When you walk with a cup of coffee(diameter 8 cm) at just the right pace of about 1 step per second, the coffee builds up its "sloshing" until

When you walk with a cup of coffee(diameter 8 cm) at just the right pace of about 1 step per second, the coffee builds up its “sloshing” until eventually, after a few steps, it starts to spill over the top. What is the speed of the waves in the coffee?