Current through Register Vol. 39, No. 6, September 16, 2024
(a) For the purposes of this Rule:
(1) "CCR" means Coal Combustion
Residuals.
(2) "CCR unit" means any
CCR landfill, CCR surface impoundment, or lateral expansion of a CCR unit, or a
combination of more than one of these units, based on the context of the
paragraph(s) in which it is used. This term includes both new and existing
units, unless otherwise specified in this Subchapter or the Dam Safety Law of
1967. For the purpose of this Rule, the term only applies to CCR dams and
surface impoundments.
(3) "Dam"
means a structure and appurtenant works erected to impound or divert
water.
(4) "Design flood" means the
flood hydrograph that is used during an engineering assessment of the CCR
unit.
(5) "Liquefaction" means a
phenomenon whereby a saturated or partially saturated soil loses strength and
stiffness in response to an applied stress, usually earthquake shaking or other
sudden change in stress condition, causing it to behave like a
liquid.
(6) "Probable Maximum
Flood" or "PMF" means the theoretically largest flood resulting from the most
severe combination of meteorological and hydrological conditions that could
conceivably occur in the drainage basin. The PMF is the runoff resulting from
the Probable Maximum Precipitation.
(7) "Probable Maximum Precipitation" or "PMP"
means the theoretically greatest depth of precipitation for a given duration
that is physically possible over a given storm area at a particular
geographical location at a certain time of the year. Estimates of rainfall
amounts and distributions associated with the PMP can be found at the following
locations: http://www.nws.noaa.gov/oh/hdsc/PMP_documents/HMR51.pdf and
http://www.nws.noaa.gov/oh/hdsc/PMP_documents/HMR52.pdf
(8) "Toe" means the point of intersection
between the upstream or downstream face of a dam and the natural
ground.
(9) "100-year flood" means
a flood that has a 1-percent chance of recurring in any given year. Rainfall
amounts for the 100-year flood can be found at:
https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html.
(10) "1000-year flood" means a flood that has
a 0.1-percent chance of recurring in any given year. Rainfall amounts for the
1000-year flood can be found at:
https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html.
(b) This Rule shall apply to a CCR
unit that meets one or more of the following:
(1) has a dam height of 25 feet or more above
the downstream toe of the structure and has a storage volume of 50 acre-feet or
more, unless the unit is exempt by
G.S.
143-215.25A;
(2) contains residuals to an elevation of
five feet or more above the downstream toe of the structure and that has a
storage volume of 20 acre-feet or more;
(3) contains residuals to an elevation of
greater than or equal to 20 feet above the downstream toe of the structure;
or
(4) has been classified as high
hazard (Class C) according to Rule .0105 of this Subchapter.
(c) Inspections and Structural
Stability Assessments of CCR units shall be completed as follows:
(1) At intervals not exceeding seven days, a
qualified engineer, or a person under his or her responsible charge, shall
inspect the discharge of all outlets of hydraulic structures that pass
underneath the base of the CCR unit for discoloration of discharge or changes
in flow.
(2) A qualified engineer,
or a person under his or her responsible charge, shall conduct monitoring of
all instrumentation supporting the operation of the CCR unit no less than once
per month according to the standards listed under
40 CFR
257.83(a), which is hereby
incorporated by reference, including subsequent amendments and additions. A
copy of this document may be obtained at no cost at
https://www.ecfr.gov/cgi-bin/text-idx?tpl=/ecfrbrowse/Title40/40cfr257_main_02.tpl.
(3) During the annual inspections of all CCR
units, a qualified engineer, or a person under his or her responsible charge,
shall conduct a visual inspection of hydraulic structures underlying the base
of the CCR unit in order to maintain structural integrity by being kept free of
deterioration, deformation, distortion, bedding deficiencies, sedimentation,
and debris.
(4) A qualified
engineer, or a person under his or her responsible charge, shall conduct
structural stability assessments and shall document whether the design,
construction, operation, and maintenance of the CCR unit is consistent with the
provisions of 40 CFR
257.73(d) and
257.74(d), which
is hereby incorporated by reference, including subsequent amendments and
additions, the NC Dam Safety Law of 1967, and the rules of this Subchapter. The
structural stability assessment shall be completed by a qualified engineer once
every five years and submitted to the Department for review for consistency
with this Subchapter and the Dam Safety Law of 1967.
(d) All CCR dams described in Paragraph (b)
of this Rule shall have a spillway system with capacity to pass a flow
resulting from a design flood as specified in the Minimum Spillway Design Flood
for CCR Units table provided in this Paragraph, unless the applicant provides
calculations, designs, and plans, prepared in accordance with
generally-accepted engineering standards, to show that the design flood can be
stored, passed through, or passed over the CCR unit without failure occurring.
