Maintenance of nonconforming use parcels. Parcels containing a nonconforming use of land or water may be maintained or repaired, including grading, paving and surfacing, or the repair and replacement of bumper or wheel stops, fences, screening and drainageways, provided that the amount of land, water or storage (i.e., vehicles, equipment and/or materials) devoted to such nonconforming use as it existed prior to the date that the nonconformity came into being is not extended, enlarged or moved.