All sites shall be graded in accordance with site plans and subdivisions as approved by the Planning Board. Gravity (stone construction) retaining walls shall not be greater than four feet in height. Walls constructed of railroad ties shall not be greater than two feet in height. Retaining walls greater than four feet in height shall be of reinforced concrete and shall be designed by a professional engineer. Wall plans, elevations and design computations shall be submitted to the Planning Board's Engineer for approval. Shoulder areas behind roadway curbs shall have a maximum slope of 5% from the top of curb. Any project which involves the disturbance of more than 5,000 square feet of ground is subject to soil erosion and sediment control plan certification from the Bergen County Soil Conservation District. The applicant shall include such a plan in its submission to the Planning Board.