Site Description. CCR Rule Initial Inflow Design Flood Control System Plan (cont.) 2

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2 Site Description Kentucky Utilities Company (KU) owns and operates Ghent Gypsum Stack, a CCR surface impoundment, at the Ghent Generating Station in Carroll County, Kentucky. The impoundment is permitted to operate by the Kentucky Division of Water (KDOW). Gypsum Stack is located entirely on the Ghent Generating Station property. The Ghent Generating Station has five CCR impoundments onsite that are hydraulically interdependent: Ash Treatment Basin #1 (ATB1), ATB2, Secondary Pond, Gypsum Stack, and Gypsum Reclaim Pond. The Gypsum Stack is hydraulically connected to the other impoundments and it is important to understand the system as a whole. The Gypsum Stack discharges into the Gypsum Reclaim Pond which then discharges into the Secondary Pond. The Secondary Pond also receives inflows from Ash Treatment Basin (ATB) #1. ATB2 is located upstream of ATB1. ATB2 s decant pipeline discharges into ATB1. Subsequently, ATB1 discharges into the Secondary Pond. Flows that activate ATB2 s auxiliary spillway leave the system and do not enter into ATB1. Normal flows from the Secondary Pond exit over a stop log riser and flow through a pipe to a concrete drainage structure that combines the Secondary Pond outflow with other sources of water before being discharged to the Ohio River through a 72- inch diameter pipe. The Gypsum Stack is operated using the gypsum stacking method and currently has two cells: the north cell and the south cell. The two cells share a common outer dike that is formed by embankments to the north and east, a natural hillside to the south, and the ATB1 eastern embankment to the west. The north and south cells are separated by an internal divider dike with an approximate current top elevation of 553. The dike around the south cell has an approximate current top elevation of 553 and the dike around the north cell has an approximate current top elevation of 545. Each cell is operated with a minimum of five (5) feet of freeboard. The principal spillway system is a network of four decant structures. Surface water from rainfall and pumped process water exits the Gypsum Stack through the east and west decant systems. Decant structures control the water surface elevation and are adjusted as required by the stacking operations. Each decant structure has an open face with grooves for the insertion of steel stop logs as required. The decant structures discharge through the 24-inch HDPE decant pipe system into the Gypsum Reclaim Pond. Analyses presented herein were developed based on a combination of field inspections, site surveys, and documentation (including historical construction plans and reports) provided by KU. Existing conditions used for analysis were based on an aerial LiDAR and a bathymetric survey both completed in 2015 and provided by KU. The topography was supplemented with GIS contours obtained from the Kentucky Division of Geographic Information (DGI). Inflow Design Flood Control System 40 CFR (a) requires that the owner or operator of an existing or new CCR surface impoundment or any lateral expansion of a CCR surface impoundment must design, construct, operate, and maintain an inflow design flood control system to adequately manage flow into and CCR Rule Initial Inflow Design Flood Control System Plan (cont.) 2

