tank over flow calculation

Hi,

We have an irrigation system which consists of a wet well and 3 submersible pumps. The wet well is supplied by a lake. We are in turn to identify the water rise in wet well during stabilization between lake and wetwell  due to  sudden stoppage of outflow from wet well (Due to submersible pump trip ), a calculation has been performed and the  modeling approach are as follows:

• Lake has been modelled using a reservoir element with a constant HGL of 74m.
• Wet well has been modelled as a tank with a capacity of 320 m^3
• Wet well is assumed to have liquid up to 74m initially.
• Two connection pipes of 800mm diameter have been taken from the lake (through a brake chamber) and connected to the wet well.
• Invert level of the outlet pipe from lake is 69.8 m.
•  invert level of the inlet pipe to wet well is 69.5 m.
• Rapid pump trip case (Considering the inertia of the pump as 0 kgm^2) has been performed.
• The maximum HGL observed in the wet well is found to be 74.17m.The graph of the HGL in the tank is attached for your understanding.

We can understand the oscillation of HGL due to stabilization of liquid between lake and wet well. But oscillation is not dying out throughout the simulation time which is strange. We expect it should stabilize to 74m at a point. Please help us with a reason for prolonged oscillation of HGL without decrease in HGL.

Thanks 

Simulation file.rar