The regulations cover roof drainage. surface drainage, subsurface drainage, stormwater drainage and pumped systems.
ROOF DRAINAGE:
This calculates the maximum roof area that can drain to varying gutter cross sectional areas and downpipe sizes. A 1:20 year regional storm event that is based on an average rain intensity over a 5 minute duration is used as the base for eaves gutter calculations and a 1:100 year rain intensity is used as the base for box gutter calculations. Roof drainage regulations extend no further than the downpipes.
SURFACE DRAINAGE:
Surface water drains via grates set in paths etc and via surface gutters. This water must divert to a silt pit before connecting to a subsurface stormwater pipe.
SUBSURFACE DRAINAGE:
Agricultural (Ag) pipes must also divert to a silt pit before connecting to a subsurface stormwater pipe.
STORMWATER DRAINAGE:
Pipes are laid wth a minimum gradient that varies with the pipe size and collect the roof, surface and subsurface water. Because pervious and paved surface areas vary as well as the drained subsurface areas, calculations must be made to design the stormwater pipe size to cope. Stormwater pipe systems operate with gravity flow and should not retain water between rain periods. This also provides regular pipe flushing every time it rains.
RAINWATER HARVESTING CHARGED SYSTEMS:
These are not covered under stormwater drainage because:
- A subsurface charged system cannot accept surface and subsurface water nor should this water be diverted to a tank in any case.
- Storm water and water in a charged pipe have different flow characteristics. For example, a gravity drained stormwater pipe at the head of the system can drain with a flushing flow with just a small amount of inflow whereas a full flow charged pipe has a much slower flow velocity for the same amount of inflow. Using large pipes at the head of a charged system builds up sludge with accompanying bacterial activity that depletes the waterr of oxygen when water is retained in the pipes between rain events. This is the biggest problem with charged systems.
- Stormwater pipe sizing regulations promote serious sediment build up in charged rainwater harvesting pipes.
- 90mm pvc stormwater pipe is not suitable for use in a charged system...bjut it is allowed.
- Additional regulations need to be applied to the best practice use of flexible couplings on vertical risers and overflow pipes.