It looks like you are telling Arc that you are trying to input a z-factor in the slope tool, but not actually defining the variable. Removing the extra comma from that line of code should help, unless of course you do want to specify the variable in which case you need to put something in there after the comma:
out_slope = Slope(input_raster, out_measurement )
or
out_slope = Slope(input_raster, out_measurement, slope )
Also make sure that your output raster name is unique - you cannot have two datasets with the same name in the same GDB. I suggest using string formatting to add the timestep number to the end of your name:
out_measurement = "PERCENT_RISE_{0}".format(actual_timestep)