Stormwater Attenuation and Soakaway Tanks: Effective Solutions for Flood Prevention

It is to provide a facility so that the storm system is not overwhelmed. It is a facility to catch water and store it and will be released at a controlled rate, rather than the extra storm water just going down the pipe at one time. It is a way to hold, store and release water at a controlled rate to not overwhelm drainage systems.

There is no one answer, it depends on the size of the plant and the size of the load. It is not just the weight of the plant itself, it is the weight of the material they could be carrying. The amount of cover will vary depending on the size and loads of the vehicles. With tracked plant, it does tend to be slightly larger surface area in contact with the tank, which means a bigger boot space.

It can be reduced, probably an equivalent weight the load vehicle cover. You can reduce it slightly with track plant because of the larger surface area in contact with the tank. However, there isn’t one area we would do this. If you have a particular machine in mind you can send us the specifications, our technical team help and give you the cover levels or what spreader plates are needed. We can calculate all this for you.

Unlike a soakaway system, they can be much closer to a building. The main thing is that the building load or the foundation load should have no effect on the tank. If you were to draw a 45-degree angle from the foundation, that line should not cross your tank. This as well depends on the tank depths. But they can be close to the building.

Like any drainage, it will need maintenance. However, it is no different or more arduous than any drainage. The majority of drainage systems will require periodic flushing or some maintenance. Therefore, should form part of your regular maintenance plan.

There are two main types of branches attenuation: inspectable tanks and non-inspectable. For non-inspectable tanks, the maintenance usually involves a good silt trap before the tank.

This is so the maintenance would be emptying out the silt trap periodically and after any large storm event, which may form part of your facilities management plan. If it's an adoptable tank, it might form part of your water board's responsibility.

Inspectable tanks though are available for if you need that extra level of maintenance, if you expect it to be a heavy site, it is going to see a lot of silt and a lot of debris. The maintenance can be putting a jet periodically through the run of the tank and giving it a proper flush. Camera inspections are available for inspectable tanks.

Maintenance will be required for all stormwater tanks but the level of such will depend on the type of tank you have and who's looking after it as well. Whether it is been adopted by your water board, it tends to be a bit stricter than a private company looking after it.

The main tests which need to be carried out are carried out tests up to CIRIA guidelines. CIRIA who writes the SuDS manual. It is a guidance for sustainable drainage systems. We test in accordance with CIRIA guidelines by independent test facilities. We test short-term compressive strengths, long-term peak. Long-term peak is the amount that these tanks are affected over their life, this is to make sure that we design all of our tanks with an excess of 50-year design life. Long-term peak tests are carried out over about 10,000 hours, to make sure that we are confident in the results that are done by independent test facilities. Compression and peak are two of the main tests which most customers would have heard of. We test up to CIRIA guidelines and also have full BBA approval on the majority of the EcoBloc range, we are also testing up to BBA standards as well, and that's independently verified by the British Board of Agreement.

Void ratio is the volume of water that can be stored in the tank. There are two types of volume, gross volume, and net volume. If you think you have attenuation create the 1m x 1m x 1m. That is the area of volume of that space is 1m cubed, most attenuation creates for structural stability have something in the middle. For example with a couplet max system, you have an area inside the system that's going to be taking up plastic. That is essentially the area within that 1m cubed, as an example, which won't store water, because a percentage of that area will be taken up by the plastic. Void ratio is basically the percentage of empty space, therefore the percentage of water that can be held. Most crate systems are about 5% plastic inside, so for every 1m cubed of water, 1m cubed of space would hold 950 litres of water rather than 1000 litres of water. That is a 95% void ratio. Graf EcoBloc Maxx holds 96% void ratio, so we can hold 960 litres of water for every meter cubed of space. Our flex and light systems can go up to 97, depending on circumstances, which is a good void ratio. This can help mass plan the design of tanks. If it's a level 22 tank, that's not going to make a big difference, but if you have a tank which is designed at a 95% void ratio, which is 1000 cubed or 1500 cubed. Because of our improved void ratio, we can shave quite a bit. We can hold the same amount of water in less space, in less crates. Which is, of course, saving. But there are lots of positive elements in the higher void ratio. Void ratio is basically the volume of water that can be held. Gross volume is the area, and in terms of basic length, width, by height. Volume ratio is the actual amount of water that can be stored in that space.

