The setting is a quiet residential road in Nottingham, UK, consisting of 67 properties. A grass verge with occasional mature trees runs the entire length of the road. There is limited parking pressure with most homes having provision for off-street parking. The road is adjacent to the Day Brook, a heavily modified watercourse which has poor water quality due, in part, to numerous sources of urban diffuse pollution. In addition there are a total of 972 properties which fall within the Day Brook floodplain. Previous fluvial events have led to property flooding downstream.

The scheme was developed with the vision to encourage and support change locally in dealing with issues around the water environment. The pilot project aimed at achieving a deliverable project that could be assessed over time and draw together statutory authorities to learn about Sustainable Drainage Systems (SuDS) and evoke change. The scheme was to be designed to achieve the following objectives:

  • Document and evaluate the design and construction of a series of rain gardens within an existing highway setting.
  • Maximise surface water interception, attenuation and infiltration.
  • Test the effectiveness of rain gardens in managing surface water from the public highway.
  • Encourage participation from local residents in the design and future management of the rain gardens.
  • Evaluate the effectiveness of the scheme as an engagement tool around the sources of urban diffuse pollution and flood risk.
  • Highlight the role that retrofit SuDS can play in improving the quality and reducing the volume of surface water flowing to urban watercourses.

The scheme required the collaboration of a number of partners and stakeholders over a tight timescale in a location where services, mature trees and highways constraints were all important. Partner contributions included:

Groundwork: Developed the outline and detailed designs, helped secure support for the scheme from City councillors, worked with the Highway Design team, managed contract negotiation and implementation of the scheme, led community consultation and facilitated the residents’ liaison group.

Nottingham City Council: Assisted with the design and technical development of the scheme, undertook a safety audit of the rain garden design, oversaw rain garden construction as Highway Authority and provide ongoing maintenance of the rain gardens.

The Environment Agency: Provided the capital funding through the Midland’s MURCI Waters programme, provided technical guidance on water quality and diffuse pollution and is leading the ongoing evaluation.

Severn Trent Water: Built a surface water hydraulic model of the scheme and are assisting with ongoing evaluation.

Varying support for the scheme amongst residents and general lack of understanding of how surface water contributes to flooding and poor water quality and safety concerns were important considerations during the design and stakeholder engagement process.

A total of 21 linear rain gardens (total of 148m2) were constructed within the grass verge, allowing for the constraints of access, below ground services, street furniture and trees. They were designed to capture runoff from 5,500m2 of highway from a total surface area of 7,100m2. The remaining surface area could not be incorporated into the scheme due to a number of mature trees clustered in one section of Ribblesdale Road.

The scheme was designed to manage surface water runoff from a 1:30 year storm event and to always intercept and treat the, often more polluted, first flush of highway runoff. Existing highway gullies have been retained to allow for overflow when the rain gardens reach capacity. Proprietary water attenuation cells were a key part of the initial design as they provide significantly higher void space capacity than clean stone. However, budget constrains meant that the use of proprietary cells was reduced and replaced by stone fill in a number of gardens.

The existing and predicted maintenance regimes were reviewed prior to construction. It was expected that maintenance of the rain gardens would be limited to an annual trim of the vegetation, with occasional mulching and clearing of the inlet. As the rain gardens were constructed within existing grass verges, the reduction of grass cutting was expected to off-set the cost of the new maintenance regime.

The scheme developed and delivered 21 retrofit rain gardens, through partnership, which was part of a vision to improve the management of water contributing towards improved water quality, flood risk and establishing improvements to green streets in the urban environment. The scheme also increased awareness of water quality and flood risk issues by partners and by members of the local community (the beneficiaries of the scheme).

The scheme has initially shown improved surface water capture and infiltration, leading to reduced pressure on downstream sewers and watercourses. The partners involved will work to promote the multiple benefits of retrofit rain gardens and use the results of the pilot, which will be evaluated further during 2014-15, to influence future surface water and fluvial flood defence schemes.

The use of a landscape-led multidisciplinary team resulted in increased understanding of the catchment-based approach to improving the water environment and demonstrated multiple benefits of collaborative working. The case study shows that retrofitting of SuDS in a highways setting with tight spatial constraints can be achieved in a cost-effective way, providing multiple benefits.

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