How can I model the following scenarios in HAMMER for transient analysis?
1) Pipe break
2) Hydrant flushing
For both of this cases you can use discharge to atmosphere - D2A element, more details below.
Here is technote on modeling D2A element, please go through it for its different applications.
Modeling Reference - Discharge To Atmosphere
Here is similar forum discussion on modeling pipe break for your reference.
D2A Element for Pipe Breakage
Information about modeling hydrant opening / closure in Hammer.
Transient simulation for a hydrant closure or opening
Bentley Technical Suppport
Answer Verified By: Parsa Pezeshk