3 out of the CCR unit associated with a particular IDF based on the hazard potential of the CCR surface impoundment. The Ghent Gypsum Stack was determined under (a)(2) to be a high hazard potential CCR surface impoundment in accordance with classification definitions provided in 40 CFR ; therefore, the IDF is the probable maximum flood (PMF). Inflow Methodology This study uses standard U.S. Department of Agriculture, Natural Resources Conservation Service (NRCS) methodology as described in the National Engineering Handbook, Part 630 Hydrology. The NRCS methodology uses several parameters, which include basin area, land usage, basin lag, and rainfall distribution to determine runoff rates and volumes. Autodesk s Storm and Sanitary Analysis 2015 (SSA) computer modeling program was used for the hydrologic analysis of the system. The SSA model was used to compute the runoff hydrograph for each cell of the Gypsum Stack. Contributing Drainage Basin The contributing drainage to the Ghent Gypsum Stack was delineated based on the LiDAR survey contours of the Ghent Generating Station. The drainage basins were delineated separately for the north and south cells and 19.8 and 23.2 acres, respectively. The drainage basins are comprised almost entirely of the Gypsum Stack itself, although a small portion of the southern hillside also drains into the south cell. Hydrologic Soil Groups The runoff calculations reflect the impact of the various hydrologic soil groups contained within the drainage basin. The soil types were determined from the NRCS Web Soil Survey for Carroll County, Kentucky. The soil survey indicated hydrologic soil types C and D along the southern hillside draining into the south cell. The Gypsum Stack itself did not have a hydrologic soil type identified and was simply identified as a pit/quarry. However, due to the land use within the drainage basin (see Curve Number Analysis, below), the hydrologic soil group does not influence runoff quantity. For conservatism in calculating the IDF runoff, antecedent moisture condition (AMC) III was used for the analysis to account for saturated soils. Curve Number Analysis The land use for the drainage basin was determined from a site inspection and aerial photos. The majority of the area is the impounded water surface and stored gypsum while the remaining contributing area consists of gravel roads with some woods along the hillside. Since the drainage basin only consists of a small amount of the wooded hillside, the entire drainage basin was assumed to have a consistent Curve Number (CN) of 98 based on the other land usages within the drainage basin. Adjusting for AMC III resulted in a CN of 99 used for the hydrologic analysis. CCR Rule Initial Inflow Design Flood Control System Plan (cont.) 3

4 Time of Concentration With a small, impervious drainage area relative to the overall size of the impoundment, the time of concentration used for the drainage basin was the NRCS Technical Release 55 minimum of 6 minutes. This also correlates with the time interval used within the SSA model hydrograph. Inflow Design Flood 40 CFR requires existing surface impoundments to adequately manage flow into and from the CCR unit associated with a particular IDF based on the classification of the structure. Since Ghent Gypsum Stack is a high hazard potential CCR surface impoundment, it must adequately manage flow associated with the PMF (40 CFR (a)(3)(i)). The Kentucky Department of Water (KDOW) Engineering Memorandum No. 5, Design Criteria for Dams and Associated Structures specifies a 6-hour duration for design rainfall events; therefore, a 6-hour rainfall duration was used for the analysis. The rainfall for this evaluation is based on the 6-hour probable maximum precipitation (PMP) rainfall depth of 27.7 inches for Carroll County taken from the KDOW Engineering Memorandum No. 2, Rainfall Frequency Values for Kentucky. The rainfall was distributed using the NRCS standard 6-hour distribution and then entered into SSA. Additional Hydrologic Contributions As discussed previously in this document, the Ghent Gypsum Stack is located in a series of hydraulically-connected impoundments (see mapping in Appendix A). Each of these impoundments is required to demonstrate compliance with applicable CCR regulations. While each impoundment is presented in its own standalone Inflow Design Flood Control System Plan, the hydraulic performance was analyzed as a system. In the case of the Gypsum Stack, no other areas or impoundments contribute to the inflow. Since the Gypsum Stack discharges into the Gypsum Reclaim Pond, the PMF runoff hydrograph from the Gypsum Stack drainage basins were added into the inflow to the Gypsum Reclaim Pond. A steady base flow was included as a Gypsum Stack inflow. The operational flows for the Ghent Generating Station were determined based on a diagram provided by KU and discussions with Ghent operations personnel. The daily plant produced flows (none of which have rainfall components) were taken from this diagram. For the Gypsum Stack, this total baseflow is 7 cubicfeet per second (cfs). The baseflows were included for the duration of the storm event. Storage Volume An existing conditions area-elevation table was developed based on the LiDAR contours and a bathymetric survey to determine the available storage volume within the Gypsum Stack. The storage available within the north and south cells was determined separately and compared to the respective runoff hydrograph volume for the IDF. Based on the surveyed configuration of the cells, each cell has more storage volume capacity than runoff volume produced by the IDF. Therefore, CCR Rule Initial Inflow Design Flood Control System Plan (cont.) 4