The requirements in the table below shall apply in place of the Minimum
Spillway Design Storm table under Rule .0205(e) of this Section.
|
Minimum Spillway Design Flood for CCR
Units
|
|
Hazard1
|
Size2
|
Spillway Design
Flood3
|
|
Low (Class A)
|
Small
|
100 year
|
|
Medium
|
100 year
|
|
Large
|
1/3 PMF
|
|
Very Large
|
1/2 PMF
|
|
Intermediate
(Class B)
|
Small
|
1000 year
|
|
Medium
|
1/3 PMF or 1000 year, whichever is larger
|
|
Large
|
1/2 PMF
|
|
Very Large
|
3/4 PMF
|
|
High
(Class C)
|
Small
|
PMF
|
|
Medium
|
PMF
|
|
Large
|
PMF
|
|
Very Large
|
PMF
|
|
1 The "Hazard"
categories in this table for CCR units are based on
15A NCAC
02K .0105 Classification of Dams and are the
same "Hazard" categories shown in the "Minimum Spillway Design Storms" table
for non-CCR dams contained in Rule .0205(e) of this Section.
2 The "Size" categories
are the same as described in the "Criteria for Spillway Design Storm Size
Classification" table found in Rule .0205(e) of this Section.
3 The "Spillway Design
Flood" specifications were derived from the combination of the more-stringent
criterion from the spillway design-flood elements of the federal CCR
regulations and the existing spillway design elements of Rule .0205(e) of this
Section.
|
(e) Structural stability assessments shall be
evaluated as follows:
(1) For purposes of this
Rule, the "critical cross sections" utilized for the required structural
stability assessments are the cross sections anticipated by the design engineer
to be the most susceptible to structural failure.
(2) CCR surface impoundments shall be
assessed under seismic loading conditions for a seismic loading event with a 2
percent probability of exceedance in 50 years, equivalent to a return period of
approximately 2,500 years, based on the USGS Seismic Hazard Maps for seismic
events with this return period for the region where the CCR unit is located.
This document is hereby incorporated by reference, including subsequent
amendments and editions. A copy may be obtained at no cost at
https://earthquake.usgs.gov/hazards/hazmaps.
(3) CCR units constructed of or founded upon
soils that are susceptible to liquefaction, as identified by a liquefaction
potential analysis, shall meet liquefaction factors of safety as required in
Part (5)(E) of this Subparagraph. The liquefaction potential analysis shall
include:
(A) soil classifications of the
embankment and foundation soils;
(B) fines content;
(C) plasticity index;
(D) water content;
(E) saturation;
(F) maximum current, past, and anticipated
phreatic surface levels within the embankment, foundation, and
abutments;
(G) location beneath or
above the natural ground surface; and
(H) penetration resistance through cone
penetration testing (CPT).
(4) Stability assessments shall be required
for CCR units with downstream slopes that may be inundated by the pool of an
adjacent water body. These assessments shall include conditions for maximum
pool loading, minimum pool loading, and rapid drawdown of the adjacent
waterbody.
(5) The safety factor
assessments shall be supported by the following engineering calculations:
(A) The calculated static factor of safety
for the end-of-construction loading condition shall equal or exceed 1.30. The
assessment of this loading condition is only required for the initial safety
factor assessment and is not required for subsequent assessments;
(B) the calculated static factor of safety
for the long-term, maximum storage pool loading condition shall equal or exceed
1.50;
(C) the calculated static
factor of safety under the maximum surcharge pool loading condition shall equal
or exceed 1.40;
(D) the calculated
seismic factor of safety shall equal or exceed 1.00; and
(E) for dams constructed of or founded upon
soils that have susceptibility to liquefaction, the calculated liquefaction
factor of safety shall equal or exceed 1.20. Post-liquefaction stability
analyses shall include characterization of the site conditions, identification
of the minimum liquefaction-inducing forces based on soil characterization,
determination of seismic effect on liquefied layers of the embankment, and
calculation of factors of safety against liquefaction for each liquefied layer
of the embankment.
(f) CCR units and surrounding areas that are
constructed of earthen material shall be designed, constructed, operated, and
maintained so that the vegetation meets the conditions outlined in the FEMA 534
guidance document entitled, "Technical Manual for Dam Owners: Impacts of Plants
on Earthen Dams" issued on September 2005. This document is hereby incorporated
by reference, including subsequent amendments and editions. A copy may be
obtained at no cost at
https://www.fema.gov/media-library/assets/documents/1027.
However, alternative forms of slope protection may be approved by the Director,
upon request by a qualified engineer through a plan submittal showing that the
proposed alternative slope protection will provide equal or better protection
from erosion than would be achieved with vegetation as specified in FEMA
534.
Authority
G.S.
143-215.25A;
143-215.26;
143-215.27;
143-215.31;
143-215.32;
143-215.34;
Eff.
January 1, 2019.