If you design the tank with access, then you can. We have a system called EcoBloc Flex Inspect, which is an acute system which has channels down the middle to allow access. It can be combined with a Maxx system or it can be just EcoBloc Flex Inspect, which means that the tank is accessible. You could do this by installing integrated access chambers under the tank. These are called video shafts, which can be seen as manholes within the tank. Rather than being outside the tank, you have a mini-manhole inside the tank to instantly access these inspection channels and drainage channels. You can also buy external inspection chambers if you would rather have that option. Would like the more inspectable piece, but would rather access it either via one of the manholes that's already there or an external inspection manhole. This all means little extra covers, extra excavation and extra manholes at that one site. But it depends on the design.

They are not suitable for man entry. The covers and access chambers are not big enough to send people into the tank for maintenance, but they are big enough for JN CCTV inspection, for regular maintenance you would need for a stormwater tank.

This is something we see frequently in design and for drainage. Permeable paving and gravel above the tanks forms part of the in-water quality element in SuDS. When the water then filters through, rather than coming into the tank with your pipes, if water comes into the tank for your permeable paving and gravel layers, then this gravel will pick up potential pollutants. The water going into the tank is then cleaner and will be cleaner when it's discharged from the tanks.

It just means a slight change to the install, and means leaving the top of the attenuation tank. You just don't put an impermeable membrane on the top, you put permeable tanks down and you bring the membrane up to the surface. But it is something we see very often.

We don't do this as standard as we have inspectable crates that can run through as an alternative to this, but there might be some systems that need this gravel and pipe. For example, our crates are a lot more open so they don't get silted up, if you have some people that have crates that aren't as open, they might get a buildup of dirt coming in which can block them up so it defeats the object of them, they will have to have the pipe and stone so that the water is going in and dirt can't get in the rest of the tank. If they need to inspect the pipe, there are lots of different alternatives to using this. It is a method that some engineers are more familiar with, whereas we have been in the market for over 10 years but the inspection run is something that we still push rather than this option, because it is a lot easier to install.

Deflection is a short term deformation on crates when they are subjected to a vehicle loading on the ground above, which means every time a vehicle. A lorry or a car goes over the tank, the load that goes over the wheel load is a concentrated load on the tank. This can cause it to deflect (dip).

Where you get this is when we are looking at minimal cover.

A lot of the time the minimum cover will be, the minimum cover, because this is where it would fail.

It has to be below a certain figure. We produce calculations in accordance with something called CIRIA-E-680, which is a set of SuDS guidelines. You must keep it below a certain figure to make sure that the ground doesn't dip or defect, which is more of an issue with tarmac surfaces, hard surfaces, because if they start to dip and slope, it will cause issues. Whereas if it was a grassed area, it is not as much of an issue.

What we do is when we have the drawings, we would look at the cover level and make sure that they are getting the cover needed for the loading.

We have different crate options. Our go-to would be either our EcoBloc Maxx or our EcoBloc light, both of which are not inspectable. What we would then do is install our EcoBloc flex inspect, which you can inspect. It allows a different setup of the crate. You can either get a jet or a camera through. Our EcoBloc flex inspect, you can have internal inspection shafts, what we call Vario shafts, which is included in the volume. You can have internal chambers where you can get access into the system without having any need for an external manhole. This helps to reduce the amount of connections and debris into the tank.

This isn’t something we offer as standard. But can be done. The things that we would have to look at is if we have put on this alternative forward, or where the pipe is situated. When it comes to install, if it is in the middle, the pipe and stone runs along the middle of the tank and you have the pressure from the crates either side and the backfill. This keeps the pipe and stone in place when you are wrapping up the tank. Whereas, if it was to be situated towards the edge of the tank, that is when you have not got that pressure coming in from either side. There can be delays on the install if anything were to happen or if it is not keeping it in place. The best thing to do is send us over the drainage drawings and we can then have a look to see what we would put forward and what the best solution is. In short term, yes, it can be done.

You can have internal corners. It is something that we try to avoid. If we have drainage drawings, we will always try and look whether we can go deeper to avoid them. But they can be done. It just creates weaker points, if they can be avoided, then it's best to. However, with our heat weld engineers, they can still make sure that it's heat welded and watertight.