5 the Gypsum Stack has adequate hydrologic and hydraulic capacity to meet the requirement of 40 CFR (a). Outlet Control Structures Surface water from rainfall and pumped process water exits the Gypsum Stack through the east and west decant systems. Decant structures control the water surface elevation and are raised and lowered as required by the stacking operations. Each decant structure has an open face with grooves for the insertion of steel stop logs as required. The decant structures discharge through the 24-inch HDPE decant pipe system into the Gypsum Reclaim Pond. Since the Gypsum Stack maintains enough storage capacity to store the entire IDF runoff hydrograph, the hydraulics of the decant system are not critical. Results The IDF hydrograph was computed within the SSA computer program using the above described parameters and the resulting volume compared to the available storage volume within the impoundment. The results indicate that the Gypsum Stack will adequately manage flow associated with the PMF into and out of the Ghent Gypsum Stack CCR unit. CCR Unit Discharge 40 CFR (b) requires that discharge from a CCR impoundment must be handled in accordance with the surface water requirements under CFR CFR requires that a facility shall not cause a discharge of pollutants in violation of the NPDES program under section 402 of the Clean Water Act (CWA), shall not cause a discharge of dredged material or fill material in violation of the Section 404 of the CWA, and shall not cause non-point source pollution in violation of the Water Quality Management plan under Section 208 of the CWA. The outflow from the Gypsum Stack combines with other flows which collectively discharge through Ghent Generating Station s Outfall #001, which is permitted through the Kentucky Pollutant Discharge Elimination System (KPDES). The permitted outfall is monitored for compliance with the generating station s individual KPDES permit which includes the same requirements as outlined in the above paragraph and provides regulatory oversight that the requirements of 40 CFR (b) are met. CCR Rule Initial Inflow Design Flood Control System Plan (cont.) 5

6 APPENDIX A: HYDROLOGIC ANALYSIS Overall Site Map Basin Topographic Map Basin Land Use Map Hydrologic Soil Group Information Curve Number Analysis

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10 85 1' 24'' W Hydrologic Soil Group Carroll, Gallatin, and Owen Counties, Kentucky 85 0' 47'' W 38 45' 11'' N 38 44' 52'' N ' 11'' N 38 44' 52'' N 85 1' 24'' W N Map Scale: 1:4,040 if printed on A landscape (11" x 8.5") sheet. Meters Feet Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 16N WGS ' 47'' W Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/19/2016 Page 1 of 4

11 Hydrologic Soil Group Carroll, Gallatin, and Owen Counties, Kentucky MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Rating Polygons A A/D B B/D C C/D D Not rated or not available Soil Rating Lines A A/D B B/D C C/D D Not rated or not available Soil Rating Points A A/D B B/D C C/D D Not rated or not available Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:15,800. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Carroll, Gallatin, and Owen Counties, Kentucky Survey Area Data: Version 12, Sep 15, 2015 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Oct 3, 2011 Oct 4, 2011 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/19/2016 Page 2 of 4

12 Hydrologic Soil Group Carroll, Gallatin, and Owen Counties, Kentucky Hydrologic Soil Group Hydrologic Soil Group Summary by Map Unit Carroll, Gallatin, and Owen Counties, Kentucky (KY612) Map unit symbol Map unit name Rating Acres in AOI Percent of AOI Pt Pits, quarries % WhA Wheeling silt loam, 0 to 2 percent slopes B % Totals for Area of Interest % Description Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms. The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows: Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission. If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. Rating Options Aggregation Method: Dominant Condition Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/19/2016 Page 3 of 4

13 Hydrologic Soil Group Carroll, Gallatin, and Owen Counties, Kentucky Component Percent Cutoff: None Specified Tie-break Rule: Higher Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/19/2016 Page 4 of 4

