Plumbing after a Hawkesbury flood

Why floods here are different

Most river valleys widen towards the sea. The NSW SES explains that the Hawkesbury-Nepean does the opposite: narrow sandstone gorges between Sackville and Brooklyn act as choke points, so floodwater flows into the valley faster than it can leave, backs up and rises rapidly. It likens the valley to a bathtub with five taps and one plug hole.

Two consequences matter for a house. There may be only a few hours between heavy rain and being told to evacuate, and in most cases you won’t see flooding in your neighbourhood before that. And a home that stays dry can still be isolated, as significant floods can disrupt roads and essential services such as electricity, sewerage and water.

Flood words on Hawkesbury pages

Probable maximum flood

What flood experts have determined is the largest possible flood for an area. If your house is within it, the SES says, you are within the floodplain.

1 in 100 year flood

One of the two flood sizes on the council’s Flood Extent Maps, which show the approximate reach of riverine flooding for it and for the probable maximum flood.

Flood Planning Level

The level, in metres AHD (Australian Height Datum), from which the minimum floor levels of buildings are set for flood protection. It is part of the council’s Flood Advice for a property.

Minor, moderate, major

The Bureau of Meteorology’s three categories for the consequences of regional flooding, as the SES describes them.

The council’s Flood Advice comes in three types: basic, with the “adopted 1:100 ARI flood level” and an estimate of land levels; detailed, with flood event levels, likely velocity, hazard categories and hydraulic classification; and one for complying development certificates. Its land levels are approximate, and the council says accurate figures need a site survey by a registered surveyor.

Before the next one

The SES suggests finding your utilities now and learning how to turn off the mains supply for power, gas and water, and turning off gas and electricity before you evacuate. On a block with a bore, NSW Health advises raising the bore head above ground level and sealing it, so floodwater and surface runoff can’t flow into the source.

When the water goes down

Wait for the SES to say it is safe to return, and don’t enter a damaged building unless authorities say it is safe. Leave any damaged or disconnected supply off; have electrical connections and appliances that got wet inspected by a licensed electrician, and gas appliances and bottles exposed to floodwater inspected for safety, before they are used. Take photos of the damage, keep receipts for repairs, and watch out for home repair scams.

Before you book repairs, check the plumber’s licence: see checking a plumber.

Water you might drink

NSW Health says rainwater tanks, and tanks holding bore water, that floods have affected are likely to contain harmful microorganisms, and the water won’t be suitable for normal use. Bring safe drinking water with you when you return, and use a town supply, or another safe source, until the tank is dealt with. NSW Health explains how to empty, clean and disinfect an affected tank on its cleaning rainwater tanks after floods page, and gives the chlorine dose by tank size on its rainwater tanks page. Your local public health unit is on 1300 066 055.

Polluted stormwater and floodwater are among the things NSW Health lists as able to contaminate groundwater, and it recommends that bore water is not used for drinking, cooking or personal hygiene without testing and treatment. See bores and groundwater.

Septic systems on the floodplain

A septic tank is, in the words of the Easy Septic Guide, “a living ecosystem where bacteria digest waste”, which can become sick if it is flooded. Hawkesbury City Council approves and inspects on-site systems in its area, and its Customer Service Centre is on (02) 4560 4444. More in septic and on-site sewage.