River Quaggy Case Study Disadvantages

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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

When the Romans built Londinium circa AD 50, they chose a good place to start. The land on the north side of the river latterly known as the Thames sloped upwards; that on its swampy southern bank did not. The threat of flooding seems not to have been lost on those imperial adventurers whom Boris Johnson has termed a “bunch of pushy Italians”. Today, the high ground we call the City of London would still stay mostly dry if the river burst its banks. The same cannot be said of much else of the capital lying close to its aquatic spine.

The Environment Agency’s “at risk” list includes the Houses of Parliament, Whitehall, City Hall, Canary Wharf, Westminster Abbey, the Tower of London, Kew Gardens, the O2 Arena, 51 railway stations, 35 Underground stations, eight power stations, more than 1,000 electricity substations, 400 schools, 16 hospitals and over half a million of Greater London’s roughly 3.3 million homes – not to mention 1.5 million of its people. Large areas of Southwark, Lambeth, Tower Hamlets, Hammersmith, Fulham, Wandsworth, Barking, Dagenham, Woolwich and Newham could find themselves under water, along with many settlements along the estuary in Essex and Kent.

Put it this way: Londoners have a lot to thank the Thames’s flood defences for. Chief among these is, of course, the Thames Barrier, the huge, vaguely sci-fi string of mini-piers and silver pods that spans the eastern gateway to the capital between New Charlton and Silvertown.

Last winter, from early December 2013 to the end of February last year, its steel gates were closed a record-shattering 50 times, preventing the river from running riot. Previously, the barrier had closed only 124 times since it began operating in 1982. The agency described this sharp increase in demand as a “blip” and, apart from routine testing, the barrier hasn’t been closed since. However, during its lifetime there’s been a strong, overall upward trend: it was closed four times in the 1980s, 35 times in the 90s, and 75 times in the 2000s. There have been 65 closures since 2010, suggesting this climb is continuing.

Is a watery end nigh? Not in the foreseeable future, says the agency – but the need to look far ahead was recognised early this century with a project entitled Thames Estuary 2100, established to manage flood risk through to 2100. A plan produced in November 2012 primarily addressed the biggest threat: surges in the twice-daily North Sea tides that drive water past Southend and Sheerness, right through the centre of London and out the other side as far as Teddington in its south-western suburbs.

… the greatest tide that ever was remembered in England to have been in this river, all Whitehall having been drowned

Samuel Pepys, 1663

The plan set out options for a whole new flood barrier, the most promising location (option 3.2) being further east, beyond London’s boundaries at Long Reach, Dartford. But it anticipates the present barrier continuing to do its job until 2070, four decades longer than originally thought, thanks to improved maintenance and revised predictions about sea levels.

The exceptional winter of 2013/14 did, though, prompt the agency to review its impacts on the barrier’s present and future operations. A report produced last August noted that the first closure of the period, completed during the first half hour of 6 December, had protected London from “the highest tide seen at Southend in over 30 years of operations”.

The other exceptional element of the 50 closures cluster was that the great majority – 41 of them – were prompted not by tidal surges from the east, but by prolonged heavy rainfall in the Thames catchment area to the west of the capital. The resulting increases in “fluvial flow” downstream created the second type of reason for closing the barrier – protecting vulnerable locations just upstream from Teddington, such as the village of Thames Ditton and the residential Trowlock Island.

These are put at risk when the pressure of the tidal Thames waters arriving from the other direction further boosts already high water levels. But that risk is reduced if, over on the opposite side of the city, the barrier is closed just after low tide. This effectively turns the London stretch of the river into a temporary reservoir with spare room into which the extra fluvial waters can freely pass. Once the tide has receded and levels on either side of the barrier match, the barrier is opened again, and the contents of the “reservoir” released to resume their journey towards the sea.

The impacts report said that while vital maintenance work on the barrier had been able to continue throughout the three demanding months, “if 50 closures was to become the norm this would start to impact on the reliability of the barrier.” It concluded that a repeat was unlikely, noting that solar and lunar conditions contributing to higher tides will not recur before 2032 “then not again until 3182”. But it also underlined the expectation that the barrier will need to be closed more often against tides, and consequntly will be able to come to the Teddington area’s aid less frequently. A new scheme to augment the locks and weirs there is being worked on.

Computer-aided design didn’t exist [when the barrier was built]. It was put together from 13,000 pen and ink drawings

Tidal defences manager Andy Batchelor

Localised flooding in London from Thames tributaries may be increasing as a risk in its own right, as experts told the London Assembly’s environment committee last January. It is estimated that 24,000 properties are under significant threat. Parts of Kingston, Barnet and Waltham Forest were top of the list. In response, the London Rivers Action Plan, supported by the mayor, has been restoring rivers previously constrained in artificial channels, giving them greater width and a safety margin for flooding into surrounding green space, improving habitats for plants and wildlife in the process. The river Quaggy, which passes through Lewisham, Greenwich and Bromley, is cited as a good example.

