The diagram below illustrates three stages that can occur in a stopbank during a flood event.
The first stage is seepage where water seeps slowly out onto the ground surface.
During the second stage, we sometimes see pressure domes. This is where the topsoil actually lifts off the silt and forms a "blister" on the ground surface and water seeps through the lifted grass layer.
The third stage is where we get more concentrated seepage points that "boil" sand out with them. It is the washing out of sand from the foundations of the stopbank that can potentially lead to stopbank failure.
Environment Waikato responds to seepage by monitoring and then sand-bagging around the seepage areas to allow water to build up inside the bagged area. This slows down or stops the seepage and ensures that sand is not eroded from the stopbank foundations.