Technical Memorandum

Transcription

1 Tucson Office 3031 West Ina Road Tucson, AZ Tel Fax Technical Memorandum To: Kathy Arnold From: Greg Hemmen, P.E. Company: Rosemont Copper Company Date: April 2, 2010 Re: Rosemont Waste Rock Storage Area Stormwater Management CC: David Krizek, P.E. (Tetra Tech) Doc #: 090/ Introduction This Technical Memorandum discusses stormwater management and the design of detention ponds and stormwater conveyance channels for the final reclaimed surface of the Waste Rock Storage Area associated with the proposed Rosemont Copper Project (Project) in Pima County, Arizona. The analyses presented herein were performed on the base concept of the Rosemont Ridge Landform shown on Figure 1. Should modifications occur to this base concept design, the general stormwater analysis presented herein, and the corresponding layout of the detention ponds and stormwater conveyance channel design, would still be applicable assuming the configuration of basin areas and channel lengths were comparable to those analyzed herein. In general, stormwater traveling overland, down side-slopes, or in swales and channels throughout the Waste Rock Storage Area will be routed to a series of proposed detention ponds located on wide benches that are hydraulically linked to downstream detention ponds and ultimately to perimeter containment areas by riprap drop chutes. The perimeter containment areas (PCAs) are generally sited between the toe of the Rosemont Ridge Landform and an adjacent natural ridgeline. The design method for the stormwater detention ponds and perimeter containment areas was based on the Natural Resources Conservation Service (NRCS) curve number approach. This hydrologic analysis was used to estimate total stormwater runoff volumes. The NRCS method for determining peak stormwater runoff, along with Manning s open-channel flow equation, was used in the hydraulic analysis associated with the conveyance channels. Conveyance channels are part of the drainage benches located in the west portion of the Waste Rock Storage Area. Tetra Tech prepared a technical memorandum titled Rosemont Dry Stack Tailings Facility Drainage Bench Analysis (Tetra Tech, 2010b) that further discusses the design of these drainage benches. 2.0 Hydrologic Methodology Overview (NRCS Method) The NRCS method was developed for general hydrologic analysis and allows for various storm distributions and durations to be analyzed. The NRCS approach is applicable to the analysis of large complex watershed systems, such as mining sites, where landscape conditions may

2 change over time. Due to its widespread acceptance, the NRCS method has been incorporated into many hydrologic modeling programs. The analysis was performed using HEC-HMS, which is a hydrologic modeling software package developed by the U.S. Army Corps of Engineer s for general applications. HEC-HMS allows for the analysis of more complex/integrated systems, i.e., multiple sub-basins, reservoir, and channel routing, etc. The primary input variables for the NRCS method required for determining total stormwater runoff volumes and peak flows are: Precipitation; Storm distribution; Curve number; Basin delineation or area; and Time of concentration or lag time. The NRCS method utilized by HEC-HMS employs these parameters to develop a specific basin s relationship between runoff and time to produce a hydrograph curve. The area under the hydrograph curve represents the basin s expected total runoff volume and the apex signifies the basin s estimated peak flow rate. The basin s resultant values for total runoff volume and peak flow were used for the design of the detention ponds and conveyance channels. These input variables are presented in the following sections and are discussed in greater detail in the technical memorandum titled Rosemont Copper Project Hydrology Method Justification (Tetra Tech, 2010a). 2.1 Precipitation Precipitation was acquired from the NOAA Atlas 14 Point Precipitation website. The coordinates used to obtain precipitation data for the Project site were N W, at an elevation of 4,429 feet above mean sea level (amsl). This point is located northeast of the Waste Rock Storage Area. The mean NOAA Atlas 14 precipitation values are typically used with the NRCS analysis. 2.2 Rainfall Storm Distributions The NRCS method allows for many precipitation patterns to be applied to the watershed. The following return periods were analyzed: The 500-year, 24-hour storm event; The 1,000-year, 24-hour storm event; and The General Probable Maximum Precipitation (PMP). The storms analyzed for estimating total runoff volume and peak runoff using the NRCS method were the 24-hour NRCS Type II Distribution and the 72-hour General PMP Distribution. Table 1 2

