Technical Memorandum Mine Plan of Operations Stormwater Assessment

Transcription

1 Tucson Office 3031 West Ina Road Tucson, AZ Tel Fax Technical Memorandum Mine Plan of Operations Stormwater Assessment To: Kathy Arnold From: David R. Krizek, P.E. Company: Rosemont Copper Company Date: March 05, 2010 CC: Mike Zeller (Tetra Tech) Doc #: 062/ Introduction This Technical Memorandum presents a Stormwater Assessment for the Mine Plan of Operations (MPO) being considered in the US Forest Service Environmental Impact Statement (EIS) for the proposed Rosemont Copper Project (Project). This analysis quantifies the potential impact of the MPO on downstream stormwater flows and average-annual runoff. In order to determine the potential stormwater impact associated with the MPO, predictions were made for the 100-year regulatory flood-peak [in cubic feet per second (cfs)] and the average-annual runoff (in acre-feet) at a common point associated with the affected drainages. The affected drainages on the east side of the Santa Rita Mountains converge at the United States Geological Survey (USGS) Gauging Station No before storm flows pass beneath State Route 83 (SR 83) in a double-barrel box culvert. Per information associated with the station, the contributing watershed area is calculated to be 14 square miles in size. Figure 1 shows the watershed areas contributing to this gauging station. Stormwater runoff from the contributing watershed area associated with Sycamore Tailings, which is part of the Sycamore Tailings and Barrel Waste Alternative, does not flow to the USGS Gauging Station, since it is located on the west side of the Santa Rita Mountains. A separate analysis was made for this contributing watershed area in the overall stormwater assessment associated with the Sycamore Tailings and Barrel Waste Alternative. 2.0 Pre-Mining/Baseline Hydrology Figure 1 shows the pre-mining or baseline watershed conditions associated with the Mine Plan of Operations. These contributing watershed areas drain to the USGS Gauging Station prior to storm flows passing beneath SR 83. This baseline assessment is assumed to be the same for

2 all the alternatives, including the MPO, for storm flows generated on the east side of the Santa Rita Mountains. An additional baseline assessment was made for the Sycamore Tailings and Barrel Waste Alternative. This is the only alternative with facilities on the west side of the Santa Rita Mountains. 2.1 Methodology Flood-Peaks Baseline 100-year regulatory peak-discharge values for the MPO were determined using Computer Program HEC-HMS, Version 3.4 [United States Army Corp of Engineers (USACE), 2009]. HEC-HMS software is both a flood-peak and volume-estimator procedure prepared by the U.S. Army Corps of Engineers, and is used to calculate 100-year regulatory peakdischarges and runoff volumes emanating from watersheds in Pima County which are generally greater than 640 acres in size. Use of HEC-HMS is in accordance with Pima County Regional Flood Control District (District) Technical Policy, Tech-015, Section A.3 (2007b), which states: For Watersheds > 1 square mile and < 10 square miles: HEC-1, HEC-HMS (including Geo-HMS) or the Pima County Hydrology Procedures may be used. If the Pima County Hydrology Procedures are used, the Time of Concentration must be < 180 minutes and detention and retention must be negligible. HEC-1 or HMS (including Geo-HMS) is preferred (emphasis added). In order to ascertain the reasonableness of results using HEC-HMS, the computed regulatory (100-year) peak discharge values were compared to the regulatory peaks using results from application of the USGS Regional Regression Equation (RRE) for Region 13 (USGS, 1997) which encompasses the Project Site associated with the MPO. In order to validate use of the USGS Region 13 RRE to calibrate 100-year regulatory peak-discharge values for watersheds at the Project Site, 100-year regulatory peak-discharge values for two (2) nearby USGS stream gauges (Barrel Canyon and Davidson Canyon) were computed by Tetra Tech using results obtained from Bulletin 17B statistical analysis (USGS, 1982), and then compared to results using the USGS Region 13 RRE at these same two (2) gauges. A comparison of the results using Bulletin 17B statistical analysis to results using the USGS Region 13 RRE indicates that the use of the USGS Region 13 RRE is very appropriate for calibration of 100-year regulatory peak-discharge values emanating from contributing watershed areas at the Project Site associated with the MPO (see Attachment 1 for computational details). Based upon calibration using USGS Region 13 RRE 100-year regulatory peak-discharge values, it was determined that, rather than a 3-hour thunderstorm distribution, a NRCS Type II 24-hour temporal storm distribution (NRCS, 1973) could be used in the HEC-HMS model, in combination with the applicable National Oceanic and Atmospheric Administration (NOAA) Atlas hour mean-precipitation value (NOAA, 2004) and application of the appropriate HYDRO- 40 Areal Reduction Factor (ARF) (NOAA, 1984). Consequently, an areally-reduced NOAA Atlas 14 mean-precipitation value of 4.23 inches (using a 0.89 ARF) was selected for use in the HEC-HMS model, because it was found that when compared to USGS Region 13 RRE results, the application of the 24-hour upper 90% confidence-interval precipitation value (i.e., an areally-reduced value of 4.82 inches), which the District suggests be used in their Technical Policy, Tech-010 (2007a), leads to a significant over-prediction of regulatory peaks at the USGS Gauging Station. 2

