Argonaut Drainage Alternatives Report - TownNewsbloximages.newyork1.vip.townnews.com/ledger.news/content/tncms/...1 1 Introduction The Argonaut Mine Tailing Storage Site in Jackson,

Prepared for:

California Department of Toxic Substances Control

Prepared by:

March, 2017

Note: URS Corporation Americas (URS) is under contract with the California Department of Toxic Substances Control. AECOM aquired URS in 2015 so all references to AECOM include URS.

Table of Contents 1 Introduction ....................................................................................................................... 1 2 Alternatives Analysis ........................................................................................................ 2 3 Conclusions and Recommendations ................................................................................ 27

List of Tables

Table 1 Hydrology Results ............................................................................................................. 7 Table 2 Alternatives Analysis Summary ...................................................................................... 27

List of Figures

Figure 1 Vicinity Map ..................................................................................................................... 2 Figure 2 Alternatives Analysis Map ............................................................................................... 4 Figure 3 Argonaut Drainage Basins ................................................................................................ 6 Figure 4 Alternative 1 ..................................................................................................................... 9 Figure 5 Alternative 2 ................................................................................................................... 13 Figure 6 Alternative 3 ................................................................................................................... 17 Figure 7 Alternative 4 ................................................................................................................... 21 Figure 8 Alternative 4 ................................................................................................................... 22 Figure 9 Alternative 5 ................................................................................................................... 25

Appendices

Appendix A Hydrology and Hydraulic Analysis

Appendix B Cost Estimates

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1 Introduction

The Argonaut Mine Tailing Storage Site in Jackson, California includes 3 dams; one Concrete Multi-Arch (CMA) dam at the downstream end of the site and two earthen dams upstream of the CMA. The CMA dam was built in 1916 and is located at the corner of the Argonaut Drive and Sutter Street in Jackson, California. The dam is about 420 to 450 feet long and 46 to 50 feet tall at its highest point. The dam consists of 13 contiguous arches and includes tie beams for bracing between buttress walls for six of the tallest arches.

The United States Army Corps Engineers (USACE) performed geotechnical and structural evaluations of the Argonaut Mining tailing dams and found that the CMA dam to be structurally weak and unstable (USACE, 2015). Based on this finding, the Department of Toxic Substances Control (DTSC) has undertaken a stability and retrofit design project for the Argonaut Dam.

Previously, runoff from the site would both pool behind the dam and flow through a small hole in the concrete shell on the right side of the dam. In heavy runoff events flow would overtop the dam. Recently, DTSC had a berm installed behind the dam to pool runoff away from the CMA structure. Additionally, both a pipe culvert and temporary pump station were installed to route runoff around the dam. The preferred retrofit design consists of placing compacted earth fill on the downstream side of the dam which necessitates that runoff must not be allowed to flow through and over the dam. This study gives the results of the alternatives evaluated to safely convey runoff around the retrofitted dam.

The Argonaut dam vicinity map is shown in Figure 1.

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Figure 1 Vicinity Map

2 Alternatives Analysis

As part of the stability and retrofit of Argonaut CMA Dam the following five alternatives (shown in Figure 2 were evaluated:

Alternative 1 – Concrete Spillway on the left abutment of the dam releases runoff to existing downstream infrastructure.

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Alternative 2 – Improvements to the existing stormwater system downstream of the dam in conjunction with spillway releases from the dam.

Alternative 3 – Stormwater detention in watershed upstream of the dam in conjunction

with spillway releases from the dam. Alternative 4 – Diversion channel upstream of Argonaut Dam with a box culvert under

Hoffman Street and flow discharged to Jackson Creek downstream of Jackson. Note that the remaining small runoff requires a small pipe spillway still required on left abutment of the retrofitted dam.

Alternative 5 – A piped diversion of runoff of much of the site through Hoffman Ridge to

bypass the majority of stormwater around the dam and the City of Jackson stormwater infrastructure. Note that the remaining small runoff requires a small pipe spillway still required on left abutment of the retrofitted dam.

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Figure 2 Alternatives Analysis Map Previous studies that were reviewed prior to beginning of this analysis included RTI and CDM Smith’s Flow Modeling and Damage Estimates for the Argonaut Mine Dam Failure Study (RTI and CDM Smith, 2016). This study addressed inundation of the City of Jackson due to a failure of the concrete multi-arch dam and the resulting tailing “mud” flow. This study included an estimate of peak, 100-year runoff to the dam site, however, that estimate was intended for a different purpose and could not be used as part of this project study. The computed peak, 100-

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year event identified in the RTI and CDM Smith study was more than 200 cfs compared to our peak, 200-year flow of 140 cfs. The primary reason the high RTI and CDM Smith estimated peak flow was their assumption that the watershed was completely impervious whereas infiltration in pervious areas such as open fields was used in this evaluation. AECOM, formerly URS, also reviewed the Amador County FEMA Flood Insurance Study (FIS) (FEMA, 2016) for this area to determine previously estimated peak flows and the limits of floodplains and determined this site is not in the floodplain. AECOM reviewed the topographic survey (prepared by KPFF in June, 2016) which shows one-foot contour intervals, spot elevations, and site features of the Argonaut basin and some adjacent properties. The topographic survey prepared for this project does not include the existing drainage system down-stream of the site. Therefore, AECOM utilized aerial mapping and GPS to determine the location of some of the downstream drainage facilities to allow our hydraulic analysis of the existing system. In addition, to evaluate the diversion option (Alternate 5), USGS topographic mapping (which is based on 20-foot contour intervals) was used for areas outside the on-site topographic footprint. AECOM staff made a field visit on May 31, 2016 to investigate the existing, down-stream stormwater drainage system, gather notes on hydrologic and hydraulic conditions, and photograph existing drainage system components. The hydrologic conditions of the Argonaut Dam drainage area and downstream drainage areas were investigated to estimate the peak discharge for a 200-year event. Figure 3 presents the drainage basins names and areas. Drainage area 5 (or 5A and 5B) are the basins that drain to the dam. Basins 1 through 4 are downstream of the dam and combine with basin 6 at Jackson Creek. Drainage basin 6 is the North Fork Jackson Creek basin which was included in the hydrologic modeling to allow comparisons of peak flows downstream of the site. All areas contributing runoff to the downstream drainage system have been accounted for in this analysis.

In addition to controlling the stormwater from the dam site, seepage through the dam will have to be discharged via an underdrain system that will carry potentially hazardous groundwater under Argonaut Road into the stream on the east side of Argonaut Drive. The discharge from the dam will be routed under Argonaut Road via a single or double 36-inch culvert while the seepage will be discharged via a separate pipe and its outfall point will be different than the 36-inch culvert(s). We anticipate that the subdrain system in the fill will connect to the existing, 12-inch culvert under Argonaut Drive.Until a passive treatment system is developed for the seepage the two discharges will comingle in the stream downstream of the property.

