Flood Information
Water level and rainfall stations can provide early warning of a flood risk, allowing action such as road closures, flood proofing or even evacuation to be taken, thus reducing the risk to life and property.
Please click here for a summary of current conditions »».
Flooding
On the northern beaches flooding generally occurs quickly. This is known as flash flooding. If you live in a flash flood area, planning can be difficult as warnings may not be possible. It is important to understand the conditions which can cause flash flooding and act early.
East coast lows can also cause flooding around lagoons. These low pressure systems also raise ocean water levels and can last for several days. Look out for severe weather warnings - ie 150mm of rain over 24 hours.
Flash Flooding
When flash flooding is likely, leave low-lying homes and businesses well before any flooding begins. Evacuation is the best action to take, but only if it is safe to do so.
If you are trapped by rising floodwater, seek refuge in the highest part of a sturdy building. Stay there and call '000' (triple zero) if you need rescue.
Look out for severe thunderstorms and heavy rainfall - ie 70mm of rain in 3 hours. That's about 20mm in 1 hour. Keep an eye on your local rain gauge for this.
Rainfall
Several factors contribute to flash flooding on the northern beaches. The two key elements are rainfall intensity and duration. Intensity is the rate of rainfall, and duration is how long the rain lasts.
Rainfall amounts provide an indication of the likelihood of flooding. These are basic trigger levels that may vary from place to place across the . It is important to note that if the rainfall exceeds these levels then the degree of flooding could be much worst. These are not maximum levels of rainfall but simply an indication of how much rain will cause flooding.
Water Levels
MHL maintains a network of automatic water level recorders across the northern beachescatchments. Data is typically recorded at 15 minute intervals, stored on-site and uploaded in near real time to the internet. This time series water level data is used for a number of purposes :
- monitoring water quantity
- informing decisions on day-to-day management
- understanding the effects of storm events in our local area
Lagoon Flooding
The four lagoons on the northern beaches can fill up like a bath tub when it rains. If it continues to rain, they will overflow and cause flooding to surrounding land.
The Northern Beaches Council manages the entrances of the lagoons to reduce the risk of flooding.
Once the water reaches a specific level, the council opens the entrance to let the water drain into the sea.
The opening levels for each lagoon are:
- Narrabeen Lagoon 1.3m AHD
- Dee Why Lagoon 2.2m AHD
- Curl Curl Lagoon 2.2m AHD
- Manly Lagoon 1.4m AHD
Look out for (Bureau of Meteorology) severe weather warnings and keep an eye on your local lagoon water level »»
The council mechanically opens the lagoons when the water levels are within these ranges.
The lagoons are mechanically opened to reduce the flood risk to surrounding properties. Several other natural factors will affect whether the council mechanically opens the lagoons. These include the amount of rainfall forecast, the tide and the height of the sand berm at the entrance.
It is important to note that in times of very heavy rainfall and/or strong ocean swell, mechanically opening the lagoon may not prevent flooding.
In very large storm events, it may not be possible for the council to open the entrance to the lagoons. In this case, flooding of properties is possible and you should work through your home or business FloodSafe Plan.
Waves and Sea Level
MHL measures the ocean water level for Sydney at HMAS Penguin at Middle Head. Sydney generally has two high tides and two low tides per day and the range of these tides is usually between 0m and 2m above mean sea level. Predicted tidal levels can be forecast based on recorded tide levels. The actual tide may differ from the predicted tide due to meteorological and oceanographic conditions.
MHL collects offshore wave data at seven continuously recording sites off the NSW coast. The ocean wave monitoring sites are based on the Waverider buoy system developed by the Dutch company, Datawell. Waverider buoys are located off Byron Bay, Coffs Harbour, Crowdy Head, Sydney, Port Kembla, Batemans Bay and Eden. To provide deepwater wave data, the buoys are typically moored in water depths between 70 and 100 metres, between 6 and 12 kilometres from the shoreline.
Coastal Erosion
Sandy beaches can be easily moved by both wind and waves. As a result, these beach systems are continuously changing in response to the actions of wind and waves. This is a natural process driven by the weather that causes beaches to build up or erode away.
Extreme weather events can cause erosion that damages buildings and coastal structures such as surf clubs, beach access pathways, stormwater pipes, car parks, and boat ramps.
Look out for (Bureau of Meteorology) warnings such as Damaging Surf and Dangerous Surf.
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