$($a$)$ Referring to the table below for the annual extreme sea level data in Hong Kong, the $50-$year design water level at North Point is $3\mathrm{.}45m.$ A coastal seawall near North Point is designed for the current $50-$year return period water level. What will be the new

design return period for this seawall after a century if water level rises at a rate of $31mm$ per decade?

Please provide detail steps with calculation