Md. Code Regs. 26.20.21.08

Current through Register Vol. 51, No. 21, October 18, 2024

Section 26.20.21.08 - Impoundments

A. General Requirements. The requirements of this section apply to both temporary and permanent impoundments. Impoundments shall be designed and constructed to ensure:

(1) Compliance with USDA, Natural Resources Conservation Service, Maryland Conservation Practice, Standard Pond Code 378 (January 2000), as incorporated by reference in COMAR 26.17.02.01 a -1B(2);

(2) Compliance with requirements of COMAR 26.17.04.05 a if the embankment is more than 15 feet in height as measured from the upstream toe of the embankment to the crest of the emergency spillway;

(3) Impoundments meeting the Class (b) or (c) criteria for dams in "Earth Dams and Reservoirs", TR-60 shall comply with "Minimum Emergency Spillway Hydrologic Criteria" table in TR-60 and the requirements of this regulation;

(4) The minimum elevation at the top of the settled embankment shall be 1 foot above the water surface in the pond with the emergency spillway flowing at design depth;

(5) The height of the embankment is a minimum of 5 percent over the design height to allow for settlement, unless it has been demonstrated to the Bureau that the material used and the design will ensure against all settlement;

(6) The top width of the embankment is not less than the quotient of (H+35)/5, when H is the height, in feet, of the embankment, as measured from the upstream toe of the embankment;

(7) Perimeter slopes are stable, are not steeper than 2:1, and are protected against erosion where water enters the impoundment;

(8) The combined upstream and downstream side slopes of the settled embankment are not less than 5:1 with neither slope steeper than 2:1;

(9) The crest of the emergency spillway is a minimum of 1 foot above the crest of the principal spillway;

(10) A cut-off trench is installed before initiating the placement of fill material for the embankment with side slopes not steeper than 1:1;

(11) The embankment foundation area is cleared of all organic material and the entire area scarified;

(12) The embankment begins by placing and spreading fill material at the lowest point of the foundation, is brought up in horizontal layers not to exceed 8 inches as required to facilitate compaction, and is compacted as specified in the design approved by the Bureau;

(13) Fill material used in the construction of the embankment is free of sod, large roots and other vegetative matter, frozen soil, and coal processing waste;

(14) The embankment slope is protected against surface erosion and against damage caused by sudden drawdown;

(15) The embankment faces and surrounding areas are vegetated, except that the faces where water is impounded may be riprapped or otherwise stabilized in accordance with accepted design practices; and

(16) Newly constructed areas and those areas disturbed by the construction are vegetated immediately upon completion of the construction.

B. Stability.

(1) Impoundments meeting the Class (b) or (c) criteria for dams contained in "Earth Dams and Reservoirs", TR-60 or the size or other criteria of 30 CFR § 77.216(a) shall have a minimum static safety factor of 1.5 for a normal pool with steady state seepage saturation conditions and a seismic safety factor of at least 1.2.

(2) Impounds not included in §B(1) of this regulation, except for coal mine waste impounding structures, shall be constructed to achieve a minimum static safety factor of 1.3 for a normal pool with steady state seepage saturation conditions.

(3) Unless otherwise determined by the Department based on site specific conditions, impoundments meeting the criteria of §§A(1) and B(2) of this regulation shall be considered to meet a minimum static safety factor of 1.3 for a normal pool with steady state seepage condition, if the:

(a) Embankment is not constructed of organic soil or clay of high plasticity;

(b) Embankment is constructed of fill material that contains sufficient moisture content to achieve proper compaction; and

(c) Existing slope of the foundation area is not steeper than the slope provided for the soil type to be used to construct the embankment in the chart at §B(4) of this regulation.

(4) Soil Classification Chart.

Soil Type	Classification	Slope
Clay (low to medium plasticity)	CL	30 percent
Clay-silt mixture	CL-ML	30 percent
Silt	ML	30 percent
Clayey sand	SC	25 percent

(5) An indication of sufficient moisture content in the fill material referenced in §B(3)(b) of this regulation is that the soil should form a ball which does not readily separate when kneaded by hand. The engineer may also specify other methods of testing moisture content.

C. Freeboard.

(1) Impoundments shall have adequate freeboard to resist overtopping by waves and by sudden increases in storage volume.

(2) Impoundments meeting the Class (b) or (c) criteria for dams in "Earth Dams and Reservoirs", TR-60 shall comply with the freeboard hydrograph criteria in "Minimum Emergency Spillway Hydrologic Criteria" table in TR-60.

D. Foundation.

(1) 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.

(2) For an impoundment meeting the Class (b) or (c) criteria for dams contained in "Earth Dams and Reservoirs", 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.

(3) For impoundments not meeting the size or other criteria of 30 CFR § 77.216(a), foundation investigation, as well as any necessary laboratory testing of foundation material, may be required to establish the stability of the foundation.

E. Spillways.

(1) An impoundment shall include either a combination of principal and emergency spillways or a single spillway configured as specified in §D(2) of this regulation, designed and constructed to safely pass the applicable design precipitation event specified in §D(3) of this regulation.

(2) The Department may approve a single open-channel spillway that is:

(a) Of nonerodible construction and designed to carry sustained flows; or

(b) Rock or grass-lined and designed to carry short-term, infrequent flows at nonerosive velocities when sustained flows are not expected.

(3) The required design precipitation event for impoundments meeting the spillway requirements of this section are:

(a) For an impoundment meeting the size or other criteria of 30 CFR § 77.216(a), a 100-year, 24-hour event, or larger event specified by the Department;

(b) For impoundments not meeting the size or other criteria of 30 CFR § 77.216(a), a 25-year, 24-hour event, or larger event specified by the Department; or

(c) For impoundments meeting the Class (b) or (c) criteria for dams in "Earth Dams and Reservoirs", TR-60, in accordance with the emergency spillway hydrograph criteria in the "Minimum Emergency Spillway Hydrologic Criteria" table in TR-60, or larger event specified by the Department.

F. Impoundments meeting the size or other qualifying criteria of 30 CFR § 77.216(a) shall comply with 30 CFR § 77.216 and this regulation. The plan required to be submitted to the district manager of the Mine Safety and Health Administration (MSHA) under 30 CFR § 77.216 shall also be submitted to the Department as part of the permit application.

G. All temporary and permanent impoundments shall be designed by, or under the supervision of, a professional engineer, using current, prudent, engineering practices and design criteria, and the design certified by a professional engineer.

H. Impoundments shall be routinely maintained during the mining operations in the following areas:

(1) Vegetative growth shall be cut where necessary to facilitate inspection and repair;

(2) Ditches and spillways shall be cleaned;

(3) Combustible material present on the surface, other than material like mulch or dry vegetation used for surface stability, shall be removed; and

(4) Other appropriate maintenance procedures shall be followed.

I. Plans for an enlargement, reduction in size, reconstruction, or other modification of dams or impoundments shall be submitted to the Department and shall comply with the requirements of this regulation.

J. Except when a modification is required to eliminate an emergency condition constituting a hazard to public health, safety, or the environment, the Department shall approve the plans before modification begins.

Md. Code Regs. 26.20.21.08

Regulations .08 were previously codified as COMAR 08.13.09.24 a A_I, respectively. Recodification occurred in April, 1993. For a history of these regulations before April, 1993, see "Administrative History of COMAR 08.13.09 before April, 1993" which follows COMAR 26.20.01.
Regulation .08 amended effective March 14, 2005 (32:5 Md. R. 582)