Thread: Drainage problem.

Originally Posted by nogoodnamesleft

Hi there all you clever folks .....I'm having a problem with my local council regarding flooding, basically they are trying to palm me off with some bs "solution" that I know by common sense, will be completely innefective ...unfortunately, I'm useless at maths, so I'm unable to work this out myself ....would one of you kind people help me out with a calculation please ?

During the flooding, the water remains "standing" at the same depth for some hours...I have calculated the volume of the "standing" water to be 140m3 (although, that's probably irrelevant) ...there are 2 x 6inch diameter drainage pipes at the bottom of this "tank" draining this water out.

For the purpose of this equation, we can assume that the pipes draining the water away are running full bore at 100%, completely unhindered.

What I need to know is......

1) what is the flow rate of the incoming water (bearing in mind it's keeping the "standing" water at the same depth, as it's being drained by the two 6 inch pipes) ?

2) what diameter of drainage pipe would be required at the bottom of the "tank" to keep it empty in the first place ?

Please dont be concerned with all the potential things that could cause the answer to vary ......I just need a reasonably calculated answer on a simple in/out basis !!!

I hope its possible to work that out ....maybe I've just made you laugh ??

It would be really helpful, if I could find that out, so - thanks in advance

The basic calculations for this can be found on this Ask Dr Math page. But that still does not solve your problem as you need to know the pressure drop between the ends of the pipe (we would know the pressure at your end if we knew the depth of the standing water, and we could assume that the pressure at the other was atmospheric) we also need to know the length of the pipe.

So: tell us the depth of the water and the lengths of the pipes and you might get an answer

CB

Originally Posted by nogoodnamesleft

Hi there all you clever folks .....I'm having a problem with my local council regarding flooding, basically they are trying to palm me off with some bs "solution" that I know by common sense, will be completely innefective ...unfortunately, I'm useless at maths, so I'm unable to work this out myself ....would one of you kind people help me out with a calculation please ?

During the flooding, the water remains "standing" at the same depth for some hours...I have calculated the volume of the "standing" water to be 140m3 (although, that's probably irrelevant) ...there are 2 x 6inch diameter drainage pipes at the bottom of this "tank" draining this water out.

For the purpose of this equation, we can assume that the pipes draining the water away are running full bore at 100%, completely unhindered.

What I need to know is......

1) what is the flow rate of the incoming water (bearing in mind it's keeping the "standing" water at the same depth, as it's being drained by the two 6 inch pipes) ?

2) what diameter of drainage pipe would be required at the bottom of the "tank" to keep it empty in the first place ?

Please dont be concerned with all the potential things that could cause the answer to vary ......I just need a reasonably calculated answer on a simple in/out basis !!!

I hope its possible to work that out ....maybe I've just made you laugh ??

It would be really helpful, if I could find that out, so - thanks in advance

My advice is to engage the services of a water engineer. MHF cannot provide a reliable answer to this problem.

Ok thanks .....The depth of the standing water is 0.5 mtr .... the pipes are approx 15 metres long (before they drop into the huge mains sewer) ......I understand that nobody can give an accurate answer as such, because there are so many potential aspects that could vary the answer, but even a rough idea based on the basic information would be a great help ....I just have no idea about maths really, so it would be a great help and much appreciated ...thanks

Originally Posted by nogoodnamesleft

Ok thanks .....The depth of the standing water is 0.5 mtr .... the pipes are approx 15 metres long (before they drop into the huge mains sewer) ......I understand that nobody can give an accurate answer as such, because there are so many potential aspects that could vary the answer, but even a rough idea based on the basic information would be a great help ....I just have no idea about maths really, so it would be a great help and much appreciated ...thanks

Plugging in the relevant values to the equation given by Dr Math gives a single pipe will drain 4 cubic metres per second, assuming there is no fall between the ends of the pipe.

But as MrF says, get a professional assessment if you want to use this in support of you argument with your council. The numbers in this post carry no warranty and cannot support a legal argument, and come with no guarantee of correctness.

CB

excuse my ignorance here ....does that mean you could give me any indication of an ansewr to my initial questions ? ...namely - what is the flow rate of the incoming water and ....what size pipe would be required for it to drain without any standing water ???

To let you understand the situation ....the only reason I'm asking this, is for the purpose of being able to initiate the conversation ! ....after all THEY are supposed to be the experts, but I know for sure, what they are proposing is nothing more than a shot in the dark, in the vain hope that I (the uneducated public) wont know any better.....if I at least have some idea, then I can ask them for their calculations proving otherwise ....which I know damn well, they haven't done !

Originally Posted by nogoodnamesleft

excuse my ignorance here ....does that mean you could give me any indication of an ansewr to my initial questions ? ...namely - what is the flow rate of the incoming water and ....what size pipe would be required for it to drain without any standing water ???

If the water is standing at the same depth for a number of hours then over that period the rate of inflow is equal to the rate of outflow or close to. Any larger outflow pipe will have drained the area as any increase in pipe diameter will increase the flow rate (assuming the problem is not that the outflow/s are blocked, in which case larger outflows are quite likely also to have been blocked)

With less head (as when filling) the outflow will have been less, hence the space will fill until there is sufficient head for the rates to match.

Why the area should subsequently drain is anybodies guess, presumably the inflow rate fell off or a blockage in the outflow was dislodged.

To let you understand the situation ....the only reason I'm asking this, is for the purpose of being able to initiate the conversation ! ....after all THEY are supposed to be the experts, but I know for sure, what they are proposing is nothing more than a shot in the dark, in the vain hope that I (the uneducated public) wont know any better.....if I at least have some idea, then I can ask them for their calculations proving otherwise ....which I know damn well, they haven't done !

.. and what will you do when they show you their calculations (you will still be Voltaire to their Euler)


CB (-still not giving quotable advice)

Lol .....nothing is blocked, it's all been surveyed to the max ....basically they are trying to get out of it on the cheap, by adding another connection to another existing 6 inch pipe ...saying this will solve it, but also fitting flood protection doors, taking away the use of one of the pipes that currently drains it (as it's inside the building) ....therefore, I'm no better off than before !

they just want to avoid spending the money to fix it properly.

I wasn't looking for "quotable advice" ....just a "ballpark" to confirm what I already know in common sense, as I really can't do maths..... and I want to avoid the huge cost of commissioning experts right now, if I don't need to

And they wont "show me their calculations" because I know for a fact they haven't done any .....thats really the point !!!

Thanks anyway