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Figure 3 Argonaut Drainage Basins

To evaluate the proposed alternatives the amount of runoff expected to both reach the dam and continue downstream into Jackson’s stormwater system was evaluated. The 0.5% chance of exceedance storm (200-year event) was chosen as the design storm. The 200-year peak flow to reach the CMA dam was determined to be 140 cubic feet per second (cfs) Downstream flows at junction 3 (Jackson Creek) would be 167 cfs. The recently constructed berm behind the dam could act to reduce these peaks due to storage behind the berm which would attenuate the peak

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flows. However, AECOM did not consider this storage in this study because the structural stability of the berm is unknown and its performance in holding back runoff during a large rainfall event can’t be assured. The berm is considered to be an existing feature that won’t affect the stability of the dam or the detention volume behind the dam. Table 1 shows the results of the hydrologic analysis, detailed methods, inputs, assumptions, and results are presented in Appendix A. Table 1 Hydrology Results Alternative #1 - Spillway Only This alternative consists of construction of a concrete chute spillway on the left abutment of the retrofitted dam which would convey runoff around the retrofit dam and into the City’s downstream stormwater conveyance system. As shown on Figure 4, a 325-foot long rectangular concrete channel would have to be constructed. This 5-foot wide channel would vary in depth ranging from 3-feet to 7-feet deep. Using the computed peak discharges from the hydrologic analysis, AECOM prepared a hydraulic model using the United States Army Corps of Engineers’ (USACE’s) Hydrologic Engineering Centers River Analysis System (HEC-RAS) (USACE, 2010) software to analyze the proposed spillway. A more detailed description of this model is shown in Appendix A.

The spillway would flow supercritical nearly the entire distance and would go through hydraulic jump at the end transitioning into subcritical flow prior to entering the culverts under Sutter Street. For the design flow of 140 cfs, velocities in the steep portion of the spillway would reach 30 feet per second but would slow to less than 5 feet per second in the flat, downstream section. The hydraulic evaluation of the channel is detailed in Appendix A. To determine if this flow could then be conveyed easterly to Jackson Creek AECOM completed a hydraulic analysis of the downstream stormwater system. In addition to the field visit where the downstream pipe and culvert system was investigated, AECOM reviewed the construction plans prepared by Caltrans (Caltrans, 2014) for recently constructed storm drain improvements near the Highway 49 and Sutter Street intersection and incorporated these improvements into our analysis. DTSC requested a copy of the drainage study for these improvements from Caltrans; however, Caltrans did not complete a study for this project.

AECOM staff met with the Jackson’s City Manager, Michael Daly, regarding the project and Caltrans’ planned improvements at the intersection of Highway 49 and Sutter Street. In addition, AECOM staff spoke with the City Engineer, Gary Ghio, regarding any existing hydrologic or hydraulic models or as-built drawings. Mr. Ghio stated that he wasn’t aware of any as-built drawings of the existing drainage system and that he did not have possession of FEMA’s hydrologic or hydraulic models. The results of the hydraulic analysis of the existing downstream stormwater system show that the maximum peak flow that could be safely conveyed from the dam into the system was 48 cfs. As noted previously, the project would currently deliver 146 cfs to the system. Therefore, Alternative #1 was not further evaluated. The estimated cost for Alternative #1, without improvements to the downstream system, is $1.0 million as shown in Appendix B.

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The chute spillway will be designed to minimize operations and maintenance by limiting debris and sediment flows into the spillway. This will be accomplished by including a low-level inlet with a trash rack and a high-level inlet which would release flows if the low-level inlet becomes blocked. In addition, a pervious barrier fence may also be installed so debris in the runoff can be captured prior to entering the inlet to the spillway. It is recommended that DTSC staff inspect the inlet and spillway at the beginning of the rainy season each year and when heavy rains are expected. The inlet, spillway, and energy dissipation structure should be cleared of all debris and sediment during these inspections.

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Figure 4 Alternative 1

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Alternative # 2 – Downstream Infrastructure Improvements This alternative consists of constructing the Alternative #1spillway and improving the downstream stormwater system to handle the estimated flows. This alternative was evaluated through a hydraulic analysis to determine the capacity of the existing system and to determine the extent of improvements to the system to safely convey the entire 200-year flow that would be discharged through the dam spillway. A detailed description of this hydrologic and hydraulic analysis for Alternative 2 is presented in Appendix A. To carry the design storm (200-year event) nearly the entire downstream stormwater system would have to be replaced with larger pipes, culvert inlets, new manholes, and additional drainage inlets. Additionally, stormwater conveyance from the dam spillway would require an additional 36” culvert under Argonaut Road to increase capacity of the stormwater system. The maximum flow the downstream system can convey is 48 cfs while the addition of the spillway will convey 146 cfs into this system. The estimated cost of this alternative is $2.0 million as shown in Appendix B. However, in addition to the direct cost of this alternative, the environmental and public impacts due to construction of improvements within City and State right-of-way would be greatest for this alternative. Impacts such as utility interruption, dust, traffic congestion and lack of access should be expected. The chute spillway will be designed to minimize operations and maintenance by limiting debris and sediment flows into the spillway. This will be accomplished by including a low-level inlet with a trash rack and a high-level inlet which would release flows if the low-level inlet becomes blocked. In addition, a pervious barrier fence may also be installed so debris in the runoff can be captured prior to entering the inlet to the spillway. It is recommended that DTSC staff inspect the inlet and spillway at the beginning of the rainy season each year and when heavy rains are expected. The inlet, spillway, and energy dissipation structure should be cleared of all debris and sediment during these inspections.

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Alternative # 3 – Detention and Spillway This alternative consists of construction of a smaller version of the Alternative #1 spillway with the addition of surface detention upstream of the remediated CMA dam. To reduce the peak flows released downstream of the dam (140 cfs) to a level that the downstream system can convey requires a 90 cfs reduction. To achieve this, nearly 6.7 acre-feet of storage is required. For the purposes of this evaluation, a wall on top of the CMA dam constructed of reinforced concrete would be constructed that would be about 7-feet high (5-feet deep of runoff with 2-feet of freeboard) that would act to attenuate runoff by creating the necessary 6.7 acre-feet of storage. The 6.7 acre-feet of storage would occur at the design storm (200 year) and would be much less for lower frequency events. As noted in Appendix A, the hydrograph of the 200-year storm would only impound significant water for less than 8 hours. As shown on Figure 6, the wall would have to be raised to an elevation of 1,373 which includes the 2-feet of freeboard. The estimated cost for this Alternative is $1.8 million as shown in Appendix B. The chute spillway will be designed to minimize operations and maintenance by limiting debris and sediment flows into the spillway. This will be accomplished by including a low-level inlet with a trash rack and a high-level inlet which would release flows if the low-level inlet becomes blocked. In addition, a pervious barrier fence may also be installed so debris in the runoff can be captured prior to entering the inlet to the spillway. It is recommended that DTSC staff inspect the inlet and spillway at the beginning of the rainy season each year and when heavy rains are expected. The inlet, spillway, and energy dissipation structure should be cleared of all debris and sediment during these inspections. The City of Jackson is planning to extend Sutter Street in the future which may encroach on the dam, depending on alignment of the street. If the wall is constructed and road goes through the dam site the road would have to constructed to go over the wall and would have to be above the 200-year water surface elevation (1371.7). The addition of the wall would necessitate a very steep grade on the road (approximately 20%) that probably would not be acceptable to the City of Jackson. For the purposes of this study, AECOM has assumed that the road extension would not encroach on the dam because it would be routed just south of the dam. AECOM also investigated the performance of this alternative if much of the upstream watershed was capped in the future making those capped areas impervious. This would result in much higher runoff rates and amounts. The peak flow determined in the previously prepared CDM Smith’s Flow Modeling and Damage Estimates for the Argonaut Mine Dam Failure Study of 200 cfs would result in a peak flow from the Argonaut Dam in 54 cfs while storing 14.5 acre-feet. It would not overtop the proposed wall but would encroach into the freeboard.

