Drainage works on fall. Water leaves your property, joins the street system, and eventually discharges somewhere lower. In most of Sydney that somewhere lower is simply lower, and it stays lower, which is why the whole thing is reliable enough that nobody thinks about it.
Around here the discharge point is a channelised creek running into a tidal cove. The Hawthorne Canal was originally the natural Long Cove Creek, cleared of mangroves from January 1890 and straightened over the following years into what is now a 1.9 kilometre stormwater channel. It discharges into Iron Cove, and Iron Cove is tidal. So the level at the bottom of the system is not fixed. It moves, twice a day.
Because the outlet level rises and falls with the tide, the same rainfall has two different outcomes depending on when it lands. Rain on an outgoing tide drains away. The same rain at high water has far less head to work with, and the system backs up toward the properties feeding it. So "it only happens sometimes" is a genuine diagnosis here rather than a shrug.
- Note the time and date every time it happens, not just that it did
- Note whether the rain was unusually heavy or ordinary
- A yard that floods in a light storm and not a heavy one is a timing pattern
- Low-lying properties near the channel have the smallest margin
- A partly blocked private line is invisible until the two coincide
On this page
1. The bottom of the system moves
This is the whole mechanism and it is simple once it is said out loud. A drainage system relies on a difference in height between where water starts and where it discharges. That difference is what makes it flow.
In most places the discharge point is a fixed level, so the available head is a constant and the system either copes with a given rainfall or it does not. It is predictable, which is why drainage problems elsewhere are usually about volume or blockage.
With a tidal outlet, the available head changes through the day. At low water there is more of it and the system performs at its best. At high water there is less, and the same volume arriving has less to push it along.
That is the part people find hardest to accept, and it is the most useful thing on this page.
2. The same storm, two outcomes
The practical consequence is a pattern that looks irrational and is not.
- An ordinary downpour on a falling tide drains away and nobody notices anything.
- The same downpour at high water has far less head to work with, and the system backs up toward whatever is feeding it.
- So a light storm can cause a problem a heavy one did not. Which is exactly the observation that makes people assume an intermittent blockage.
- And a repair can appear to work simply because the next few storms happened to land at the right time.
That last one is worth dwelling on, because it is how money gets spent on the wrong thing and then appears to have been well spent.
3. Record the time, not just the event
This is the single most useful thing a household here can do, and it takes seconds.
- The date and the time it started.
- Roughly how heavy the rain was, in your own words.
- Where the water went and how far it got.
- When it receded, and whether that lined up with the rain stopping.
- Anything you noticed about the channel or the street system if you could see it.
- A photograph, dated.
Two or three events recorded that way answer a question that no amount of inspection on a dry day can. If the pattern lines up with timing rather than with rainfall, that is a diagnosis, and it is one you produced for nothing.
4. And the reason not to relax about it
Everything above could be read as "nothing is wrong, it is just the tide". That would be the wrong conclusion and it is worth being clear about why.
A partly blocked private stormwater line is invisible until the conditions coincide. Ninety per cent of the time there is enough margin that a restricted line still copes, so nobody knows it is restricted. The one occasion it matters is the occasion when the margin was already small.
So timing and a defect are not alternatives. They are two things that combine, and the useful position is to remove the one you control. Your own pits, grates, downpipes and lines are worth having clear and worth having filmed once, precisely because you cannot do anything about the other half.
Yard flooding on and off?
Get the timing recorded and the private line checked. Together they answer it.
5. Low-lying properties feel it first
The closer a property sits to the channel and the lower its levels, the smaller the margin between draining and not.
That is not a defect in those properties and it is not a reason to avoid them. It is a reason for their owners to be more diligent about the half they control, because they have less room for a restricted line to hide in.
It is also a reason to think about levels before any work that changes them. Paving, landscaping, a new path or a deck can all raise ground around a property, and where the margin is already small that matters more than it would elsewhere.
6. What is yours and what is not
- Yours: the pits, grates, downpipes and stormwater lines inside your boundary. Clear them, know where they discharge, and have them filmed once.
- Not yours: the street system and the channel itself. Council and the relevant authority own those, and they are the people to ask about them.
- Nobody's: the tide. It is worth saying plainly, because a great deal of money gets spent trying to fix something that is not a fault.
We can clear and film what is inside your boundary and tell you honestly whether it is contributing. We cannot change the level of a cove, and we would rather say so than sell you an investigation into the weather.
- The local stormwater channel was straightened from a natural creek and discharges into a tidal cove.
- So the level at the bottom of the system rises and falls twice a day.
- The same rainfall has different outcomes depending on when it lands.
- A light storm causing a problem a heavy one did not is a timing pattern, not a fault.
- A partly blocked private line stays invisible until the margin is already small.
- Record the date and time. Two or three events answer what dry-day inspection cannot.
Stormwater and the tide FAQs
Why did a light storm flood the yard when a heavier one did not?
Almost certainly timing. With a tidal outlet the available head changes through the day, so the same volume of water behaves differently depending on when it arrives. That pattern is genuinely diagnostic and it is worth recording.
Does that mean nothing is wrong?
Not necessarily, and it would be the wrong conclusion to draw. A restricted private line copes fine most of the time and shows itself only when the margin is small. Timing and a defect combine rather than being alternatives.
What can I actually do about it?
Remove the half you control. Clear your pits and grates, know where every downpipe discharges, and have the private lines filmed once. You cannot change the tide, so it is worth being certain about everything else.
Should I raise it with Council?
The street system and the channel are Council's and the relevant authority's, so anything about them is a question for them. A dated record of events makes that conversation far more useful than a general complaint.
Is my property worse because it is low?
It has a smaller margin, which is a reason for more diligence about your own drainage rather than a problem in itself. It is also worth thinking about before any work that raises ground levels around the house.
Can you tell me when high water is?
We are not going to publish tide times or predict events. What we would say is that if you record the date and time when something happens, the pattern becomes obvious quickly and it is yours rather than something anybody has to sell you.