Introduction

A beautiful, biodiverse, productive and super-hard-working climate resilient landscape that puts the community and rainwater at centre stage.

The Problem

This cohousing development is located on a steeply sloping, north-west facing site that descends 11m to the Bourne Brook – a tributary of the River Colne that passes through the historic city of Colchester.  Colchester has a rich heritage of textile manufacturing and the development incorporates a historic mill and associated Grade 2 Listed mill pond that were built in 1845. This presented both significant topographic challenges as well as the importance of respecting and celebrating the historic fabric of the site.

A site masterplan layout of 23 houses had already been developed by the architects Anne Thorne Architects, and landscape architects Studio Engleback when Robert Bray Associates took over the design of the landscape and introduced integrated rainwater management.  As such one of our main challenges was to incorporate visible and nature-based surface water management into the existing layout in ways that brought delight, biodiversity, ease of maintenance, a special sense of place, and tangible value for the community as well as support the leisure and life activities that they wanted to carry out in the landscape, such as food growing and swimming.

The solution, philosophy, process and benefits

The solution embodies our ‘whole fabric’ approach to managing rainwater whereby we challenge all parts of buildings and landscape to manage and utilise rainfall in multiple, distributed, and diverse features rather than starting with a single point of collection as is typical in most development in the UK.  It also demonstrates our commitment to creative solutions to functional problems that are designed to deliver multiple co-benefits to people and wildlife but turning the water management approach on its head compared to conventional thinking.  ‘Drainage’ has, for centuries, been treated as a problem-solving exercise whereas we as landscape architects consider the rain than lands on our urban fabric as a resource and opportunity – an opportunity for more storm and drought resilient, climate adaptive, dynamic, verdant, health-giving, wildlife-friendly, and meaningful places. Working from the top down, following the flow of raindrops through the landscape:

– THE ROOFS:

Although the new terrace roofs were designed and planning-approved with a slight pitch, they had already assumed they were green roofs, with planting and substrate proposed by leading green roof exponent, Gary Grant.  We developed a method of converting this into a blue-green roof, capable of managing the 1 in 10 rainfall event on the roof where the rain lands, by utilising chevron baffles bonded to the roofing membrane and ultra-low-flow, flow controls.  These hold back rainfall behind each baffle, spreading it across the whole roof area, maximising volume losses through evapotranspiration, giving time for substrate and plants to hydrate, and slowly discharging residual rainfall to the rest of the landscape.  This solution took pressure off the rest of the landscape on this steeply sloping site by effectively managing a significant proportion of rainfall on the roofs, allowing us to keep subsequent features shallow, accessible, and comfortable in the landscape.

The roof planting is designed to be drought tolerant and wildlife-friendly utilising a range of native meadow species and habitat features such as logs and boulders.
Following our philosophy of avoidance of river-polluting misconnections, where new toilets, sinks, and appliances are connected to surface water pipes instead of foul sewer pipes, all rainwater pipes from the roofs are not allowed to enter the ground.  Instead, they end in a shoe at ground level that allows water flow to drop onto surface channels or rills that then carry those flows to the next part of the system.  This also helps keep water flow visible as much as possible making the system legible, and easy and intuitive to maintain by the community themselves.

– PATHS & VEHICULAR SURFACES:

Rain landing on path areas either falls into surface rills or channels that lead to raingardens or planted permeable surfaces or flow directly into the same.  Access road areas flow to meadow-planted swales that run alongside the road, or into the meadow-planted permeable median strip that runs the lengths of the road.  Parking and loading areas are formed in the same meadow-planted permeable paving using our own specification of structural and permeable growing and sub-base medium, allowing sufficient flows to pass through the paving, treating the cocktail of runoff pollution and passing into sub-base attenuation zones.  Trees planted adjacent to the permeable parking areas enjoy large-volume ‘Stockholm-style’ structural tree pits that extend beneath the parking, ensuring sufficient resources, hydration, and gas exchange for long-term health.

