Close×

Getting across a river bar requires patience and preparation, says Alan Lucas.

Barred river and lake entrances can be intimidating at times, but by following basic rules successful crossings can be made conditional only on your patience. As inconvenient as it may be, the most important rule is to remain in port or at sea if your chosen bar has ‘closed out’ to bad weather or heavy swell.

A bar is a shallow, sandy silt-bank that accumulates outside the entrance of a large body of water. It is typically associated with a basin of deep water between it and the end of the breakwaters. Deeper gutters sometimes scour out to one or both sides of the bank where they can provide the safest approach channels. More commonly, silt-banks accumulate to one side of an entrance leaving just one approach channel diagonally out from the breakwaters towards the most prevalent swell direction. Such channels can be so static as to have fixed leading beacons showing their best depths.

Breakwaters are man-made rock walls directing their bodies of water into the sea. They are the result of channel rationalisation during the late 1800s to the mid 1900s, their purpose being to transform lazy waterways and their sprawling sandbank deltas into commercially useful ports. Engineers of the day cleverly harnessed the power of the streams by training them to self-flush during ebb tides and floods, thus keeping their depths more reliable and entrances navigationally safer.

Sand bars forming off their entrances are nature’s payback. Fast flowing water is successful in scouring deeper channels and carrying the resultant silt out to sea but it dumps much of it immediately outside the entrance leaving one or more gutters as already described and illustrated.

The positions of these gutters rarely shift but their depths vary according to the vagaries of nature, floods scouring them deeper and droughts making them indolent.

There is also the swell factor: an area’s prevailing swell direction keeps things tolerably static whereas periods of uncharacteristic swell behaviour, driving straight into an entrance, can cause navigational chaos and produce breaking seas right across a bar with or without associated wind.

Bar depths relate to waterway volume: A big, deep river, for example, typically has a bar depth of 4 to 6 metres, spring low tide, with possibly 10 or more metres in its entrance basin. A small river, on the other hand, may have just one to two metres on its bar LWS, with a basin of only two or three metres.

Timing is everything

Without local knowledge and quiescent sea conditions, an ebb tide is not the time to cross a bar. Not only do waves stack up and break more readily but the ebb stream slows incoming vessels and holds them in potential surf conditions for much longer. A rising tide is the safest time to cross a bar: not just for the deeper water and faster entry it promises but for the calming effect of a tidal stream running fair with the ocean swell.

Timing a bar crossing for the run-in stream is all important, and getting it right demands an understanding of the ‘over-run’ factor. This phenomenon is ignored or misunderstood by many novice sailors who experience unnecessarily frightening moments as a result. Over-run is caused by tide levels equalising somewhere between the entrance and the headwaters, which creates seemingly irrational behaviour at the entrance. But, as confusing as it seems, this is not irrational behaviour - it is perfect hydraulic logic. It works like this:

At the entrance of any large body of water, low and high tide predictions comply with the tide tables, but upstream where an incoming tide meets an out going tide there is a collision and mingling of old and new water until height equalisation throughout the entire waterway is achieved.

Exactly where equalisation occurs varies according to the different tide levels of the day and is in any case irrelevant to the subject here, the only information the sailor needs is the actual time that equalisation happens at the entrance and this is always around three hours later than tide predictions. In other words slack water at the entrance usually happens midway between low and high tides - and vice versa.

For this reason, bars should never be crossed earlier than three hours after low tide. In fact, some waterway over-runs lasts only two hours, but without local knowledge three hours should be the minimum. When high tide is reached at the entrance, the in-going stream continues for another three hours, making it theoretically safe to enter up to three hours after high tide, but don’t push the limit. Cross the bar between three hours after low tide and no later than one hour after high tide.

Riding the wave

Entering a barred waterway, different vessels behave differently in breaking waves. Yachts, with their large rudders, ride a breaking wave with far more controllability than motorboats, whose small rudders need prop-power for steerage. When exiting a bar, roles reverse, motorboats being able to power through breaking waves, whereas an underpowered yacht is likely to be stopped dead in her tracks then fall away and turn beam on to the offending wave. Her ballast keel will almost certainly prevent her from rolling over, but down flooding is possible if too many apertures are open.

Catamarans have the best of both worlds: rudders large enough to steer off a breaking wave and an engine in each hull to assist the steering when steaming against the waves. Runabouts are vulnerable on a bar if badly handled, but have the speed and manoeuvrability to watch, wait and grab opportunities unavailable to slower boats.

The bottom line, regardless of the type of vessel involved, is that bars can be dangerous and should be treated with caution. Don’t enter or depart a barred waterway during adverse weather, and when conditions are deemed right, wait for the fourth hour of a rising tide to ensure that the out-going over-run has turned - or is at least slack. Only in calm weather may all of the above be ignored.

 

comments powered by Disqus