Case study:Quaggy Flood Alleviation Scheme

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Location: 51° 27' 21", 0° 1' 41"
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Project overview

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Status Complete
Project web site
Themes Flood risk management
Country England
Main contact forename Toni
Main contact surname Scarr
Main contact user ID User:Ascarr
Contact organisation Environment Agency
Contact organisation web site http://www.environment-agency.gov.uk
Partner organisations London Borough of Lewisham, Quaggy Waterways Action Group
This is a parent project
encompassing the following
projects
Manor Park, Quaggy channel improvements, Sutcliffe Park
Overview

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.

Lessons learnt

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Image gallery


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

Catchment

River basin district Thames
River basin London

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
Ecoregion Great Britain
Dominant land cover Suburban
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

Name River Quaggy
WFD water body codes GB106039023290
WFD (national) typology Calcareous
WFD water body name Quaggy
Pre-project morphology
Reference morphology
Desired post project morphology
Heavily modified water body Yes
National/international site designation
Local/regional site designations
Protected species present No
Invasive species present Yes
Species of interest Japanese knotweed (Fallopia japonica), Himalayan balsam (Impatiens glandulifera )
Dominant hydrology
Dominant substrate
River corridor land use
Average bankfull channel width category
Average bankfull channel width (m)
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m3/s)
Average channel gradient category
Average channel gradient
Average unit stream power (W/m2)


Project background

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

Mitigation of a pressure
Hydromorphology
Biology
Physico-chemical
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Measures

Structural measures
Bank/bed modifications
Floodplain / River corridor
Planform / Channel pattern
Other
Non-structural measures
Management interventions
Social measures (incl. engagement)
Other


Monitoring

Hydromorphological quality elements

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

Biological quality elements

Element When monitored Type of monitoring Control site used Result
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Physico-chemical quality elements

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Any other monitoring, e.g. social, economic

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


Monitoring documents



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Supplementary Information

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