3 summarizes the storms that were considered for design purposes. For detailed information regarding the development and analysis of these storms, refer to the technical memorandum titled Rosemont Copper Project Hydrology Method Justification (Tetra Tech, 2010a). Table 1 Design Storms Parameters NRCS Storms Return Period (years) 1, N/A Duration (hours) Precipitation (inches) Distribution NRCS Type II NRCS Type II General PMP 2.3 Rainfall Losses - Curve Number The NRCS has developed a widely used curve number (CN) procedure for estimating runoff from storm events. The NRCS method incorporates this curve number procedure. Rainfall initial losses depend primarily on soil characteristics and land use (surface cover). The NRCS method uses a combination of soil conditions and land use to assign runoff factors (curve numbers). Curve numbers represent the runoff potential of a soil type (i.e., the higher the curve number, the higher the runoff potential). For hydrologic calculations used to determine the runoff potential, the NRCS classifies soils as A, B, C, or D based on their hydrologic soil group. Type A soils, such as sandy soils, have a very low runoff potential. Heavy clay and mucky soils, as well as shallow/rocky soils, are a type D soil, and have a very high runoff potential. Soil groups at the Project site were determined from the NRCS Soil Survey Geographic Database (SSURGO) data set. The curve number of 85 was selected for all the basins associated with the base concept design of the Waste Rock Storage Area. This curve number selection is considered conservative for the basins associated with the Waste Rock Storage Area whose proposed soils are anticipated to be type C. 2.4 Drainage Basin Delineations As shown on Figure 1, separate drainage basins were analyzed within the proposed Waste Rock Storage Area and surrounding perimeter areas, including the upstream Pit Diversion watershed basin. Each basin was delineated with its unique contributing watershed. 2.5 Rainfall Runoff Volume The NRCS method determines rainfall runoff volume using the following relationships: ( P 0.2S) Q = P + 0.8S 2 3

4 1000 S = 10 CN Where: Q = The accumulated runoff volume in inches; P = The accumulated precipitation in inches (NOAA Atlas 14 Point Estimates); S = The maximum soil water retention parameter in inches; and CN = The curve number. The initial abstraction, I A, is the total amount of precipitation (in inches) that infiltrates and is absorbed into the soil before runoff begins. The initial abstraction is related to the maximum soil water retention parameter by the expression 0.2*S. For determining a basin s total runoff volume, the accumulated runoff volume after soil losses, Q, is converted to feet then multiplied by the planar acreage of the basin area to obtain the value in acre-feet (ac-ft). 2.6 Time of Concentration / Lag Time The time of concentration (Tc) used in the NRCS method was determined by considering the most hydraulically distant flow path for each basin and was calculated using the sum of the travel times for each flow segment within the basin. The travel time is the ratio of flow length to flow velocity and defined by the following relationship: Where: Tc = n i= 1 L v i i Tc = The time of concentration, in minutes; n = The number of flow segments; L i = The length of the flow path, in feet, for the i th segment; and V i = The estimated velocity, in feet per second, for the i th segment (ft/s). The limited drainage bench areas within the Waste Rock Storage Area generally comprise two (2) flow segments: an overland flow path down the side-slope and flowing into a proposed V- channel with 3H:1V (Horizontal to Vertical) side-slopes. The velocity of the overland segment was estimated by the following equation: Where: V = a*s 0.5 V = the estimated velocity, results in ft/s; 4

