N.D. Admin Code 69-05.2-16-09 - Performance standards - Hydrologic balance -Sedimentation ponds
1. General
requirements. Sedimentation ponds must be used individually or in series and:
a. Be constructed before any disturbance of
the undisturbed area to be drained into the pond.
b. Be located as near as possible to the
disturbed area and out of perennial streams, unless approved by the commission.
c. Meet all the criteria of this
section.
2. Sediment
storage volume. Sedimentation ponds must provide adequate sediment storage
volume. Sediment storage volume must be determined using the universal soil
loss equation, gully erosion rates, and the sediment delivery ratio converted
to sediment volume, using either the sediment density or other empirical
methods derived from regional sediment pond studies if approved by the
commission.
3. Detention time.
Sedimentation ponds must provide the required theoretical detention time for
the water inflow or runoff entering the pond from a ten-year, twenty-four-hour
precipitation event (design event). The theoretical detention time must be
sufficient to achieve and maintain applicable effluent standards. The
calculated theoretical detention time and all supporting materials must be
included in the permit application.
4. Dewatering. The stored water must be
removed by a nonclogging dewatering device or a conduit spillway approved by
the commission, and have a discharge rate to achieve and maintain the required
theoretical detention time. The dewatering device may not be lower than the
maximum elevation of the sediment storage volume.
5. Each operator shall design, construct, and
maintain sedimentation ponds to prevent short circuiting to the extent
possible.
6. The design,
construction, and maintenance of a sedimentation pond or other sediment control
measures do not relieve the operator from compliance with applicable effluent
limitations.
7. For sedimentation
ponds designed to contain the runoff from a ten-year, twenty-four-hour
precipitation event (design event), there must be no spillway outflow as a
result of runoff from the design event or lesser runoff events, unless multiple
runoff events occur before the pond can be dewatered in accordance with
approved plans in the permit.
8.
Sediment must be removed from sedimentation ponds on a periodic basis in order
to maintain an adequate storage volume for the design event.
9. An appropriate combination of principal
and emergency spillways or a single spillway must be provided to safely
discharge the runoff from a twenty-five-year, six-hour precipitation event for
a temporary impoundment, a fifty-year, six-hour precipitation event for a
permanent impoundment, or a larger event specified by the commission. The
spillways must be capable of safely discharging the required event when the
impoundment is at high water elevation. Commission approval of open channel
spillway grades and allowable velocities must be obtained and velocities must
be nonerosive. Earth or grass lined spillways may be used only where sustained
flows are not expected.
10. The
minimum elevation at the top of the settled embankment must be one foot [30.48
centimeters] above the water surface in the pond with the emergency spillway
flowing at design depth. For embankments subject to settlement, this minimum
elevation requirement applies at all times.
11. The constructed height of the dam must be
increased a minimum of five percent over the design height to allow for
settlement, unless it has been demonstrated to the commission that the material
used and the design will ensure against settlement.
12. The minimum top width of the embankment
may not be less than the quotient of (H+35)/5, where H is the height, in feet,
or (H+10.7)/5, where H is the height, in meters, of the embankment as measured
from the upstream toe of the embankment.
13. The upstream side slope of the settled
embankment may not be steeper than a horizontal to vertical ratio of 3:1 and
the downstream side slope of the settled embankment may not be steeper than a
horizontal to vertical ratio of 2:1.
14. The foundation area must be cleared of
all organic matter, all surfaces sloped to no steeper than a horizontal to
vertical ratio of 1:1, and the entire foundation surface scarified. Cutoff
trenches must be installed if necessary to ensure stability.
15. The fill material must be free of sod,
large roots, other large pieces of vegetative matter, and frozen soil, and in
no case shall coal processing waste be used.
16. The placing and spreading of fill
material must be started at the lowest point of the foundation. The fill must
be brought up in horizontal layers in the thicknesses required to facilitate
compaction and meet the design requirements of this section. Fill adjacent to
structures, pipe conduits, and drainfill or antiseep collars must be compacted
to a density equal to that of the surrounding fill by hand tamping or by using
manually directed power tampers or plate vibrators. Compaction must be
conducted as specified in the approved design. In lieu of the specific design
requirements of this subsection and subsections 11 through 14, the operator may
demonstrate that the design of the structure has a minimum static safety factor
of 1.3 for a normal pool with steady state seepage saturation conditions.
