|(a) The final cover system shall be composed of no less than two feet of soil. The first 18 inches or more of cover shall be of clayey soil, classification sand clayey (SC) or low plasticity clayey (CL) as defined in the "Unified Soils Classification System" developed by the United States Army Corps of Engineers, compacted in layers of no more than six inches to minimize the potential for water infiltration. A high plasticity clayey (CH) soil may be used; however, this soil may experience excessive cracking and shall therefore be covered by a minimum of 12 inches of topsoil to retain moisture. Other types of soil may be used with prior written approval from the executive director. (b) The final six inches of cover shall be of suitable topsoil that is capable of sustaining native plant growth and shall be seeded or sodded immediately following the application of the final cover in order to minimize erosion. (c) Side slopes of the final cover for all above-ground disposal areas (aerial fills) shall not exceed a 25% grade (four feet horizontal to one foot vertical). Side slopes for the final cover in excess of 25% may be authorized by the executive director, provided that controlled drainage such as flumes, diversion terraces, spillways, or other acceptable methods are incorporated into the final cover system design in the site development plan and submitted to the executive director for review and approval. The final cover for the topmost portion of a unit or facility shall have a gradient of not less than 2.0% and not greater than 6.0%, and shall possess a sufficient minimum grade to preclude ponding of surface water when total fill height and expected subsidence are taken into consideration. (d) The executive director may approve an alternative final cover design that: (1) achieves an equivalent reduction in infiltration as the clayey soil cover infiltration layer specified in subsection (a) of this section; and (2) provides equivalent protection from wind and water erosion as the topsoil layer specified in subsection (b) of this section. (e) No later than 60 days prior to the initiation of closure activities, the owner or operator shall submit the design and specifications for the closure of these municipal solid waste (MSW) landfill units or MSW sites to the executive director for review and approval. The final cover shall be installed no later than October 9, 1993. (f) After completion of closure, the owner or operator of these MSW landfill units or MSW sites shall comply with the post-closure care requirements for this final cover, as detailed in §330.463(a) of this title for the duration of the post-closure period for these units or sites.