In terms of how flexible tanks are constructed using geocellular crates, you can have any design. Obviously, the regular shapes are a lot easier for the contractors to excavate, because there's nothing worse when you have different shaped tanks with lot of internal corners. We understand sometimes it has to be done, with all the new works that are going on, especially when they're in crowded areas, you have to avoid certain things and pile caps. When that happens, you don't have any option. We will always try and put forward the best design, but they have dimensions that you have to stick to, you cannot cut crates. We would always put forward an alternative to what's been specified. But, there is no limit, it's just whether we can make it a better solution or not.

The advantage of installing an underground storage tank for attenuation purposes is that it maximises land utilisation. Unlike ponds or swales, which take up valuable surface area, an underground tank allows for the use of space above it for essential amenities such as car parks, roadways, green spaces, and even parks. This is especially beneficial in densely populated areas where land is limited but development is necessary.

By placing the tank underground, the space is not lost or wasted. It remains available for various purposes while fulfilling the attenuation function. Underground tanks can be designed to be either light duty or heavy duty, depending on the specific requirements and location.

The alternative of using ponds or swales often results in empty spaces that serve their purpose only during storm events. They can appear as grassy holes or reed-filled areas, providing little value or wildlife habitat most of the time.

In many cases, people may not even realise that attenuation tanks are present beneath their back gardens or even in park spaces. These tanks can be seamlessly integrated into various locations, including roundabouts, without compromising valuable surface space.

The key advantage of underground storage tanks is the efficient use of available land, ensuring that no space is wasted and that development can proceed without sacrificing essential amenities.

When sizing our tanks, we don't specifically include a buffer volume like you would with a wastewater tank for accommodating extra visitors. It's a different process. Our engineers take various factors into account. They calculate the current runoff rate from the site, the amount of non-absorbable runoff, and consider the downstream system. They also consider Climate Change by applying a safety factor of around 30-50% to account for anticipated increased rainfall.

The tanks are designed to handle rare storm events, such as one in 30 or one in 50-year storms, and accommodate the projected changes in weather patterns. So while there may be some additional volume in the tank, it's not a buffer in the traditional sense. Our goal is to match the tank size as closely as possible to the drainage plan we receive, considering all these factors beforehand.

The purpose of flow control in an attenuation tank is to store water and prevent overwhelming storm sewers and drains. It slows down the flow of water exiting the tank, ensuring it is slower than the incoming flow. This gives the tank time to drain gradually and prevents flooding or backup issues downstream.

Flow control devices come in different types. For smaller tanks, a simple orifice plate with a smaller hole than the incoming pipe can be used. Larger tanks may utilize bespoke vortex flow controls designed to match the tank's dimensions and flow rates. These devices create a swirling motion to slow down the water, and their size is determined to match the capacity of the downstream system.

Overall, flow controls are crucial in regulating the water flow and ensuring it doesn't exceed the capacity of the drainage system.

The maintenance schedule for treatment plants can vary based on whether it's a domestic or commercial system. For domestic systems, we recommend that homeowners have a general understanding of how the system works and perform regular checks. It's also advisable to have a professional service provider inspect the system once a year to ensure it's working properly and the effluent quality meets standards.

Commercial systems, on the other hand, typically require ongoing maintenance and have designated personnel responsible for their care. These individuals should have a good understanding of the system, its operation, and troubleshooting procedures. Commercial treatment plants usually undergo maintenance twice a year, and if additional modules are present for phosphorus removal, maintenance may be required more frequently, potentially three to four times a year.

Maintenance also includes the desludging of the tank, which has a designated volume specified in the technical data sheets. The interval for desludging can range from 6 to 12 months, depending on factors such as the number of people served and the size of the treatment plant.

It's important to consider these maintenance requirements to ensure the optimal performance and longevity of the treatment plant.

A percolation test measures how well the soil drains and determines its suitability for a soakaway or an attenuation tank. The test involves digging a trial trench of specific dimensions and timing the water as it drains. The pit is filled to the expected tank level, and the time it takes for 25% and 75% of the water to disappear, as well as the emptying time, is recorded. The test is repeated three times for consistency, and the average of the results is taken.

