Axisymmetric SEEP/W model on a reclaimed island

Greetings of the day,

There is a island created by reclaiming the fill materials on the river bed. The location is slightly upstream of a large dam. The island is located at the bank of the river, circular in shape and there is no portion in common to the bank of river. The island is created by gabions and soil reinforcements. There are 3 levels of horizontal drains (gravel materials) for quick dissipation of pore water. The height of the island is 21m. When the dam is filled at it's maximum the island will be submerged to 19m of it's height. 

So, While doing the drawdown condition analysis in axisymmetric model how to provide the boundary conditions ? 

The rise in water level from 0m to 19m will take place in a period of 2 months & the drawdown will happen in 3 months period. 

Thank you for your time.