Whenever the tract to be subdivided is of such unusual topography, size or shape that the strict application of the subdivision requirements would result in substantial hardship or whenever the strict application of the subdivision requirements would be contrary to the public interest, the Council, upon recommendation of the Commission, may vary or modify such requirements so that the subdivider is allowed to develop his property in a reasonable manner; provided, such variance or modification will not have the effect of nullifying the intent and purpose of the subdivision regulations for the City or of interfering with carrying out the comprehensive plan.