The area between the breaking wave furthest out to sea and the shoreline (or, in the case of an offshore reef, where waves dissipate) is called the surf zone.
As waves break over a sandbank, the water surges upwards, creating a region of water that is higher than mean sea level (MSL). This water needs to return back to MSL due to the effect of gravity. It does this by flowing into deeper channels in the surf zone that can run along a beach and offshore. The depth of water above MSL is called the ‘set-up’. As the set-up increases, the pressure on that water to return to MSL increases, and this can result in faster flow rates in longshore and rip currents.
Offshore, there is no set-up; sea level is at MSL. On a sandbank, set-up is high and gravity has a strong effect on the water that is above MSL. The effect will be to push water through rip currents and other channels that have a lower set-up.