5 a = the overland flow coefficient, conservatively estimated at 7.0; and S = the slope of the flow path, in feet per foot (ft/ft). The velocity of the channel segment was estimated utilizing an iterative approach between the applied HEC-HMS and Manning s equation for open-channel flow. An initial time of concentration was generated from an assumed velocity for the channel segment to obtain an estimate of the peak flow rate. This value was then used in Manning s formula presented in the following section to refine the assumed velocity. The overland flow velocity and channel velocity were summed to give the total time of concentration for each basin. This time of concentration was then converted into a lag time, which is equal to 0.6*Tc, and input into HEC-HMS. 3.0 Hydraulic Methodology Overview (Manning s Formula) The geometry of a typical drainage bench includes a stormwater conveyance V-channel with 3H:1V side-slopes and a total design flow depth of 4.5 feet in addition to an access road and an outer berm. This design is discussed in further detail in Tetra Tech s technical memorandum titled Rosemont Dry Stack Tailings Facility Drainage Bench Analysis (Tetra Tech, 2010b). The hydraulic analysis entailed utilizing Manning s open-channel flow equation together with the drainage bench s hydrologic analysis in order to appropriately determine the peak flow rate and check the corresponding flow depth in the proposed V-channel. Manning s formula for openchannel flow is as follows: Where: A R S Q = n 1 2 Q = the channel flow rate, in cubic feet per second (cfs); A = the cross sectional area of flow, in square feet (ft 2 ); R = the hydraulic radius of flow, in feet; S = the longitudinal slope of the flow path for the channel, universally 2% or 0.02 ft/ft; and n = Manning s roughness coefficient for the channel, conservatively estimated at (unitless). A roughness coefficient of assumes a large earthen drainage ditch. 4.0 Hydrologic and Hydraulic Analysis Results In general, and as shown on Figure 1, stormwater traveling overland, down side-slopes, or in swales and channels throughout the Waste Rock Storage Area will be routed into a series of detention ponds that are configured on wide benches. These benches are assumed to be graded in a manner to evenly and effectively distribute stormwater into a linked succession of 5

6 detention ponds. Detention ponds on upper benches are also hydraulically connected to lower benches and detention ponds by riprap drop chutes intended to pass potential overflow. Similarly, stormwater generated from the perimeter areas located around the toe of the Waste Rock Storage Area, including the upstream Pit Diversion basin, will be routed directly into containment areas. The containment areas also contain potential overflow from the detention ponds on up-gradient benches. The basins located along the toe of the Waste Rock Storage Area are known as PCAs. The detention ponds proposed on the wide benches associated with the base concept design of the Waste Rock Storage Area were designed to contain the anticipated runoff volume generated from a 500-year, 24-hour storm event. Additionally, the PCAs located between a natural ridge and the toe of the Rosemont Ridge Landform were analyzed in terms of controlling runoff from the General PMP storm event. Table 2 summarizes the NRCS parameters utilized for determining the total runoff volume generated from the relevant design storms: Table 2 NRCS Parameters Parameters 500-year, 24-hour General PMP P (inches) CN S (inches) I A (inches) Q (inches) The total runoff volume of a contributing basin, given as acre-feet (ac-ft), was calculated for both the 500-year, 24-hour and General PMP design storms by multiplying the plan acreage of the basin area by the accumulated runoff volume after soil losses, Q, which was then converted to feet. The detention ponds located on the wide benches of the Waste Rock Storage Area were generally designed to contain the 500-year, 24-hour storm volumes generated from a given collective of basins. The PCAs were generally expected to contain the General PMP storm volumes also produced from a given combination of basins. Table 3 summarizes the volumetric analysis for the various basins examined herein together with the estimated storage capacities of the corresponding detention ponds. Figure 1 illustrates the flow of stormwater throughout, and around, the Waste Rock Storage Area. Figure 2 represents typical cross sections of the Waste Rock Storage Area. 6

7 Table 3 Summary of the Waste Rock Storage Area Volumetric Analysis Total Runoff Volume Total Storage Capacity Post Event Storage Capacity Available Waste Rock (WR) 500-year, General Detention 500-year, General PCA Storage Area Area 24-hour PMP Ponds 1 PCA 24 hour ID Basin Name (acres) (ac-ft) (ac-ft) (ac-ft) (ac-ft) (ac-ft) (ac-ft) -- WR Top Deck - S PMP 2,3,4,5 -- WR Top Deck - N WR Bench Total PCA 1 PCA Basin PCA 2 Pit Diversion PCA Basin Total PCA 3 PCA Basin PCA 4 WR Bench 2 - S PCA Basin Total PCA 5 PCA Basin PCA 6 PCA 7 WR Bench 2 - N SE Tails Bench WR Bench PCA Basin Total SE Tails Bench WR Bench SE Tails Bench WR Bench PCA Basin Total PCA 8 PCA Basin Notes: 1. Detention ponds are designed at: 8-feet deep for the WR Top Deck, WR Bench 1, and WR Bench 3; 6-feet deep for WR Bench 2 and WR Bench 5; and 4-feet deep for WR Bench Excess runoff volume conveyed to PCA 1 for either storm event will be routed either to the Flow-through Drain System or into the Open Pit at this location. 3. Excess runoff volume conveyed to PCA 2 for the General PMP storm event will be routed through PCA Basin 4 s drainage bench V-channel to PCA Excess runoff volume potential conveyed to WR Bench 1 and PCA 6 beyond the General PMP storm event could be routed to PCAs 4 and PCA 3 is designed with an embankment (2:1 side-slopes, 50 feet wide crest) to contain the runoff volume from PCA Basin 3. 7

