Glenbrook River Flood Alleviation Scheme

Ariel view of Lowry, Belfast
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Case study snippet

East Belfast has experienced a number of severe flood events over recent years with approximately 285 homes at risk of flooding from the Glenbrook River. Amey was appointed to review proposals for new flood defences, develop the design, complete procurement and manage construction.

Given the urban location, Amey designed a solution that minimised disruption to residents. The restricted nature of the site and associated construction restraints called for careful consideration and the final design comprised both precast and cast insitu sections to aid the speed of construction but also allowed flexibility for the contractor on site.

The challenge

DFI Rivers appointed Amey to review and update a proposed flood defence design, procure the construction contractor and manage construction. The proposed scheme comprised the realignment of the existing culvert and the construction of new flood defences where the height of the existing watercourse bank is inadequate.

On receiving this brief, Amey appointed Waterco as a sub-consultant to bring in complementary expertise and undertake a joint review of the existing hydraulic model. The model needed to be updated to revise the design level for a Q100 flood level, or 1% Annual Exceedance Probability event (which is a flood event that has a 1% chance of the event happening every year for the next 100 years). In addition, 20% was added to the flows to account for climate change (as per the DfI document “Technical Flood Risk Guidance in relation to Allowances for Climate Change in Northern Ireland”, published in Feb 2019) and an allowance of 600mm freeboard was included, as per DfI guidance. This secured the design against future climate change impacts.

The scheme’s location within the predominantly urban area of East Belfast and the restricted nature of the site meant that emphasis was placed on developing a constructible design and engaging with residents at an early stage.

Our approach

Amey’s solution included the replacement of two existing 900mm culverts with concrete box culverts (1.2m high x 2.2m wide). An existing open concrete trapezoidal channel would be replaced with a U channel (2m high x 2.2m wide). An L shape retaining wall (2.9m high) would extend from the end of the U channel to tie in with an existing culvert. The combined use of precast and cast in-situ sections aided the speed of construction and allowed flexibility onsite.

An extensive public consultation programme during the design stage secured agreement from local residents for a 2.5m wide working corridor on either side of the existing watercourse and a package of accommodation works drawings and schedules was provided to residents in the 65 properties directly affected.  

The scheme gained statutory approval from the Drainage Council in Winter 2019. Amey’s design team prepared a tender package for the procurement through an NEC 3 Option A contract. This contract was recommended to the Client, as it reduces the extent of risk for the client. The scheme was advertised for tender in September 2020. Construction began in March 2021 and was completed in July 2022. An Amey NEC Project Manager and Supervisor oversaw construction.


A holistic approach to the design was taken from project initiation, where there was a focus on resident engagement, constructability, site safety and reducing construction impact. The multi-disciplinary team were encouraged during collaborative meetings to explore ways to optimise the design with this key focus in mind. 

Resultantly incorporated into the design:

  • Review of anticipated dig depths. Proposed levels were kept similar to existing invert levels to minimise excavations. This reduced working from height risk and increased site safety. Secondly, it minimised temporary works and thus costs for the client.
  • Numerous design options were reviewed, including precasting bases and walls separately and precast L sections with a cast-insitu joint in the base. The final design considered weight v length of the section. This would enable the precast elements to be placed using smaller construction plant. The use of precast elements also realises carbon savings compared to cast-insitu. Using a combination of precast and insitu sections further aided the speed of delivery while also giving contractors flexibility on site.
  • Along with designing elements that required smaller plant, through designing the proposed route mainly online, the working corridor was able to be reduced to 5m. The utilisation of an online solution meant that construction took place within the existing channel, while the river continued to flow via temporary pipes and over-pumping. This minimised the impact and accommodation works required for local residents, thus aiding stakeholder buy-in.
  • The resultant levels and route were modelled to confirm the capacity and client brief were met.
  • Value was added when the contractor adopted many of the construction methodologies derived during the design stage. One example included the reduced use of over-pumping by installing a temporary 900mm pipe in the existing channel and surrounding it with stone to allow the water to flow below the works (temporary access roads) and thus only needing over-pumping at the excavation rather than from the source.

Upgrades to flood defences will give ongoing increased protection to approximately 285 homes. In addition, our consultative approach and commitment to community engagement kept residents informed and invested in our solution.

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