Drainage systems in Bali — French drains, channels and soakaways

Tropical rain math

A serious December cell drops 60–100 mm of rain in an hour. On a 300 m² roof-and-terrace villa that’s up to 30,000 liters arriving in sixty minutes — a water truck every six minutes, parked on your garden. Drainage designed for polite European rain (the default, because that’s what most imported details assume) simply surrenders. Everything we size — pipe diameters, channel widths, soakaway volumes — starts from Bali’s numbers. It’s why our systems look slightly overbuilt in August and exactly right in February.

Reading water before moving soil

The site visit for drainage is a detective job: stain lines on walls, moss maps, where the lawn squelches in dry season (groundwater) versus after rain (surface water), what the uphill neighbour’s new roof does to your boundary. In Canggu the usual villain is a high water table plus paved-over paddy; in the hills near Ubud it’s concentrated runoff arriving from above with real velocity. Different villains, different systems — a French drain for one, a catch-and-channel interception for the other. Diagnosis is free; guessing is expensive.

The French drain, done properly

Everyone sells “French drains”. The version that works for decades: trench to a surveyed fall (min 1%), geotextile wrapping the gravel so Bali’s fine silt can’t clog it in two seasons, perforated pipe at the bottom (holes down — yes, down), washed gravel, and a destination that can actually receive the water. The version that fails: a gravel trench with no fabric, no fall and no destination — a buried bathtub. The price difference is maybe 20%; the lifespan difference is tenfold.

Soakaways, and when they’re the wrong answer

Where ground can drink — most of the island’s volcanic soils — a structured soakaway is the elegant end point: your roof water recharges the same aquifer your well drinks from. But in Sanur and low coastal Canggu, wet season groundwater rises to where the soakaway floor is already underwater, and pouring rain into saturated ground is theatre. There the honest design is storage plus a pump pit with a verified discharge route. We’ll tell you which physics your plot obeys before quoting either.

Landscape drainage design for villa gardens

Garden drainage is designed like a small catchment, not added pipe by pipe. We start from the levels: lawns and planting beds fall 1–2% away from the house, terraces and paths shed water sideways into channels, and no surface drains toward a door. Where water collects, catch pits with grated tops take it underground; French drains along the uphill boundary and at the foot of every terrace or retaining wall intercept seepage before it reaches the backfill. Every pipe run keeps at least a 1% fall to one destination — a soakaway sized to the roof and paved area, or a pump pit where the garden sits too low.

Two neighbours in the garden get their own rules. The septic system stays separate: roof and yard water never enters the tank or its soakaway, and storm drains keep clear of the soakaway field. The pool gets a deck channel so rain does not wash garden soil into the water, and its backwash and overflow get a planned discharge instead of the lawn — something we set out together with the pool excavation. Designed before landscaping, this costs a drawing and a day of levels; retrofitted, it costs the lawn.

Costs, and the cheap moment to act

A full villa perimeter system — 60 m of French drain, two catch basins, one soakaway — typically lands between IDR 37 and 57 million. The same scope after a flood has already been through the living room costs the same, plus the floor. The cheap moment is dry season: machines move freely, lawns recover fast, and you beat the November queue. Full rates on the pricing page; scheduling logic in the rainy season guide →