Exactly half of the Thames barrier’s 174 lifetime closures have been to help alleviate river flooding. But tides, the biggest worry, are its core business. Surges begin with bands of low atmospheric pressure originating in the Atlantic, which have the effect of raising sea levels. After passing north of Scotland, these high-water humps swing south down England’s east coast before funnelling into the Thames’s open mouth, sometimes heightened further by strong winds. “A surge tide entering the estuary can increase water levels by one to three metres,” the TE2100 plan explains.

The planners have put mitigating the effects of climate change at centre stage. They expect it to produce raised average sea levels, surge tide levels and wave heights in the coming decades, albeit by less than previously thought. Meanwhile, land levels in the south-east of England are falling – only by about 1.5mm a year, but over the course of a century it all adds up. In addition, development along the river’s banks is slowly invading its space, leaving less room within which its waters can expand.

The TE2100 plan sets out an action programme, already being put into effect, for building resilience into the floodplain environment. Setting new buildings back from the waterfront helps, as a buffer of green can double as an overspill facility. The space in front of the barrier’s visitor centre exemplifies the principle: when the barrier shuts out a high tide, the resulting build-up can put its picnic tables under water. The chancellor George Osborne allocated £196m for the Thames Estuary programme in December.

In the barrier’s operations centre, which looks out across that famous row of stainless steel-clad shells, tidal defences manager Andy Batchelor emphasises the integration of many elements is key to maintaining a safe level for the Thames. To the east they include a barrier at Barking Creek where the River Roding meets the Thames and, beyond the Greater London boundary, a floodgate at Tilbury Dock and the finely named Fobbing Horse barrier at Canvey Island.

Less conspicuous but just as vital to the system are the over 400 gates and smaller structures protecting homes, business and other buildings along the river’s length, and the more than 185 miles of embankments and walls. Hanging on the operations centre wall, a photograph shows the Queen’s visit there for at the barrier’s official opening in 1984.

As Batchelor observes, the power station look of the original control consoles is long gone. Today, the space is hushed and computerised, a monitoring hub for its own data and more from the Met Office and the UK National Tidegauge Network, enabling forecasts of dangerous conditions up to 36 hours in advance. Local flood warnings are issued too, enabling residents of places such as Richmond to anticipate water spilling across the towpath and up to their front gates.

“We’re modelling what is going to happen and what operations might be required to manage the water at different points along the river,” Batchelor explains. “If the level is going to be anything higher than 450mm from the top of the wall in the centre of London, then we will shut the barrier.”

Without it, London would get its feet wet and much worse. Recorded history of the Thames banks bursting goes back a long way. In December 1663 Samuel Pepys recorded “the greatest tide that was ever remembered in England” and “all White Hall having been drowned, of which there was great discourse”. The last major flood affecting central London occurred in January 1928, when defences in Chelsea and Victoria were overwhelmed at dead of night, with a section opposite the Tate Gallery in Millbank collapsing. Among the paintings damaged as a result were works by Turner, famous for his boiling seascapes.

Parliament and the Tower were swamped too, as were the then fairly new Blackwall and Rotherhithe tunnels and, most tragically, the basement homes of some of the capital’s poorest residents, largely in Southwark and on the Westminster side of Lambeth Bridge. Fourteen people drowned and thousands had to leave their homes. It occurred thanks to a perfect combination of storms, with a tidal surge meeting a doubling of inflow from the west caused by heavy rain and snow thawing in the Cotswolds, where the Thames has its source.

The catalyst for the barrier’s construction was the North Sea flood disaster of winter 1953, when a huge tide saw sea levels rise by more than 18 feet and overwhelm coastal defences in Scotland, Belgium and, most horrifically, the Netherlands where 1,836 lives were claimed.

In eastern England there were 307 fatalities, as large parts of Lincolnshire, Norfolk, Suffolk and Essex were washed over. Canvey Island’s sea walls collapsed, leading to 58 of those deaths, and a further 35 perished at Jaywick in Clacton. The waters coursed upstream through the then busy docks, swamping factories, gasworks and electricity generating stations from Tilbury all the way to the East End, where the Thames spilled on to the streets of West Ham and into 1,000 homes. The centre of London just about escaped, but there are photographs of people in Richmond marooned on towpath benches. There were no warning systems. Victims were caught completely unawares.

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