An Island Bay section I inspected sat at close to 30 degrees, with the house perched on a bench cut into the slope decades ago. The stormwater line ran downhill from the house in a dead straight trench, then simply stopped at a rusted grate half-buried in blackberry, thirty metres short of the street. Nobody had walked that line in years. Whatever the pipe was supposed to connect to further down the hill, it was not connecting to it now, and every drop of roof and paved-area water from that property was finding its own way from that point on.
Why steep sections change the stormwater question
On flat ground, stormwater mostly asks one question: is there enough pipe capacity for the volume. On a steep Wellington section it asks a second question that matters just as much: does the water have a controlled path the whole way down, or does it get free somewhere partway and start cutting its own channel through the batter. A pipe that terminates mid-slope, an open channel that was fine when it was built but is now half-collapsed, or a soakage area on a bench that was never checked against the clay below it are all versions of the same failure — water with a plan for the first half of the journey and none for the second.
Where the water is actually meant to end up
A properly designed system on a steep section carries roof and surface water in piped form for as much of the descent as practical, because open channels on a 20 to 30 degree slope erode quickly once vegetation cover is disturbed and rainfall intensity picks up. The pipe should discharge either to a council stormwater main at the street, to an engineered soakage system sized for the specific soil it sits in, or to a stable, armoured outlet if surface discharge onto lower ground is unavoidable. What it should never do is discharge onto an unprotected batter face and rely on the slope itself to disperse it, which is effectively what that abandoned Island Bay line had been doing for years by default.
What goes wrong when the path is broken
A broken or disconnected stormwater run on a slope tends to fail quietly at first. Water finds a low point, starts a rill, and that rill deepens every wet season because nothing is stopping it from concentrating in the same channel each time. Wellington’s weathered greywacke clay loses strength fast once it stays saturated, so a channel that looks like minor erosion in year one is often the early stage of a shallow slip by year three or four. The regional council holds the technical guidance on stormwater design and soakage assessment for exactly this reason, since what counts as adequate capacity depends heavily on the specific catchment, slope, and soil type rather than a single rule that applies everywhere.
What I look for before recommending a fix
On an inspection I walk the full stormwater path from roof to final discharge point wherever I can physically get to it, not just the section near the house, because that is where problems like the Island Bay grate hide. Erosion channels, sediment fans at the base of a slope, and vegetation growing unusually lush in one strip are all signs water is running somewhere it should not be. Where subsoil drainage is also part of the picture, the same sizing principles that apply to soakpits on flatter ground still hold, just working against a steeper hydraulic gradient and less margin for error if a pipe is undersized or badly laid.
Frequently asked questions
Why is stormwater design different on a steep section compared with a flat one? The water needs a controlled path for the entire descent, not just enough pipe capacity, because any point where it escapes onto an open slope can start eroding a channel that worsens every wet season.
Can an old surface stormwater channel on a slope still be adequate? Sometimes, if it is stable, armoured against erosion, and sized for current rainfall intensity, but many older channels were built for lighter catchments than they now carry and need reassessing.
What is the first sign a slope’s stormwater system is failing? A rill or erosion channel that reappears in the same spot after rain, or unusually lush vegetation in one strip, both point to water concentrating somewhere it should be piped instead.