If you didn’t manage to attend our webinar about the Codes for Adoption you can either view a recording by clicking here or read this in-depth article by Michael Micklethwaite from our Civils team.
So… Codes for Adoption, we should all know this document… right? As of the 1st of April 2020, the document became mandatory so of course we are all using it… No?! Well let me introduce you to this document.
The code itself is quite vast and covers all the legal covenants required by the water companies, but right now I want to focus on just one part of this vast document and this is Appendix C of the document known as the ‘Design and Construction Guidance’. This is the document that now replaces Sewers for Adoption 7th edition, so I suppose we could call this Sewers for Adoption 8th edition, even though it is not called Sewers for Adoption 8th edition and I am guessing that is for a specific reason, that being Sewers for Adoption was guidance, this document however is mandatory so I think OFWAT wanted to make a clear step change with this document, but as already mentioned and I just wanted to reiterate, this document is now mandatory as of the 1st April 2020.
If you have not found this document then it can be found here… https://www.water.org.uk/sewerage-sector-guidance-approved-documents/
Currently we have been using Sewers for adoption 6th and 7th edition, some water authorities preferring one edition over the other and some authorities liking bits from both, and for any current
schemes or any scheme submitted for planning, prior to the 1st April 2020 then those standards will still apply, however for new schemes moving forwards, then the new Design and Construction
Guidance should be employed.
Over the years we have seen lots of changes across all the water companies and particularly with surface water drainage and management, as the risks and awareness of flooding increases, then we are constantly seeking new methods of sustainable drainage therefore, I feel there was a need for the Sewers for Adoption guidance to evolve and embrace a number of these new concepts, of course we have had The SUDS Manual for a good few years now and it contains and awful lot of technical data on the different types of SUDS elements and the design and detailing of SUDS. This new Design and Construction guidance finally brings the two documents together.
Let’s take a deeper look into The Design and Construction Guidance document itself and without going through every page I will highlight some of the more interesting changes and additions to this document, from its predecessor.
First of all, the document is broken down into six clear chapters which are as follows:
1. PART A – GENERAL
2. PART B – DESIGN AND CONSTRUCTION OF NEW FOUL SEWERS AND LATERAL DRAINS
3. PART C – DESIGN AND CONSTRUCTION OF NEW SURFACE WATER DRAINAGE SYSTEMS
4. PART D – PUMPING STATIONS
5. PART E – CIVIL ENGINEERING SPECIFICATIONS
6. PART F – MECHANICAL AND ELECTRICAL SPECIFICATION FOR SMALL PUMPING STATIONS
As we delve into Part A of the document I just want to take just one step backwards first and discuss the normal process when planning the surface water strategy and the best location for discharging the surface water, in this regard we have the SUDS hierarchy which is as follows:
BACK TO SOURCE (INFILTRATION)
SURFACE WATER BODY (RIVER / STREAM)
SURFACE WATER SEWER / HIGHWAY DRAIN
The above hierarchy has always steered us away from using public sewers in all cases meaning we have had to consider other methods of surface water disposal which would then have to remain private, due to not being classed as a ‘sewer’.
This is where I want to jump back into Part A of the document now and one of the main things that stands out, is the number of definitions that can be found here has increased. Definitions such as ‘freeboard’ and ‘effective discharge’ which are all terms directly relevant to SUDS features. In addition, it also explains the definition of a Section 104.
Section 104 of the Water Industry Act 1991 (WIA) only applies to the adoption of assets predominantly used for the drainage from buildings or paved areas belonging to buildings.
In the introductory section of the document it also tells us that…
Compliance with this document is mandatory except in areas identified as being permitted local practises.
But the more prevalent changes in the document is in the ‘adoption’ of ‘SUDS’ elements and how this is achieved, if we recall back to the SUDS hierarchy and the term ‘sewer’ we can now see in the new document that this terms has been upgraded. Let us now consider the next two statements from the document.
A successful drainage strategy should include considerations of future maintenance. Only certain types of SUDS components are sewers and are therefore adoptable.
And briefly jumping into Part C of the document which is relative to the Design of Surface Water components, we retrieve the following quote that together with the above define what is a ‘sewer’
and that even as SUDS features, they can be adopted.
A component is potentially adoptable as a sewer (or lateral drain) if all of the following apply (please note that this is a non-exhaustive list and not all structures that meet the following criteria will be adoptable):
1. It is constructed for the drainage of buildings and yards appurtenant to buildings;
2. It has a channel (a depression between banks or ridges with a definite boundary;
3. It conveys and returns flows to a sewer or to a surface water body or to groundwater, and
4. It has an effective point of discharge, which must have lawful authority to discharge into a watercourse or other water body 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.
So, this begs the big question! Just which SUDS will be adopted, and which ones will not be adopted?! Let’s start by taking the SUDS manual which has become more and more prevalent, mainly due to the local authorities referencing it and imposing parts of the manual, but now the Water Authorities have also to start using this guidance as it is now quoted in the Design and Construction Guidance. Therefore, the SUDS manual seems the best place to start. I have reviewed each SUDS element found in the manual and listed them out in a table below, even including the SUDS elements that would plainly not be adoptable, and then in accordance with the above definition of a sewer we will see which could qualify.
In the above table I have considered what was previously considered for adoption in the older Sewers for Adoption 6th and 7th editions, then we have the new Design and Construction Guidance (DCG). I have also included the Local Authority (LA) in there and then just for completeness I have put a ‘private’ column at the end to pick up the remainder, although as it can be seen from the table any one of them can remain private if we wish to maintain the asset privately.
