The nature of a disaster is such that there can't be a specific set of criteria that can be used "to gauge the severity of the incident." Every disaster presents its unique set of circumstances; in the aftermath of such an event, the immediate need is to assess the situation and start meeting needs. The severity of the incident can be rated later, when the crisis has been mitigated.
After doing initial evaluation and determination of the safety of addressing those in immediate need, resources to help in addressing those needs should be contacted and brought in as and where required. Coordination of help efforts becomes an important issue at this point, but it is often difficult to carry out an organized plan for relief efforts due to the urgency of the circumstances being addressed.