Surface water drainage calcs

[eandg]

Any easy ways to calculate/worked examples I can follow?

[Dave Jones]

http://www.thewatercalculator.org.uk/

 

[ADLIan]

I think that’s water use rather than drainage

[Jilly]

I still haven't resolved my SuDS requirements, its a palaver. Some companies will provide the calculations if you buy their (expensive) soakaway creates. The Susdrain.org website is useful, but the calculations are for larger developments and not completely relevant. It might be worth looking on the planning applications of properties in your local council to see how they have discharged their planning conditions. 

[jamieled]

The UK suds website has all the tools you should need for your runoff and storage calcs. The ciria suds manual has most of the detail. If you have any specific questions on the methods then ask away.

[saveasteading]

It is difficult. There are programs to calculate it, but it is basically, simply  the iteration of different  rainfall possibilities against outflow, and how much is left to store.

If you can handle the logic then you can do it yourself. If not then you may not get the right answer by buying a program.

You will probably end up buying the exensive crates anyway, so might as well get their free design, with the benefit that bco will accept it.

You can sometimes save by reducing the crates volume by using natural storage, eg ponds, swales, even barrels.

[eandg]

3 hours ago, jamieled said:

The UK suds website has all the tools you should need for your runoff and storage calcs. The ciria suds manual has most of the detail. If you have any specific questions on the methods then ask away.

Will have a look, thanks. 

[Gus Potter]

43 minutes ago, saveasteading said:

It is difficult. There are programs to calculate it, but it is basically, simply  the iteration of different  rainfall possibilities against outflow, and how much is left to store.

If you can handle the logic then you can do it yourself. If not then you may not get the right answer by buying a program.

@eandg try and get your hands on a copy of BRE disgest 365 Soakaway design..wait here on BH! It's an 8 page document with some worked examples. Also it's simple enough to help you get a handle on the basics. Once you grasp this it will help you form a view as to what will and won't suit your site.

 

If you get the bug you can automate it with excel.. and play!

 

Now you may have a discharge constraint. You have probably come across flow control valves, say the Vortex / Hydro brake type and had a glance at the cost. With a bit of careful design you can do this on a small scale using a plate with a hole drilled in it.. an orifice plate.. and you find example orifice calcs to work out the flow. What you do need to do it to add some other bits.. a good filter to stop the small hole getting blocked and an over spill.

 

I did a job a while ago using this on a small industrial unit using an oriface plate and a well designed grill filter, silt trap. So long as you can justify the flows and design it so it can be maintained the authority seemed happy, so was I with my effort. Did the hole thing for less than £ 150 quid from memory.

 

 

[Bramco]

One thing to do is to check with the local drainage board what their accepted maximum greenfield runoff rate is.  Then check on the Suds site https://bit.ly/3rKBtli what the rate is for your location, it's based on soil types etc. In our case this was less than the maximum.  Our engineer gave me the expected runoff rate for the newly developed area of our build.  It turned out with a bit of maths that the newly developed bit would put the overall runoff rate 6% above the maximum greenfield rate.   

 

We wrote an email to the drainage board, making a song and dance about the environmentally crazy idea of sticking a load of plastic crates in the ground when if we simply discharged straight into the dyke in front of the plot, we would have a negligible impact on the chance of flooding.  They agreed.

 

We then presented that email to the local CC flood authority team and they agreed.  We'll need to get permission to put a head wall in the dyke but don't see that as an issue. We've already had permission to build a culvert.  Interestingly, the flood authority in a separate email conversation pointed out that if we caused any damage through flooding that could be attributed to us then it would be a civil matter for the properties/land owners down stream.  Fortunately they all discharge into the dyke anyway, so we're not expecting any issues on that front.

 

Our plot is 1 acre and the newly developed bit probably about 200m2 with the house and patios.

 

Simon