By letter filed with the city clerk, the city manager may designate, subject to city council approval, a qualified city administrative officer to be acting city manager during his temporary absences or disabilities. The city manager or the city council may revoke such designation at any time and another person may be designated acting city manager to serve during such times. If the city manager fails to make such designation, the city council may appoint an acting city manager to serve during such time. The city council may remove an acting city manager at any time.