Does PP check for Soakaway size?

[Junglejam]

We are nearing completion of our self build and one of the last major tasks is to install the soakaway for rainwater.  Prior to starting our build we had to carry out a percolation test and provide a scheme for dealing with the surface water and this was submitted and Discharge of Condition was approved by the Council as part of our PP.  However, our percolation test result required a whooping 40m3 of soakaways to cater for a roof size of 120m2 plus 45m2 for the double garage. I've spoken to the firm who carried out the calculations a couple of times and they verified their calcs are correct. Installing a green roof or rain harvesting tank would not offset this at all. Building control will return to check for underground drainage of pipes and inspection chambers but are not really interested in the number of crates installed. My question is: when does planning get involved with this? Do they check what is installed before I cover over....? 

 

 

 

[Mr Punter]

You should carry out the drainage works in accordance with the approved scheme you submitted.

 

Planning do not check what you have done.

[Moonshine]

1 hour ago, Junglejam said:

We are nearing completion of our self build and one of the last major tasks is to install the soakaway for rainwater.  Prior to starting our build we had to carry out a percolation test and provide a scheme for dealing with the surface water and this was submitted and Discharge of Condition was approved by the Council as part of our PP.  However, our percolation test result required a whooping 40m3 of soakaways to cater for a roof size of 120m2 plus 45m2 for the double garage. I've spoken to the firm who carried out the calculations a couple of times and they verified their calcs are correct. Installing a green roof or rain harvesting tank would not offset this at all. Building control will return to check for underground drainage of pipes and inspection chambers but are not really interested in the number of crates installed. My question is: when does planning get involved with this? Do they check what is installed before I cover over....? 

 

is there any option to get permission from your local water authority to put the surface water down the sewer.

 

Our earth on our land is very good to build on, but crap at drainage, we were able to get permission from our local water authority to drain into the combined sewer in the road.

[Ferdinand]

2 hours ago, Mr Punter said:

You should carry out the drainage works in accordance with the approved scheme you submitted.

 

Planning do not check what you have done.

 

Surely it would be BCO that would check?

[Mr Punter]

3 minutes ago, Ferdinand said:

 

Surely it would be BCO that would check?

 

 

Yes, but according to the OP they are not interested in the number of crates installed.

[Ferdinand]

1 minute ago, Mr Punter said:

 

 

Yes, but according to the OP they are not interested in the number of crates installed.

 

I was being a little lazy. Thanks for pointing that out.

[gravelrash]

40m3 is very excessive. I calculated for 150m2 roof 1 in 10 year 6hour storm and with 40% climate increase, I created a wild detention pond, it has a standing pond level with a banking around sides and can accept the storm flow if required. Pond liner sits halfway up banking and maintains level by natural percolation through soil banking, when it floods it will dissipate at its own speed through ground percolation.

 

'Rainfall Calculations and soil infiltration.
Calculations for 1 in 10 year storm with 6 hour
change show a required hard surface water
(In accordance with the Wallingford Procedure)
R=0.35, Z1 360min= 33.4mm x Z2 (1.22) x 1.4
56.7mm/6hr x 150m2 = 8505lts rainfall/6hr'

 

This was accepted and passed by planning and BC.  what is your percolation like? 

[rh2205]

Depends on resources but it’s planning that often check soakaway sizes to current planning policy, the size is based on the local policy requirements in the local plan & how much flood risk issues there are, bizarrely BC does not take into account the relevant storm periods other than usually a very basic 1in10 year return period and often ignores the impact of climate change. Probably an intentional government loophole so no one really has to protect others properly from flooding. Im talking cumulatively as the Pitt review showed a long time on it’s own a single development is negligible but as we know there are lots of infills & extensions these days.

[Junglejam]

The calculations were reached through timing of water infiltration of soil (water does pool after down pours) and 1 in100 year event of flooding. 

There is no road drainage or sewers nearby as we are out in the sticks. However, there is an open ditch that runs along two sides of the property (just outside the plot) and onto the lane and this could act as a storm flow if connected. I suppose what I really need to know - do planning check whether I install all 200 crates or not!? 

 

[gravelrash]

Planning will not check physically they just want the calcs and design to sign off the decision and conditions, inspection is the Building Inspectors job...if he can be arsed. Have you asked the BI if he needs to see open trench and crates? Just mention over phone if he needs to before you back cover...if he is happy not to inspect then just stick a few in and pipe off to drainage dyke.

[Blooda]

We're building on Clay - which goes at least 5m deep. 

Our ground investigation guy, said the ground was not suitable for soak away, and are nowhere near a ground water sewer or suitable watercourse.

The Water Authority, [Severn Trent] accepted this, and accepted this as a reason to allow rainwater to be put into the fowl sewer.  It was a condition that the flow was attenuated to less than 2 l/s [If I remember].

 

My Engineer suggested an attenuation tank made up of soak away crates with membrane and then some sort of vortex valve.  All of which he charged me in looking at and doing the calcs.  The solution was more suited to a development of multiple houses rather than a single house.

 

We ended up going to Rainwaterhavesting, who did the calcs for free, the solution was cheaper, which included attenuation and also rainwater harvesting to toilets.

 

 

 

Edited November 24, 2021 by Blooda