HomeMy WebLinkAboutDRC-2009-006364 - 0901a0688014d81arDii
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DENISO
MINES
November 18.2009
Senl Via Electronic Mail (PDF) and Federal Express
Mr. Dane Finerfrock,
Executive Secretary,
Utah Radiation Control Board.
Utah Department of Environmental Quality,
Division of Radiation Control
168 North 1950 West
SaU Uke City, Utah 84114-4850
Dear Mr. Finerfrock:
Denison Mines (USA) Corp.
1050 17th Street, Suite 950
Denver, CO 80265
USA
Tel: 303 628-7798
Fax: 303 389-4125
www.denisonmines.com
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NOV 2009
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Re: White Mesa Mill-Amendment Request Relating to Freeboard Limit Calculations for
Tailings Cells and Comrol of Storm Water from Cell 3
Please find attached a letter leport authored by Geosyntec Consultants which was prepared in
Older lo respond to DRC's request for informatioji dated October S, 2009 pertaining to fhe
ahove-captioned subject.
If you should have any questions regarding this report please contact Mr. Harold Roberts or me.
Yours very truly.
>ENisoN MINES (USA) CORP.
Steven D. Lajidau
Manager, Environmental Affairs
CC Ron Hochstein
Harold Roberts (w/o attachments)
David Frydenlund
David Turk
Page 1 of 21
Written by: S. Yeo Date: 10/29/09 Reviewed by: G. Corcoran Date: 11/9/09
Client: DMC Project: White Mesa Mill Project/
Proposal No.:
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ANALYSIS OF SURFACE WATER HYDROLOGY
WHITE MESA MILL, BLANDING, UTAH
OBJECTIVE
The objective of this surface water hydrology analysis is to evaluate surface water
management associated with the 6-hour Probable Maximum Precipitation (PMP) event
in Cells 2, 3, 4A, and 4B at the White Mesa Mill site near Blanding, UT. The analysis
includes Cells 2, 3, and 4A inflow and outflow hydrograph evaluations, freeboard limit
establishment in Cell 4A and Cell 4B, stormwater discharge rates, and stormwater
volumes.
Engineering analyses are performed for the following conditions:
• Case 1: Current conditions – Cell 3 partially filled with tailings, interim cover
placed on portions of Cell 3, and Cell 4A accepting surface water runoff from
Cells 2 and 3;
• Case 2: Interim conditions – Cell 3 filled with tailings and interim cover
placement complete, Cell 4A accepting surface water runoff from Cells 2 and 3;
and
• Case 3: Interim conditions – Cell 3 and Cell 4A filled with tailings and interim
cover, and Cell 4B accepting surface water run off from Cells 2, 3 and 4A.
DESIGN CRITERIA
Based on the Utah Department of Transportation (UDOT) Drainage Manual (2004),
Spillway capacity calculations (Geosyntec, 2007), and the Probable maximum
precipitation (PMP) event computation (Geosyntec, 2009), the following conclusions
are summarized and presented with a schematic site plan in Figure 1:
• The PMP for the site 6-hour storm event is 10 inches (in.);
• The watershed areas of Cells 2, 3, 4A, and 4B are 87 acres (ac), 83 ac, 42 ac,
and 42 ac respectively;
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• Total watershed area is approximately 254 ac;
• The discharge rates for Cells 2, 3, and 4A were calculated to be 1283 cubic feet
per second (cfs), 1224 cfs, and 620 cfs, respectively.
• Spillways were installed between Cells 2 and 3, and Cells 3 and 4A. A spillway
was designed between Cells 4A and 4B and will be installed with the
construction of Cell 4B; and
• The spillways between Cells 3 and 4A and between Cells 4A and 4B were
designed for peak discharge rates of 3050 cfs and 3264 cfs, respectively.
Existing Cell 1
Existing Cell 2
Existing Cell 3
Cell 4A Cell 4B
SUMMARY
Watershed Area
- Cell 2: 87 ac
- Cell 3: 83 ac
- Cell 4A: 42 ac
- Cell 4B: 42 ac
- Total: 254 ac
Discharge Given
- Cell 2: 1283 cfs
- Cell 3: 1224 cfs
- Cell 4: 620 cfs
PMP for 6-hr event:
10 in.
