The site wide drainage strategy carried out by ourselves identified thirteen separate surface water networks. Each network would be designed to mimic the existing, naturally occurring, catchments as far as was practically possible. Each network would allow attenuation in the form of multiple basins and flow restrictions prior to connecting to one of the existing water courses.
Foul water was identified as needing pumping stations to drain each of the development phases due to the site levels and locality of existing public sewers capable of receiving the estimated flows.
The detailed design of the phase 1 infrastructure, located centrally within the development, has been progressed further to the above-mentioned overall drainage strategy, this comprised of the following:
Surface water drainage designs; Phase 1 contained 5 separate surface water networks and 1 foul water network, the largest of which is network 7 and is the focus of the below notes.
This network was designed to serve the main spine road, providing access to phase 1, along with the school and care cells SC1 and Cell K, and 9 residential cells A, B, CH, D, E, G, H, I and J.
- The drainage designs were developed whilst working closely with the landscape architects to ensure that existing trees and hedgerows were maintained whilst ensuring that the landscape planting strategy within the SuDS features were designed to provide the maximum biodiversity possible.
- Close dialogue with the LLFA and NWL was paramount in the development of the drainage strategy and detailed designs. Multiple meetings were held right from the concept stage and through the detailed design stages to ensure the most sustainable and functional designs were progressed and allow approval through the planning process and Section 104 technical submission.
- Through the work with the other disciplines described above it was concluded that not every cell would drain to its own individual basin. In these cases the cells were drained to strategically placed basins that would serve a number of cells and spine road areas.
- Through the processes described above it was identified that seven basins would be utilised to accommodate the volumes associated with the runoff from phase one.
- Dry detention basins were chosen over ponds or wetlands due to the proximity of Durham Tees Valley Airport and the risk of gathering birds and subsequent risk of bird strikes. To increase biodiversity as much as possible, whilst avoiding this risk, small ponds/micropools, were detailed into the design of each basin. With varying depths of water owed to undulating pond floors, the shallow pools were planted suitably, maximising their biodiversity potential whilst deterring flocking birds.
The foul water drainage designs needed carful consideration in relation to phasing/build programme and adoption of the system by NWL. The following notes give a brief outline of how designs developed.
- The existing topography and the site layout were closely analysed together to establish the most economical route for the carrier pipe network and pumping station itself.
- Due to the size of the phase close collaboration with NWL was essential from concept through to the more detailed designs. Multiple meetings with NWL helped to develop the initial concept, taking on board certain comments and requests from NWL and securing the final designs.
