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Lamb Drove - SuDS residential scheme

Location

Lamb Drove is located within Cambourne, a new settlement 13km west of Cambridge (see Fig 1 and 2).


Fig 1: Location of Cambourne (click for link)

Description

Lamb Drove is a residential development of 35 affordable homes (built by Cambridge Housing Society) on a one hectare site. The aim of this unique and pioneering project was to showcase practical and innovative Sustainable Water Management Techniques (incorporating SuDS and property flood resiliance and resistence) within residential developments.

The project intended to demonstrate that SuDS are a viable and attractive alternative to more traditional forms of drainage and to deliver practical solutions for new housing areas.

Main SuDS used

A range of SuDS measures were used to demonstrate different available techniques and the application of ‘management train’ from ‘prevention’ to ‘site control’ and ‘regional control’ techniques. The measures include:

water butts;
permeable paving;
a green roof;
swales;
detention and wetland basins; and
a retention pond.

Permeable Paving – The paving within the adoptable roads and in some of the car parking areas is of permeable construction (see Fig 2 and 3). Gaps between the paving allow water to enter porous storage zones and to filter out pollutants.


Fig 2: Permeable Paving	Fig 3: Permeable Paving

Green Roof - A small demonstration green sedum roof is included to reduce and treat runoff (see Fig 4 and 5).


Fig 4: Green roof on bike shed	Fig 5: Green roof on bike shed

Swales – Most of the excess water from the site will be fed into a series of shallow open channels, further slowing the flow of water and continuing the water treatment process (see Fig 6 and 7).


Fig 6: Swales (artist impression)	Fig 7: Swales

Detention Basins – Sculpted depressions in open spaces help to slow down the runoff rate and store water on a temporary short-term basis during extreme events(see fig 8 and 9).


Fig 8: Detention basin	Fig 9: Detention basin

How it works

The concept of the management train was adopted for the site. The management train uses drainage techniques in series to improve the water quality and controls the quantity of runoff incrementally by reducing pollution, flow rates and volumes. It mimics as much as possible the natural pattern of drainage prior to development and uses a hierarchy of techniques within an integrated framework, to manage runoff as close as possible to the source.

Water management was considered from the point at which it falls on land and buildings to the point at which it leaves the site.

At Lamb Drove measures have been kept simple, natural and visible. The aim is to control the runoff starting as close as possible to its source. The use of source control features within the housing development areas manages most pollution and deals with the day-to-day runoff storage requirements.

Roofwater is collected in water butts, for use in gardens, or flows directly to grass swales or under-drained swales. Rain falling on roads or paths will pass through the permeable block paving, where it is filtered and stored in the permeable layer of crushed rock below, or flows into under-drained swales where water is treated and conveyed. Water then travels downstream through a series of swales, detention basins and wetlands till it reaches a final retention pond. Here it is stored before being released to a local drainage ditch outside the development site.

When the capacity of source control measures are exceeded the excess water is safely stored and treated in larger SuDS features integrated within public open space until the flood risk has passed. Such measures also contribute to the provision of green space, green infrastructure, visual amenity and promoting wildlife. Throughout the site, blockage free and simple flow control structures as well as overland flow paths were provided to cater for more extreme flood events.

Specific details

The driving force behind the project is the growing pressure for new housing in the UK. Cambridgeshire in particular is a key growth area under the government’s housing strategy, with up to 50,000 new houses planned to be built by 2016. It is also a relatively low-lying county where flooding in river valleys and urban watercourses is a major concern.

The project was part of a European funded programme (FLOWS), which featured 40 projects throughout Germany, the Netherlands, Norway, Sweden and the UK.

Design and construction

Once Lamb Drove had been chosen as the site for the project (in January 2004) it took just over two years to design and then install the system, with residents moving in during spring/early summer of 2006. Royal Haskoning were employed as drainage consultants and used their expert advice and guidance to help design the SuDS scheme at Lamb Drove.

Project time-scale constraints meant that the design principles and layout of the site were already set and the SuDS had to be built around it

Fig 10 illustrates the initial SuDS proposals developed for Lamb Drove. It clearly shows the range of SuDS measures (permeable paving, swales, detention basin etc) and demonstrates the principles of the management train and source control, detailed above.

The system and measures that were chosen were derived from the site topography and the pre-development drainage patterns, which ensure two key overland flood routes through the site.


	Fig 10: Initial SuDS proposal (courtesy Haskoning UK Ltd) Click on plan for detailed view

In order to provide a 1:100 year return period volume with an extra 20% allowance for probable climate change, it became apparent that the peripheral public open space would be required to achieve a reasonable hierarchy of storage. Therefore recreational space in the LAP area within the site and in adjacent greenway and golf course were designed to contain SuDS.

The use of public open spaces, including adjacent, off-site open spaces, provides a viable solution to the problem of accommodating softer landscaped SuDS within housing developments. This will become more of an issue due to the Government’s policy within PPS3 of higher density developments (minimum of 30 dwellings/ ha net)

Also in the early stages of the design, it was found that the full attenuation of runoff to a 1:100 year standard, with allowance for climate change, could be accommodated within the site boundary. However, this would have required the use of below ground storage techniques, which are costly and relatively more difficult to manage and maintain.


