Bar Crossing Weather
A sailing yacht navigating a bar crossing on the Australian coast. Photo: Kevin Green
This feature article originally appeared in AFLOAT Magazine and is the latest edition of Weather with Malcolm Riley exploring marine weather, storm tracking and offshore sailing conditions.
There was a tragic incident at the Ballina Bar recently. Barways are locations where a number of forces and processes combine which can make crossing them hazardous. They are usually locations where mariners transit either to or from the open sea. Combinations of waves, tide, river outflow, wind, underwater terrain and human or natural terrain can all combine.
I can break these various elements down as follows:
Waves
The waves can be either sea (wind waves) or swell (waves generated a large distance away). I will concentrate on swell waves as they are generally the problem when crossing a bar. When waves break they either spill, plunge or surge.
Where the sea bottom is a slow gradual slope up to the shore the break will often be a spilling wave. Spilling waves break and then keep breaking for a considerable distance. This diffuses the wave’s energy over a large area. Where the sea bottom is relatively deep up until the shore, the wave will initially be surging (not breaking) until quite close to the shore. When the sea floor rises more steeply a plunging wave often forms. When the swell reaches waters that is one half of its wavelength, the wave will stand up (begin to surge). As the water depth continues to fall, the wave will stand up more and eventually spill or plunge. Plunging waves can have very steep faces and the wave rises to such a height that it collapses over itself.
In open water, a swell wave with a 10 second wave period (time between two crests) will be travelling at about 30 knots (around 56 km/h). As the wave reaches a depth of one half of its wavelength, friction from the sea floor will slow the wave down and the energy then translates to make the wave higher. Swell waves do travel in sets, where a group of waves will appear that are larger than the waves that have been mainly predominant. A set of waves can be anything from as low as three and up to 18 waves. Exactly how or what size the waves in a set are or how frequent the sets appear is dependent on how the waves were formed; probably thousands of kilometres away.
Each of the three waves (spill, plunge or surge) can have their own dangers. Surging and plunging waves have a lot of force associated with them. The force of a wave is highly variable, depending on the speed and size of the wave, but if you look at just over a 1,000 kilograms per cubic metre of wave it will give an idea how powerful waves can be. These forces may only be applied for fractions of a second. The force is enough to change the direction of a moving vessel markedly. The spilling wave releases the energy over a large area and this can be seen in a large area of whitewater. The forces applied on a vessel are much less. However, whitewater is a mixture of air and water, so the white water is less dense, so a displacement vessel would sit lower and propellers and rudders would be less efficient.
There is a good wave calculator that is worth having a play around with on the Swellbeat website.
Tide
The tide flowing in and out of a river bar is a large factor. The two main factors are changing depth and current flow. Current during an outgoing tide will tend to make any wave coming in stand up more with steeper faces, resulting in a greater likelihood of surging or plunging waves. An incoming tide where the current travels with the waves tend to see the wave crests get further apart and even reduce a little in size. An incoming tide is more likely to lead to spilling waves.
The other factor is water depth, the higher the tide, the greater the water depth. This changing water depth will move the area where the waves are likely to break in towards the shore on high tide and further outwards on low tide. The recognised best time to cross a bar is around slack water near the high tide, preferably just before high tide.
River Flow
This is really a subset of tide. Rivers emptying their water to the sea will add to the current on an outgoing tide making the current tide plus river flow. In an incoming tide, somewhere along the river the tide and river flow may cancel each other out, albeit for a brief period of time. All this depends on the state of the river. If there has been recent heavy rainfall, the amount of water coming out of the river will be much greater and the amount of current it will add to an outgoing tide will be large. A river is also fresh water and somewhere near the barway the water on the upper layers will change from fresh to brackish to salt. Fresh water is less dense, so your displacement vessel would sit lower and propellers and rudders would be less efficient, although this is not a great consideration.
Wind
Wind in many ways acts like a current. If the wind is blowing offshore into the swell, it will helping to form plunging waves. The wind blowing onshore, with the swell tends to flatten the crests and make them more likely to spill when breaking.
Underwater Terrain
The shape and undulations of the sea floor entering the barway can make waves stand up in some places and not others, due to the depth. The changing depths may cause waves to break at one point of the bar and not others. Local knowledge is the key here. Once in the barway, unless it is uniformly dredged, there will be some changes in depth. Consider a large rock or shallow area, even if it is many metres below your keel. Any current that passes over this obstruction will speed up, just like when you put your finger over the end of the garden hose. This can cause an unexpected surge in the current.
Human or Natural Terrain
Many barways have human constructed rockwalls or breakwaters. In some places these are naturally occurring rock. Most of the time the swell that is coming into a barway does not come straight in, there is usually some kind of angle to the waves. This means that waves are more likely to break or hit on one side of barway. If the side that the waves hit is solid, (human or natural obstruction) waves will bounce off this wall, causing smaller secondary waves that move in a direction different to the regular waves. These secondary waves push up against one of the regular waves, increasing the wave heights briefly and causing a very choppy channel.
Know Before You Go
I am not going to tell anyone how to cross barways, there is plenty of information from people with more skin in the game and knowledge than I. But remember, there are many factors at play in these stretches of water and you need to consider them all and how your vessel is likely to react.
There are plenty of barway videos out there and I thought I would add a couple. One is me bringing the Lady Nelson into Lakes Entrance in Victoria. Whilst the conditions are not bad, you can see how much we moved about; we are not being towed, just following a ports vessel. The other is one of two guys and their dog coming a cropper in a tinny at Noosa bar. All ended well, but I think if I was the dog I would have bitten the driver once he got to shore.
Watch: Lady Nelson entering Lakes Entrance, Victoria
Watch: Two guys and their dog at Noosa Bar
This feature article originally appeared in July 2026 AFLOAT Magazine. To explore more stories like this, browse the AFLOAT Magazine archive or view the latest edition of AFLOAT Magazine.
