What?
6 new bungalows
Where?
Walsden, Todmorden
When
Completed March 2019
Design and implementation
For the construction of the access roads, gardens and surface water collection for the 6 new dwellings, a sustainable drainage system was required by the statutory authorities and indeed requested by the client.
An attenuation system with a slow release mechanism was the only viable solution to this complicated site. A series of oversized drainage chambers providing 2 cubic meters, an attenuation below the parking surface providing 75 cubic meters and a vortex flow control unit were used to achieve an acceptable result. Permeable tarmac and pavement surfaces were also implemented to avoid excessively loading the drainage system during periods of heavy rainfall.
Benefits and constraints
The site was fringed by a small stream (or torrent during heavy rains), a former canal basin and evidence of historic attempts to improve the stability of this seemingly flood prone development site. A small combined sewer was located near the site and was the only feasible route for the foul drainage and surface water drains, however Yorkshire Water wanted to restrict the flow into their drain, to limit the potential issues that may cause downstream in Todmorden and Hebden Bridge, which have both experienced flooding in the last months and years.
Further opportunities
The site was strewn with glacial deposits and areas of peat, sand and clay. The bearing capacity of the soil was poor and needed improving. We needed to adopt an holistic approach to combating the poor soil and the drainage limitations. In consultation with drainage engineers and highways consultants, we developed a system that would allow the containment of surface water to be released at a slow rate of 2.5l / second, and one which would stabilise the parking and pedestrian areas.
Costs
It is important to note that without these SuDS measures, the site could not have been developed. We had explored alternatives for the ground and surface water prior to arriving on the SUDs solution, including discharging the water into the canal. This would have been a very expensive and time consuming option. We were told by the Canal and Rivers Trust that to get the relevant permissions would take at least 18 months and the likely cost of the work would be £30k plus legal expenses – i.e. the SuDS system saved both time and money.
Figures identified associated with the costs of the attenuation system and the vortex flow control were approximately £9k. In addition, the system adopted to stabilise the road and increase attenuation capacity was approximately £3k. There were additional costs for the consultants and load bearing tests.
The client was CVCLT along with John Eastwood Homes. Storah Architecture as lead consultant, David Hare Construction were the main contractors. Mason Clark Associates were the lead drainage consultant and Tensar advised and supplied the ground stabilising materials.