For tank lengths exceeding 10 meters, the process is repeated at intervals to ensure consistent infiltration rates. Calculations are then done using the recorded values to determine the median value and the soakaway area. It involves working out volumes, surface areas, and timing calculations, which can be facilitated with provided documents.

The key is to conduct the test accurately, record measurements diligently, and consult with a qualified professional for further assistance in the calculations.

To size a stormwater attenuation or soakaway tank, we need specific information. It's not just about the volume needed; the dimensions are crucial. Length, width, and height are the key factors to determine the appropriate base plates, end plates, and ventilation services required. Providing a drainage drawing is the best option as it gives us a comprehensive understanding of the project. It allows us to calculate the necessary components and provide an accurate price.

Additionally, knowing the location of the tank is important. Whether it will be situated under a park, green space, driveways with light car loading, or more heavily trafficked areas like roads or car parks, helps us design the right plate system to meet your requirements.

If a drainage drawing is not available, providing the length, width, and height of the tank is crucial. This information allows us to propose a suitable solution based on the available space. However, if more detailed calculations are required, we may request additional information during the order stage.

In summary, the dimensions of the tank, along with the location and a drainage drawing if possible, are the key pieces of information we need to properly size and propose an appropriate stormwater attenuation or soakaway tank.

Stackable crates offer several benefits when installed. Firstly, they significantly reduce installation time as workers can quickly feed the crates into place. The system is designed to be user-friendly, making it easy to handle and install. Additionally, crate systems can be tailored to fit into unique spaces, including odd shapes or L-shaped configurations. This customisation ensures optimal use of available space. By using stackable crates, you can save time, energy, and reduce the carbon footprint associated with the installation process. The simplicity of the system, with its convenient 90-degree turn, makes it straightforward to use and eliminates the risk of errors. Overall, stackable crates provide an efficient and time-saving solution for stormwater management installations.

There can be a build-up of silt at the bottom of the tank when using blocks or crates, but it depends on the design and maintenance plan. A properly sized silt trap installed before the tank can catch most of the silt. Regular maintenance, including emptying the silt trap, is crucial. For adoptable tanks or situations with a high possibility of silt accumulation, an inspection or jetting channel can be designed into the tank. This allows for periodic high-powered jets to flush out the silt. The level of silt accumulation will vary based on the effectiveness of the maintenance plan and the quality of the silt trap. Some tank designs also incorporate inspection chambers or integrated shafts to facilitate easier maintenance. Overall, a well-designed maintenance plan and the inclusion of proper silt traps can help manage and reduce silt build-up in the tank over time.

To ensure a successful installation of the tank, there are specific requirements for the excavation. First, we need the excavation to be dug according to our provided drawings, with a 500-millimetre working border around the edges. This space allows our team to access the sides and install the tank effectively.

The excavation should be graded, stepped, or leveled to provide a safe entry and exit. It's crucial to prepare the bottom of the excavation with a layer of shingle or gravel, ideally 50 to 100 millimetre thick. The preferred material is pea gravel or pea shingle, around 10 millimetre in size. This material compacts well and doesn't contain excessive fines that could wash away or create voids under the tank.

If pea gravel is not available, alternative materials may be considered, but our technical team will need to review photos to determine their suitability. It's essential that the material is non-angular to prevent any damage to the tank's membrane or textile, as these tanks are designed to store water. Materials like sharp sand or 6F2 are not suitable for the base of the excavation.

In summary, providing the required working space, grading the excavation, and using the appropriate non-angular shingle or gravel will ensure a proper foundation for the tank installation.

The recommended material for the base and backfill of a stormwater tank is non-angular pea gravel or shingle. It's crucial that the material is non-angular and doesn't contain excessive fines or sharp objects. Typically, a size range between 8mm and 16mm is suitable, and non-angular options are commonly used.

The minimum amount of cover you can have on top of a tank is 250 millimeters, suitable for pedestrian-only access without any vehicle loading. The maximum amount of cover depends on the type of crates used. Light-duty crates require less cover, while heavy-duty crates can accommodate greater loads. The maximum recommended cover above the tank is 2.75 metres. For specific projects, site-specific calculations can be done to determine the optimal amount of cover based on loading requirements. The information regarding minimum and maximum cover will be provided in the spec sheets or can be obtained through site-specific calculations based on the drainage drawing.