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Alternative #4 – Upper Diversion to Hoffman Ridge This alternative consists of diverting flows from the upper portion of the watershed and was evaluated in response to a suggestion by the EPA. This alternative would include a diversion channel in the watershed upstream of the CMA dam with a box culvert under Hoffman Street and a channel carrying flows all the way to Jackson Creek. This system would carry approximately 115 cfs (200-year event) which would reduce the peak flows released downstream of the dam to about 27 cfs. Since the existing downstream drainage system can convey about 48 cfs, the diversion channel would eliminate the need modify the existing downstream drainage system and would eliminate any detention behind the dam. A plan and profile view of this system are shown on Figure 7 and Figure 8. The estimated cost for this Alternative is $2.6 million as shown in Appendix B but would require purchasing an easement and/or private land. The chute spillway will be designed to minimize operations and maintenance by limiting debris and sediment flows into the spillway. This will be accomplished by including a low-level inlet with a trash rack and a high-level inlet which would release flows if the low-level inlet becomes blocked. In addition, a pervious barrier fence may also be installed so debris in the runoff can be captured prior to entering the inlet to the spillway. It is recommended that DTSC staff inspect the inlet and spillway at the beginning of the rainy season each year and when heavy rains are expected. The inlet, spillway, and energy dissipation structure should be cleared of all debris and sediment during these inspections.

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Alternative #5 – Lower Diversion Through Hoffman Ridge This alternative consists of a diverting the majority of the Argonaut dam watershed runoff through the ridge to the south and was evaluated as an alternative to constructing the spillway and creating capacity problems in the City’s downstream system. The diversion pipe was sized to convey the 200-year flow which is less than the flow that would be discharged through the spillway (115 cfs versus 140 cfs) because runoff would have to back up behind the existing berm to start spilling through the diversion pipeline. As shown on Figure 9, a 42-inch drain line would be constructed through Hoffman ridge that would convey the runoff to a swale south of town. This line would be installed via jack and bore through the ridge and extend to where it daylights on the slope towards Jackson Creek. These flows will continue down an existing drainage swale and then into Jackson Creek, thereby bypassing the City’s system. This alternative greatly reduces peak flows in the City’s stormwater system reducing peak flows from 167 cfs to 31 cfs. A small area between the berm and the dam would not be drained through the diversion so this water would have to be picked-up via a small diameter spillway pipe that would convey the remaining runoff around the dam. The estimated cost for this alternative is $3.2 million as shown in Appendix A but would require, like Alternative 4, obtaining an easement or purchasing of private lands. The chute spillway and the inlet to the diversion will be designed to minimize operations and maintenance by limiting debris and sediment flows into the spillway. This will be accomplished by including a low-level inlet with a trash rack and a high-level inlet which would release flows if the low-level inlet becomes blocked. In addition, a pervious barrier fence may also be installed so debris in the runoff can be captured prior to entering the inlet to the spillway. In addition, a pervious barrier fence may also be installed so debris in the runoff can be captured prior to entering the inlet to the spillway. It is recommended that DTSC staff inspect the inlet, spillway and diversion inlet at the beginning of the rainy season each year and when heavy rains are expected. The inlet, spillway, diversion inlet, and energy dissipation structure should be cleared of all debris and sediment during these inspections.

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3 Conclusions and Recommendations Alternative #1 - Spillway Only, was deemed not viable because the downstream drainage system only has capacity for 48 cfs whereas the project would deliver 140 cfs to the system. Alternative # 2 – Downstream Infrastructure Improvements, consists of constructing the spillway and improving the downstream system to handle 140 cfs and is a viable option although significant public disruption along Sutter Street and Highway 49 would occur. Nearly the entire downstream infrastructure would have to be replaced with larger pipes, culvert inlets, new manholes, and additional drainage inlets. The estimated cost of this alternative is $2.0M. Alternative # 3 – Detention and Spillway, consists of constructing the stormwater spillway with detention upstream of the dam and is the least costly of the alternatives and would reduce peak discharge flows down to 48 cfs which the downstream infrastructure can convey. To achieve this, about 6.7 acre-feet of storage is required. The estimated cost for this alternative is $1.8M. Alternative #4 – Upper Diversion to Hoffman Ridge, consists of diverting the majority of runoff through an upper basin interceptor channel to Hoffman Ridge which would eliminate the need for detention. This alternative may potentially be the EPA’s future plan and will not be considered further. At this time this alternative is higher in cost and would require leasing or purchasing private land. The estimated cost for this alternative is $2.6M. Alternative #5 – Lower Diversion Through Hoffman Ridge, consists of diverting a significant amount of the Argonaut basin runoff through Hoffman Ridge to the south is the most expensive alternative but greatly reduces peak flows in the City’s system downstream. The estimated cost for this alternative is $3.2M. Table 2 presents an overview of the pros and cons of each alternative: Table 2 Alternatives Analysis Summary

Alternative Estimated Cost

Issues and Discussions

Alternative #1 – Spillway Only

$1.2M Pro: Simple solution – all stormwater is discharged around the remediated dam Con: Downstream drainage system does not have capacity for the increased runoff. Constructing the spillway would result in surface flooding in Jackson. This is not a viable option.

