Open space. No less than 30% of the site shall be maintained in nonvehicular open space. The developer shall be required, where possible, to preserve or incorporate natural features, such as woods, streams and open space areas, which add to the overall cohesive development of the garden apartment and overall community development. However, all conditions deemed hazardous by the Board of Supervisors, including natural feature hazards, are to be eliminated or all precautions deemed appropriate by the Board of Supervisors to reduce the hazard are to be provided by the developer.