3 On the other hand, using the NRCS TYPE II 24-hour temporal storm distribution with an areallyreduced NOAA Atlas hour mean precipitation value of 4.23 inches produces a regulatory peak that, while still larger than the USGS Region 13 RRE prediction, falls within general agreement with standard predictive results. It is further noted that use of the 24-hour precipitation value produces a larger flood volume than does a 3-hour thunderstorm, providing an additional design factor-of-safety from a volumetric standpoint Average-Annual Runoff An extensive precipitation/runoff data-collection effort was conducted by Tetra Tech prior to computing baseline average-annual runoff values for the MPO. Six (6) primary sources were relied upon to obtain precipitation/runoff data. These sources were: 1. The USGS; 2. The Arizona Department of Water Resources (ADWR); 3. The United States Department of Agriculture (USDA); 4. The USDA National Research Conservation Service (NRCS); 5. The National Oceanic and Atmospheric Administration (NOAA) Western Regional Climate Center (WRCC); and 6. The NOAA Hydrometeorological Design Studies Center. [See References Section at the end of this Technical Memorandum]. Upon completion of the collection effort and review of available data, Tetra Tech determined that the technical approach contained within USGS Open-File Report , titled A Proposed Streamflow-Data Program for Arizona (Moosburner, 1970) was well suited for purposes of computing baseline average-annual runoff from streams entering, traversing, and exiting the Project Site. This decision was made because the procedure in the referenced document uses average-annual precipitation as well as the contributing watershed area (and in some locations, mean watershed elevation) to estimate baseline average-annual runoff, in acre-feet, emanating from streams in Arizona. In the case of the Project Site associated with the MPO, this distinction is important because within similar higher-elevation watersheds baseline average-annual runoff has been shown to be correlated to watershed area, average-annual precipitation, and mean watershed elevation. Because USGS Open-File Report (Moosburner, 1970) was published 40 years ago, Tetra Tech contacted personnel in the USGS Tucson office to see if the document had been updated or refined, considering that there are now many more years of precipitation/runoff data collected by the USGS. Through this contact, it was discovered that the report had not been updated. However, Tetra Tech was able to obtain an updated USGS data set (preliminary in nature), for which some watershed data extend through water year This updated data set was then used by Tetra Tech to conduct its own regression analyses for the purpose of developing a multi-variable relationship, to estimate baseline average-annual runoff values associated with the MPO, which is functionally similar to the multi-variable relationships which are found in USGS Open-File Report (Moosburner, 1970). In this way, the reliability of 3