Alternative #2 – Spillway and Downstream Improvements

$2.0 M Pro: All stormwater is discharged around the dam and conveyed through Jackson to Jackson Creek with improvements to all stormwater infrastructure Con: Major infrastructure upgrade is required

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Con: Environmental and public impacts due to construction of improvements within City and State right-of-way; includes utility interruption, dust, traffic congestion and lack of public access. Con: Not compatible with EPA’s future plans for capping some areas of the mine

Alternative #3 – Spillway and Detention

$1.8M Pro: All work is within the Argonaut basin. Pro: Discharge is constrained to meet Jackson’s downstream stormwater infrastructure Pro: Construction integrated with dam remediation Pro: Lowest cost Pro: Compatible with EPA’s future plans for capping some areas of the mine Con: Some, but limited, stormwater detention up to 5-feet deep upstream of dam (less than 8 hours) Con: Need to raise existing crest during remediation

Alternative #4 – Divert Flows from the Upper Watershed through Ridge

$2.6M Pro: Eliminates impacts to Jackson’s downstream stormwater Infrastructure Pro: Eliminates the need for raising the dam Pro: May be part of EPA’s future solution Con: Need access and/or easement to private property Con: More complicated construction Con: High Cost

Alternative #5 – Diversion Through Ridge

$3.2M Pro: Eliminates impacts to Jackson’s downstream stormwater Infrastructure Pro: Eliminates the need for raising the dam Con: Need access and/or easement to private property Con: More complicated construction

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Con: Still requires a small spillway system at the dam Con: Highest Cost

Because each of the four viable alternatives (numbers 2, 3, 4, and 5) would safely convey the 200-year runoff event and would not increase peak flows into the downtown area of Jackson the least costly alternative (Alternative #3), which can be designed and constructed within the current Argonaut basin, appears to be the preferred alternative.

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References Caltrans, 2014. State of California Department of Transportation, District 10. Construction Plans for Improvements to the Intersection of Highway 49 and Sutter Street. FEMA, 2016. United States Federal Emergency Management Agency. Flood Insurance Study for Amador County RTI and CDM Smith, 2015. Flow Modeling and Damage Estimates for the Argonaut Mine Dam Failure Study USACE, 2015. Structural and Geotechnical Evaluation of Argonaut Dam

APPENDIX A HYDROLOGY AND HYDRAULIC ANALYSIS

Table of Contents 1 Introduction ....................................................................................................................... 1 2 Hydrologic Analysis .......................................................................................................... 4 3 Hydraulic Analysis .......................................................................................................... 14 4 References ....................................................................................................................... 21

List of Tables

Table 1 Storm Depths for Different Frequencies ............................................................................ 6 Table 2 Runoff Curve Numbers ...................................................................................................... 8 Table 3 Overland Travel Times ...................................................................................................... 8 Table 4 Shallow, Concentrated Flow Travel Times ....................................................................... 9 Table 5 Channel Travel Times ........................................................................................................ 9 Table 6 Lag Time Summary ........................................................................................................... 9 Table 7 Existing Conditions Stage-Storage-Discharge Curve ...................................................... 10 Table 8 Developed Conditions Stage-Storage-Discharge Curve .................................................. 11 Table 9 Hydrology Results ........................................................................................................... 11 Table 10 HEC-RAS Results.......................................................................................................... 17 Table 11 Existing System Capacity .............................................................................................. 19 Table 12 Normal Depth System Capacities .................................................................................. 19 Table 13 Design Capacities .......................................................................................................... 20 Table 14 Diversion Pipe Sizing .................................................................................................... 20

List of Figures

Figure 1 – Vicinity Map .................................................................................................................. 1 Figure 2 Alternatives Analysis Map ............................................................................................... 3 Figure 3 Argonaut Drainage Basins ................................................................................................ 5 Figure 4 100-year 24-hour Isohyets ................................................................................................ 7 Figure 5 10-year 24-hour Isohyets .................................................................................................. 7 Figure 6 Proposed Detention Pond Inflow Versus Outflow Versus Stage Results ...................... 13 Figure 7 HEC-RAS Hydraulic Profile of the Spillway ................................................................. 15

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1 Introduction

The purpose of this hydrologic and hydraulic analysis is in support of the alternatives analysis of drainage improvements as part of the dam retrofit project of Argonaut Dam located in Jackson, California. The Argonaut dam vicinity is shown in Figure 1.

Figure 1 – Vicinity Map

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The alternatives evaluated are the following and are shown in Figure 2.

Alternative 1 – Concrete Spillway on the left abutment of the dam releases runoff to existing downstream infrastructure.

Alternative 2 – Improvements to the existing stormwater system downstream of the dam

in conjunction with spillway releases from the dam.

Alternative 3 – Stormwater detention in watershed upstream of the dam in conjunction with spillway releases from the dam.

Alternative 4 – Diversion channel upstream of Argonaut Dam with a box culvert under

Hoffman Street and flow discharged to Jackson Creek downstream of Jackson. Note that the remaining small runoff requires a small pipe spillway still required on left abutment of the retrofitted dam.

Alternative 5 – A piped diversion of runoff of much of the site through Hoffman Ridge to

bypass the majority of stormwater around the dam and the City of Jackson stormwater infrastructure. Note that the remaining small runoff requires a small pipe spillway still required on left abutment of the retrofitted dam.

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Figure 2 Alternatives Analysis Map

AECOM reviewed the topographic survey (prepared by KPFF in June, 2016) which shows one-foot contour intervals, spot elevations, and site features for the mine site and some adjacent properties. The topographic survey prepared for this project does not include the existing drainage system down-stream of the site. Therefore, AECOM utilized aerial mapping and GPS to determine the location of some of the downstream drainage facilities to allow our hydraulic analysis of the system. In addition, to evaluate the diversion option, USGS topographic mapping

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(which is based on 20-foot contour intervals) was used for areas outside the on-site topographic footprint. AECOM staff made a field visit on May 31, 2016 to investigate the existing drainage system, gather notes on hydrologic and hydraulic conditions, and photograph existing drainage system components. To evaluate these alternatives the amount of runoff expected to both reach the dam and continue downstream into Jackson was evaluated. The 0.5% chance of exceedance storm (200-year event) was chosen as the design storm. 2 Hydrologic Analysis

The hydrologic conditions of the Argonaut Dam were investigated to estimate the peak discharge for a 200-year event. The peak discharge was calculated using the U.S. Corps of Engineers’ HEC-HMS rainfall-runoff modeling program (USACE, 2015) incorporating the National Resource Conservation Service (NRCS) Unit Hydrograph method described in Technical Release 55 (TR-55) (NRCS, 1986) to model the rainfall-runoff relationships of the drainage areas. NRCS methods were used because there are no stream gages in the area to estimate peak flow by probabilistic methods. The hydrologic model input parameters required to estimate the peak discharge are described below.

Drainage Basins

The contributing drainage basins were delineated from topological data from the National Elevation Dataset (http://ned.usgs.gov/) (USGS, 2016) and within the vicinity of the project site one- foot contour data was used based on the topographic survey by KPFF dated June, 2016. The drainage basins are largely undeveloped with sparse vegetative ground cover and total approximately 3.6 square miles. Figure 3 presents the drainage basins names and areas. Drainage areas 5A and 5B are the basins that drain to the dam. Basins 1 through 4 are downstream of the dam and combine with basin 6 at Jackson Creek. Drainage basin 6 is the North Fork Jackson Creek basin which was included in the hydrologic modeling to allow comparisons of peak flows downstream of the site.