17. If a proposed impoundment can
meet the size and other criteria of
30 CFR
77.216, the following additional requirements
must be met:
a. An appropriate combination of
principal and emergency spillways must be provided to safely discharge the
runoff from a one-hundred-year, six-hour precipitation event, or a larger event
as specified by the commission.
b.
The embankment must be designed and constructed with a seismic safety factor of
1.2 and a static safety factor of at least 1.5 for a normal pool with steady
state seepage saturation conditions, or a higher safety factor as designated by
the commission.
c. Appropriate
barriers must be provided to control seepage along conduits that extend through
the embankment.
d. The criteria of
the mine safety and health administration as published in
30 CFR
77.216 must be met.
18. If an impoundment meets the class B or C
criteria for dams (those located where failure would be expected to cause loss
of life or serious property damage) in the United States department of
agriculture, natural resource conservation service technical release no. 60
(TR-60), Earth Dams and Reservoirs, 1985, the following apply:
a. The emergency spillway must comply with
freeboard hydrograph criteria found in the minimum emergency spillway
hydrologic criteria table in TR-60, or greater event specified by the
commission.
b. The embankment must
be designed and constructed with a seismic safety factor of 1.2 and a static
safety factor of at least 1.5 for a normal pool with steady state seepage
saturation conditions, or a higher safety factor as designated by the
commission.
c. Foundation testing,
as well as any necessary laboratory testing of foundation material, must be
performed to determine design requirements for foundation stability.
d. Appropriate barriers must be provided to
control seepage along conduits that extend through the embankment.
19. Impoundment inspections.
a. A registered professional engineer, or
other specialist under the direction of a registered professional engineer,
shall inspect each impoundment as required under subdivisions b, c, and
d. The registered professional
engineer and specialist must be experienced in the construction of
impoundments.
b. Inspections must
be made regularly during construction. Reports for each inspection during
construction must be prepared and maintained at or near the minesite.
c. After impoundment construction
is complete, the registered professional engineer shall promptly provide the
commission a certified report that the impoundment has been inspected during
construction on a regular basis and it has been constructed as designed and
according to the approved plan and this chapter. The report must include a
summary of the construction inspections, final storage capacity, and if
applicable, a discussion of any deviations from the approved plan.
d. After impoundments are operational, they
shall be inspected at least yearly until removal of the structure or release of
the performance bond. A registered professional engineer shall promptly provide
the commission with an annual impoundment inspection report addressing any
appearance of instability, structural weakness, or other hazardous conditions,
depth and elevation of any impounded water, existing storage capacity, any
existing or required monitoring procedures and instrumentation, and any other
aspects of the structure affecting stability. A single report can address more
than one impoundment; however, the condition of each impoundment must be noted
separately.
e. A copy of the
inspection reports must be maintained at or near the minesite.
20. The embankment, including the
surrounding areas and diversion ditches disturbed or created by construction,
must be stabilized with respect to erosion and sudden drawdown by a vegetative
cover or other means immediately after the embankment is completed. The active
upstream face where water will be impounded may be riprapped or otherwise
stabilized to protect the embankment from erosion and sudden drawdown. Areas
where the reestablishment of vegetation is not successful or where rills and
gullies develop must be repaired and revegetated according to section
69-05.2-15-06.
21. In addition to the requirements of
subsection 19, all impoundments meeting the criteria of subsections 17 and 18
must be examined according to
30 CFR
77.216-3. Other impoundments must be examined
at least quarterly by a qualified person for appearance of erosion, structural
weakness, and other hazardous conditions. The annual inspection required by
subsection 19 will be considered one of the examinations required by this
subsection.
22. Plans for any
enlargement, reduction, reconstruction, or other modification of dams or
impoundments must be submitted to the commission. Commission approval of these
plans is required before modification begins, unless a modification is
necessary to eliminate a hazard to public health, safety, or the environment.
23. Sedimentation ponds or other
sediment control measures may not be removed until authorized by the commission
and the disturbed area has been stabilized and revegetated. The ponds or other
measures may not be removed sooner than two years after the last augmented
seeding unless the last augmented seeding is a supplemental seeding into an
established vegetation stand that is effectively controlling erosion. When the
pond is removed, the affected land must be reclaimed, unless the pond has been
approved by the commission for retention. If the commission approves retention,
the pond must meet all the requirements for permanent impoundments of section
69-05.2-16-12.
Notes
General Authority: NDCC 38-14.1-03
Law Implemented: NDCC 38-14.1-24
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