INFLOW DESIGN FLOOD CONTROL SYSTEM PLAN INFLOW DESIGN FLOOD. Chesapeake Energy Center CCR Surface Impoundment: Bottom Ash Pond

Transcription

1 INFLOW DESIGN FLOOD CONTROL SYSTEM PLAN INFLOW DESIGN FLOOD CONTROL SYSTEM PLAN Chesapeake Energy Center CCR Surface Impoundment: Bottom Ash Pond Submitted To: Chesapeake Energy Center 2701 Vepco Street Chesapeake, VA Submitted By: Golder Associates Inc W. Laburnum Avenue, Suite 200 Richmond, VA April 2018 Project No <fl"golder Associates

2 April 2018 Project No Table of Contents 1.0 Certification Introduction Inflow Design Flood Control System Plan Hazard Potential Classification Inflow Design Flood Inflow and Outflow Control Conclusions... 3 Tables Table 1 CEC HEC-HMS Output Appendices Appendix A Precipitation Frequency Data Server (NOAA Atlas 14) Appendix B Bottom Ash Pond Hydraulic Modeling Analysis i (/}{;older Associates

3 Inflow Design Flood Control System Plan Chesapeake Energy Center Bottom Ash Pond April 2018 Project No CERTIFICATION This Inflow Design Flood Control System Plan for the Chesapeake Energy Center s Bottom Ash Pond was prepared by Golder Associates Inc. (Golder). The document and Certification/Statement of Professional Opinion are based on and limited to information that Golder has relied on from Dominion and others, but not independently verified, as well as work products produced by Golder. On the basis of and subject to the foregoing, it is my professional opinion as a Professional Engineer licensed in the Commonwealth of Virginia that this document has been prepared in accordance with good and accepted engineering practices as exercised by other engineers practicing in the same discipline(s), under similar circumstances, at the same time, and in the same locale. It is my professional opinion that the document was prepared consistent with the requirements in of the United States Environmental Protection Agency s Standards for the Disposal of Coal Combustion Residuals in Landfills and Surface Impoundments, published in the Federal Register on April 17, 2015, with an effective date of October 19, 2015 (40 CFR ), as well as with the requirements in resulting from the EPA s Hazardous and Solid Waste Management System: Disposal of Coal Combustion Residuals From Electric Utilities; Extension of Compliance Deadlines for Certain Inactive Surface Impoundments; Response to Partial Vacatur published in the Federal Register on August 5, 2016 with an effective date of October 4, 2016 (40 CFR ). The use of the word certification and/or certify in this document shall be interpreted and construed as a Statement of Professional Opinion, and is not and shall not be interpreted or construed as a guarantee, warranty, or legal opinion. Daniel McGrath Print Name Associate and Senior Consultant Title Signature Date 1 c!/j{iolder :Associates

4 Inflow Design Flood Control System Plan Chesapeake Energy Center Bottom Ash Pond April 2018 Project No INTRODUCTION This Inflow Design Flood Control System (FCS) Plan was prepared for the Chesapeake Energy Center s (CEC) inactive Coal Combustion Residuals (CCR) surface impoundment known as the Bottom Ash Pond (BAP). This FCS Plan was prepared in accordance with 40 CFR Part 257, Subpart D and is consistent with the requirements of 40 CFR The CEC, owned and operated by Virginia Electric and Power Company d/b/a Dominion Energy Virginia (Dominion), is located in the City of Chesapeake, Virginia at 2701 Vepco Street. The CEC includes an existing, inactive CCR surface impoundment, the BAP, as defined by the Disposal of Coal Combustion Residuals from Electric Utilities; Final Rule and Direct Final Rule (40 CFR 257; the CCR rule). This FCS Plan has been developed based on the existing BAP topography as of January 15, INFLOW DESIGN FLOOD CONTROL SYSTEM PLAN 3.1 Hazard Potential Classification As indicated in Golder s Hazard Potential Classification Assessment, the BAP is assigned a Significant hazard potential rating per 40 CFR Inflow Design Flood According to 40 CFR (a)(3)(ii), a hazard potential rating of Significant requires an evaluation of the 1000-year storm event. Per the NOAA Atlas-14, provided in Appendix A, the 1000-year event rainfall total for the 24-hour duration is 14.6 inches. 3.3 Inflow and Outflow Control Inflow to the BAP is primarily stormwater runoff from the adjacent inactive CCR landfill, totaling approximately 23.1 acres of contributing area. The majority of stormwater arrives at the BAP in the northeast corner from the eastern perimeter channel. Other than maintaining pre-established runoff control measures on the landfill, there are no inflow control measures proposed. The BAP s primary outlet for stormwater is a 30-inch corrugated high-density polyethylene (CHDPE) pipe installed in the western embankment. This pipe drains to the adjacent existing sedimentation basin. The BAP normally does not maintain a pool of water due to the free-draining nature of the bottom ash, for which a small allowance has been made in the hydraulic model. The stage-storage curve for the BAP was developed using the January 15, 2018 topography, and shows that the BAP has approximately 13 acrefeet of available water storage volume at the embankment crest. The BAP stormwater system was modeled in the U.S. Army Corps of Engineers Hydrologic Engineering Center s Hydraulic Modeling System (HEC-HMS), and the analysis is included in Appendix B. The analysis was conducted using the 24-hour, 1,000-year event, which was modeled as 14.6 inches of rain Based on 2 c!ijgoldµ Associates

