Stormwater and drainage on a sloping block

Every Adelaide council requires a stormwater management plan as part of the development application on sloping sites. The plan shows how peak flow from the developed site will be captured and released.

The typical rule: peak stormwater discharge from the developed site must not exceed the pre-development peak discharge. This is called "no net increase" and is a common council requirement across Australia.

The site must direct stormwater to a legal point of discharge. Options include: connection to the council stormwater main in the street; connection to an on-site infiltration or detention system; or connection to a natural drainage line where legally permitted.

A hydraulic engineer prepares the stormwater management plan. On simple sites this is included in the standard design fee. On complex sites (steep, poorly-drained, or requiring detention systems) a specialist engineer is engaged.

The roof is the largest impervious surface on the site. All rain falling on the roof must be captured by gutters and directed through downpipes to the stormwater system.

Gutter sizing must match the roof area. Standard 150mm quad gutters handle typical residential roofs. Steeper roofs and larger areas may require oversized gutters or box gutters.

Downpipe sizing and location must be adequate for the calculated peak flow. Typical practice: one downpipe per 60 to 100 square metres of roof, positioned at the ends of gutter runs.

A rainwater tank buffers the peak flow and stores water for garden use, toilet flushing, and (with plumbing modifications) laundry use. South Australia mandates rainwater tanks on new builds; taking advantage of this for stormwater buffering is efficient.

Water flowing across the block surface (from rain, from neighbouring properties, from the upslope side of the block) must be intercepted before it reaches the building.

Surface drainage: grade the site to shed water away from the building. Typical grading is 1 in 20 (5 percent) for landscaped areas near the home, sloping away from the building.

Sub-soil drainage: install a perforated pipe at the interface between the natural ground and any retained soil or building foundation. This intercepts groundwater flow and directs it to the stormwater system.

On steep sloping blocks, both surface and sub-soil drainage are typically required. Sub-soil drainage alone catches groundwater; surface drainage catches surface water.

South Australian regulations require a rainwater tank for new residential buildings, with a minimum capacity typically around 1,000 to 5,000 litres depending on plot size and roof area.

For genuine stormwater buffering benefit, tank capacity should be significantly larger. Common practice on sloping blocks is a 5,000 to 15,000 litre tank connected to the toilets and laundry as well as the garden taps.

Position the tank downhill from the collection point but uphill from the point of use if possible. This allows gravity delivery to garden taps without a pressure pump. For internal use, a pressure pump is required.

Tank overflow must connect to the stormwater system. During heavy rain the tank fills quickly and then overflows; the overflow route must not damage neighbouring properties or cause erosion.

Stormwater systems require periodic maintenance. Neglected systems become the largest source of long-term issues on sloping-block homes.

Annual tasks: clean gutters and downpipes of leaves and debris; check any leaf guards or first-flush diverters; inspect visible drainage outlets for blockage; check the rainwater tank for sediment build-up.

Every 5 to 10 years: flush the sub-soil drainage system; inspect below-ground stormwater pipes for damage; clean out any pit or interceptor devices.

On sloping blocks, keep the drainage plan and any as-built drawings safe. Future owners will need them for maintenance and for any subsequent building work.

If drainage problems develop (damp in walls, subsidence in the garden, unexpected water pooling), engage a specialist drainage consultant early. Diagnosing and remediating drainage issues is much cheaper than dealing with the structural consequences of ignored drainage.