  Distributed control agents have been advocated as an effective means for
improving the resiliency of our physical infrastructures under unexpected
events. Purely local control has been shown to be insufficient, centralized
optimal resource allocation approaches can be slow. In this context, we put
forth a hybrid low-communication saturation-driven protocol for the
coordination of control agents that are distributed over a physical system and
are allowed to communicate with peers over a "hotline" communication network.
According to this protocol, agents act on local readings unless their control
resources have been depleted, in which case they send a beacon for assistance
to peer agents. Our ripple-type scheme triggers communication locally only for
the agents with saturated resources and it is proved to converge. Moreover,
under a monotonicity assumption on the underlying physical law coupling control
outputs to inputs, the devised control is proved to converge to a configuration
satisfying safe operational constraints. The assumption is shown to hold for
voltage control in electric power systems and pressure control in water
distribution networks. Numerical tests corroborate the efficacy of the novel
scheme.