Not to Scale
Figure 1. Schematic site plan
Spillways
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DESIGN HYDROGRAPH
A design hydrograph for Cell 3 is developed using the Rational Method (RM) described
in the UDOT Drainage Manual for areas as large as 300 acres (UDOT, 2004). The RM
formula is expressed as follows:
Q = C I A
Where: Q = peak discharge, in cubic feet per second (cfs)
C = runoff coefficient proportion of the rainfall that runs off the surface (no
units)
The runoff coefficient, C is based on the percent of impervious area, soil types,
and slope for the drainage area under consideration. For this evaluation, a C
value for light industrial (0.6) is used (UDOT, 2004; Attachment A). This is a
conservative value since the site has little impervious area.
I = average rainfall intensity for a duration, in inch per hour (in/hr)
The PMP evaluated using “Hydrometeorological Report No. 49: Probable Maximum
Precipitation Estimates, Colorado River and Great Basin Drainages” (Hansen, et. Al.,
1984) was 10–in. for the 6-hour storm event. Based on the PMP event computation
(2009), the hourly increments are arranged as follows:
Hour: 1 2 3 4 5 6
Incremental
Rainfall (in.): 0.1 0.4 8.3 0.8 0.2 0.1
In addition to the hourly increments, 15-min incremental PMP values for hour 3, the
highest hour, is calculated using Table 4.5 in Hansen, et al. (1984, Attachment B), as
shown in Table 1. Table 1 also presents intensity and discharge rates from Cells 2, 3,
and 4A. Hydrographs of each cell are depicted in Figure 1. Peak flows from Cells 2, 3,
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and 4A shown in Table 1 are consistent with the previous discharge analyses performed
for the spillway calculations (Geosyntec, 2007).
Table 1: Increment, intensity, and flows of each cell
Hour Increment
(in) Intensity (in/hr) Cell 2 (cfs) Cell 3 (cfs) Cell 4A (cfs)
0 0 0 0 0 0
1 0.1 0.1 5.2 8 4
2 0.4 0.4 20.9 17 8
2.25 6.14 24.56 (= 6.14 x 4) 1282.1 1224 619
2.5 1.25 5.0 (= 1.25 x 4) 261.0 248 126
2.75 0.50 2.0 (= 0.50 x 4) 104.4 99 50
3 0.42 1.68 (= 0.42 x 4) 87.7 83 42
4 0.8 0.8 41.8 45 23
5 0.2 0.2 10.4 12 6
6 0.1 0.1 5.2 4 2
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CASE 1
Introduction
Case 1 presents the current site conditions: Cell 3 is partially filled with tailings and the
most eastern portion of the cell has partial interim cover placement. As illustrated in
Figure 3, precipitation falling on the eastern portions of Cell 2 is expected to sheet flow
to Cell 3 based on existing topographies provided by DMC. The eastern portion of Cell
3 is graded to drain towards the tailings depression in the central portion of Cell 3. The
western portions of Cells 2 and 3 are graded to drain towards the central portion of Cell
3 and the Cell 3 to Cell 4A spillway. It is assumed that the greatest depth location
(GDL) along the Cell 3 southern berm is behind the Cell 3/4A spillway due to the
current grading conditions and depressions in the vicinity of the spillway (Figure 4).
Therefore, surface water from Cells 2 and 3 is conveyed to Cell 4A.
Figure 2: Hydrographs with 6-hour PMP
0
200
400
600
800
1000
1200
1400
0123456Time (hr)
Fl
o
w
(
c
f
s
)
Cell 2
Cell 3
Cell 4A
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Volume Storage in Cell 3 and Hydrograph for Outflow
Based on the existing topography and the hydrograph of Cell 3 inflow, the stormwater
storage volume is evaluated. The lowest area in Cell 3 is at Elev. 5602 ft MSL and the
Cell 3/4A spillway elevation is 5604 ft. The approximate storage areas at elevations
5602 ft and 5604 ft are 12 ac and 22 ac, respectively, as shown in Figure 4, resulting in
an approximate water storage volume of 253 ac.ft (10,995,488 cu. ft) in Cell 3. Figure
5 presents the storage volume of stormwater from the PMP, as a function of time in Cell
3.
