When should I use "Transition Start" versus "Warm Start" for the "Start type" calculation option with the Implicit dynamic solver?
Each system responds to calculation options differently. By default the numerical solver assumes a dry condition to start the simulation; i.e., all inflows are zero and all depths are zero in all elements at the beginning (the pipes are empty and dry). For situations in which the user wants a non-zero starting flow or the catchments’ hydrology calculations to give none-zero flows, the model provides two options to start the simulation: Warm Up Time for a Warm Start or Transition Start for a transitional start.
When using Transition Start, the model initiates the simulation from zero flows and immediately reaches the actual non-zero flows in the first time step and continues the simulation.
When using Warm start, the model estimates initial depths based on non-zero inflows and uses a user-specified warm-up time (entered in the Warm Up Time field) to approach steady state conditions prior to starting the actual simulation.
Note that if the model is a looped network then the warm start will not be used, because of the difficulty in estimating the non-zero initial conditions for complex looped networks. Also, other hydraulic effects such as attenuation would not be able to be accounted for. So, for example if you are looking at the flow at a downstream point in the network at the end of the simulation, it may not match the flow at the beginning of the simulation even if loading pattern multipliers are the same at the beginning and end, because of attenuation effects that happen later on in the simulation. (see second link listed below)
If you experience initial instability in the model and different warm up times don't make the results better, try transition start. If a warm up time of up to a few hours doesn't help, it is probably not reasonable to increase it further. You can read more about the process of tweaking calculation options in the Help topic "Troubleshooting Dynamic Wave model Calculations" or the link in the See Also section below.
Troubleshooting unstable SewerGEMS and CivilStorm results using the Implicit solver
Conduit flow at start of simulation lower than expected with Implicit solver