Results instability- Explicit Solver

Hello,

I'm simulating a storm network in sewergems platform. My network is connecting with a pond and the variation of  HGL of the pond and the U/S node of the incoming channel is shown below.

 I'm using explicit solver, with  6 sec routing time step with Slot surcharge method. what can be the reason for this nature of instability in the model?

BR

Nishadi