Can the flood vent requirements be satisfied when only one wall is accessible below the base flood elevation?
Yes. Per FEMA's Technical Bulletin 1, Openings in Foundation Walls and Walls of Enclosures Below Elevated Buildings (FEMA, 2008), although openings should be installed on at least two sides of each enclosed area, it is not required. The only requirement is that there is a minimum of two openings on exterior walls to allow floodwaters to enter directly. In order to meet the requirement, the openings must be located so that the portion of the opening intended to allow for inflow and outflow is below the base flood elevation (BFE).
FEMA recommends, however, that openings be installed on at least two sides of each enclosed area to decrease the chances that all openings could be blocked with floating debris and to allow for more evenly distributed inflow of floodwater and draining of the enclosed area. It is recommended that the openings be reasonably distributed around the perimeter of the enclosed area unless there is clear justification for putting all openings on just one or two sides (such as buildings set into sloping sites).
FEMA. 2008. Openings in Foundations Walls and Walls of Enclosures in Special Flood Hazard Areas in Accordance with the National Flood Insurance Program. NFIP Technical Bulletin 1. Washington, DC. August 2008.