The important natural features of the premises involved shall be preserved in the design of the development. Natural features which shall be protected, including the natural terrain, wetlands, wooded areas, scenic vistas, natural drainageways and bodies of water. A development proposing soil removal or placement shall only be permitted to significantly alter or encroach existing natural features of the premises if the Land Use Board finds that such alteration, disturbance, movement or placement of soils is the minimum necessary to allow the use of the land for the intended permitted purpose and that there is no alternative to the development design which would eliminate or mitigate any adverse impact on the soils and their natural features. Where such alternative(s) exists, they shall be utilized to reduce the need for the proposed soil removal or disposition or placement. "Significantly alter or encroach existing natural features" shall be deemed to be the removal or placement of soils involving 700 or more cubic yards.