Why it always seems to happen in the same months
The worst rain on this side of the peninsula is not the monsoon most people have heard of. The northeast monsoon that floods Kelantan and Terengganu from November largely spends itself on the far side of the range.
What catches Negeri Sembilan and the Klang Valley is the inter-monsoon — roughly April to May, and again from late September into November — when the afternoon thunderstorms are heaviest. These are short, violent falls of rain. A drain does not have to cope with a whole day of it. It has to cope with twenty minutes of it.
Six reasons a drain floods, and how to tell which one you have
Silt, and more of it than you think. Drains fill from the bottom up, and they lose capacity faster than they lose depth — the flow depends on both the area of the channel and how efficiently that channel is shaped, so a U-drain silted a third of the way up has lost considerably more than a third of what it could carry. This is the most common cause by a distance, and the cheapest to put right if it is caught before the silt sets hard.
The drain has lost its fall. Drains work on gravity alone. If a section has settled, or never had enough gradient to begin with, water stops moving — and standing water drops its silt, which flattens the fall further. It gets worse on its own. This one you can check yourself: look at the drain on a dry day, two days after rain. If water is still lying in it, that length has lost its fall. A drain with proper gradient is dry between storms.
Something is blocking it. Rubbish and cut vegetation, kitchen grease hardening at a gully trap, tree roots through a cracked joint, or sand washed down from construction work upstream. Sand from an active site is the one people miss — it arrives faster than anything else and it packs hard.
The drain is now too small for what drains into it. This is the answer nobody wants, and increasingly it is the real one. The drain has not changed; the land above it has. Where there was soil and scrub soaking up rain there is now roof, road and apron, and almost all of it runs off. A drain sized for kampung land twenty years ago is being asked to take the runoff of a housing scheme. Nothing is broken — it is simply the wrong size now.
The structure has failed. Cracked walls, a collapsed invert, an undermined base, a headwall pulling away. Worth knowing: where a drain keeps cracking in the same place, the drain is usually the victim rather than the cause. The ground behind it is moving, and the answer is a retaining structure, not another new drain.
The problem is downstream, outside your boundary. Your drain is fine. What it discharges into is blocked, silted or submerged, so the water has nowhere to go and backs up to you. This is the one that cannot be fixed by working on your own length — and it is why we walk the line before quoting rather than after.
Is it a drain problem or a sewer problem?
These two get confused constantly, and calling the wrong party costs days.
- Stormwater — the longkang. Rainwater. Open drains, gully traps and culverts, discharging eventually to a stream or river. Muddy or clear water, worst during and just after rain, overflowing outward across ground and roads.
- Sewage. Wastewater from toilets, kitchens and bathrooms, in closed pipes to a treatment plant. Where an area is on public sewerage that is usually IWK's network rather than the council's, though not every area is connected the same way.
The quick test. If it smells foul, backs up into the house through a floor trap or a toilet, and does it whether or not it has rained, it is sewerage — and a drainage contractor is the wrong first call. If it appears when it rains, runs outward rather than upward, and smells of little, it is stormwater.
The two do meet. A cracked sewer pipe running beside a drain, or an illegal connection discharging into a stormwater drain, produces something that looks like a drainage problem and is not.
What an honest repair involves
It starts with walking the line, not with a price. Upstream to find where the water is actually coming from, downstream to find where it is being held up. Quoting a length of drain without doing that is guessing — and a new drain in the wrong place does not remove a flood, it moves it to whoever is downhill of you.
Desilting and regrading. Where the drain is structurally sound and has simply silted or settled, clearing it and re-establishing the fall restores it. It is the cheapest real fix there is. Ask what happens to the silt — it has to be carted away and disposed of, and a quotation that ignores that is not complete.
Rebuilding the failed length, not the whole drain. A collapsed or undermined section usually needs recasting, but it is rarely all of it that has gone. Be wary of a price to rebuild everything when thirty metres is what failed.
Increasing capacity, where the catchment justifies it. A deeper or wider drain, a larger culvert, a box culvert where a pipe has been outgrown. This is real work and sometimes the only answer — but it should follow evidence about what actually drains into it.
Stabilising the ground. Where movement behind the drain is what keeps breaking it, the drain gets rebuilt once instead of annually.
Before you accept anyone's quotation
A drainage quotation should let you check it without being an engineer. Look for:
- The length, and the size and type of drain — not "repair drain", but the metres and the section.
- What happens to the silt and the spoil. Disposal is a real cost. If it is missing it will reappear later, or it will be dumped somewhere it should not be.
- Reinstatement. If the road, apron or driveway has to be broken to reach the drain, who puts it back, and is it in the price?
- Something about levels or fall. A drain rebuilt to the same flat profile that failed will fail again.
- Traffic management, if it is roadside work in a live area.
- What is excluded. Everything not written down is a variation waiting to happen.
If you are holding a quotation right now, ours included, our quotation checker walks through this item by item and tells you what is missing from it.