Soakaways

[saveasteading]

| am currently calculating soakaway sizes for rain and foul. Scottish Standards section 3.

 

Can anyone explain the logic of why the soakaway area for rainwater is half of the effective depth, and only to the sides (as if nothing runs out of the bottom!).

Meanwhile for treated foul water outfall, the bottom area is the criterion.

 

I don't have  any problems with this, but would like to understand any logic, if there is.

 

[Temp]

Think in England you include the bottom for rainwater soakaways. 

 

The half the depth issue might be something to do with water pressure in the hole?

[saveasteading]

I don't think I have ever designed a soakaway in England, because the ground has not allowed it, so haven't done a comparison. It is all about attenuation storage and slow release that still causes flooding.

Our Scottish project is on pure sand so is well suited.

 

Scotland's rules on rainwater seem much more relaxed than England's which is perhaps why the standards seem a bit half-baked. 

I worried about this, but a little  research shows that Scottish rainfall is much more predictable, and generally less than is thought.

I have compared two towns that interest me.

 

Rainfall in Inverness is low, totalling 723 mm in a typical year. Precipitation is distributed evenly, falling over about 143 days per year. 

Royal Tunbridge Wells. The rainfall here is around 737 mm | over about 99 days

 

This is only part explanation for why TW has frequent flooding problems: the storms are just more intense very locally.