Nonconforming fences and hedges. Any fence or hedge existing on the date of adoption or amendment of this section and not in conformance with this section may be maintained. If such a fence is damaged by fire, flood, explosion, earthquake, war, riot or natural occurrence, except for normal maintenance and repair, the fence may be restored to its original size, location and the use that it had immediately before the damage or destruction occurred. It may be increased in size larger than it was before destruction if necessitated by state or federal requirements. The total lifetime structural repairs or alterations of such a fence, not including those caused by a natural occurrence described above, shall not exceed 50% of the assessed value of the fence at the time of its becoming a nonconforming fence unless it is permanently changed to conform to the requirements of this section.