Hydro Brake - sounds scary!

[Oldsteel]

I am at the spec stage of my build (no we haven't turned a sod yet) and my surface water drainage report says, because I am on clay on a sloping site, I can discharge surface water into the stream at the bottom of the site. Sounds simple but just like many other things 'engineers' add huge cost and complicate matters. The drainage engineer has specified a '1500mm hydro brake manhole' halfway down the run to the stream which is intended to restrict the outflow to 2l/sec (two litres per second) 'with a 1.8m design head - a buffer tank in case of storm water'. I have just visited a friend building a house in the next village and he said he had to dig out an area about 1.5m deep 'the size of a small swimming pool' (and it was, he showed me the area which essentially sat underneath the whole area of his back terrace) which he called an 'attenuation tank' which is designed to do the same thing. His was constructed of plastic on a pea shingle bed. Looking online the hydro brake manholes are constructed out of concrete and look to be about 10 feet deep. All in all, this looks very costly either way. Has anyone else come across this and can anyone suggest, erm, ways around it? I suggested to the drainage engineer that we put in a pond to act as the buffer tank as a way to avoid this expensive  lump of concrete, but he said we would still need the Hydro Brake in the outflow pipe from the pond!  I haven't yet got any quotes for the Hydro Brake but it looks like a single truck to get it to site and a crane to place it, let alone the cost to make it.

[scottishjohn]

the deciding factor is gong to be the size of outlet pipe from your atenuation tank and the head --how deep the tank is -that will decide how much water  + how fast the water can  exit the tank 

sure someone on here will have the formulae for working that out  ,once you know how much water you are expecting to control from your surface /roof drainage

 

 

 

[Oldsteel]

OK so its a case of reading. The drainage layout diagram was accompanied by 6 charts which described flows vs rainfall, which each of 6 charts describing a period of time ranging from 15 minutes to 600 minutes entitled 'Storm sewer design by the  modified rational method'. Needless to say I hadn't studied these in detail. Upon further inspection of each one of these charts I note that the spec of the Hydro-brake appears to be the same: Hydro-Brake Optimum Manhole: 4 DS/PN 1.003. Volume 4.7m3. So, if my calculations are correct with a diameter of 1200mm then the manhole would be 4m deep. A sizeable piece of concrete!

So am I stuck with this or is there another way?

PS. The design criteria seems to be taking the worst storm in the past 100 years in our area and adding 40% for climate change. 

Edited November 1, 2019 by Oldsteel

[Temp]

Is the stream 4m below the location of the hydro-brake?

 

We have an underground rainwater storage tank made of fibreglass. I wonder if one of those could be used for the hydro-brake? Google 6000L underground rainwater tank.

 

For those you need to dig a big hole and put concrete in the bottom to bolt the tank down to prevent it floating when empty. Then back fill.

[Temp]

Seems you can get  ombined rainwater harvesting and attenuation tanks from 1500L upwards..

 

https://www.stormsaver.com/products/attenuation/stormsaver-combined-rainwater-harvesting-and-attenuation

 

Or perhaps you just need a few crates and some pipe!..

 

https://www.stormsaver.com/products/attenuation/stormsaver-attenuation-crates

 

Edited November 1, 2019 by Temp

[scottishjohn]

do the maths  2l per sec is 7200 litres an hour --  7.2 cubic metres -- so how long are they trying to hold it back for

[Dreadnaught]

My SE initially specified a hydrobrake and, when I looked it up, I was surprised at how much it cost (around £1,000 if I recall correctly). In the end I agreed with my SE to submit the drainage calculations to the council without the hydrobrake on the off chance they would approve them without it and fortunately they did. However, following our heroic SUDS efforts, the calculated maximum run-off rates were not hugely in excess of the mandated figure which is presumably why.

[Jeremy Harris]

2 hours ago, Dreadnaught said:

My SE initially specified a hydrobrake and, when I looked it up, I was surprised at how much it cost (around £1,000 if I recall correctly). In the end I agreed with my SE to submit the drainage calculations to the council without the hydrobrake on the off chance they would approve them without it and fortunately they did. However, following our heroic SUDS efforts, the calculated maximum run-off rates were not hugely in excess of the mandated figure which is presumably why.

 

Sounds a lot of money for a device that is pretty simple and has no moving parts.  In essence it's much like a plug hole turned on its side, that's designed to always create a flow-limiting vortex when it gets totally submerged.  Just had a look at the design calcs, and it seems a pretty easy thing to design and fabricate for any limiting flow rate.  I can't find list prices, though, for off-the-shelf units.

 

We had concerns expressed about surface water drainage, with good reason, as the stream alongside us has been known to flood in very heavy rain.  I used the rainfall intensity data in Part H3, together with the surface areas involved, to work out the size of attenuation tank we needed, added a bit to make it a nice round number of heavy duty crates, and then just buried the crates, wrapped in suitable permeable membrane, under the drive.  The one area of permeable soil we had in one corner of the hole where the crates are (the rest was clay) gave a suitable permeability in terms of both allowing the attenuation tank to drain within a reasonable time, and in terms of not releasing water that ultimately flows underground to the stream at a rate that's high enough to add to storm surge related flooding.