14 85 1' 14'' W Hydrologic Soil Group Carroll, Gallatin, and Owen Counties, Kentucky (Gyp Stack South Drainage Basin) 85 0' 47'' W 38 45' 1'' N 38 44' 47'' N ' 1'' N 38 44' 47'' N ' 14'' W N Map Scale: 1:3,010 if printed on A landscape (11" x 8.5") sheet. Meters Feet Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 16N WGS ' 47'' W Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/19/2016 Page 1 of 4

15 Hydrologic Soil Group Carroll, Gallatin, and Owen Counties, Kentucky (Gyp Stack South Drainage Basin) MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Rating Polygons A A/D B B/D C C/D D Not rated or not available Soil Rating Lines A A/D B B/D C C/D D Not rated or not available Soil Rating Points A A/D B B/D C C/D D Not rated or not available Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:15,800. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Carroll, Gallatin, and Owen Counties, Kentucky Survey Area Data: Version 12, Sep 15, 2015 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Oct 3, 2011 Oct 4, 2011 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/19/2016 Page 2 of 4

16 Hydrologic Soil Group Carroll, Gallatin, and Owen Counties, Kentucky Gyp Stack South Drainage Basin Hydrologic Soil Group BrD Hydrologic Soil Group Summary by Map Unit Carroll, Gallatin, and Owen Counties, Kentucky (KY612) Map unit symbol Map unit name Rating Acres in AOI Percent of AOI Brashear silty clay loam, 12 to 20 percent slopes EfE3 Eden flaggy silty clay, 20 to 30 percent slopes, severely eroded C % D % Pt Pits, quarries % Totals for Area of Interest % Description Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms. The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows: Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission. If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/19/2016 Page 3 of 4

17 Hydrologic Soil Group Carroll, Gallatin, and Owen Counties, Kentucky Gyp Stack South Drainage Basin Rating Options Aggregation Method: Dominant Condition Component Percent Cutoff: None Specified Tie-break Rule: Higher Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/19/2016 Page 4 of 4

18 Hazen and Sawyer 5775 Peachtree Dunwoody Road, Suite D-520, Atlanta, GA Project Name: Ghent CCR Compliance Calculated By: K. Ray Basin: ATB2_BasinA-Truck Date: Unload Area to ATB2 6/14/2016 Hazen No.: Manager: J. Powers Curve Number Analysis Curve Number (CN) 1 Percent of Total (% AREA) CN x % AREA Land Use Description Soil Group (acres) Open Space A (Fair Condition) B C % 38.8 D Woods/Forest A (Fair Condition) B C Gravel Areas, Lakes, & Impervious Areas Total Overall Basin Curve Number D A B C % 49.9 D acres sq-mi 89 AMC II 95 AMC III 1 From USDA NRCS Part 630 Hydrology National Engineering Handbook, Chapter 9

19 Hazen and Sawyer 5775 Peachtree Dunwoody Road, Suite D-520, Atlanta, GA Project Name: Ghent CCR Compliance Calculated By: K. Ray Basin: ATB2_BasinB-Direct Date: Drainage Basin to ATB2 6/14/2016 Hazen No.: Manager: J. Powers Curve Number Analysis Curve Number (CN) 1 Percent of Total (% AREA) CN x % AREA Land Use Description Soil Group (acres) Open Space A (Fair Condition) B C % 8.5 D Woods/Forest A (Fair Condition) B C Gravel Areas, Lakes, & Impervious Areas Total Overall Basin Curve Number D A B C D % acres sq-mi 96 AMC II 98 AMC III 1 From USDA NRCS Part 630 Hydrology National Engineering Handbook, Chapter 9

20 Hazen and Sawyer 5775 Peachtree Dunwoody Road, Suite D-520, Atlanta, GA Project Name: Ghent CCR Compliance Calculated By: K. Ray Basin: ATB1_Basin-Direct Date: Drainage Basin to ATB1 6/14/2016 Hazen No.: Manager: J. Powers Curve Number Analysis Curve Number (CN) 1 Percent of Total (% AREA) CN x % AREA Land Use Description Soil Group (acres) Open Space A (Fair Condition) B C D Woods/Forest A (Fair Condition) B C % 2.4 Gravel Areas, Lakes, & Impervious Areas Total Overall Basin Curve Number D % 21.1 A B C D % acres sq-mi 92 AMC II 97 AMC III 1 From USDA NRCS Part 630 Hydrology National Engineering Handbook, Chapter 9