8 4.1 Waste Rock Top Deck South Self-Contained According to the results indicated in Table 3, and as represented on Figure 1, the WR Top Deck South basin, which is the most upstream portion of the Waste Rock Storage Area, has enough detention pond storage capacity to contain the General PMP storm event. 4.2 Waste Rock Top Deck North and Waste Rock Bench 1 Self-Contained As indicated in Table 3 and on Figure 1, the next basin downstream is the WR Top Deck North, whose detention pond storage capacity is sufficient to contain the 500-year, 24-hour storm event, but not the General PMP event. The excess runoff volume from this basin is designed to overflow into WR Bench 1, whose surplus detention pond storage capacity, after retaining its own General PMP runoff volume, is intended to contain the remainder of WR Top Deck North s runoff volume from a General PMP storm event. 4.3 PCA Basin 1 Reporting to PCA 1 PCA Basin 1 is located in the steep northwest portion of the Waste Rock Storage Area adjacent to the Open Pit. As indicated in Table 3, the runoff volume reporting to PCA 1 is expected to be greater than PCA 1 s storage capacity for both design storms. However, the additional volume is designed to be routed either into the flow-through drain system or into the Open Pit at this location. The flow-through drains are discussed in further in Tetra Tech s technical memorandum titled Rosemont Flow-through Drain Design (Tetra Tech, 2010c) and in another technical memorandum titled Rosemont Flow-through Drain Sizing (Tetra Tech, 2010d). 4.4 Pit Diversion and PCA Basin 2 Reporting to PCA 2 As indicated in Table 3 and on Figure 1, the runoff volume reporting to PCA 2 is generated from the upstream Pit Diversion basin and PCA Basin 2. The storage capacity for PCA 2 will contain the 500-year, 24-hour storm event from both basins but, will not contain the General PMP event. The excess runoff volume produced from the General PMP storm event is designed to be routed onto a bench flowing to PCA 4. PCA 4 has adequate storage capacity to contain the General PMP from PCA Basin 4 and overflow volume from PCA PCA Basin 3 Reporting to PCA 3 As indicated in Table 3 and on Figure 1, PCA 3 is designed with an embankment to contain the runoff volume from PCA Basin 3. The estimated runoff volume reporting to PCA 3 from PCA Basin 3, for both the 500-year, 24-hour and the General PMP design storms, is anticipated to be less than PCA 3 s capacity. 4.6 Waste Rock Bench 2 South and PCA Basin 4 Reporting to PCA 4 As indicated in Table 3, WR Bench 2 South has a detention pond storage capacity that is sufficient to contain the 500-year, 24-hour storm event but not the General PMP event. The General PMP excess runoff volume from WR Bench 2 South is designed to overflow via a riprap drop chute into PCA 4. As discussed above, PCA 4 has adequate storage capacity to contain the General PMP from PCA Basin 4 and overflow volume from PCA 2. 8