The main tool we use for welding the tanks is called a wedge welder. It is used to overlap the membrane sections and seal them together. We also use a heat gun for sealing the plastic on adaptor plates. The wedge welder is a specialist equipment that provides superior welds compared to tape. It is the best tool for the job and ensures high-quality results, which is why we offer a guarantee on our welding.

When it comes to attaching side plates or end plates to an attenuation tank, it's a straightforward process. With the Max system, you don't need side plates for every crate. You simply build the tank to its final shape and then attach the side plates along the outer edges for stability and to prevent backfill pressure on the membrane. It's as simple as clipping them onto the tank.

For the Reflect system, which includes the eco block flex crates with two solid sides, you need to turn the crates as you go. For example, if a crate is on a corner, you turn it so that the solid sides align with the tank's edge, and then you add plates to the corners.

If you need further clarification or assistance, we are available for one-on-one discussions, phone calls, or video calls to guide you through the process.

Yes, construction vehicles can drive over the tank once it's installed, but it's important to ensure that the site provides sufficient cover to support the weight of heavy vehicles. This consideration is crucial, especially during the construction phase when vehicles may be driving over the tank. If there are cranes or heavy equipment involved, we need to assess the specifications and possibly use spreader plates to distribute the weight appropriately. Each case is unique, so we need specific information to provide the right guidance.

The proximity of an attenuation tank to a building depends on its depth, ground cover, and foundation design. Shallow tanks with minimal cover can be placed closer to buildings, while deeper tanks typically need to be located further away. The critical factor is that the tank should not bear any structural load from the building's foundations. To determine the suitable placement, imagine drawing a 45-degree line from both sides of the building's foundation. This line should not intersect with any part of the attenuation tank. In some cases, tanks can be directly beneath buildings, such as in parking structures where space is limited. However, careful design ensures that the tank does not affect the support columns or foundations. The specific placement will vary depending on the tank shape and building's foundation design.

It very much depends on the crate type. We have a rain bloc crate, which is a traditional one-piece crate, which can go as far as 7 meters underground, but it will depend on the soil type essentially. Heavier clay soils have more of a lateral loading and more loading on the tank, and so couldn't go as deep.

But in perfect conditions, a rain bloc crate can go as far as 7 meters underground. Graf EcoBloc Maxx, a maximum of 5 meters for the soils that we deal with. But again, quite often the actual loading, where we put in the figures and the cover labels and everything, it's more like somewhere between 1.5 - 2.5. It is very site specific. But the absolute maximum, I would say, for an EcoBloc range is about 5 meters underground. But often the surrounding conditions won't support that. Again, we have to do calculations for every site, which we will do, but that is your maximum labels.

Yes, they command what we would call the water quantity part of SuDS. SuDS has four elements; bioimmunity, biodiversity, water quality and water quantity. And so water quantity is where the SuDS element with attenuation tanks comes in just because essentially it's dealing with that quantity of water. An attenuation tank does not constitute a SuDS system, it's part of a SuDS system. A SuDS system is a entire holistic design to now protect them in the same biodiversity of the site and now for future generations. But an attenuation tank is a very important part of SuDS to cope with that water quantity.

I mean, it's really, there's a few components. The first thing you do is when you've excavated a nice, level flat base now, so the tank can sit nicely on the surface. You would start with a layer of geotextile. We'd use like 300 gram geotextile, which would protect the tank from any potential sharp stones or anything like that in the soil when it's back-filled.

Then you would put down a layer of impermeable geomembrane on the base, which is the black plastic liner which essentially makes the tank a tank. And then, that's when you start adding the crates. So the crates would be installed in the format of the design, which has been specified, has been approved from a contractor. So as the crates are in the required shape, essentially, it's almost like wrapping a big present. The black plastic is brought up the sides and a lid piece is put on, and then is welded shut, to create a sealed tank system.

Obviously, before that final seal, we'd add things like pipe connections in, so that water can enter in and go out the tanks, whereas kind of in stubs, black pipe stubs put on, so that the tank can be joined into a single pipework. Then once it's all been sealed up, once the black membrane has been sealed, usually using something like a wedge welder, which creates a nice double seam, tight double seam on the top of the tank, then we put a final layer over the top and right down the sides of that geotextile material. Again, to protect the top and sides also, from sharp stones and things like that.

