Section 816.46 - Hydrologic balance: Siltation structures(a) For the purpose of this section only, disturbed areas shall not include those areas-(1) In which the only surface mining activities include diversion ditches, siltation structures, or roads that are designed constructed and maintained in accordance with this part; and(2) For which the upstream area is not otherwise disturbed by the operator.(b)General requirements.(1) Additional contributions of suspended solids sediment to streamflow or runoff outside the permit area shall be prevented to the extent possible using the best technology currently available.(2) All surface drainage from the disturbed area shall be passed through a siltation structure before leaving the permit area, except as provided in paragraph (b)(5) or (e) of this section. The requirements of this paragraph are suspended effective December 22, 1986, per court order.(3) Siltation structures for an area shall be constructed before beginning any surface mining activities in that area, and upon construction shall be certified by a qualified registered professional engineer, or in any State which authorizes land surveyors to prepare and certify plans in accordance with § 780.25(a) of this chapter a qualified registered professional land surveyor, to be constructed as designed and as approved in the reclamation plan.(4) Any siltation structure which impounds water shall be designed, constructed and maintained in accordance with § 816.49 of this chapter.(5) Siltation structures shall be maintained until removal is authorized by the regulatory authority and the disturbed area has been stabilized and revegetated. In no case shall the structure be removed sooner than 2 years after the last augmented seeding.(6) When siltation structure is removed, the land on which the siltation structure was located shall be regraded and revegetated in accordance with the reclamation plan and §§ 816.111 through 816.116 of this chapter. Sedimentation ponds approved by the regulatory authority for retention as permanent impoundments may be exempted from this requirement.(c)Sedimentation ponds.(1) When used, sedimentation ponds shall-(i) Be used individually or in series;(ii) Be located as near as possible to the disturbed area and out of perennial streams unless approved by the regulatory authority, and(iii) Be designed, constructed, and maintained to-(A) Provide adequate sediment storage volume;(B) Provide adequate detention time to allow the effluent from the ponds to meet State and Federal effluent limitations;(C) Contain or treat the 10-year, 24-hour precipitation event ("design event") unless a lesser design event is approved by the regulatory authority based on terrain, climate, other site-specific conditions and on a demonstration by the operator that the effluent limitations of § 816.42 will be met;(D) Provide a nonclogging dewatering device adequate to maintain the detention time required under paragraph (c)(1)(iii)(B) of this section;(E) Minimize, to the extent possible, short circuiting;(F) Provide periodic sediment removal sufficient to maintain adequate volume for the design event;(G) Ensure against excessive settlement;(H) Be free of sod, large roots, frozen soil, and acid- or toxic-forming coal-processing waste; and(I) Be compacted properly.(2)Spillways. A sedimentation pond shall include either a combination of principal and emergency spillways or single spillway configured as specified in § 816.49(a)(9) .(d)Other treatment facilities.(1) Other treatment facilities shall be designed to treat the 10-year, 24-hour precipitation event unless a lesser design event is approved by the regulatory authority based on terrain, climate, other site-specific conditions and a demonstration by the operator that the effluent limitations of § 816.42 will be met.(2) Other treatment facilities shall be designed in accordance with the applicable requirements of paragraph (c) of this section.(e)Exemptions. Exemptions to the requirements of this section may be granted if-(1) The disturbed drainage area within the total disturbed area is small; and(2) The operator demonstrates that siltation structures and alternate sediment control measures are not necessary for drainage from the disturbed area to meet the effluent limitations under § 816.42 and the applicable State and Federal water quality standards for the receiving waters.48 FR 44051, Sept. 26, 1983, as amended at 53 FR 43605, Oct. 27, 1988; 59 FR 53029, Oct. 20, 1994; 75 FR 60275, Sept. 29, 2010; 79 FR 76231, Dec. 22, 2014; 81 FR 93392, Dec. 20, 2016; 82 FR 54979, Nov. 17, 2017