I was given the task of producing high-level design documents, describing flows of a new development project.
The project's specifications contains a few flow-charts of the project's use cases.
These tend to be packed with data, including branches (decisions that determine multiple paths) and loops (flows that return to a previous state).
Attempting to describe these using traditional sequence diagrams hit a well, as (to me) it seems that these are better used for things that are more "single direction" in their nature, with no control flows, etc.
What is the optimal way of creating high-level design diagrams showing flow of data between server-client, including different branches of execution and cycles?
Sequence diagram could meet you requrements.Use combined fragments. For branches (decision) there is alternative type (ALT) combined fragment with several sections. For loop (cycles) also use combined fragment but of (Loop) type. Data flow is described in form of message arguments. Communication direction corresponds to message direction.
It is not significant what level of detail you need to design. UML diagrams coul be used for any level you choose.