Then it's back-filled essentially, so it's a very simple process, but hard to explain. One of those things you probably need to see.

It's really not recommended. No. I mean, I'd say, probably, the basic answer is no. But we'll never say never. If you had a big enough soakaway, and a clay with poor soil, well then theoretically, you could. But before any soakaway, a soakaway is always the preferable option with designers. Essentially, it's always looked at first, and there will be a percolation test done on the soil, to see whether it's practical.

But also, if you have to have a massive attenuation tank to soak away a small amount of water, then it's really not a practical solution, because the soil is just not permeable enough, to drain it. So an attenuation tank becomes the best solution, to essentially carry the water down, and dispose of it through other water courses, or through the storm sewer system.

Fairly simple, really that so these tanks should sit empty majority of the time, or at least not completely full. So, the air which is in the tank, half the time does fill up or there's a storm event if it's not filling up the entire tank and the air which is contained within the structure needs to escape. So, you've got a vent pipe up to the surface and usually run, the customer will run a vent pipe from the top of the tank to the surface and usually, at a safe area, like (?0.37) or with a grated cover. So that when water enters the tank, the air has somewhere to escape to. And that is as simple as that essentially, so as not to compromise the tank welding or anything like that or the cover. So, when water comes in, it needs to leave and so the vent pipe allows it to do that.

Well, I'm going to just start by expressing first the differences and the nations of the UK, like the three main nations that's in Northern Ireland as a completely different set of bodies and everything. But in Scotland, SUDS are a legal requirement for new development, except for single dwellings, small single dwellings and things like that, so there is a framework there. But it just says that you have to incorporate it into your development.

Wales, it's also mandatory, but again, it goes slightly further again where you have to have your SUDS plan approved by the national approving body and before construction can begin, has to be in before construction can begin, and it must meet the specific Wales national standards. England is the only the [inaudible 00:01:32] it's kind of recommended. The guidance states that it's mandatory for... It's not mandatory, sorry, for planning applications, but they highly recommend it for large developments unless it's inappropriate to do so.

Again, that is a bit wooly essentially, as what is inappropriate. That will vary per council area potentially, that could vary per developer, whether you could put cases for when something's appropriate when it isn't. Is that cost, is it space? It's a bit of a gray area. So I mean also, England being the largest nation of the UK, to get back to the question, if it was to become mandatory, it would mean that every new site essentially then it would have to have a SUDS system designed and in place and it would have to be considered at the early stage of development. And SUDS [inaudible 00:02:25] best when it's considered early, designed early. Have be better prepared, contractors have to be better prepared, sites have be better prepared.

Everything would have to go in as per the planning application without deviation. It would also be much and have a fantastic effect on the water quality, quantity, et cetera going forward. Every site would need to consider the runoff, but it could potentially... It's one of those until it happens, we don't know. But it could cause delays potentially in state statin. Would the planning bodies have enough gear adopted and all these extra applications coming in? Could some sites not be approved? Who knows, potentially. That's the kind of downside. But really the upside is massive if we were to put SUDS in every development.

We suggest to say, as we've talked about the four elements of SUDS before, the water quality element, which is environmentally conscious people, water quality should be in everybody's mind. Water quantity to reduce flood risk for pre-existing developments and people living in pre-existing development where SUDS was a thing, putting in a development should not increase the risk of flood to other people already living in the area. And the amenity in biodiversity, which just makes the spaces better for the people in the environment.

It's one of the impact will gain, like everything, with upon the impact it'll have in the construction industry. It will depend on the amount of planning if for work done by the governing bodies before they make the regulation if they choose to do so. Actually, it's only under review at the minute, but the environmental uptick could be massive going forward.

Because I guess what it would mean is most kind of projects now usually look at probably maybe the quantity side in England. Obviously in Scotland and Wales, it's all mandatory anyways, there's not much difference there. Especially in England, from the sounds it's just the runoff that's thought about, not kind of the quality biodiversity immunity. It's more we just need to deal with the runoff to make sure there's no flooding, but they don't think about the other aspects to SUDS.

Very true, and I must say that as much, as I said, it's a bit of a silly legislation, majority of sites and developers in these days are thinking about a SUDS system. They're already considering it, whether it becomes [inaudible 00:05:06] to get them approved, if the regulation changes might be the consideration. But most systems, most large developments certainly are considering SUDS at the moment as you see.