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Figure 3 Argonaut Drainage Basins Rainfall

There are two rainfall parameters required for the NRCS method including rainfall depth and distribution type. The 2-, 5-, 10-, 25-, 50-, 100- and 200- yr 24-hour design storm depths for the project area was obtained from the National Oceanic and Atmospheric Administration

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(http://www.noaa.gov/) (NOAA, 2016). The storm depths for different flood frequencies were presented in Table 1.

Table 1 Storm Depths for Different Frequencies

Flood Frequency Precipitation

(in)

2 YR 2.7

5 YR 3.4

10 YR 4.0

25 YR 4.8

50 YR 5.3

100 YR 5.9

200 YR 6.4 Note: Storm depths for the project area was obtained from the National Oceanic and Atmospheric Administration (http://www.noaa.gov/) (NOAA, 2016). Rainfall distribution per TR-55 “Rainfall” section, page 1-1.

The rainfall distribution type was determined by using a geographical boundaries map of rainfall distributions included in TR-55. An SCS Type IA temporal storm distribution was used for this area based on the geographical location.

In this report the only 100-yr and 10-yr storm Isohyets were included. Figure 4 and Figure 5 shows the 100-year and 10-year 24 hour storm Isohyets.

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Figure 4 100-year 24-hour Isohyets

Figure 5 10-year 24-hour Isohyets

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Runoff Curve Number and Lag Time

Runoff curve numbers were developed to determine runoff losses due to infiltration and interception per TR-55. The runoff curve numbers were determined by reviewing available ground cover and soil group data. The National Land Cover Database (http://www.mrlc.gov/) (USGS, 2016) was used to determine the ground cover types within the drainage areas. Review of aerial imagery and a field investigation was completed to verify the ground cover types within the drainage areas. The NRCS Web Soil Survey Website (http://websoilsurvey.sc.egov.usda. gov/App/HomePage.htm (NRCS, 2016) was used to determine the hydrologic soil group classifications within each drainage area. The soil types within the study area are classified primarily as hydrologic soil group D. Table 2 gives the resulting runoff curve numbers. In the developed condition the Argonaut watershed may be capped, however, for the purposes of this report, it was assumed that a cap will not change any existing infiltration conditions.

Table 2 Runoff Curve Numbers

Basin ID Area (Ac)

Runoff Curve Number

1 18 84 2 8 82 3 5 82 4 6 87 5 113 87

5A 91 87 5B 22 84 6 2,173 76

Note: Runoff curve numbers were developed per Chapter 2, Estimating Runoff, of TR-55. Ground cover types were determined using the National Land Cover Database (http://www.mrlc.gov/) (USGS, 2016). Hydrologic soil groups were determined using the NRCS Web Soil Survey Website (http://websoilsurvey.sc.egov.usda. gov/App/HomePage.htm (NRCS, 2016).

Lag times were calculated by following the TR55 guidelines. Table 3 presents the overland travel times; Table 4 presents the shallow, concentrated travel times, Table 5 presents the channel travel times, and Table 6 gives the lag time summary.

Table 3 Overland Travel Times

Basin

n value

L S 2Yr/24hr

Rainfall Depth Overland Travel

Time (tr)

(ft) (ft/ft) (in) (minutes) 1 0.40 300 0.02 2.69 56 2 0.24 260 0.02 2.68 33 3 0.11 60 0.02 2.68 6 4 0.24 270 0.02 2.68 35 5 0.15 120 0.02 2.67 12

5A 0.15 120 0.02 2.67 12

5B 0.15 40 0.02 2.67 5

6 0.24 290 0.02 2.78 36

Note: Overland travel times were determined from Chapter 3, Sheet Flow, page 3-3 of TR-55.

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Table 4 Shallow, Concentrated Flow Travel Times

Basin L Slope Average Velocity Shallow Concentrated

Travel Time

(ft) (ft/ft) (ft/s) (min) 1 370 0.133 5.88 1 2 410 0.189 7.02 1 3 750 0.128 7.00 2 4 900 0.082 5.60 3 5 830 0.048 3.54 4

5A 830 0.048 3.54 4

5B 550 0.176 6.78 1

6 4,160 0.053 3.71 19 Note: Shallow, Concentrated travel times were determined from Chapter 3, Shallow Concentrated Flow, page 3-3 of TR-55.

Table 5 Channel Travel Times

Basin

Cross-Sectional

Area

Wetted Perimeter

Hydraulic Radius (R)

Manning’s “n”

Length Slope Average Velocity

Travel Time

(ft2) (ft) (ft) (ft) (ft/ft) (ft/s) (minutes)

1 7 9 0.752 0.012 900 0.065 26.1 1 2 2 9 0.188 0.04 750 0.082 3.5 4 3 88 35 2.493 0.045 600 0.104 19.5 1 4 N/A 5 57 2 1.967 0.03 700 0.084 22.5 1 5 180 4 3.604 0.04 600 0.073 23.7 0 5 180 4 3.604 0.04 350 0.014 10.3 1

5A1 57 2 1.967 0.03 700 0.084 22.5 1 5A2 180 4 3.604 0.04 300 0.073 23.7 0 5B 180 4 3.604 0.04 500 0.014 10.3 1 6A 72 2 2.119 0.04 5,450 0.055 14.3 6 6B 108 3 2.680 0.04 4,800 0.019 9.8 8 6C 198 4 3.740 0.04 3,200 0.021 13.0 4 6D 312 5 4.756 0.04 2,850 0.015 12.8 4

Note: Channel travel times were determined from Chapter 3, Open Channels, page 3-3 of TR-55.

Table 6 Lag Time Summary

Basin Computed Total Travel Time Lag Time Lag Time

(minutes) (minutes) (hr) 1 17 10 0.2 2 38 23 0.4 3 8 5 0.1 4 37 22 0.4 5 18 11 0.2

10

Basin Computed Total Travel Time Lag Time Lag Time

(minutes) (minutes) (hr) 5A 17 10 0.2

5B 7 4 0.1

6 77 46 0.8 Note: Lag times were determined by summing the overland, shallow concentrated, and channel travel times multiplied by 0.6 per TR-55.

Storage

Under existing conditions, runoff would be attenuated below the dam and behind Argonaut Road because the existing 36” culvert would not have capacity to discharge the entire 139 cfs without significant hydraulic head. This would lead to the runoff backing up behind the road. AECOM took this into account in our hydrologic model using the stage-storage-discharge curve shown in Table 7.