21 Hazen and Sawyer 5775 Peachtree Dunwoody Road, Suite D-520, Atlanta, GA Project Name: Ghent CCR Compliance Calculated By: K. Ray Basin: Secondary Pond Date: Direct Drainage Basin 6/14/2016 Hazen No.: Manager: J. Powers Curve Number Analysis Curve Number (CN) 1 Percent of Total (% AREA) CN x % AREA Land Use Description Soil Group (acres) Open Space A (Fair Condition) B % 6.7 C D % 30.0 Woods/Forest A (Fair Condition) B C Gravel Areas, Lakes, & Impervious Areas Total Overall Basin Curve Number D A B C % 53.5 D acres sq-mi 90 AMC II 96 AMC III 1 From USDA NRCS Part 630 Hydrology National Engineering Handbook, Chapter 9

22 Hazen and Sawyer 5775 Peachtree Dunwoody Road, Suite D-520, Atlanta, GA Project Name: Ghent CCR Compliance Calculated By: K. Ray Basin: Gypsum Stack Date: South 6/14/2016 Hazen No.: Manager: J. Powers Curve Number Analysis Land Use Description Soil Group Curve Number (CN) 1 (acres) Percent of Total (% AREA) CN x % AREA Open Space A (Fair Condition) B C D Woods/Forest A (Fair Condition) B C D Gravel Areas, Lakes, & A Impervious Areas B C D % 98.0 Total 23.2 acres sq-mi Overall Basin 98 AMC II Curve Number 99 AMC III 1 From USDA NRCS Part 630 Hydrology National Engineering Handbook, Chapter 9

23 Hazen and Sawyer 5775 Peachtree Dunwoody Road, Suite D-520, Atlanta, GA Project Name: Ghent CCR Compliance Calculated By: K. Ray Basin: Gypsum Stack Date: North 6/14/2016 Hazen No.: Manager: J. Powers Curve Number Analysis Curve Land Use Description Soil Group Number (CN) 1 (acres) Open Space A (Fair Condition) B C D Woods/Forest A (Fair Condition) B C Gravel Areas, Lakes, & Impervious Areas Total Overall Basin Curve Number Percent of Total (% AREA) CN x % AREA D A B C % 98.0 D acres sq-mi 98 AMC II 99 AMC III 1 From USDA NRCS Part 630 Hydrology National Engineering Handbook, Chapter 9

24 Hazen and Sawyer 5775 Peachtree Dunwoody Road, Suite D-520, Atlanta, GA Project Name: Ghent CCR Compliance Calculated By: K. Ray Basin: Gypsum Reclaim Date: Pond 6/14/2016 Hazen No.: Manager: J. Powers Curve Number Analysis Curve Number (CN) 1 Percent of Total (% AREA) CN x % AREA Land Use Description Soil Group (acres) Open Space A (Fair Condition) B C D % 19.9 Woods/Forest A (Fair Condition) B C Gravel Areas, Lakes, & Impervious Areas Total Overall Basin Curve Number D A B C D % acres sq-mi 95 AMC II 98 AMC III 1 From USDA NRCS Part 630 Hydrology National Engineering Handbook, Chapter 9

25 Hazen and Sawyer 5775 Peachtree Dunwoody Road, Suite D-520, Atlanta, GA Project Name: Ghent CCR Compliance Calculated By: K. Ray Basin: Gyp SW Bypass Date: 6/14/2016 Hazen No.: Manager: J. Powers Curve Number Analysis Curve Number (CN) 1 Percent of Total (% AREA) CN x % AREA Land Use Description Soil Group (acres) Open Space A (Fair Condition) B C D % 41.4 Woods/Forest A (Fair Condition) B C Gravel Areas, Lakes, & Impervious Areas Total Overall Basin Curve Number D % 7.4 A B C D % acres sq-mi 89 AMC II 96 AMC III 1 From USDA NRCS Part 630 Hydrology National Engineering Handbook, Chapter 9