9 4.7 PCA Basin 5 Reporting to PCA 5 As indicated in Table 3 and similar to PCA 3, the runoff volume reporting to PCA 5 from PCA Basin 5 for both design storms is estimated to be notably below PCA 5 s capacity. 4.8 Waste Rock Benches 2 North and 3 with PCA Basin 6 Reporting to PCA 6 As indicated in Table 3, WR Bench 2 North has a detention pond storage capacity that is adequate to contain the 500-year, 24-hour storm event but not the General PMP event. The General PMP excess runoff volume from WR Bench 2 North is designed to overflow via riprap drop chutes onto WR Bench 3 and ultimately into PCA 6. WR Bench 3, which also receives SE Tailings Bench 6 s runoff volume, has a detention pond storage capacity that is large enough to contain the 500-year, 24-hour storm volume produced from both basins but not the General PMP storm event. Like WR Bench 2 North, the General PMP excess runoff volume from WR Bench 3 is planned to overflow via a riprap drop chute into PCA 6, whose surplus pond storage capacity is also anticipated to contain PCA Basin 6 s runoff volume from both the 500-year, 24- hour and General PMP events. 4.9 Waste Rock Benches 4 and 5 with PCA Basin 7 Reporting to PCA 7 As indicated in Table 3 and on Figure 1, WR Bench 4, which also receives SE Tailings Bench 7 s runoff volume, has a detention pond storage capacity that is adequate to contain the 500- year, 24-hour storm volume produced from both basins but not the General PMP storm event. The General PMP excess runoff volume from WR Bench 4 is designed to overflow via a riprap drop chute onto WR Bench 5 and ultimately into PCA 7. WR Bench 5, which also receives SE Tailings Bench 8 s runoff volume, has a detention pond storage capacity that is enough to contain the 500-year, 24-hour storm volume produced from both basins but not the General PMP storm event. Similar to WR Bench 4, the General PMP excess runoff volume from WR Bench 5 is intended to overflow via a riprap drop chute into PCA 7, whose surplus storage capacity is also anticipated to contain PCA Basin 7 s runoff volume from both design storms PCA Basin 8 Reporting to PCA 8 As indicated in Table 3 and like PCAs 3 and 5, the runoff volume reporting to PCA 8 from PCA Basin 8 for both design storms is expected to be less than PCA 8 s capacity PCA Summary In general, and with the exception of PCA 1, PCAs 2 through 8, and their respective contributing basins, are designed in series to contain the projected stormwater volumes produced from upstream basins in the Waste Rock Storage Area during the General PMP storm event. PCAs 2 through 8 and their respective contributing basins are also intended for design redundancy in the event that potential excess runoff volume may be generated from an event beyond the General PMP storm. 9

10 4.12 Waste Rock Drainage Benches The design event used to calculate peak flows for the drainage benches within the Waste Rock Storage Area was the 500-year, 24-hour storm. In addition to this design storm, and for comparison purposes, the 1,000-year, 24-hour event was also used in the analysis of the drainage benches. Table 4 summarizes the results of the peak runoff analysis and the resultant flow depth for the proposed V-channel with 3H:1V side-slopes corresponding to the four (4) drainage benches located in the Waste Rock Storage Area: Table 4 Summary of the Waste Rock Storage Area V-channel Analysis Waste Rock (WR) Storage Area Drainage Bench Name 1,000-year, 24-hour NRCS Type II Storm Peak Flow Flow Rate Depth (cfs) (ft) 500-year, 24-hour NRCS Type II Storm Peak Flow Flow Rate Depth (cfs) (ft) WR Top Deck Drainage Bench NW WR Drainage Bench NW WR Drainage Bench PCA Basin 4 Drainage Bench Note: NW WR Drainage Bench 1 produced comparable peak flow rates to NE Tailings Bench 2, which was used as the basis for the V-channel s design in the Technical Memorandum titled Rosemont Dry Stack Tailings Facility Drainage Bench Analysis (Tetra Tech, 2010b). According to the results provided in Table 4, NW WR Drainage Bench 1 yielded the greatest flow magnitude. The corresponding V-channel flow depth for the 500-year, 24-hour design storm event associated with this bench is approximately 3.5 feet. By accounting for an additional one (1) foot for freeboard as the prescribed design basis for these drainage channels, the total design flow depth is 4.5 feet and the top width of the proposed V-channel with 3H:1V sideslopes is 27 feet. The peak flow and depth results for NW WR Drainage Bench 1 are consistent with and comparable to the controlling NE Tailings Bench 2. NE Tailings Bench 2, the basis for the V-channel configuration, was designed with one (1) foot of freeboard and adequate safety factors to convey the peak runoff should the V-channel s volumetric capacity be reduced by 30% due to sedimentation. The design basis for the V-channel configuration is discussed in greater detail in the technical memorandum titled Rosemont Dry Stack Tailings Facility Drainage Bench Analysis (Tetra Tech, 2010b). Furthermore, the remaining drainage benches and their respective V-channels analyzed herein had lower peak flow rates and thus shallower flow depths than NW WR Drainage Bench 1. Attachment 1 contains the output results together with the data and calculations developed for this analysis. 5.0 General Summary Based on the analysis and the results discussed herein, the detention ponds on the Waste Rock Storage Area benches are sufficient to contain the runoff volume generated from a 500-year, 24-hour storm event. 10

