in the park now the River Quaggy runs though it, because of increased biodiversity, better opportunities for recreation, and the peacefulness and relaxation of being near water.

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Image gallery

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Catchment and subcatchment

Catchment

Subcatchment

River name Quaggy
Area category 10 - 100 km²
Area (km2)
Maximum altitude category 100 - 200 m
Maximum altitude (m) 135

135 m
0.135 km
13,500 cm

Dominant geology Calcareous
EcoregionGreat Britain
Dominant land coverSuburban
Waterbody ID GB106039023290

Other case studies in this subcatchment: Colfes School, Lidl, Manor House Gardens, Manor House Gardens Gauging Station, Manor Park, Mottingham Farm, Quaggy channel improvements, River Quaggy- Chinbrook meadows, Sutcliffe Park, Sydenham Cottages Nature Reserve

Site

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Project background

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Reach length directly affected (m) 4300

4,300 m
4.3 km
430,000 cm

Project started
Works started
Works completed
Project completed
Total cost category more than 10000 k€
Total cost (k€) 18,000

18,000 k€
18,000,000 €

Benefit to cost ratio
Funding sources Environment Agency

Cost for project phases

Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname
Investigation and design
Stakeholder engagement and communication
Works and works supervision
Post-project management and maintenance
Monitoring

Reasons for river restoration

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Measures

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Structural measures

Bank/bed modifications
Floodplain / River corridor
Planform / Channel pattern
Other

Non-structural measures

Management interventions
Social measures (incl. engagement)
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Monitoring

Hydromorphological quality elements

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quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Biological quality elements

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quality elements

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Physico-chemical quality elements

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quality elements

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Any other monitoring, e.g. social, economic

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Monitoring documents

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Additional documents and videos

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Additional links and references

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Supplementary Information

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Overview of the Quaggy Flood lleviation Scheme
Sutcliffe Park - How it works

River management case studies

HARD Engineering in the Mississppi
  • The US needed to prevent the yearly floods and tame the river to make it navigable in order to develop
  • Before management schemes were implemented the river constantly shifted its channel and eroded its banks
  • They used stone dykes to trap sediment and provoke the river to erode vertically so that the channel was deep enough for paddle steam boats to use
  • More wing dykes were constructed along with reserviors, levees and channel straightening, channelisation (concrete matressing) and dregding were also used -----> this all made the river faster as they increased the gradient along the rivers long profile
  • All of this management, like all river management in the America, was completed by the US Army Corps of Engineers and costs $180 million a year in maintainence as the force of the water sweeps away thousands of dollars worth of management each year. It is hard to manage rivers as they constantly change (in a state of dynamic equilbrium) and so management techiniques are based on guess work and trialed in labs. Some people, though, think that management of the Mississippi has made the floods worse......
    • 1993 = 3 months of torrential rain -----> defences were not designed for the such large size of flood that occured and the local people chose not to pay for the levees to be heightened. The levees failed. However, many think that if the levees didn't fail then the flooding would have been worse as they are believed to constrict water movement, block up the channel and increase pressure.
    • Floodplain development has raised the flood risk as concrete increases surface runoff by reducing infiltration. Also the removal of vegetation reduces the interception store and, because there is nothing to trap the sediment, can raise the level of the river bank. Drains etc, which are designed to imitate the natural processes like throughflow,are a lot more efficient and so the water enters the channel quicker. Therefore scientists conclude that floodplains should not be built on as they are a natural flood defence that is supposed to flood.
SOFT Engineering in the River Rhine
  • The high flooding of the River Rhine in 1993 and 1995 , in combination with the growing awareness of global climate change, made the public and respective authorities realise that constantly raising the height of levees and dykes, for example, is neither economically or environementally sustainable and that, instead, it is more appropriate to allow the river more room so that it can deal with a higher discharge at a lower water level. This reflects a new philosophy that we should adapt to the shape and behaviour of the river basins nto alter them to suit us. This has been approached by:
    • Landuse change and relocation of habitats - not allowing building developments to be constructed on flood plains as they are supposed to flood
    • Floodplain land use zoning - land is being zoned for uses that will not be damaged by winter floods like forests and parks etc.
    • Afforestation - the planting of trees has increased the interception store, prevented the net movement of sediment and so reduced the amount of water and sediment reaching the river
    • Room for the River scheme which includes:-
      • An increase in water meadows which can be allowed to flood when necessary. The sealing of the soil surface with tarmac or concrete in vulnerable areas is being limited to slow the water run off into the rivers
      • Ground coverage of vegetation with woodlands and grasslands is being increased
      • The use of fertilisers on soil is being carefully monitored because these affect the soil structure and its ability to retain water
      • To allow more space for trees on the floodplain, metres of silt accumulated over many years has been stripped and deep trenches constructed
  • All of these soft engineering methods have increased the time taken for water to enter the channel, reduced the amount of water that does enter the channel, created a channel that has a larger cross section and so can accomodate a larger volume of water and moved people away from the most vulnerable areas - remeber that disasters, like a flood, only occur when people come in close contact with a risk!
  • Some hard management options are still being used though like the building of flood relief channels to siphon off the Rhine flood water when the delta becomes overloaded, making the course of the river straighter and shorter and increasing the height of some of the levees.
SOFT Engineering in the River Quaggy
  • The River Quaggy runs through southeast London and since the 1960's it has been heavily managed by building artifical channels and culverts to divert the flow beneath the surface as it passed through Greenwich.
  • The areas of Lewisham and Greenwich have become more densely populated and the flood risk has increased, due to the continued development, and so more is needed to be done to protect the surroundign area. Further widening and deepening of the channel were considered but instead teh Environment Agency decided a softer option was most appropriate. A solution was proposed by the local residents, who formed the Quaggy Waterways Action Group, that would improve the local environment whilst also provided protection against floods.
  • The plan was to bring the river back above ground once again , cutting a new channel through Sutcliffe Park, and creating a new multi-functional open space. This method improves both the flood management and quality of the park. A culvert did remain to take soem of the excess water, during times of flood, underground but a new lake was built to allow the are to deal with the majority of the excess water when the river floods.
  • The park itself was lowered and shaped to create a new floodplain where watercould naturally collect, instead of rushing downstream through the previous artifical channels to flood Lewishantown centre. The parks flood storage capacity is equivalent to 35 Olympic swimming pools, has reduced the risk of flooding for 600 homes and businesses in the local area and created a diverse environment for wildlife
  • By reducing the river to a more natural course and including a flood storage area, the scheme has created a wetland environment with reedbeds, wildflower meadows and trees. This scheme won the Natural Environment category in the 2007 Waterways Renaissance Awards and the Living Wetland Award.
This is the remainder of the river case studies we need to know for the exam. I don't know if any of you feel the same, but after going through the mock yesterday, I realised that it is so much easier to do well if you know the case studies really well. So next I think I will go through the population case studies as there are rather a lot of them - the population policies and migration case studies are already on here........

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