I don't want anyone to think watching this that the water quality in England is off the chart bad. It's not. And I don't know that for fact, so I don't know the testing, but in majority of the time it has been considered anyway just as good practice. But the fact it's just been in every development, even your smaller developers, singled, dwellings, et cetera, would need to consider potentially a SUDS system. And as you said, the quality that they're considering, the quality, the immunity, the biodiversity, it's all about the future. It's protecting these spaces for the future, protecting them for other people.

So key considerations are things like... Also, you just mentioned the soil type, other things would be things like rainfall, local rainfall for the area, climate change considerations, the size of the [inaudible 00:00:23] the size of your development. Discharge rate, allowable discharge rate into the local storm sewer system, and the slower the discharge rate allowed... The less the storm drains can cope with, the larger your tank has to be to because it has to be released more, whether it's released more slowly. You have to consider if there's groundwater in the area. You have to consider also the later use of the crates, what will it be under? Will it be under a landscaped area? Will it be under a really heavily trafficked area? There's quite a lot of different considerations to take in for design of your tank, but there are the main ones.

They can quite significantly impact. So the first impact really is, is it an attenuation system or a soak away? So, your type of soil will impact which solution, essentially, you need to think about. Well draining soils will drain, which are with very little groundwater are usually suitable for soak away systems. Obviously, you have to do our testing to test that. So that's the first way it will affect your system, but assuming it is an attenuation tank that is required, different soils can affect the tank longterm.

So thinking about something like heavy clay soil will put additional lateral loading and weightbearing on the tank over the course of its life. We design tanks for a minimum and SuDS guidance is that we should design tanks for a minimum design life of 50 years. So the heavier the soil, think about 50 years worth of potential pressure on the tank from heavy clay soils means that you have to think about the crate type that you're using. You have to think about the crate type you're using, you have to think about the depth of the tanks, the amount of loading for the cover and everything on top of it, that kind of thing.

So it will potentially, can have an impact for the longterm life of the crate. And you have to think about the backfill, obviously you should never really put [inaudible 00:01:37] material directly around the tank, it should be well created gravel material originally backfilling the tank. So it will have been affecting that kind of thing as well.

Yes, stormwater attenuation systems can be retrofitted to existing buildings if there is enough space available and the existing drainage levels can be matched. It can be a bit challenging because the drainage infrastructure is already in place, and you need to ensure that the system can hold an adequate volume within the available space. Planning and forethought are required for successful retrofitting. Additionally, other stormwater management elements like rainwater harvesting systems for irrigation or tree pits can also be added at a later stage. However, it's important to consider practical limitations and feasibility during the planning process. In summary, retrofitting stormwater attenuation systems is possible but requires careful consideration and planning.

An attenuation tank can potentially cause flooding if it's not properly maintained. Regular silt removal and flushing of the tank are important to prevent blockages. After a large storm, clearing silt traps, gullies, and channel drains is crucial. In rare cases, pipe blockages could cause surface water flooding.

However, extreme storm events are infrequent, such as one in 100 or one in 200-year events. These are rare occurrences, and most people would never experience them in their lifetime. Even in these rare cases, an attenuation tank can significantly reduce the impact of flooding compared to having no tank at all.

While Mother Nature can sometimes bring unpredictable and extraordinary events, the frequency of such occurrences is very low. Safeguarding against these rare events is challenging, but attenuation tanks provide valuable protection against typical storm events in most cases. The massive storm events seen in places like the US are unlikely to occur in the British Channel or North Sea regions.

An attenuation tank is a simple system used to hold excess water. It receives water through multiple inlets, which can include pipes and channel drains, and usually has one outlet. The tank acts as a void to allow the water to build up and prevent overwhelming downstream drains during heavy storms. The outlet typically contains a flow control device, such as an orifice plate or vortex flow control, to regulate the water's discharge rate. The tank size is designed based on factors like rainfall, surface area, and climate change considerations. If designed properly, the tank should always have the capacity to cope with the site's water volume. Offline attenuation tanks are less common and have a single inlet and outlet at the bottom, filling and draining during significant flow events. It's crucial to position the outlet at the tank's base to ensure complete drainage. Failure to do so may turn the tank into a retention tank, which can lead to lost capacity and increased flood risk.