Existing Cell 1
Existing Cell 2
Existing Cell 3
Existing Cell 4A
Not to Scale
Figure 3. Schematic surface water flow in Case 1
Water flow
N
GDL (Elev.): 5602 ft
Bottom of spillway:
(Elev.): 5604 ft
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Figure 4. Areas of Elev. 5602 ft and Elev. 5604 ft in Cell 3
Elev. 5602 ft
Elev. 5604 ft
N
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Figure 5. Volume stage of stormwater vs. time
0
0.5
1
1.5
2
2.5
0 50 100 150 200 250 300Storage (ac.ft)
Ti
m
e
(
h
r
)
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Hydrograph for Cell 3
Using RM methodology, a hydrograph for Cell 3 inflow is generated using Table 2 and
Figure 5. The Cell 3 inflow is 2506 cfs which includes discharge from Cell 2 and
precipitation falling on Cell 3. Note: the resulting Cell 3 outflow is evaluated after the
storage volume calculation resulting in a lower peak outflow (1149 cfs) than if Cell 3
storage was not available.
Figure 6. Schematic surface water flow in Case 1
0
500
1000
1500
2000
2500
3000
0123456Time (hr)
Fl
o
w
(
c
f
s
)
Cell 3 outflow
Cell 3 inflow
Page 10 of 21
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Freeboard Limit in Cell 4A
The total volume of stormwater flow entering Cell 4A is discharge from Cell 3 and the
precipitation falling across the Cell 4A area. This volume is represented by the area
beneath the 6-hour hydrograph (Figure 7).
0
500
1000
1500
2000
0123456
Time (hr)
Fl
o
w
(
c
f
s
)
Figure 7. Hydrograph of inflow Cell 4A
The stormwater volume flowing from Cell 3 to Cell 4A was found to be: 3,240,000 cu.
ft. The stormwater volume across the area of Cell 4A results in an approximate depth of
2 ft:
Depth = Volume of Runoff/Area of Cell
1.95 ft = 3,240,000 cu. ft./(38 acres x 43,560 sq. ft./acre)
1.95 ft ≈ 2 ft
Where:
Area of cell at 5590 ft MSL = 38.2 acres
Area of cell at 5588 ft MSL = 37.8 acres
Average area = 38 acres
Page 11 of 21
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Per the Cell 4A BAT Monitoring, Operations, and Maintenance Plan (2009), the
minimum free board required is 3 ft, or 5590 ft MSL; therefore, since the 2 ft required
for the 6 hr PMP event is less than 3 ft, the maximum solution elevation will be 5590 ft
MSL.
CASE 2
Introduction
Case 2 presents the interim condition: Cells 2 and 3 are filled with tailings and the 3 ft
interim cover, sloping north to south at 0.2%, is in place. In this case, Cell 4A is
accepting surface water run off from Cells 2 and 3, as shown in Figure 8. Precipitation
falling on the eastern portions of Cells 2 and 3 would sheet flow into Cell 4A.
Precipitation on the western portions of Cells 2 and 3 would sheet flow into a
conveyance structure located along the southern edge of Cell 3. The conveyance
structure would direct flow to the Cell3/4A spillway, as shown in Figure 6. It is
assumed that all surface water from Cells 2 and 3 would be diverted to Cell 4A.
Therefore it is not required to evaluate hydrograph inflow and outflow and/or volume
storage in Cell 3.