4 the older relationships in USGS Open-File Report (Moosburner, 1970) could be corroborated. A detailed map of average-annual precipitation in Arizona, obtained from the NRCS and titled Arizona Annual Precipitation ( ) (USDA NRCS, 1998), was used to determine baseline average-annual precipitation at the Project Site. Baseline average-annual precipitation was determined to be ±24 inches, although average-annual precipitation data collected for the area encompassing the MPO indicates that annual precipitation is about 18 inches, on average, than the ±24 inches indicated on the NRCS map. Accordingly, results are presented for averageannual precipitation depths of 18 inches only, and then compared to other study results of average-annual runoff. Using procedures found in USGS Open-File Report (Moosburner, 1970), Tetra Tech found that the Region 1 Seasonal Mean-Discharge Equation best applies under the MPO. This is because: In the area encompassing the MPO, an estimated 90% of average-annual runoff occurs from July through September; For the area encompassing the MPO, the use of the Region 1 Seasonal Mean- Discharge Equation produces average-annual runoff estimates that are larger than the average-annual runoff estimates produced by the Annual Mean-Discharge Equation. Using USGS Quad Map topography for the area encompassing the MPO, mean watershed elevations were determined. The USGS Open-File Report (Moosburner, 1970), Region 1, Seasonal Mean-Discharge Equation is: Q AA = ( )A 0.71 PS 2.25 Where, Q AA = Average-annual runoff, in acre-feet; A = Watershed area, in square miles; PS = Seasonal precipitation, in inches ( 0.51 of annual value); As stated previously herein, a multi-variable relationship was developed by Tetra Tech for this analysis, regressed on: USGS-supplied contributing watershed area; Average-annual precipitation; and Mean watershed elevation. This relationship is: Q AA = ( x10-06 )A P E

5 Where, Q AA = Average-annual runoff, in acre-feet; A = Watershed area, in square miles; P = Average-annual precipitation, in inches (18 inches); and E = Mean watershed elevation, in feet. 2.2 Results Table 1.0 shows the pre-mining or baseline results for the 100-year regulatory flood-peak and the average-annual runoff arriving at USGS Gauging Station No Backup data for the pre-mining stormwater analysis is provided in Attachment 1. Table 1.0 MPO Baseline Hydrology Results Baseline Conditions (DA = 14.0 square miles) Point of Concentration Peak Discharge Average-Annual Runoff USGS Gauging Station 8072 cfs 1407 acre-feet DA = Discharge Area 3.0 Post-Mining Watershed Conditions 3.1 Stormwater Controls From 2007 MPO Figure 2 shows the estimated post-mining watershed boundary associated with the Mine Plan of Operations. The Reclamation and Closure Plan (Tetra Tech, 2007) associated with the 2007 MPO was general in the planned approach to stormwater control on the outer slopes of the final landform, termed herein as the 2007 MPO Rosemont Ridge Landform (2007 MPO Landform). The 2007 MPO Landform is a consolidated and contoured earthen structure consisting of waste rock from the Open Pit; a closed Heap Leach Facility encapsulated with waste rock; and a Dry Stack Tailings Facility, also encapsulated with waste rock. The stormwater-control options for the outer slopes of the 2007 MPO Landform listed in the report included: Barber Pole Method Dendritic Pattern Retention Method Continuous soil covered or rock covered slopes Continuous soil covered or rock covered slopes with natural landform features 5