Table 7 Existing Conditions Stage-Storage-Discharge Curve

Stage Area Cumulative Volume Peak Flow

(ft2) (Acres) (ft3) (Acre-Feet) (cfs)

1324 1,220 0.028 - 0.000 0

1325 2,037 0.047 1,628 0.037 2

1326 2,991 0.069 4,142 0.095 12

1327 4,381 0.101 7,828 0.180 29

1328 5,808 0.133 12,923 0.297 48

1329 6,945 0.159 19,299 0.443 68

1330 7,961 0.183 26,752 0.614 84

1331 8,942 0.205 35,204 0.808 95

1332 10,032 0.230 44,691 1.026 104

1333 11,276 0.259 55,345 1.271 112

1334 12,423 0.285 67,194 1.543 120

1335 14,206 0.326 80,509 1.848 127

1336 15,824 0.363 95,524 2.193 134

1337 17,114 0.393 111,994 2.571 140

1338 18,411 0.423 129,756 2.979 147 Note: Stage and storage values were determined utilizing KPFF topographic survey 1-foot contour intervals. Discharges determined from HEC-RAS hydraulic modeling.

Alternative 3 provides new detention storage behind the dam. The stage-storage-discharge curve shown in Table 8 was used in the HEC-HMS hydrology model.

11

Table 8 Developed Conditions Stage-Storage-Discharge Curve

Stage Area Cumulative Volume Outflow (Weir)

(ft2) (Acres) (ft3) (Acre-Feet) (cfs)

1368 0 0.000 - - -

1369 22,960 0.527 11,480 0.264 2

1370 65,877 1.512 55,899 1.283 11

1371 131,972 3.030 154,824 3.554 25

1372 172,676 3.964 307,148 7.051 41

1373 204,533 4.695 495,752 11.381 60 Note: Stage and storage values were determined utilizing KPFF topographic survey 1-foot contour intervals. Discharges determined from HEC-RAS hydraulic modeling.

Results

Using the HEC-HMS hydrologic model, the existing conditions, peak discharge to Jackson Creek would be 1,388 cfs for the 200-year, 24-hour storm. No debris flow was included in the peak runoff estimates. Table 9 shows the result of the modeling for each alternative for the 200-year frequency. Table 9 Hydrology Results

Drainage Basin/Location

Peak Discharge (cfs)1

Existing Conditions

Developed Conditions

(Alternative’s 1 and 2)

Alternative 3 (Detention)

Alternative 4 (Diversion)


5 140 N/A N/A N/A N/A

5A N/A 115 115 115 115

5B N/A 27 27 27 27

Combined 5A and 5B

N/A 140 140 N/A 140

Proposed Detention Release

N/A N/A 44 N/A N/A

ExStorage 117 N/A N/A N/A N/A

4 7 7 7 7 7

Junction 1 (Basins 4 and 5 Combined)

123 146 50 32 7

3 5 5 5 5 5

Junction 2 (Basins 4, 5, and 3 Combined)

126 151 52 37 11

2 7 7 7 7 7

12

Drainage Basin/Location

Peak Discharge (cfs)1

Existing Conditions

Developed Conditions

(Alternative’s 1 and 2)

Alternative 3 (Detention)



1 15 15 15 15 15

Junction 3 (Basins 4, 5, 3, 2, and 1

Combined) 147 167 74 53 31

6 1,285 1,285 1,285 1,285 1,285

Junction 4 (All Basins Combined)

1,388 1,367 1,354 1,367 1,367

1. Note that peak flows are combined hydrographically and are not arithmetically added

Figure 7 shows the inflow, outflow, and stage results of the detention pond during a 200-year, 24 hour storm.

13

Figure 6 Proposed Detention Pond Inflow Versus Outflow Versus Stage Results

14

3 Hydraulic Analysis

Spillway

Using the computed peak discharges from the hydrologic analysis, AECOM prepared a hydraulic model using the United States Army Corps of Engineers’ (USACE’s) Hydrologic Engineering Centers River Analysis System (HEC-RAS) (USACE, 2010) software to analyze the proposed spillway. Figure 7 shows the planned spillway invert, top of wall, water surface elevation, and critical depth. The spillway would flow supercritical nearly the entire distance and would go through hydraulic jump at the end transitioning into subcritical flow prior to entering the culverts under Sutter Street. For the design flow of 139 cfs, velocities in the steep portion of the spillway would reach 30 feet per second but would slow to less than 5 feet per second in the flat, downstream section.

15

Figure 7 HEC-RAS Hydraulic Profile of the Spillway

16

17

Table 10 shows the results of the HEC-RAS analysis of the spillway.

Table 10 HEC-RAS Results

River Station Min Channel

Elevation Water Surface

Elevation Velocity of Channel

(ft) (ft) (ft/s)

River Sta Min Ch El W.S. Elev Vel Chnl

(ft) (ft) (ft/s)

1010 1367 1369.89 9.7

920 1365.2 1367.18 14.1

895 1363.95 1365.70 16.0

885.388* 1362.12 1363.60 19.0

875.777* 1360.3 1361.62 21.2

866.166* 1358.47 1359.69 23.0

856.555* 1356.65 1357.80 24.4

846.944* 1354.82 1355.91 25.6

837.333* 1352.99 1354.04 26.6

827.722* 1351.17 1352.19 27.4

818.111* 1349.34 1350.34 28.1 808.5* 1347.51 1348.48 28.8

798.888* 1345.69 1346.65 29.3 789.277* 1343.86 1344.80 29.7 779.666* 1342.04 1342.97 30.1

770.055* 1340.21 1341.13 30.4 760.444* 1338.38 1339.29 30.7 750.833* 1336.56 1337.47 30.9

741.222* 1334.73 1335.63 31.1 731.611* 1332.91 1333.81 31.3

722 1331.08 1331.97 31.4

713.* 1330.35 1331.28 30.2 704.* 1329.63 1330.59 29.2 695.* 1328.91 1329.90 28.3

686.* 1328.18 1329.19 27.7 677.* 1327.45 1328.48 27.1 668.* 1326.73 1327.78 26.6

659.* 1326.01 1327.08 26.2 650 1325.28 1326.37 25.8

640.* 1325.23 1326.39 24.1

630.* 1325.18 1326.42 22.6

620.* 1325.13 1330.75 5.0

Note: * Interpolated Cross-Section

18

Downstream Drainage System

The hydraulic modeling of the downstream system was completed for two reasons: 1) To determine the capacity of the existing system and 2) To determine the extent of improvements to the system to safely convey the entire 200-year flow that would be discharged through the dam spillway. AECOM used Manning’s equation for both open-channel flow and, where necessary, for pressure flow. Table 11 shows the maximum flow that each pipe segment can convey without overtopping the grate of the upstream structure (pressure flow). Segments 2 and 3 limit the overall system to 48 cfs, much less than the 134 cfs that would inundate this system in a 200-year event. If the system is not under pressure, segments 2 and 3 limit the system’s capacity to only 36 cfs as shown in Table 12. To carry the design storm (200-year event) nearly the entire system would have to be improved. Table 13 shows the design sizes and capacities. Diversion The diversion pipe through the ridge and under Hoffman Road was sized convey the 200-year flow. AECOM used Manning’s equation for open-channel flow with the results shown in Table 14. The peak, 200-year flow is less than the flow that would be discharged through the spillway (115 cfs versus 140 cfs) because runoff would have to back up behind the existing berm to start spilling through the diversion pipeline. This would attenuate the flows out through the diversion. In addition, a small diameter pipe will have to be constructed on the left dam abutment to drain the small area between the berm and the dam.