5 Inflow Design Flood Control System Plan Chesapeake Energy Center Bottom Ash Pond April 2018 Project No this analysis, the BAP inflow design flood control system is capable of adequately managing the inflow from the 1,000-year event without overtopping the embankment, and has adequate spillway capacity to manage the resulting outflow. The following table summarizes the results of the HEC-HMS analysis for the 1,000-Yr storm event. Qin (CFS) Bottom Ash Pond Primary Max Hw Qout (Ft El*) (CFS) Table 1: CEC HEC-HMS Output BA Infiltration (CFS) Qin (CFS) Sedimentation Basin Max Hw (Ft El*) Qout (CFS) * Top of berm elevation = 20.0 feet 4.0 CONCLUSIONS Through work performed by Golder, both field inspection and document review, it is our opinion that the BAP inflow design flood control system has sufficient capacity for the 1000-year storm event, as required by 40 CFR c!ijgoldµ Associates

6 APPENDIX A Precipitation Frequency Data Server (NOAA Atlas 14)

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8 PDS-based depth-duration-frequency (DDF) curves Latitude: , Longitude: s::. 25 a. QI -0., C a -"' 15 u!!! Average recurrence interval (years) C C E E.;., 6... ~ ~ C C C ~ ~ ~.s::;.s::;.s::;.s::;.s::; >- >- >- >- >- E E E "' "'"' "' "' ",I,.l,..,:, ;., ~,I, sf,..: "' "' "... Duration >- >- >- >- 1i' " "' "'"' C 6 J. 0 " "' <t"' s::. 25 -a. QI -0 C , a -"' 15 u!!! Duration 5-<nln - 2-day 10-mln - 3-day 15-mln 4-day - 30-mln 7-day - 6G-mln 10-ctay - 2-nr - 20-day - 3-flr - 30-day - 6-flr 45-day - 12-hr - 60-day - 24-hr Average recurrence interval (years) NOAA Atla~ 14, Volume 2, Version 3 Created (GMT): Tue Dec 5 20:54:

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10 APPENDIX B Bottom Ash Pond Hydraulic Modeling Analysis

