Polk County Bank of Johnston has agreed to pay an $18,095 civil penalty for violating provisions of federal flood protection laws and banking regulations. The Federal Deposit Insurance Corp. said in a document made public today that Polk County Bank made loans secured by buildings or mobile homes in flood-designated areas without determining whether the collateral was covered by flood insurance. In addition, it did not advise borrowers of the need to obtain private flood insurance, according to the order, which was signed June 16. The bank waived its right to a hearing on the charges.