26 APPENDIX B: HYDRAULIC ANALYSIS SSA Analysis Output Storage Volume Analysis

27 Project Description File Name _Ghent_SSA_CCR Plans.SPF Project Options Flow Units... Elevation Type... Hydrology Method... Time of Concentration (TOC) Method... Link Routing Method... Enable Overflow Ponding at Nodes... Skip Steady State Analysis Time Periods... Analysis Options CFS Elevation SCS TR-55 User-Defined Hydrodynamic YES NO Start Analysis On... Jan 01, :00:00 End Analysis On... Jan 12, :00:00 Start Reporting On... Jan 01, :00:00 Antecedent Dry Days... 3 days Runoff (Dry Weather) Time Step :00:00 days hh:mm:ss Runoff (Wet Weather) Time Step :00:00 days hh:mm:ss Reporting Time Step :01:00 days hh:mm:ss Routing Time Step... 1 seconds Number of Elements Rain Gages... Subbasins... Nodes... Junctions... Outfalls... Flow Diversions... Inlets... Storage Nodes... Links... Channels... Pipes... Pumps... Orifices... Weirs... Outlets... Pollutants... Land Uses... Rainfall Details Qty SN Rain Gage Data Data Source Rainfall Rain State County Return Rainfall Rainfall ID Source ID Type Units Period Depth Distribution (years) (inches) 1 Rainfall Time Series PMP SCS_6-Hr_TII Cumulative inches User Defined

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29 Subbasin Summary SN Subbasin Area Weighted Total Total Total Peak Time of ID Curve Rainfall Runoff Runoff Runoff Concentration Number Volume (ac) (in) (in) (ac-in) (cfs) (days hh:mm:ss) 1 ATB1_Basin :06:00 2 ATB2_BasinA :06:00 3 ATB2_BasinB :06:00 4 GypReclaim_Basin :06:00 5 GypStackNorth_Basin :06:00 6 GypStackSouth_Basin :06:00 7 GypStackSWBypass_Basin :06:00 8 SecATB_Basin :06:00

30 Project Name: Ghent Generating Station - Gypsum Stack North Cell Subject: Existing Pond Volume Analysis Hazen Project #: Date: 10/08/16 Performed By: KJR Contour EL (ft) Incremental Area (sf) Available Storage Incremental Volume (cf) Cumulative Volume (cf) Cumulative Volume (cy) , , , ,554 11, , , ,817 32, , ,857 1,526,674 56, , ,299 2,198,973 81, , ,967 2,902, ,516 PMP runoff from SSA model convert PMP runoff from SSA model pumped inflow based on 6 hr storm and 7 cfs total water volume to be stored Storage Available to top closed contour 545 excess storage available ac-in 1,982,198 cf 151,200 cf 2,133,398 cf 2,198,973 cf 65,575 cf

31 Project Name: Ghent Generating Station - Gypsum Stack South Cell Subject: Existing Pond Volume Analysis Hazen Project #: Date: 10/08/16 Performed By: KJR Contour EL (ft) Incremental Area (sf) Available Storage Incremental Volume (cf) Cumulative Volume (cf) Cumulative Volume (cy) ,509 21,790 21, , , ,780 8, , , ,655 29, , ,555 1,516,210 56, , ,649 2,268,859 84, , ,235 3,057, , ,045, ,956 3,869, ,298 PMP runoff from SSA model convert PMP runoff from SSA model pumped inflow based on 6 hr storm and 7 cfs total water volume to be stored Storage Available to top closed contour 553 excess storage available ac-in 2,322,583 cf 151,200 cf 2,473,783 cf 3,057,094 cf 583,311 cf