	Fig 11: Final design of SuDS system (courtesy Haskoning UK Ltd) Click on plan for detailed view

The adopted final drainage strategy (see Fig 11) shows a number of key changes to the system. This includes the addition of a further detention basin in order to reduce the discharge rate from the site. Undertaking the changes from the initial proposals to the final design had resource implications. Therefore the lessons learnt from this project include the need for confirmation from all the key stakeholders and organisations early in the process and well before the final design if signed-off.

Achievements

Amongst the many achievements of this project are the following:

Successfully incorporated and intergraded a comprehensive SuDS scheme within a residential environment, with positive reactions from both the residents and stakeholders.
Demonstrates practical, innovative and cost effective drainage techniques, in both relatively low and high-density developments.
Substantial improvement in the biodiversity, ecology and subsequent quality of life at Lamb Drove compared to typical residential developments.
The sculptured swales and detention basins have resulted in a visually enhanced and attractive landscape. This provides increased amenity and social value to residents and the local community.
The use of SuDS has resulted in an improved quality of water leaving the site compared with traditional piped drainage systems.
The estimated cost savings due to SuDS is approximately £11k. In addition, each house will have two water butts to collect rainfall from the roof, which can be used for watering gardens and other applications for which rainwater is suitable. Omission of the new storm sewer connection should give some financial benefits (approximately £30/year/household) to the residents as it avoids the annual payment of storm water disposal changes to the sewerage undertaker.
The project received a commendation award at the RTPI national planning awards 2006 and has been cited in the PPS25 Practice Guide

Lessons Learnt

Key lessons that have been learnt from undertaking this project include:

Many aspects of SuDS can be installed and maintained at a lower cost compared to more traditional forms of drainage. The most cost-effective SuDS measures are likely to be achieved through landform and other treatments within the soft landscaped areas, as the long-term management and maintenance of these measures can be effectively incorporated within landscape and wildlife management regimes.
Need to keep SuDS simple, as this will reduce maintenance costs and the likelihood of system being well maintained into the future. Also a key advantage of swales and other surface-based SuDS is that problems can be observed and dealt with as they arise.
In high density developments, SuDS are likely to rely more heavily on special treatments of hard areas (such as permeable paving or green roofs) with potentially higher costs and different maintenance requirements.
SuDS can be incorporated into any terrain as they do not necessarily depend upon infiltration into porous sub-strata. SuDS can equally well be incorporated into the surface landscaping of impervious ground such as clay similar to Lamb Drove.
The full attenuation of the 1 in 100 year storm within the development site boundary is both costly and difficult to achieve without the use of multi-use open spaces (e.g. public open space).
SuDS should be implemented and integrated from the beginning of the design phase as part of a holistic approach to sustainable development. This should be at Master Planning stage and preferably as part of the Development Plan.
High quality specification and quality control in the construction phase is important to reduce whole life costs and future problems.
SuDS calls for highly integrated project management and structured communications. It is vital that there is close engagement and cooperation with all parties. Clear performance standards/criteria should be set from the start, and everyone should be aware and sign up to them. The design brief should be clear, precise and simple.
Input of specialist SuDS drainage expertise was effective from the FLOWS team but there needs to be wider understanding and expertise amongst other stakeholders.

The key lessons from this project have been gathered together into a Best Practice Guide. This can be downloaded from the FLOWS website (www.flows.nu). A CD of the document is also available by contacting Cambridgeshire County Council.

Other Issues

Maintenance and Adoption - “Softer” measures (e.g. swales and detention basins) will be undertaken by Cambridge Housing Society (general site care) and the Cambridge Wildlife Trust (off-site care). This involves a monthly site visit to undertake grass cutting, litter picking and inlet/outlet inspections. The permeable paving has been adopted by Cambridgeshire County Council (with an agreed commuted sum payment) and maintenance includes biannual suction sweeping.

Safety – Initially there were concerns from some parties due to the perceived notion that there would be lots of standing water within the system. However, education about the true nature of SuDS (e.g. that there would only be a small amount of water in them and that this would only be in heavy rainfall events), together with ROSPA (Royal Society for the Prevention of Accidents) undertaking a very successful and transparent safety audit, reassured stakeholders of the lack of risk.


	Fig 12: Display Board on site

Education – In order that residents and wider community are aware of the SuDS measures at Lamb Drove they have all been presented with information on the system. In addition a site interpretation board has also been erected on site (see fig 12).

Monitoring – Cambridgeshire County Council have secured funding for the ongoing monitoring of the system in order to demonstrate the long term impact of SuDS on both the quantity and quality of water leaving the site as well as environmental and social implications. Royal Haskoning is currently preparing monitoring proposals.

For further information on the project see the Cambridgeshire County Council website

The information on the Lamb Grove project was provided by:
Wendy Hague, Cambridgeshire County Council, Box No: RES1507, Room C013 Castle Court, Shire Hall, Castle Hill, Cambridge, CB3 0AP

Fola Ogunyoye, Haskoning UK Ltd, Rightwell House, Bretton Centre, Peterborough, Cambridgeshire, PE3 8DW

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