Page 12 of 21
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Channel Sizing in Cell 3
A trapezoidal channel is proposed along the Cell 3 southern berm, as shown in Figures
8 and 9. The proposed 1800 ft long channel will convey surface water from the western
portions of Cells 2 and 3 with a slope of 0.2% towards the spillway. Using the peak
runoff data, the required flow depth was calculated using the Mannings’ equation:
Q = (1.49/n)*R2/3*S1/2*A
Where:
Q = discharge (cfs)
n = roughness coefficient, 0.025 for soil material
R = hydraulic radius (ft)
S = channel slope (ft/ft)
Existing Cell 1
Existing Cell 2
Existing Cell 3
Cell 4A
Not to Scale
Figure 8. Schematic surface water flow in Case 2
Water flow
Proposed
channel
N
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A = flow area (ft2)
And the selected channel size is:
Bottom width: 20 ft
Side slopes: 4 Horizontal:1 Vertical (H:V)
(See Attachment C)
Based on the UDOT Drainage Manual (UDOT, 2004), freeboards are calculated as
follows:
]2[6
1 2
max g
VYFB+=
Where:
FB = freeboard (ft)
Ymax = maximum depth of flow (ft)
V = average velocity of flow (ft/sec)
g = acceleration due to gravity = 32.2 ft/sec2
Expected peak discharge, freeboard, and total design depth were calculated using the
spreadsheet included as Attachment C, and are summarized as follows:
Table 2. Minimum freeboard and maximum depth along the channel
Location (from
west end)
Expected peak discharge
and sub-area (cfs)
Actual
Depth (ft)
Freeboard
(ft)
Max. Design
Depth (ft)
Phase I @ 600 ft 60
(sub-area A) 1 0.1 1.1
Phase II @ 1200 ft 320
(sub-areas A & B) 2.5 0.5 3
Phase III @ 1800 ft 690
(sub-areas A, B, & C) 3.8 0.7 4.51
Note: (1) GDL along the Cell 3 southern berm.
Page 14 of 21
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Existing Cell 2
Existing Cell 3
Not to Scale
Figure 9. Schematic surface water flow in Case 2; (a) plan view with sub
watershed and (b) detail view of channel
Surface water flow
1.1-ft depth
@ 600 ft
3-ft depth
@ 1200 ft
4.5-ft depth
@ 1800 ft
(a)
(b)
Water flow Sub-area C
Sub-area B
Sub-area A
Spillway
0-ft depth
N
Page 15 of 21
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Freeboard Limit in Cell 4A
Stormwater inflow to Cell 4A is comprised of discharge from Cell 3 and precipitation
falling on Cell 4A (the area beneath the Cell 4A hydrograph, Figure 10).
The volume was found to be: 5,940,000 cu. ft. The stormwater volume across the area
of Cell 4A results in an approximate depth of 4 ft:
Depth = Volume of Runoff/Area of Cell
3.61 ft = 5,940,000 cu. ft./(37.8 acres x 43,560 sq. ft./acre)
3.61 ft ≈ 4 ft
Where:
Area at 5590 ft MSL = 38.2 acres
Area at 5586 ft MSL = 37.3 acres
Average Area = 37.8 acres
Per the Cell 4A BAT Monitoring, Operations, and Maintenance Plan (2009), the
minimum free board required is 3 ft (5590 ft MSL). Because 4 ft is greater than the
Figure 10. Hydrograph of inflow Cell 4A
0
500
1000
1500
2000
2500
3000
3500
0123456Time (hr)
Fl
o
w
(
c
f
s
)
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minimum freeboard requirements of 3 ft, the maximum allowable level of solution is
5589 ft MSL (4 ft of freeboard).
CASE 3
Introduction
Case 3 presents future site conditions: Cell 3 is filled with tailings and 3 ft interim cover
sloping north to south at a slope of 0.2%. Cell 4A is filled with tailings and has partial
3 ft interim cover placed, sloping north to south at a slope of 0.2%. In this case, the
precipitation falling on the western portions of Cells 2 and 3 would be expected to sheet
flow directly into the proposed Cell 4B. Precipitation falling from the eastern portions
of Cells 2 and 3 would sheet flow into Cell 4A. Sheet flow would continue across Cell
4A into a proposed trapezoidal conveyance structure along the southern berm to direct
flow to the Cell 4A/4B spillway (Figures 11 and 12).
Existing Cell 1
Existing Cell 2
Existing Cell 3
Cell 4A
Not to Scale
Figure 11. Schematic surface water flow in Case 3
Water flow
Cell 4B
N
Page 17 of 21
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Channel Sizing in Cell 4A
A trapezoidal channel is proposed along the southern berm in Cell 4A, as shown in
Figure 11. The proposed 1550 ft channel will convey surface water from Cells 2, 3, and
4A to the Cell 4A/4B spillway with a 0.4% slope. The channel size is based on the peak
runoff of 2124 cfs. The selected channel size is as follows:
Channel size (see Attachment D)
Bottom width: 40 ft
Side slope (south): 8(H): 1(V)
Side slope (north): 5(H): 1(V)
Table 3. Maximum depths and minimum freeboards along the channel
Location
(from
east end)
Expected peak
discharge and
sub-area
(cfs)
Actual
Depth
(ft)
Freeboard
(ft)
Max.
