SECTION 12.517. Hydrologic Balance: Permanent and Temporary Impoundments

(a) General Requirements. The requirements of this subsection apply to both temporary and permanent impoundments.

(1) Impoundments meeting the significant or high hazard class criteria of dams in the U.S. Department of Agriculture (USDA), Natural Resources Conservation Service Technical Release No. 60 (210-VI-TR60, July 2005), Earth Dams and Reservoirs, shall comply with the table of Minimum Auxiliary Spillway Hydrologic Criteria in Technical Release No. 60 (TR-60), which is incorporated by reference, and the requirements of this section.

(2) An impoundment meeting the size or other criteria of 30 CFR 77.216(a) shall comply with the requirements of 30 CFR 77.216 and of this section.

(3) The design of impoundments shall be certified in accordance with §12.190(a) of this title (relating to Reclamation Plan: Ponds, Impoundments, Banks, Dams, and Embankments) as designed to meet the requirements of this part using current, prudent, engineering practices and any design criteria established by the Commission. The qualified professional engineer shall be experienced in the design and construction of impoundments.

(4) Stability.

(A) An impoundment meeting the significant or high hazard class criteria for dams in TR-60, or the size or other criteria of 30 CFR 77.216(a) shall have a minimum static factor of 1.5 for a normal pool with steady state seepage saturation conditions, and a seismic safety factor of at least 1.2.

(B) An impoundment not included in subparagraph (A) of this paragraph, except for a coal mine waste impounding structure, shall have a minimum static safety factor of 1.3 for a normal pool with steady-state seepage saturation conditions or meet the requirements of §12.190(c) of this title (relating to Reclamation Plan: Ponds, Impoundments, Banks, Dams, and Embankments).

(5) Impoundments shall have adequate freeboard to resist overtopping by waves and by sudden increases in storage volume. Impoundments meeting the significant or high hazard class criteria for dams in TR-60 shall comply with the freeboard hydrograph criteria in the Minimum Auxiliary Spillway Hydrologic Criteria table in TR-60.

(6) Foundations.

(A) Foundations and abutments for an impounding structure shall be stable during all phases of construction and operation and shall be designed based on adequate and accurate information on the foundation conditions. For an impoundment meeting the significant or high hazard class criteria for dams in TR-60, or the size or other criteria of 30 CFR 77.216(a), foundation investigation, as well as any necessary laboratory testing of foundation material, shall be performed to determine the design requirements for foundation stability.

(B) All vegetative and organic materials shall be removed and foundations excavated and prepared to resist failure. Cutoff trenches shall be installed if necessary to ensure stability.

(7) Slope protection shall be provided to protect against surface erosion at the site and protect against sudden drawdown.

(8) Faces of embankments and surrounding areas shall be vegetated, except that faces where water is impounded may be riprapped or otherwise stabilized in accordance with accepted design practices.

(9) An impoundment shall include either a combination of principal and auxiliary spillways or a single spillway configured as specified in subparagraph (A) of this paragraph, designed and constructed to safely pass the applicable design precipitation event specified in subparagraph (B) of this paragraph.

(A) The Commission may approve a single open-channel spillway that is of nonerodible construction and designed to carry sustained flows or earth- or grass-lined and designed to carry short-term, infrequent flows at non-erosive velocities where sustained flows are not expected.

(B) Except as specified in subsection (c)(2) of this section, the required design precipitation event for an impoundment meeting the spillway requirements of this paragraph is:

(i) for an impoundment meeting the significant or high hazard class criteria for dams in TR-60, the auxiliary spillway hydrograph criteria in the Minimum Emergency Spillway Hydrologic Criteria table in TR-60, or greater event as specified by the Commission;

(ii) for an impoundment meeting or exceeding the size or other criteria of 30 CFR 77.216(a), a 100-year, 6-hour event, or greater event as specified by the Commission; and

(iii) for an impoundment not included in subparagraphs (A) and (B) of this paragraph, a 25-year, 6-hour event, or greater event as specified by the Commission.

