I had once gave a good answer (IMHO) for this similar issue.
Shortly speaking, if you need to design High Level Architecture Diagram, you are less required to have skills at good diagram structure such as UML. High Level Architecture Diagram should be simple and cannot be simpler, represent the business process and understandable by anyone, even the management. It should explain how the business flows, and handling for every cases available, even for negative case (error or exception in business level).
It will help the management understand the requirement clearer and pitfalls for such decisions, and help to redesign early.
For Low Level Architecture Diagram, you are required to have a decent skill for UML diagram (don't know if there is other format out there). It is required so you can catch any technical issue such as authentication and race conditions, and design pitfalls.