11 Additionally, PCAs, are generally located between the toe of the Rosemont Ridge Landform and a natural ridgeline. PCAs are generally designed to mitigate and control runoff volumes produced from a combination of basins during the General PMP storm event. The design basis for the drainage benches in the Waste Rock Storage Area is the 500-year, 24- hour design storm with one (1) foot of freeboard. Additionally, the proposed V-channel with 3H:1V side-slopes and a total design flow depth of 4.5 feet is adequate to convey the estimated stormwater runoff generated from the NRCS Type II 500-year, 24-hour design storm for all drainage benches within the Waste Rock Storage Area. Furthermore, the proposed drainage channel cross section is designed with sufficient redundancy to safely transmit stormwater runoff from the 500-year, 24-hour event with a 30% loss of volume due to sedimentation. 6.0 REFERENCES National Oceanic and Atmospheric Administration (NOAA) (2009). NOAA Atlas 14 Precipitation Frequency Atlas of the United States. Viewed November 6, Natural Resource Conservation Service (NRCS) (2009a). Soil Survey Website. Viewed August 25, Natural Resource Conservation Service NRCS (2009b). Web Soil Survey Website. Viewed August 25, Tetra Tech, Chee, R., and Hemmen, G. (2010a). Rosemont Copper Project Hydrology Method Justification. Technical Memorandum to Kathy Arnold (Rosemont Copper Company). Technical Memorandum dated January 27, Tetra Tech, Hemmen, G. (2010b). Rosemont Dry Stack Tailings Facility Drainage Bench Analysis. Technical Memorandum to Kathy Arnold (Rosemont Copper Company). Technical Memorandum dated January 28, Tetra Tech, Salguero, M., and Carrasco, J. (2010c). Rosemont Flow-through Drain Design. Technical Memorandum to Kathy Arnold (Rosemont Copper Company). Technical Memorandum dated April 5, Tetra Tech, Salguero, M., and Chee, R. (2010d). Rosemont Flow-through Drain Sizing. Technical Memorandum to Kathy Arnold (Rosemont Copper Company). Technical Memorandum dated April 5,

12 FIGURES

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15 ATTACHMENT 1 OUTPUT RESULTS

16 Attachment 1 of Technical Memorandum Titled Rosemont Waste Rock Storage Area Stormwater Management Summary of V-channel Hydrologic and Hydraulic Analysis Calculations and Results CAD Data and Calculations 1,000-year 24-hour NRCS Type II Storm 500-year 24-hour NRCS Type II Storm A A A L1 H1 S1 L2 H2 S2 V1-sht* T1-sht V2-ch T2-ch Tc Tlag Qpeak Vol. D-3H:1V V2-ch T2-ch Tc Tlag Qpeak Vol. D-3H:1V Waste Rock Area Basin Name (ft2) (ac) (mi2) (ft) (ft) (ft/ft) (ft) (ft) (ft/ft) (ft/s) (min) (ft/s) (min) (min) (min) (cfs) (ac-ft) V-ch (ft) (ft/s) (min) (min) (min) (cfs) (ac-ft) V-ch (ft) WR Top Deck Drainage Bench 888, , NW WR Drainage Bench 1 2,775, , NW WR Drainage Bench 2 2,096, , PCA Basin 4 Drainage Bench 1,495, , *Denotes the overland flow coefficient, a, was conservatively estimated at 7.0 for the calculation: V = a*s