19

Table 11 Existing System Capacity

Node Upstream

Pipe Invert

Down- stream Pipe

Invert Grate

Elevation

Max Capacity Under

Pressure (cfs)

Pipe Diameter

(in)

Pipe Length

(ft)

Pipe Slope (ft/ft)

Manning's n

Hydraulic Radius, Rfull, (ft)

Flow Area,

Afull (ft2) Velocity

Friction Slope, S

Friction Loss, Hf

Minor Loss, Hm

Hydraulic Gradeline (ft)

1 1212.60 1217.50 1223.93 149 48 141 3.48% 0.015 1.000 12.57 11.9 1.425% 2.009 2.18 1223.8

2 1218.50 1219.83 1224.43 48 36 90 1.48% 0.015 0.750 7.07 6.8 0.686% 0.617 0.00 1224.4

3 1219.83 1219.88 1224.52 48 36 13 0.38% 0.015 0.750 7.07 6.8 0.686% 0.089 0.00 1224.5

4 1219.88 1220.22 1227.50 108 36 86 0.40% 0.015 0.750 7.07 15.3 3.472% 2.986 0.00 1227.5

5 1220.22 1222.37 1229.00 103 36 48 4.49% 0.015 0.750 7.07 14.6 3.158% 1.513 0.00 1229.0

6 1222.37 1225.50 1230.70 60 36 157 1.99% 0.015 0.750 7.07 8.5 1.072% 1.682 0.00 1230.7

7 1225.50 1235.00 1240.29 84 30 173 5.49% 0.015 0.625 4.91 17.1 5.554% 9.608 0.00 1240.3

8 1235.00 1260.00 1265.00 106 27 157 15.92% 0.015 0.563 3.98 26.7 15.513% 24.355 0.00 1264.6

9 1260.00 1274.75 1278.30 67 27 179 8.24% 0.015 0.563 3.98 16.9 6.198% 11.094 2.20 1277.9

Note: Existing system capacity was determined utilizing Manning’s Equation for gravity flow and Manning’s assuming a Preissmann Slot for pressure flow.

Table 12 Normal Depth System Capacities

Node Upstream

Pipe Invert

Down- stream Pipe

Invert Grate

Elevation

Pipe Diameter

(in)

Pipe Length

(ft)

Pipe Slope (ft/ft)

Manning's n

Assumed Depth (ft) theta

Flow Area Velocity

Hydraulic Radius

Maximum Capacity

Normal Depth (cfs)

1 1212.60 1217.50 1223.93 48 141 3.48% 0.015 4.00 3.14 12.57 11.9 1.00 233

2 1218.50 1219.83 1224.43 36 90 1.48% 0.015 3.00 3.14 7.07 6.8 0.75 70

3 1219.83 1219.88 1224.52 36 13 0.38% 0.015 3.00 3.14 7.07 6.8 0.75 36

4 1219.88 1220.22 1227.50 36 86 0.40% 0.015 3.00 3.14 7.07 15.3 0.75 36

5 1220.22 1222.37 1229.00 36 48 4.49% 0.015 3.00 3.14 7.07 14.6 0.75 123

6 1222.37 1225.50 1230.70 36 157 1.99% 0.015 3.00 3.14 7.07 8.5 0.75 82

7 1225.50 1235.00 1240.29 30 173 5.49% 0.015 2.50 3.14 4.91 17.1 0.63 84

8 1235.00 1260.00 1265.00 27 157 15.92% 0.015 2.25 3.14 3.98 26.7 0.56 107

9 1260.00 1274.75 1278.30 27 179 8.24% 0.015 2.25 3.14 3.98 16.9 0.56 77

Note: Existing system capacity was determined utilizing Manning’s Equation for gravity flow.

20

Table 13 Design Capacities

Node Upstream

Pipe Invert

Down- stream Pipe

Invert Grate

Elevation

Peak 100-Year Flow

(cfs)

Pipe Diameter

(in)

Pipe Length

(ft)

Pipe Slope (ft/ft)

Manning's n

Hydraulic Radius, Rfull, (ft)

Flow Area,

Afull (ft2) Velocity

Friction Slope, S

Friction Loss, Hf

Minor Loss, Hm

Hydraulic Gradeline (ft)

1 1212.60 1217.50 1223.93 149 48 141 3.48% 0.015 1.000 12.57 11.9 1.425% 2.009 2.18 1223.8

2 1218.50 1219.83 1224.43 134 48 90 1.48% 0.012 1.000 12.57 10.7 0.738% 0.664 0.00 1224.5

3 1219.83 1219.88 1224.52 134 48 13 0.38% 0.012 1.000 12.57 10.7 0.738% 0.096 0.00 1224.6

4 1219.88 1220.22 1227.50 134 42 86 0.40% 0.012 0.875 9.62 13.9 1.503% 1.293 0.00 1225.8

5 1220.22 1222.37 1229.00 134 42 48 4.49% 0.012 0.875 9.62 13.9 1.503% 0.720 0.00 1226.6

6 1222.37 1225.50 1230.70 134 42 157 1.99% 0.012 0.875 9.62 13.9 1.503% 2.360 0.00 1228.9

7 1225.50 1235.00 1240.29 134 36 173 5.49% 0.012 0.750 7.07 19.0 3.421% 5.918 0.00 1234.8

8 1235.00 1260.00 1265.00 134 30 157 15.92% 0.012 0.625 4.91 27.3 9.045% 14.201 0.00 1249.0

9 1260.00 1274.75 1278.30 134 30 179 8.24% 0.012 0.625 4.91 27.3 9.045% 16.191 5.79 1271.0

Note: Design capacities eres determined utilizing Manning’s Equation for gravity flow and Manning’s assuming a Preissmann Slot for pressure flow.

Table 14 Diversion Pipe Sizing

Upstream Pipe Invert

Downstream Pipe Invert

SD Pipe Flow (cfs)

Pipe Diameter

(in) Pipe Length

(ft)

Pipe Slope (ft/ft)

Manning's n Depth (ft)

Flow Area Velocity

Hydraulic Radius

1370.00 1362.00 115 45 899 0.89% 0.012 3.50 10.73 10.7 1.09

Note: Diversion pipe size was determined utilizing Manning’s Equation for gravity flow.

