Castle Irwell

Castle Irwell is a visually stunning residential development in Salford’s Lower Broughton. Housing up to 500 new homes, the beautifully landscaped location provides a selection of high-quality two, three and four-bedroom residences a stone’s throw away from Manchester city centre.

The riverside setting, captivating green spaces and abundant natural splendour make Castle Irwell a tranquil oasis for those looking to escape the hustle and bustle of the city centre without losing the convenience of close proximity to it.

With conversations on how to build green space into our urban environments becoming more prevalent in the national conversation, Castle Irwell serves as a perfect example of how to do it viably and authentically.

Castle Irwell – The project specifics

Previously a student village for Salford University, the Greenfield site for Castle Irwell was acquired by our client Salboy and given planning for a residential development featuring up to 500 homes.

The construction was separated into three phases. With phase one complete, Phase two nearing completion and phase three about to go to planning, the project is well underway and is bringing a significant housing boost to the area of Lower Broughton.

Offering nine different house types, the site features beautifully modern townhouses arranged in a courtyard style. At the centre of the development, a large green public open space provides room for residents and visitors to enjoy nature.

The River Irwell weaves around the development, running flush against the site boundaries in two locations. While adding to the environmental allure of the location, the proximity to the river raised some engineering challenges to overcome.

The engineering challenge: Building on an active flood plain

The site of the Castle Irwell development is on an active floodplain. The results of this were laid bare in 2015 when the River Irwell bursts its banks during the Boxing Day floods. This rainfall event led to significant flooding across nearly 400 homes in the Lower Broughton area.

While the chances of the site flooding again are considered low, a workable engineering solution was required to ensure the development was safe.

The solution

We worked closely with the Environment Agency to establish the risks of the site and to understand the requirements we would need to meet to safeguard against them.

It was established early on that the finished floor level of every house would need to be at least 600mm above the 1-in-100-year climate change level and no more than 600mm below the 1-in-1000-year level. Not only that, but we had to ensure the presence of the development did not cause offsite flooding to the neighbouring buildings.

JBA Consulting – a specialist flood modeller – were instructed to produce a detailed flood model to enable us to see how flood waters would behave across the Castle Irwell site.

The results of this modelling allowed the flood flow routes across the site to be identified.

The plan was to allow the water to be channelled in the same way with the addition of the new houses. To achieve this, we needed to consider how adding structures and increasing site levels would decrease the volume capacity across the development.

Taking what we understood about the flooding, we developed our masterplan and JBA updated the flood model to reflect it. This gave a clear picture of how those changes would affect the behaviour of the water should flooding occur. Importantly, it it validated whether the FFLs of the dwellings met the Environment Agency’s requirements.

To combat the reduced volume capacity, we made use of the green space at the centre of the development. The majortiy of the time, the landscaping of the site will function purely as a beautiful green space for the community to enjoy. In the unlikely event of a flooding event, this landscaping will provide flood compensation, protecting the homes on site.

To provide the flood compensation volume in the public open space, we created depressions within them, allowing them to store flood water.

An additional layer of protection comes from the wetlands that are found at the perimeter of the development. These wetlands are around 1.7m deep but can provide additional storage above their designed maximum water level in the event of a flood.

We liased closely with the contractor and provided a digital 3D triangulation of the depressions and wetlands that was used by a semi-autonomous excavator to accurately form the features.
With the site’s flooding issues fully considered, it was possible to achieve all the Environment Agency’s requirements to mitigate flood risk.

The engineering challenge: Designing a drainage solution

Another engineering challenge of the site had to do with drainage. The site is largely flat and as we all know, drainage is governed by gravity. Matters were made slightly more complicated by the fact that the site was subject to restricted run-off rates because of its Greenfield status. This left us with a relatively small discharge area relative to the size of the area.

As with all matters of drainage, we tried to come up with a solution that was as high up the drainage solution hierarchy as possible. With that in mind, the most obvious solution was to direct the runoff to the River Irwell as it flows along site boundary.

The Solution

Our solution to the drainage issue was multifaceted and involved several methods coming together. To begin with, the primary runoff was to be delivered back into the Irwell via a discharge pipe and headwall placed on the river bank. Because of the greenfield runoff limitations, we had to come up with a way to keep this discharge rate within acceptable limits, which ultimately meant we had to also supplement this strategy with additional water storage options on site.

This was ultimately an exercise in slowing down the movement of water on siteand reducing the volume run off wherever possible.

The first thing to consider was the variable level of the river. If the river rose to a level above the discharge pipe outfall, this would ultimately hamper water flow, meaning water could back up on the Castle Irwell development. The alternative of raising the pipe was unfeasible as we had to position the pipe deep enough under the ground to meet minimum cover requirements and to allow a sufficient fall for the gravity system.

When we coupled the low greenfield runoff rate with the chance of the river constricting the discharge rate further, it became clear we had to design and implement some water storage options on site.
The question is how and where would we do this? It’s always preferable to adopt a sustainable drainage system (SUDS) but it was also important to deliver a solution that is viable for the developer.

While it would have been easy to stick a big tank in the middle of the development, it wasn’t the most efficient way. So we did what we do best and went to work designing for efficiency.

The wetlands that we created at the perimeter of the development were an obvious candidate to store excess water. Although they would already be holding water year-round, they could take on additional volume in the event of high rainfall. A pipe would carry runoff to the wetland areas, and if the levels of these got too high, the water could overtop the wetlands into one of the depressions in the landscape via a runoff pipe.

Although the wetlands were lined to enable a normal water level to be maintained, the infiltration rates within the rest of the public open space were good. This allows any water that enters the offline basin to dissipate overtime and therefore reduces the overlay run off volume.

Further surface water attenuation has been provided within the courtyards of each of the housing plots. A number of the plots utilised permeable paving systems that allowed water to seep between the grit filled gaps between blocks and into the coarse graded aggregate sub base below that features a 30% void ratio to store the runoff. Each of the permeable paving systems has a discharge pipe to the primary network with the pass forward flow suitably restricted to store water at the head of the network which helped to alleviate any flood risk downstream due to the surcharged outfall. Dependent on the ground conditions the permeable paving systems were detailed to suit by allowing infiltration when possible which helped to reduce the overall runoff volume.

The Client’s view

“The Castle Irwell development will bring high-quality homes at a reasonable price to Salford’s Lower Broughton. Our vision has always been for the site to deliver a green and tranquil residential space while retaining the convenience of being close to the city.

Renaissance’s expertise allowed us to realise that vision while fully addressing the challenges of the site”

Nick Russell, Salboy

Client: Salboy
Location: Salford
Value: £120m