The Winter Water Table on the Swan Coastal Plain
Between June and September the ground water in Perth's low-lying suburbs rises to the surface. Understanding where and when it happens decides which jobs you can do in winter and which you should leave until spring.
Perth's rain falls mostly between May and September, and on the Swan Coastal Plain there is nowhere for it to go but down into the sand. The water table rises, in places by more than a metre, and by July the bottom of a 600 mm trench in Bayswater, Canning Vale or Byford is a pond.
Where it bites hardest
The plain is a series of low dune ridges with wet depressions between them. The worst-affected areas are the old wetland chains: Bayswater and Maylands along the river, the Jandakot and Canning Vale flats, Beeliar and Cockburn, the Wanneroo and Gnangara lake chains, and almost everything around Byford, Mundijong and Serpentine on the Peel plain. Higher dune suburbs (Karrinyup, Mount Lawley, the Spearwood ridge) drain freely all year.
Your council's stormwater and flood mapping, or simply the presence of a lake or drain nearby, tells you which you are on.
What it does to a job
- Trenches fill. Sand below the water table is slurry; walls collapse instantly and pipe bedding floats.
- Compaction fails. You cannot compact saturated sand for a slab or driveway base.
- Machines sink. Lawns and paddocks that carry a 2 tonne excavator in April bog it in July.
- Concrete pours are risky. Footings poured into water are weak and non-compliant.
Planning around it
- Do deep work in autumn. April and May are the best months on the coastal plain: soft ground, low water. See the WA earthmoving calendar.
- Do shallow work in winter. Reticulation at 300 mm, paving prep on high ground and garden beds are fine.
- Switch to hard ground. If you have Hills or gravel work, winter is the time for it.
- Dewater if you must. A submersible pump in a sump at the low end of the trench keeps it workable for a day. Sump the corner, not the trench line.
- Use a trencher for pipe. A skid steer trencher cuts a narrow slot that stays open long enough to drop poly in, where a bucket trench would slump.
Protecting the surface
Winter is when tracks do their damage. Fit the widest track setting your machine allows, carry timber mats or plywood for gateways, avoid turning on wet lawn, and park on the driveway. A rubber-tracked loader such as the RS06 spreads its weight better than a wheeled machine on soft ground.
Drainage: the winter job worth doing
The one excavation that makes sense in winter is drainage, because you can see where the water goes. Spoon drains, ag-drains to a soakwell on higher ground and regraded surfaces fix the problem you are standing in. Just dig from the outlet back toward the wet area so each section drains as you cut it.
Read the ground conditions by region for a broader picture of where winter access is the limiting factor across WA.