PORT DISTRICTS; MUNICIPAL CORPORATIONS; CONTRACTS WITH LABOR UNIONS; POSSIBLE LIABILITY FOR PORT'S FAILURE TO MAINTAIN OPERATIONS DURING LABOR DISPUTE (1) Port districts may enter labor agreements with unions when necessary, and may not withdraw unilaterally from such agreements in the absence of breach or legally recognized excuse for non-performance [[nonperformance]]. (2) Port districts have an independent duty to maintain services and must exercise reasonable diligence in preventing labor disputes which cause work stoppages if they are to avoid liability claims.