An operation method of a storage controller, which is configured to control a nonvolatile memory device, includes initiating a first instance of a respective reliability operation for a respective memory block included in the nonvolatile memory device, the respective reliability operation including detecting a degradation level of the respective memory block and setting a respective skip reference value based on the detected degradation level; determining whether a respective number of consecutively skipped instances of the respective reliability operation is less than the respective skip reference value; and selectively skipping or performing a next instance of the respective reliability operation based on the determination result.