Underwater Inspections are performed every sixty months or more frequently if conditions warrant. Perform an Underwater Inspection on structures where the submerged portions of the structure have a history of water depths of at least four feet year-round or where the submerged elements are in less than four feet of water, but wading would be unsafe due to channel bottom conditions, high current, localized scour, or lack of freeboard. With the transition from the Coding Guide to the Specification for the National Bridge Inventory (SNBI) in accordance with 23 CFR 650.311, the collection and use of certain SNBI data is required to establish inspection intervals and to update and implement the reduced inspection interval policies by June 6, 2024. Prior to SNBI data collection TxDOT will use equivalent criteria per the Coding Guide to determine the reduced interval inspection criteria, provided later in this section.