(10) The vertical portion of any remaining highwall shall be located far enough below the low-water line along the full extent of the highwall to provide adequate safety and access for the proposed water users.

(11) A qualified professional engineer or other qualified professional specialist under the direction of a professional engineer, shall inspect each impoundment as provided in subparagraph (A) of this paragraph. The professional engineer or specialist shall be experienced in the construction of impoundments.

(A) Inspections shall be made regularly during construction, upon completion of the construction, and at least yearly until removal of the structure or release of the performance bond.

(B) The qualified professional engineer shall promptly after each inspection required in subparagraph (A) of this paragraph provide the Commission a certified report that the impoundment has been constructed and/or maintained as designed and in accordance with the approved plan and this chapter. The report shall include discussion of any appearance of instability, structural weakness or other hazard condition, depth and elevation of any impounded waters, existing storage capacity, any existing or required monitoring procedures and instrumentation, and any other aspects of the structure affecting stability.

(C) A copy of the report shall be retained at or near the minesite.

(12) Impoundments meeting the NRCS significant or high hazard class criteria for dams in TR-60, or the size or other criteria of 30 CFR 77.216 must be examined in accordance with 30 CFR 77.216-3. Impoundments not meeting the NRCS Class B or C criteria for dams in TR-60, or subject to 30 CFR 77.216, shall be examined at least quarterly. A qualified person designated by the operator shall examine impoundments for the appearance of structural weakness and other hazardous conditions.

(13) If any examination or inspection discloses that a potential hazard exists, the person who examined the impoundment shall promptly inform the Commission of the finding and of the emergency procedures formulated for public protection and remedial action. If adequate procedures cannot be formulated or implemented, the Commission shall be notified immediately. The Commission shall then notify the appropriate agencies that other emergency procedures are required to protect the public.

(b) Permanent Impoundments. A permanent impoundment of water may be created, if authorized by the Commission in the approved permit based upon the following demonstration:

(1) the size and configuration of such impoundment will be adequate for its intended purposes;

(2) the quality of impounded water will be suitable on a permanent basis for its intended use and, after reclamation, will meet applicable state and federal water-quality standards, and discharges from the impoundment will meet applicable effluent limitations and will not degrade the quality of receiving water below applicable state and federal water-quality standards;

(3) the water level will be sufficiently stable and be capable of supporting the intended use;

(4) final grading will provide for adequate safety and access for proposed users;

(5) the impoundment will not result in the diminution of the quality and quantity of water utilized by adjacent or surrounding landowners for agricultural, industrial, recreational, or domestic uses; and

(6) the impoundment will be suitable for the approved postmining land use.

(c) Temporary Impoundments.

(1) The Commission may authorize the construction of temporary impoundments as part of an underground coal mining operation.

(2) In lieu of meeting the requirements of subsection (a)(9)(A) of this section, the Commission may approve an impoundment that relies primarily on storage to control the runoff from the design precipitation event when it is demonstrated by the operator and certified by a qualified professional engineer that the impoundment will safely control the design precipitation event, the water from which shall be safely removed in accordance with current, prudent engineering practices. Such an impoundment shall be located where failure would not be expected to cause loss of life or serious property damage, except where:

(A) impoundments meeting the NRCS significant or high hazard class criteria for dams in TR-60, or the size or other criteria of 30 CFR 77.216(a), shall be designed to control the precipitation of the probable maximum precipitation of a 6-hour event, or greater event as specified by the Commission; and

(B) impoundments not included in subparagraph (A) of this paragraph shall be designed to control the precipitation of the 100-year, 6-hour event, or greater event as specified by the Commission.

Source Note: The provisions of this §12.517 adopted to be effective November 4, 1997, 22 TexReg 10640; amended to be effective December 28, 2020, 45 TexReg 9503