– RAINGARDENS & BASINS:

Most planted areas in the scheme deliver valuable water management functionality in the form of shallow, accessible, and easily maintained raingardens filled with beautiful, wildlife-supporting flowering herbaceous, ornamental grass, evergreen and specimen planting, as well as native and ornamental trees.  Within these are further innovative ultra-low-flow, flow controls developed by Robert Bray Associates that allow us distribute attenuation features throughout the development in multiple shallow features rather than one or two deeper and less usable features.  This is one of the keys to unlocking the multi-functionality potential of Sustainable drainage Systems (SuDS) approaches.  Again, these flow controls are designed to be protected from blockage and very easy to access and maintain by the community.

– SWIMMING MILL POND:

Most of the network of roofs, rills, raingardens, and permeable surfaces eventually drain to the historic mill pond in front of the old mill building that has been renovated as part of the development.  By insisting that all runoff passes through nature-based, pollution-busting features prior to it reaching the swimming pond, we can ensure that the water is sufficiently clean to allow swimming – one of the ambitions of the community.

We renovated the pond with a new puddled clay lining and filled the upper, narrower parts with native marginal planting, leaving an open swimming zone near to the buildings.  A simple, visible slot weir allows the water level to rise slightly in extreme rainfall events providing further flood mitigation without compromising the day-to-day functionality of the pond.

– CONNECTING TO THE BROOK:

Any rainfall that passes through the pond and final slot weir flow control drops into a channel feature inspired by historic features found on site.  This has features to reduce flow velocity before the flow passes into the brook at the surface without conventional headwalls that can disrupt the channel morphology.

– MAINTENANCE:

The whole system is designed on our principles of ‘Passive Maintenance’ whereby the system is visible and easy to understand, blockages or other issues are vigorously avoided through design but are visible and easy to diagnose if they do occur, and everything can be easily maintained using conventional landscape maintenance techniques and equipment; without the need for costly or specialist maintenance.

The Results

“The deck beside the pond is a favourite spot to socialise; there are also quieter spaces around the pond where we can sit and relax and enjoy the varied wildlife including moorhens, dragonflies, frogs and the occasional heron.

“Rain garden beds alongside Cannock Mill Rise not only look beautiful but play an important role in the Sustainable Drainage System to decrease surface water run-off on our sloping site.” Cannock Mill Cohousing.

“The RBA’s careful design we were able to save a considerable amount of money on the substantial drain runs and plastic storage tanks previously proposed by the engineers.  The Combination of planting, rills and outlets works extremely well both from a visual and practical point of view and the cohousing community is very happy with the result.” Anne Thorne – Architect and resident of Cannock Mill Cohousing.

Summary

·      Integrated, creative, and innovative nature-based solutions saved significant construction and maintenance risks and costs.
·      The system is significantly more resilient than typical engineered approaches.
·      Rainwater is treated as a resource and an expression of nature to be celebrated; naturally irrigating the landscape, following visible channels, spouting out of walls, supporting wildlife and topping up the swimming pond.
·      Pollution is naturally cleaned and filtered by the system protecting on-site wildlife and the natural world from hydrocarbons, microplastic, heavy metals and urban heat.
·      The system is able to manage extreme storms up to the 1 in 100 event + 30% Climate Change Allowance and was verified by a specialist hydraulic engineer, McCloy Consulting, due to down-stream sensitivity to flooding.
·      Maintenance is easily carried out by the community themselves.
·      Unlike many new developments that have mono-functional SuDS ponds that are a hazard to people and wildlife, no part of this landscape is ‘taken up’ by rainwater management.  Instead the whole system is designed with multi-functionality, beauty, placemaking, and biodiversity in mind creating an exemplar to demonstrate what can be done whilst saving money.

A creative, innovative, opportunity-focussed response to our climate and biodiversity emergency.

Architecture offices involved in the design: Anne Thorne Architects LLP

Location: Mill, Cannock, Old Heath Rd, Colchester CO2 8AA, Colchester, Essex, UK

Design year: 2019

Year Completed: 2019

Project Description: Cannock Mill Cohousing Climate Resilient Landscape

Client: Cannock Mill Cohousing Community
Location: Colchester, Essex, UK
Construction Cost: £6 million (whole development)

logo-landscape-forms

LILA 2024 Sponsor

Info