6 Further discussion of these options is provided in the Reclamation and Closure Plan (Tetra Tech, 2007). As a subset of the last option, the Ridge and Valley Method was contemplated for the final reclaimed surfaces of the 2007 MPO Landform. As described in the 2007 reclamation report: In the Ridge and Valley Method, channels are constructed generally perpendicular to the crest and toe of the outer slope. The contributing slopes extend from the channel on either side, terminating at ridges (parallel to the channel) where the adjacent section then begins. These sections are then repeated across the face of the outer slope, with rounded convex summits and concave bases. 3.2 Post-Mining Stormwater Controls Applied to MPO Design work associated with the Rosemont Project has been ongoing since submittal of the Reclamation and Closure Plan (Tetra Tech, 2007). Based this updated design work, the stormwater controls described below were applied to the 2007 MPO Landform for this alternatives assessment: Stormwater drainage channels would be placed at every 100-foot vertical rise on the outer slopes of the Dry Stack Tailings Facility. Stormwater would flow off these benches to stilling pools/drop-structures located on the outer slopes of the tailings area, to natural ground, or to stormwater-control basins located on wide benches in the Waste Rock Storage Area. The predicted runoff from these drainage benches was based upon a separate Technical Memorandum titled Rosemont Dry Stack Tailings Facility Drainage Bench Analysis (Tetra Tech, 2010); Drop-structures located on the west side of the Dry Stack Tailings Facility would drain to the USGS Gauging Station location. Drop-structures would also be located on the north and west sides of the 2007 MPO Landform. Flows emanating from these drop-structures would drain to a Central Drain or to stormwater ponding areas located between the toe of the North Dry Stack Tailings Facility and adjacent, natural ridge areas; The Central Drain, or flow-through drain, is a large rock drain intended to provide a hydraulic connection between the up-gradient side of the 2007 MPO Landform and the down-gradient side; An Infiltration Drain was incorporated into the 2007 MPO Landform that is hydraulically connected to the Central Drain. For the purposes of this stormwater alternatives assessment, the Infiltration Drain is assumed to pass storm events larger than the 500-year, 24-hour storm event off the top surface while smaller events are retained on the top surface in large, depressed areas; Stormwater control basins would be constructed on wide benches in the Waste Rock Storage Area to contain up to the 500-year, 24-hour storm event. Stormwater generated from flows in excess of the 500-year, 24-hour storm event would be routed to containment areas located between the toe of the Waste Rock Storage Area and adjacent, natural ridge areas. These areas would generally be sized to 6

7 contain the Probable Maximum Precipitation (PMP) event. Stormwater routing to these perimeter containment areas would be via rocked slopes connecting the benches to the perimeter areas. 4.0 Post-Mining Hydrology Figure 2 depicts the estimated post-mining watershed area draining to the USGS Gauging Station for the Mine Plan of Operations. The contributing basin of 6.82 square miles, shown on Figure 2, is only applicable to the 100-year regulatory flood and to the average-annual runoff, based on the following assumptions: The top of the dry stack tailings is assumed to contain storm flows up to the 500- year, 24-hour storm event. Stormwater control basins in Waste Rock Storage Area are assumed to contain storm runoff from up to the 500-year, 24-hour storm event, and the perimeter areas associated with the Waste Rock Storage Area are assumed to contain storm runoff from up to the PMP event. No downstream stormwater contribution is expected from the Central and Infiltration Drains (flow-through drains) associated with average-annual conditions. Therefore, runoff from any watershed located up-gradient of the landform associated with the MPO is not expected to arrive at the USGS Gauging Station. Should runoff from a 100-year regulatory event reach the down-gradient end of the Central or Infiltration Drains, the flood-peak would be significantly attenuated, and is not expected to affect the peak value experienced at the USGS Gauging Station location. 4.1 Methodology As was done for determining baseline hydrology, post-mining 100-year regulatory peakdischarge values were determined using Computer Program HEC-HMS, Version 3.4 (USACE, 2009), as described in Section of this Technical Memorandum. The HEC-HMS model for post-mining conditions also uses a Type II 24-hour temporal storm distribution (NRCS, 1973) in conjunction with a NOAA Atlas hour areally-reduced (per HYDRO-40) mean-precipitation value of 4.23 inches (NOAA, 2004). The reasonableness of predicting peak-discharge, using the HEC-HMS model (USACE, 2009) in conjunction with a NRCS Type II 24-hour temporal storm distribution (Need Reference) and a NOAA Atlas hour areally reduced (per HYDRO-40) mean-precipitation value of 4.23 inches (NOAA, 2004), was also described in Section of this Technical Memorandum. Likewise, average-annual runoff for post-mining conditions was determined using the same procedures which were described in Section of this Technical Memorandum. 7