21

4 References

NOAA, 2016. United States National Oceanic and Atmospheric Administration (http://www.noaa.gov/) NRCS, 2016. United States Natural Resource Conservation Service Web Soil Survey Website (http://websoilsurvey.nrcs.usda.gov/app/) NRCS, 1986. United States National Resource Conservation Service. Technical Release 55, Urban Hydrology for Small Watersheds USACE, 2010. HEC-RAS Computer Software, Version 4.1.0 USACE, 2013. HEC-HMS Computer Software, Version 4.0 USGS, 2016. United States Geologic Survey National Land Cover Database (http://www.mrlc.gov/) USGS, 2016. United States Geologic Survey National Elevation Dataset (http://ned.usgs.gov/)

APPENDIX B COST ESTIMATES

SUMMARY Alternative 1 Alternative 2 Alternative 3 Alternative 4 Alternative 5

Spillway Only

Downstream Infrastructure Improvements

Detention and Spillway

Upper Diversion to

Hoffman Ridge

Lower Diversion Through Hoffman

Ridge

$1,041,601 $2,001,809 $1,843,256 $2,570,559 $3,181,182

ALTERNATIVE #1 COST ESTIMATE (SPILLWAY ONLY)

Item Description Quantity Unit Unit Cost Cost

1 Survey and Layout 3 CD $4,500 $13,500 2 Mobilization 1 LS $25,000 $25,000 3 Clear and Grubb 8,000 SF $0.50 $4,000 4 SWPPP 8,000 SF $0.50 $4,000 5 Allowance for Utility Relocation 1 LS $5,000 $5,000 6 Spillway Inlet Structure 1 EA $15,000 $15,000 7 5' Wide by 5' Deep Spillway 1 LS $515,000 $515,000 Subtotal: $581,500

8 General Condition (15%) $87,225

9 Bond & Insurance (2%) $13,375

10 Contractor Fee (8%) $54,568

Direct Cost Estimate $736,667

11 Contingency (20%) $147,333

Direct Construction Probable Cost $884,001 12 Permitting 1 LS $25,000 $25,000

13 Engineering (15%) $132,600 Project Cost: $1,041,601

ALTERNATIVE #2 COST ESTIMATE (DOWNSTREAM INFRASTRUCTURE IMPROVEMENTS)


1 Survey and Layout 3 CD $4,500 $13,500 2 Mobilization 1 LS $25,000 $25,000 3 Clear and Grubb 8,000 SF $0.50 $4,000 4 SWPPP 17,550 SF $0.50 $8,775 5 Demolition 1 LS $25,018 $25,018 6 Traffic Control 1 LS $62,000 $62,000 7 30"RCP 340 LF $275 $93,500 8 36"RCP 327 LF $310 $101,370 9 42" RCP 400 LF $380 $152,000

10 12" RCP 120 LF $62 $7,440 11 48" MH 9 EA $4,500 $40,500 12 Drop Inlets 6 EA $2,000 $12,000 13 Restore Pavement 6,360 EA $6.50 $41,340


14 Spillway Inlet Structure 1 EA $15,000 $15,000 15 5' Wide by 5' Deep Spillway 1 LS $515,000 $515,000 Subtotal: $1,116,443




Direct Cost Estimate $1,414,355

19 Contingency (20%) $282,871

Direct Construction Probable Cost $1,697,226 20 Permitting 1 LS $50,000 $50,000


ALTERNATIVE #3 COST ESTIMATE (DETENTION AND SPILLWAY)


1 Survey and Layout 3 CD $4,500 $13,500 2 Mobilization 1 LS $25,000 $25,000 3 Clear and Grubb 8,000 SF $0.50 $4,000 4 SWPPP 12,000 SF $0.50 $6,000 5 Allowance for Utility Relocation 1 LS $5,000 $5,000 6 8-Foot MSE Wall 475 LF $1,000 $475,000 8 Earthen Fill in Wall 1350 CY $35 $47,250 9 Spillway Inlet Structure 1 EA $15,000 $15,000

10 3' Deep by 3" Wide Spillway 1 LS $415,000 $415,000 11 Improve City System Culvert Inlet 1 EA $20,000 $20,000 Subtotal: $1,025,750

12 General Condition (15%) $153,863 13 Bond & Insurance (2%) $23,592 14 Contractor Fee (8%) $96,256 Direct Cost Estimate $1,299,461

15 Contingency (20%) $259,892 Direct Construction Probable Cost $1,559,353

16 Permitting 1 LS $50,000 $50,000 17 Engineering (15%) $233,903

Project Cost: $1,843,256

ALTERNATIVE #4 COST ESTIMATE (UPPER DIVERSION TO HOFFMAN RIDGE)


1 Survey and Layout 3 CD $4,500 $13,500 2 Mobilization 1 LS $27,000 $27,000 3 Clear and Grubb 205,000 SF $0.50 $102,500 4 SWPPP 205,000 SF $0.50 $102,500 5 Inlet Structure 1 EA $15,000 $15,000 6 Channel Excavation 16,000 CY $15 $240,000 7 Disposal of Spoils 16,000 CY $5 $80,000 8 5' by 5' Box Culvert 2 EA $60,000 $120,000 9 Outfall Structure 1 LS $30,000 $30,000

10 Rock Slope Protection 250 CY $65 $16,250

11 Concrete Lining for Downstream Channel

2,600 LF $100 $260,000

12 3' Deep by 3" Wide Spillway 1 LS $415,000 $415,000 Subtotal: $1,421,750

13 General Condition (15%) $213,263 14 Bond & Insurance (2%) $32,700 15 Contractor Fee (8%) $133,417 Direct Cost Estimate $1,801,130

16 Contingency (20%) $360,226 Direct Construction Probable Cost $2,161,356

17 Drainage Easement 3.5 Acres $10,000 $35,000 18 Permitting 1 LS $50,000 $50,000 19 Engineering (15%) $324,203

Project Cost: $2,570,559

ALTERNATIVE #5 COST ESTIMATE (LOWER DIVERSION TO HOFFMAN RIDGE)


1 Survey and Layout 2 CD $4,500 $9,000 2 Mobilization 1 LS $50,000 $50,000 3 Clear and Grubb 5,000 SF $0.50 $2,500 4 SWPPP 16,100 SF $0.50 $8,050 5 Traffic Control 2 CD $1,500 $3,000 6 Earthwork 1 LS $79,921 $79,921 7 42" Jack and Bore 850 LF $1,050 $892,500 8 Outfall Structure 1 LS $30,000 $30,000 9 Rock Slope Protection 190 CY $65 $12,350

10 Concrete Lining for Downstream Channel

2,600 LF $100 $260,000

11 Spillway Inlet Structure 1 EA $15,000 $15,000 12 3' Deep by 3" Wide Spillway 1 LS $415,000 $415,000 Subtotal: $1,777,321





Direct Cost Estimate $2,251,581

16 Contingency (20%) $450,316

Direct Construction Probable Cost $2,701,898 17 Drainage Easement 2.4 Acres $10,000 $24,000 18 Permitting 1 LS $50,000 $50,000


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