Is a 2100mm manhole unusual?

[Alan Ambrose]

@SBMS >>> the simulation isn't linear (i.e. half the sqm doesn't yield half the required attenuation) and is possibly more like an exponential curve

 

Yeah, I think this might actually be quite hard stats. So, with the 'common sense' adjustments you made, I'm getting about 2x the storage requirement per m^2 that you are? I'm in East Suffolk, so way over to the low rainfall/low storm east, so presumably should get fairly lenient tank volumes. It is true that my LPA doesn't seem to make much of a fuss about SuDS. Where are you on the rain map?

 

>>> Secondly - how did they calculate your discharge rate?

 

Good question, I see 2 l/s/ha mentioned all over the place and my overall plot is 0.13 ha, so that doesn't make total sense. I think it should be derived from the 'greenfield runoff rate' i.e. you are not supposed to make the situation worse than if it was a field. Maybe that explains why they adjusted yours up.

 

Jeez, this is complicated, I see the 2 l/s/ha figure on page 8/9 here:

 

https://assets.publishing.service.gov.uk/media/5a79d1a240f0b66d161ae5d2/suds-consult-annexa-national-standards-111221.pdf

 

Edited August 24 by Alan Ambrose

[saveasteading]

1 hour ago, Alan Ambrose said:

the East of UK gets v. few 50mm storms

That is what it says, based on that map, but a later one shows 'the future' and it includes central areas and eastern. 

 

It is purely my observation that floods in Hastings and Tunbridge Wells were during rain events that were only mild , just a couple of miles away. I don't know if that would show up in these maps.

 

Of course, urban areas that are not served by modern drainage will get a very quick build-up of water from roofs and roads, so 50mm is perhaps not a critical number there, but would be in flood plains.

[SBMS]

7 hours ago, Alan Ambrose said:

@SBMS >>> the simulation isn't linear (i.e. half the sqm doesn't yield half the required attenuation) and is possibly more like an exponential curve

 

Yeah, I think this might actually be quite hard stats. So, with the 'common sense' adjustments you made, I'm getting about 2x the storage requirement per m^2 that you are? I'm in East Suffolk, so way over to the low rainfall/low storm east, so presumably should get fairly lenient tank volumes. It is true that my LPA doesn't seem to make much of a fuss about SuDS. Where are you on the rain map?

 

>>> Secondly - how did they calculate your discharge rate?

 

Good question, I see 2 l/s/ha mentioned all over the place and my overall plot is 0.13 ha, so that doesn't make total sense. I think it should be derived from the 'greenfield runoff rate' i.e. you are not supposed to make the situation worse than if it was a field. Maybe that explains why they adjusted yours up.

 

Jeez, this is complicated, I see the 2 l/s/ha figure on page 8/9 here:

 

https://assets.publishing.service.gov.uk/media/5a79d1a240f0b66d161ae5d2/suds-consult-annexa-national-standards-111221.pdf

 

Yes I don’t quite understand it myself - but makes sense it’s an exponential simulation. 

We are in the north west (Lancashire) so…wet!

 

Our site area for positively drained area is 0.3ha - does that make a difference? Yours is roughly half; could that account for the 2x difference??