8 4.2 Results Table 2.0 below shows the post-mining or baseline results for the 100-year regulatory flood peak and the average-annual runoff arriving at USGS Gauging Station No Backup data for the post-mining stormwater assessment is provided in Attachment 2. Table 2.0 MPO Post-Mining Hydrology Results Post-Mining Conditions (DA = 6.82 square miles) Point of Concentration Peak Discharge Average-Annual Runoff USGS Gauging Station 3785 cfs 762 acre-feet 5.0 Conclusions The results of the baseline and post-mining hydrology assessment for the Mine Plan of Operations indicate that flood peaks generated by the 100-year regulatory event on the east side of the Santa Rita Mountains, and arriving at the USGS Gauging Station, would likely be reduced by 53.1%, when compared to pre-mining conditions. Correspondingly, the averageannual runoff would likely be reduced by 45.8%, when compared to pre-mining conditions. 6.0 References Arizona Department of Water Resources (2008). Draft State Water Atlas, Volume 8, Active Management Area Planning Area. Dated July Blakemore, T. E., Hjalmarson, H.W., and Waltemeyer, S.D., United States Geological Survey (USGS) (1997). Methods for Estimating Magnitude and Frequency of Floods in the Southwestern United States. Water-Supply Paper Eychaner, J.H., USGS (1984). Estimation of Magnitude and Frequency of Floods in Pima County, Arizona, with Comparisons of Alternative Methods. Water Resources Investigations Report Dated August Hemmen, G. Tetra Tech (2010). Rosemont Dry Stack Tailings Facility Drainage Bench Analysis. Technical Memorandum to Kathy Arnold, Rosemont Copper Company. Technical Memorandum Dated March Moosburner, Otto, USGS (1970). A Proposed Streamflow-Data Program for Arizona. USGS Open-File Report Dated August National Oceanic and Atmospheric Administration (NOAA) Technical Memorandum NWS Hydro-40, (1984). Depth-Area Ratios in the Semi-Arid Southwest United States. Dated August

9 NOAA National Weather Service Hydrometeorological Design Studies Center, (2004). NOAA Atlas 14, Volume 1. Dated NOAA Western Regional Climate Center (WRCC) / Desert Research Institute (2009). Arizona Climate Summary. Dated December NRCS Technical Paper 149, (1973). A Method for Estimating Volume and Rate of Runoff in Small Watersheds. Dated April Pima County Regional Flood Control District, (2007a). Technical Policy, Tech-010. Dated May Pima County Regional Flood Control District, (2007b) Technical Policy, Tech-015, Section A.3. Dated October Tetra Tech (2007). Reclamation and Closure Plan. Prepared for Rosemont Copper Company. Report Dated July United States Army Corps of Engineers (USACE) Hydrologic Engineering Center (2009). Hydrologic Modeling System HEC-HMS, Version 3.4. Dated August United States Department of Agriculture (USDA), National Resources Conservation Service (NRCS) (1998). Map of Arizona Annual Precipitation, Dated April USGS (1982). Guidelines for Determining Flood Flow Frequency, Bulletin 17B of the Hydrology Subcommittee, Geological Survey, Office of Water Data Coordination, Reston, Virginia. Dated March USGS (2009). Personal Communication with and Data (Preliminary) Acquisition from James Leenhouts, USGS, Tucson. December

10 FIGURES

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13 ATTACHMENT 1

14 MINING PLAN OF OPERATIONS (MPO) ALTERNATIVE BaselineConditionsModel Tetra Tech, Inc.

15 MPO Sycamore Alternative Tailings and Barrel Waste Alternative Baseline Conditions Model Tetra Tech, Inc.

16 MPO Sycamore Alternative Tailings and Barrel Waste Alternative Baseline Conditions Model Tetra Tech, Inc.

17 MPO Sycamore Alternative Tailings and Barrel Waste Alternative Baseline Conditions Model Tetra Tech, Inc.

18 ATTACHMENT 2

19 Mining Plan of Operations (MPO) ALTERNATIVE Post Mining Conditions Model Tetra Tech, Inc.

20 Tetra Tech, Inc.

21 Tetra Tech, Inc.