The Overflow Path

Engineers distinguish between the "minor system" and the "major system." The minor system is the pipes, the catch basins, the downspouts — sized for frequent storms, perhaps a one-in-five-year event. The major system is the path water takes when the minor system is full. In well-designed cities, the major system is explicit: streets crowned to carry water in their gutters, parks that double as detention basins, swales between buildings that guide flow toward rivers. In poorly designed cities, the major system is whatever happens — basements, underpasses, living rooms.

Australia's stormwater guidelines, developed through decades of flash-flood experience, require that every development demonstrate an "overland flow path" capable of conveying the one-in-one-hundred-year storm without entering buildings. Queensland's planning code specifies that floor levels must sit 300mm above the major-system flood level, and that flow paths must be free of obstructions that would divert water toward structures. The principle is simple: water will exceed your pipes; show where it goes when it does.

At the building scale, the overflow path connects the rain garden (19) to the street or to a larger infiltration basin (120). When the rain garden fills — when that 25mm design storm becomes a 50mm storm — water must have somewhere to go that isn't the foundation wall. This requires deliberate grading: a low point at the rain garden's outlet, a swale or hardened channel leading away from the building, a curb cut or driveway edge that accepts the flow without damming it. The path need not be ugly. A river-stone channel, a grass swale with a defined low line, a paved courtyard sloped to drain toward the street — all serve the purpose. What matters is that the path exists, is unobstructed, and leads away from the building and away from neighboring buildings.

The cost of not planning the overflow path reveals itself in insurance claims. A 2019 analysis by the Institute for Catastrophic Loss Reduction found that 40% of Canadian homeowner water-damage claims originated from surface water entry during heavy rainfall — not from plumbing failures or roof leaks, but from water flowing across the ground and into the building. Many of these losses occurred in homes with functioning eavestroughs and downspouts, where the problem was not collection but destination: the water went where the ground told it to go, and the ground sloped toward the house.