Design
Depth
(ft)
Approx.
Proposed
Cover
Elevation
(ft, MSL)
Designed
Elevation @
bottom of
channel
(ft, MSL)
Phase I @ 0 ft 515
(sub-areas A) 2.1 0.4 2.5 5606 5603.5
(= 5606-2.5)
Phase II @ 1550 ft 2124
(sub-areas A & B) 4.1 0.9 5.0 5602.35
5597.35
(= 5602.35-
5.0)
Page 18 of 21
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Freeboard Limit in Cell 4B
No stormwater storage is assumed in Cell 4A; therefore, the volume of stormwater
entering Cell 4B is the same as the volume calculated in Case 2 (5,940,000 cu. ft) plus
the precipitation falling in Cell 4B:
Existing Cell 2
Existing Cell 3
Cell 4A
Not to Scale
Figure 12. Schematic surface water flow in Case 3
Water flow
Cell 4B
Surface water flow
(b)
(a)
Sub-area B Sub-area A
5597.35 ft MSL
@ 1550 ft
5603.5 ft MSL
@ 0 ft
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4B Volume = 4A Volume + 4B Precipitation
4B Volume = 5,940,000 + (42 ac. x 43,560 sq. ft./ac x 10/12 ft) = 7,464,600 cu. ft.
The volume was found to be approximately 7,464,600 cu. ft. The stormwater volume
across the area of Cell 4B results in an approximate depth of 4.5 ft:
Depth = Volume of Runoff/Area of Cell
4.23 ft = 7,464,600 cu. ft./(40.5 acres x 43,560 sq. ft./acre)
4.23 ft ≈ 4.5 ft
Where:
Area at elevation 5591 ft MSL = 41 acres
Area at elevation 5586.5 ft MSL = 39.9 acres
Average area = 40.5 acres
Per the Cell 4A BAT Monitoring, Operations, and Maintenance Plan (2009), the
minimum freeboard required is 3 ft (5591 ft MSL). Because 4.5 ft is greater than the
minimum freeboard requirements of 3 ft, the maximum allowable level of solution is
5589.5 ft MSL (4.5 ft of freeboard).
CONCLUSIONS
The conclusions are as follows:
• The current site conditions are adequate to handle the 6 hour PMP event;
• The proposed site conditions with suggested conveyance channels will be
adequate to handle the 6 hr PMP event;
• Under the current conditions, the maximum height of tailings in Cell 4A should
be no greater than 5590 ft MSL;
• Under the interim Cells 2 and 3 closure conditions, Cell 4A should have a
maximum height of tailings no greater than 5589 ft MSL. Once Cell 4B is
constructed and accepting tailings, Cell 4A can be filled to capacity; and
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• Under the interim Cells 2, 3, and 4A closure conditions, Cell 4B should have a
maximum height of tailings no greater than 5589.5 ft.
REFERENCES
Utah Department of Transportation (UDOT). Drainage Manual (2004), Manual of
Instruction – Roadway Drainage (US Customary Units), Storage Facilities.
Geosyntec, 2007. Cell 4B Design Report, White Mesa Mill, Blanding, Utah, Spillway
Capacity Calculation, December.
Denison Mines Corporation (DMC), 2009. Cell 4B Lining System Design Report,
Response to DRC Request for Additional Information – Round 3 Interrogatory,
Cell 4B Design - Probable maximum precipitation (PMP) event computation
(2009), September.
Denison Mines Corporation (DMC), 2009. Cell 4A BAT Monitoring, Operations and
Maintenance Plan.
Hansen, E. Marshall, Schwartz, Francis K., Riedel, John T., 1984.
“Hydrometeorological Report No. 49: Probable Maximum Precipitation
Estimates, Colorado River and Great Basin Drainages,” Hydrometeorological
Branch Office of Hydrology National Weather Service, U.S. Department of
Commerce, National Oceanic and Atmosphere Administration, U.S. Department
of Army Corps of Engineers, Silver Springs, Md.
ATTACHMENTS
Attachment A: Recommended Runoff Coefficients (UDOT, 2004)
Attachment B: Table 4.5 in Hansen, et al. (1984)
Attachment C: Channel design in Cell 3
Attachment D: Channel design in Cell 4