Underground soakaway crates, where to position an overflow pipe.

[epsilonGreedy]

My planning permission requires installation of 4 m3 of underground roof rainwater storage as part of a SUDS runoff attenuation design. The overflow from this underground storage runs off to another open amenity pond which then ultimately overflows into regular road drainage within the village.

 

The site seller installed the main overflow pipe 110mm from the pond to my site boundary. I am now extending that pipe a few meters into my plot but I will not be installing the underground crates for another year. I need the drain extension, that I am now digging the trench for, to end up at a suitable position where it will connect to the underground crate storage.

 

How critical is the entry point positing into these plastic crates? I am not concerned about drainage flow gradient, my question relates to the connection point of the overflow pipe into the crates in plan view i.e. must it be at the corner of the 4 m3 hole or can it be any intermediate point?

[PeterW]

It can be anywhere as long as it’s slightly lower than the inflow pipe. It should also be close to the top of the crates otherwise you won’t get the benefit of the whole of the crate volume. 

[epsilonGreedy]

15 minutes ago, PeterW said:

It can be anywhere as long as it’s slightly lower than the inflow pipe. It should also be close to the top of the crates otherwise you won’t get the benefit of the whole of the crate volume. 

 

 

Just what I was hoping to hear, in fact I was digging outside in anticipation of your answer until rain stopped play a few minutes ago.

 

Hmm yes the inlet pipe height differential, that will trigger some difficult gradient calcs, I am struggling to maintain a viable gradient from the outlet to the pond.  Think I might have to raise the crates 300mm and cap them with 100mm of reinforced concrete otherwise I will end up with a strange hump in the middle of the driveway.

[PeterW]

Go deeper but narrower ..? Or even go with a wide inverted T as this is surely just storm attenuation ..?

[epsilonGreedy]

20 hours ago, PeterW said:

Go deeper but narrower ..?

 

 

That would reduce the size of the concrete cap. btw the concrete cap is a substitute for the regulation depth of standard fill over the top of the crates which is 500mm as I recall. The underground crates have to be positioned near the entrance of the drive to maintain achieve a minimum 5m distance to my garage footings and also a neighbour's garage. Because of this location I cannot discount the risk of an HGV using the top of my drive for an unplanned 3-point turn.

 

20 hours ago, PeterW said:

... Or even go with a wide inverted T as this is surely just storm attenuation ..?

 

Yes this whole headache is mandated for storm attenuation. I was going to ask for a description of the inverted T option but as I was typing my question I got it. I like it and I should be able to reduce the concrete cap to 1 m2.