The main danger
from bars is the river water rushing out. Sean remember that video of the poor guy going under the Golden gate bridge to close to that shore line were the tidle flow is extreme and he got well and truely nailed.
The list of dangers is as follows. Please add to it if I have missed anything.
Firstly, the outflow of water. This makes several dangers. The water flow changes rapidly and often times can even reverse depending where you are on the wave cycle, how deep your keel is as the wave flow can "slide" over the river current
below it. Many boats broach in this situation. they pick up speed with the wave, surf down the back then suddenly hiot the river flow against them. The hull can not handle the speed and the bow digs in and around you go. This also means you can lose stearing control and end up in a part of the bar that you don't want to be.
Wind direction. Just like wind against a tide, an good onshore wind will cause the wave to stand steeply.
Tidal flow. Tide against river flow will also make waves stand up.
Bar. This of course is a navigation
hazard. Some bars can close a mouth of to passing over till the tide is high enough. Sand bars are constantly changing position and depth
. The shape and depth
of bar alters how the wave works. Waves can often rise up very abrubtly and because the bottom is constantly changing, the wave behaviour is also constantly changing. Just because the waves behaved one way the day before, does not mean they wil behave the same the next.
and yes, the bottom causes a wave to build just like coming onto a beach.
Another major danger
is the bottom and the trough of the wave. If the bar is shallow, you can get slammed onto the bottom with great force. If that didn't cause damage, the next wave will, as you are now stationary and possible at the wrong angle to take the next wave.
Because of all the above, the sea conditions on a bar can change rapidly and to extremes. If a bar is borderline passible, it often pays to wait a while as even 30 minutes or so can make a big difference. It also pays to wait and study the bar before you cross. Look for wave cycles and what they are doing.
Sometimes bars can be hard to read. Just because a bar has low breaker sets, does not mean it is not dangerouse. Sometimes large waves can infact be quite passible. The photo above show's that. I doubt many of us would attempt waves of that size.
I stated above that I would never attempt a bar crossing. that is refering to crossing in my boat I have now. I have been over many bars in small boats, have made beach launchings in Cray boats on the east coast
(some Kiwis will know what that's about) and so now enough to knwo I aint' trying it in my current
heavey displacement 6'9" keel boat.
Plus i was younger back then :-)