Introduction

From April 2020, water and sewerage companies operating in England have begun to offer the adoption of surface water sewers, including certain SuDS features. This is put forward and agreed within the new ‘Design and Construction Guidance’ (DCG).

This is a step change from the previous regulations ‘Sewers for Adoption 7’ in which mostly belowground pipe and tanked sewer systems were previously adopted by water companies. Recent changes in guidance and approaches in how we look to manage surface water now look towards the four pillars of SuDS aiming to manage and improve: water quality; water quantity; amenity and biodiversity (The SuDS Manual C753, CIRIA). Obviously, below ground pipe and tank systems offer little benefit in improving and offering amenity and biodiversity, so a new approach to the guidance was developed in the DCG.

What is the Design and Construction Guidance (DCG)?

The DCG is the next iteration of the ‘Sewers for Adoption’ guidance to meet current best practice for the management of foul and surface water. The DCG is for “use by developers when planning, designing and constructing foul and surface water drainage systems intended for adoption under an agreement made in accordance with Section 104 of the Water Industry Act 1991”. The guidance is, SuDS features within England are potentially adoptable under the DCG if the component:

a) is constructed for the drainage of buildings and yards appurtenant to buildings;
b) it has a channel (a depression between banks or ridges with a definite boundary);
c) it conveys and returns flows to a sewer or to a surface water body or to groundwater; and
d) it has an effective point of discharge, which must have lawful authority to discharge into a watercourse or other water body or onto or into land. As with conventional piped systems, this right to discharge must be secured by the developer and transferred to the sewerage company on adoption.

What is Silva Cell?

The DeepRoot® Silva Cell® is a patented, modular, suspended pavement system that integrates water, soil and tree roots and delivers on-site stormwater management through bioretention. DeepRoot’s Silva Cell system can be designed for surface water management by constructing a bioretention system within the Silva Cell modules. A typical bioretention system consists of:

·       A ponding layer to store surface water runoff prior to treatment through the treatment media

·       Treatment media which filters the surface water and removes solids, organic and non-organic particles

·       Aggregate storage to convey the treated flow to the outlet or stored before infiltration to the surrounding soil

·       An overflow structure to prevent surface flooding during exceedance conditions

How does it meet the criteria?

The Silva Cell® system can be designed as a bioretention system for the collection, storage and treatment of surface water runoff. This is indicated within the DCG document in section C7.5.1 expanding the narrative around the bioretention system “in its simplest forms, it can be a tree pit”.

For the Silva Cell to be considered for adoption by a water ad sewerage company, please ensure the following criteria are met:

1. In addition to the items below, also refer to specific water company guidance.

2. It drains buildings and yards appurtenant to buildings (avoid collecting drainage from highway, land or groundwater)

3. It has a well defined boundary (this will be defined as the outer edge of the Silva Cells and tree pit)

4. The outflow is to a sewer, surface water body or groundwater

5. Discuss the use of permeable pavements as final cover as these are not included for adoption (permeable pavement will not be adopted but top geomembrane on Silva Cell could be a defined boundary)

6. Future maintenance costs are accounted for (Maintenance of the system is 0.5 – 1% per annum of capital cost)

7. Final option for discharge should be to an existing sewer. (if discharging to a sewer, include a non-return valve to prevent surcharging back into the Silva Cell)

8. Access to the Silva Cell is not required for maintenance, but inlet gullies used for sediment control should be accessible for cleaning.

9. Detailed design should be completed in line with Chapter 18 of the SuDS Manual (SuDS Manual Assessment Checklist (Table B.15) should be submitted with application)

10. System should be designed to draindown within 24 hours.

Data from DeepRoot

DeepRoot have created a Stormwater Design Tool which can be used to estimate the size of your Silva Cell system. It can also be used to understand the number of Silva Cell modules required for the area of bioretention required.

Need further information?

For additional support from DeepRoot, please refer to our resources section for more information on the Silva Cell or contact [email protected].

Link to Design and Construction Guidance:
https://www.water.org.uk/wp-content/uploads/2020/03/SSG-App-C-Des-Con-Guide-v-2-100320-C.pdf
Link to The SuDS Manual and other helpful documentation and resources: https://www.susdrain.org/resources/SuDS_Manual.html