Working down the table straight away we have two SUDS elements that would not be suitable for adoption, these are ‘rainwater harvesting units’ and ‘green roofs’ as they do not conform to that
definition of a ‘sewer’ and if anything they really form part of the structure of the building, especially green roofs, therefore these would not be suitable for adoption with a S104.
Then, next on the list we have ‘infiltration systems’ such as soakaways. Now soakaways are not new and have been used in new drainage systems wherever we can, but previously we would have designed the adoptable network up to the soakaway and the extent of adoption would end just before the soakaways. We can now include the soakaway itself in the extent of adoption and have them online in the adoptable system rather than adjacent to it. In the document however it does not specify as to what type of soakaway would be acceptable, rather it references the SUDS manual, so in regards to whether a precast concrete ring type of soakaway or cellular unit type of soakaway would be suitable for adoption is not clear and I can imagine that different water authorities are going to have their own preferences on this.
Then on the list we have proprietary treatment systems which could be for example vortex flow control devices such as an hydrobrake and these have always been there, and have always been
considered for adoption, so no real change here. However, during my research into this new document and my presentation of my findings, I have been asked on several occasion when it comes
to proprietary treatment systems whether this includes petrol interceptors which do fall into this category. However, unfortunately there is no mention of them within the Design and Construction
Guidance therefore I do not see these been accepted within a S104 adoption agreement.
Then we have filter strips which are defined as vegetated areas of gently sloping ground that do not have a clear defined channel or conveyance of surface water from one defined to point to another and therefore they do not adhere to that definition of a sewer and so conclusively we wouldn’t expect these to qualify for adoption under the new design and construction guidance.
However filter drains do qualify and this is because they have a clear defined channel excavated in the ground that can be lined and then filled with a permeable aggregate and could also have a pipe running along the bottom of the trench, the qualifying criteria being that they must be receiving domestic or roof drainage and therefore can be considered for adoption.
Furthermore, as per the filter drains, swales are also included, as they can be defined from one point to another and we can see a definite channel and therefore can be considered as a sewer under the new definition and therefore could be considered to be included in the adopted system.
Then we have somewhat of a surprise, but it is identified in the Design and Construction Guidance and this is Bioretention systems. A bioretention system could be for example a rain garden or advanced swales, but the key point being that they have to be receiving domestic or roof drainage and not solely highway drainage, this is one that will be interesting to see just exactly how these are used and developed as part of an adopted drainage system.
Then, following straight on from bioretention systems next on the list are trees, these are not in the Design and Construction guidance and therefore are not considered to be included in the S104
adopted system, this however refers to trees on their own not being considered for adoption but maybe if they are part of something else such as a bioretention system then they might form part of an adopted SUDS system. As with the bioretention systems, it is another one to watch and see how it develops.
The next one on the list is quite a common one which is permeable paving and does have defined channels and is a clear structure and can potentially act as an infiltration structure, but it is very much a highway first and it has been decided that this cannot be adopted by Water Authorities, so this would have to remain private or be adopted by the local highway department. This is actually happening now with some local highway authorities, where they have taken adoption of the permeable paving as highway so there are other options available, and some other positives in the adoption of SUDS elements.
The penultimate SUDS element in our table above, are attenuation tanks. Now these have always been included however it has varied around the different water companies which type of tank they would consider adopting. I don’t see this changing an awful lot in the short term but in the Design and Construction Guidance it does reference cellular storage tanks, therefore I would like to see that more water companies take on the adoption of these structures. However, I then think the stopping block on the use of cellular storage as an attenuation tank underneath an adopted highway, would then require approval from the highways department and it is common that they would not want to adopt a road with a cellular attenuation tank beneath it, but maybe that will also change in time.
Finally, I have grouped the last two SUDS elements together which are detention basins, ponds and wetlands which I believe will be the most popular change. Although it is not new in using these on a development site for the surface water attenuation, traditionally Water Companies would have stopped their extent of adoption just prior to the pond or would have requested the sewer have an overflow that runs around the pond so as to bypass it. We can now include the pond within the system and offer it all up for adoption as one system.
So just how could that look in practical terms and change how we layout and design our surface water drainage networks? So as just one example, we could have a piped system that would then discharge into a cellular attenuation tank which could be controlled with a vortex flow controller and then discharges through a swale then into a pond before outfalling into a soakaway, which I am sure has been designed somewhere. The important change here is that we can now offer this whole chain including each SUDS element between the pipework up for adoption with the Water Authorities as one, rather than having to pull the pond and swales offline and having them managed by a private management company.
In practice this should allow for the design of simpler and more efficient SUDS drainage systems.
The above is my reasonable interpretation of parts of the Design and Construction Guidance and by no means a definitive guide to the overall document. Please take time to review the Guidance for yourselves and then please consider just how will the future look?!
Just as a small epilogue to the final statement above, at the time of writing this paper on the 9th June 2020 we are obviously living in very strange times and the industry and the world has changed and somethings will have changed forever, this means that the chances are even if you have seen the above document you have not used it, I spoke with an acquaintance at one of the Water Authorities several weeks ago in regards to this document and they did had everything all ramping up for April to be trained on the use of the Design and Construction Guidance… as of this date that still has not happened so although this document brings some exciting new elements to drainage design and specifically adoption, it is possible you will not have used them, as yet.
Michael Micklethwaite, Director