After adding a surge tank to my model, there is an initial surge (starting right after the initial time step). Why is this occurring?
The problem could be that you have your "Elevation (Initial)" and "Elevation (Maximum)" properties of your surge tank set to the same elevation, while also having the "Has Check Valve?" property of the tank set to 'True'. If that is the case then all you need to do is set the "Elevation (Initial)" to a slightly smaller value than the maximum elevation. For example, if the maximum elevation is 347.21 ft, then set the initial elevation to 347.20 ft. This will still allow you to simulate the tank being full without causing the initial transient down surge at the beginning of the simulation.
The problem with having "Elevation (Initial)" and "Elevation (Maximum)" set to the same value is that the software is expecting the tank to start overflowing. However, with "Has Check Valve?" set to True, this implies that the surge tank is a one-way tank (flow can only go out to the system). If such a case, HAMMER assumes that the tank elevation will never get to the maximum elevation for overflow to occur.
Note that if you set "Has Check Valve?" to False, you will be able to enter a weir coefficient and weir length. If there is no check valve, the surge tank would be allow to overflow and the initial surge will not occur.
General HAMMER FAQs