A drainage and flood management strategy was designed to minimise both the risk of flooding and the impact to the existing downstream sewer system. The strategy was devised following consultation with the Environment Agency and was based on an agreement on the proposed discharge rate from the development site, into the existing Thames Water’s combined sewer.
In order to minimise the impact to the existing downstream drainage network, the proposed surface water discharge rate has been reduced by 80% from that of the existing surface water runoff rates, including and up to 1 in 100 year storm events. To minimise the risk of flooding and to comply with Planning Policy Statement 25 (National Planning Policy Framework), an additional 20% allowance in the rainfall intensities due to climate change, was incorporated into the design.
Runoff from the building façade is collected via channel drainage and discharged into the below ground drainage network. Considering the unique angled face of the façade, the design of a linear channel surrounding the new building was especially challenging and was developed with a leading manufacturer in order to achieve a unique and bespoke design.
Below ground attenuation tanks have been designed and installed at four locations across the site with one soakaway tank within the site boundary, providing the required attenuation volume.
The attenuation tanks are made of light geocellular units with a honeycomb structure, providing a void ratio of 95% for attenuation of surface water within the tanks. Flow control device hydrobrakes were also designed to restrict the flow at agreed discharge rates.
The site’s industrial heritage, coupled with a myriad of underground services, made the installation of the below ground attenuation tanks difficult. Ramboll implemented solutions that included the uneven orientation of tanks and the bridging over of existing pipelines.