 

Just a pity that we have a few houses further up the hill behind us who don't have something similar.  Several, fairly large area, hard surfaced drives up there feed torrents of water on to the lane, turning it into a fast flowing stream in heavy rain (and washing loads of debris down that ends up at the end of our drive).

 

 

[Oldsteel]

Thanks for all your advice. I like the idea of the combined RW harvesting and attenuation tank, as the drainage engineer had specc'ed the two separately, and considering the cost of the manhole I was going to drop the RW harvesting idea completely. As I have not yet submitted the SuDS report to the council, I will revert to the drainage engineer and ask him about the (less expensive) alternatives suggested here, in particular an attenuation tank, in whatever form suits the site. @Jeremy Harris your comments about the value of the hydrobrake is also well taken, we are halfway down a steep hill and I have no idea whether the houses at the bottom of the hill suffer from flooding due to the arrival of water from further up. Your explanation reassures me this is not simply bureaucracy gone mad!

[Oldsteel]

@Sensus I contacted the company referred by your link but it turns out they only do systems for commercial or housing estates. They did however put me on to these guys http://suds-solutions.co.uk who make a hybrid attenuation tank/harvesting solution suitable for domestic and are sending me a quote. I will update this thread with the results. By the way the concrete manhole/hydrobrake is coming in at £4500 excluding installation.

[Jeremy Harris]

Not sure if this is any help, but I did find off-the-shelf hydro brakes in the end (part way down this page): https://www.drainagesuperstore.co.uk/browse/underground-drainage/soakaway-crates.html 

 

Very expensive for what they are, I can only guess that they are not something that sells in high volume.  You could get one fabricated from stainless plate for a lot less than those prices, although I'd guess there may well then be an issue with demonstrating compliance with the spec (I suspect it's the compliance testing that makes up a fair bit of the price).  It does look as if there may be some cheaper options on that link above, though.

[Oldsteel]

The more I learn about the hydro-brake (time spent wildly out of proportion with other build aspects!) the more I think the hydro-brake is a commercial solution mis-applied to a domestic requirement. There is good video here which implies its a solution for large developments, but also shows how it works https://fpmccann.co.uk/product-category/stormbrake-gb/ . @Jeremy Harris looks like you are right, the cost starting at £950 is because each one is custom made according to the head, volume and outflow requirement. I was thinking - whats wrong with a small diameter outflow pipe to limit the flow? Pressure will increase as the head rises, and debris needs to be dealt with, but a vortex brake seems like an over-engineered solution. Sure enough I received a much better quote for an all-in-one grp attenuation tank, proprietary outflow limiter, rainwater harvester and pump from https://rainwaterharvesting.co.uk at just over half the cost of the concrete manhole and hydro-brake. All I need now is to gauge installation costs.

[Dreadnaught]

@Oldsteel I tried an attenuation tank as an option but I could not bury it on my site (no dig restrictions), it would have been sitting on the surface. It was rejected as a solution by the council SUDS-person as being a potentially temporary structure.

 

I imagine the your tank will be buried and thus will be an undoubtedly permanent solution and so cannot not fall foul of such an objection.

Edited November 5, 2019 by Dreadnaught

[Jeremy Harris]

Our attenuation tank is buried.  It's made up of 20 heavy duty crates, wrapped in permeable membrane and buried deep underneath our drive.  Total capacity is a bit under 4,000 litres.  I bought the heavy duty blue Aquacell crates cheaply from a ground works chap that had a yard full of them "left over from another job".  Think I paid a fiver each for them, plus another £20 for delivery.

[Oldsteel]

On 01/11/2019 at 17:12, Sensus said:

 

Your Engineer is right.

 

A hydrobrake itself is a pretty simple device: it's merely a carefully designed orifice that induces a vortex in the water flowing through it. The faster the water flows, the bigger the vortex, until it eventually 'chokes' at a the designed flow rate, It has no moving parts, needs no maintenance under normal operation, and shouldn't ever break down. They're absolutely routine on developer housing sites - I've used them hundreds of times (but, of course, nobody bats an eyelid when the cost is amortised over dozens of houses).

 

Have you had infiltration tests done? It may be that you have some infiltration on the site, and can make deep or over-sized soakaways work. If you can, look at doing anything possible to reduce the impermeable area.

 

Alternatively, you may wish to consider something like this. Probably more expensive than the hydrobrake manhole for initial installation, but at least you get the benefit of rainwater harvesting.

  

@Sensus Credit where its due - your last link there is what I ended up with, and its far cheaper than the hydrobrake manhole, and I get the added bonus of rainwater harvester built in. And yes I have had the tests done and the water goes nowhere, so no option for soakaway.