11 (/)\ Golder Associates CALCULATIONS Date: January 23, 2018 Made by: DPM Project No.: Checked by: KAL Subject: CEC BA Pond Inflow Design Flood Reviewed by: Analysis Project: CEC BOTTOM ASH POND EXISTING CONDITION The purpose of this evaluation is to determine the hydraulic performance of the existing Bottom Ash Pond CCR impoundment at the Chesapeake Energy Center (CEC) during the 1,000-year storm event. This evaluation is in support of the Inflow Design Flood Control System Plan, and is based on a Significant hazard potential classification as defined in of the CCR Rule. 1.0 CALCULATIONS 1.1 Pond Storage Volume The bottom ash pond storage volume was computed based on the existing conditions, as surveyed in January 2018, as partially excavated for beneficial reuse. The stored bottom ash was considered a solid, and available water storage was based on the developed surface contours. The maximum available storage in the pond is 12.9 acre-feet at elevation Overtopping occurs above elevation Attachment 1 contains the stage storage rating table used in the HMS model. 1.2 Outlet Design and Capacity The existing spillway is a 30-inch diameter, 48-foot long horizontal culvert with an invert elevation of feet that discharges into the adjacent sediment basin. There is no auxiliary spillway. The peak capacity of the culvert is 49.0 cubic feet per second (CFS). The pond normally does not retain water in a permanent pool due to the relatively free-draining nature of the bottom ash. As such, an allowance has been made for a small amount of infiltration in the model. This is consistent with the observed behavior of the pond during small storm events. Attachment 1 contains the outlet rating tables. 1.3 Storm Routing Calculations The bottom ash pond stormwater system analysis was performed using the US Army Corps of Engineers Hydrologic Engineering Center s Hydraulic Modeling System (HEC-HMS) software package (ref #1). The direct drainage area to the pond is 23.1 acres. The predominant soil types in the area are Hydrologic Soil Group (HSG) B soils. Approximately 3.2 acres of the top of the landfill is covered with a geomembrane rain cover. The bottom ash pond discharges directly to the adjacent sediment basin, so the evaluation also considers the overall water level and discharge from the sediment basin. Design Storm Per (a)(3)(ii), the impoundment is required to adequately manage flow resulting from the 24-hour, 1,000-Yr storm event. The 24-hour, 1,000-Yr storm event precipitation amount was obtained from the Precipitation Frequency Data Server (PFDS, ref #2) for Chesapeake, Virginia, as 14.6 inches. HMS Model Input Figure 1 illustrates the connectivity of the stormwater elements and the data inputs as modeled in HEC HMS. Golder Associates Inc W. Laburnum Avenue, Suite 200 Richmond, VA USA Tel: (804) Fax: (804) Golder Associates: Operations in Africa, Asia, Australasia, Europe, North America and South America Golder, Golder Associates and the GA globe design are trademarks of Golder Associates Corporation

12 FCS Analysis Calculations January 23, 2018 CEC BA Pond North Slope Ci, Top Raincover Ci, East Ch North Half West Slope East Slope Ci, West Channel South Slope Ci, Sed Basin DA Ci, Sediment Basin BA Pond Partial I!: :!!.I BA Infiltration Figure 1 CEC BA Pond HEC-HMS Model HMS Model Output The following table summarizes the results of the HEC-HMS analysis for the 1,000-Yr storm event. Table 1: CEC HEC-HMS Output Qin (CFS) Bottom Ash Pond Primary Max Hw Qout (Ft El*) (CFS) BA Infiltration (CFS) Qin (CFS) Sediment Basin Max Hw (Ft El*) Qout (CFS) * Top of berm elevation = 20.0 feet (I :: Golder Associates

13 FCS Analysis Calculations January 23, 2018 CEC BA Pond CONCLUSIONS Based on the calculations presented herein, the existing bottom ash pond and culvert outlet at the Chesapeake Energy Center can pass the 1,000-Yr event without overtopping. The adjacent sediment basin is also capable of receiving the design storm event discharge without overtopping. The 30-inch culvert outlet for the bottom ash pond is suitable for the design storm event. 3.0 REFERENCES 1) U.S. Army Corps of Engineers Hydrologic Engineering Center Hydrologic Modeling System (HEC HMS) release ) Precipitation Frequency Data Server (NOAA Atlas 14) ATTACHMENTS 1) BA Pond Stage-Storage and Culvert Outlet Rating Tables Associates

14 Chesapeake Energy Center - Bottom Ash Pond January 2018 HEC-HMS Input Tables BAP - North Section ELEVATION AREA (SF) AREA (AC) , , , , BAP - South Section ELEVATION AREA (SF) AREA (AC) , , , , , Combined Sections stage storage ELEVATION AREA (SF) VOLUME, ft^3 AC-FT TOTAL , , , , , , ,106 87, ,841 63, ,925 34, ,901 7, Drainage Areas for Partial Excavation Area Description Size, Ac. CN H L Slope, ft/ft T L, min Drains to Top Rain Cover RR Channel North Slope East Channel East Slope East Channel (half) South Slope BA Pond BA Pond Proper Self Drainage Areas for Sediment Basin Area Description Size, Ac. CN H L Slope, ft/ft T L, min Drains to West Slope West Channel Sed Basin Proper Self Golder Associates Inc. Attachment

15 Chesapeake Energy Center - Bottom Ash Pond January 2018 HEC-HMS Input Tables Assumed k for bottom ash: Pond floor constant (q): cm/s 1.64E-04 CFS/ft2 30" CPP Rating Elevation Q out (CFS) Pond Floor* Q out (CFS) *pond floor area = only northern section area due to road Sediment pond elevation-area as of 1/15/18 ELEVATION AREA (SF) AREA (ACRES) , , , , , , , , , , , Golder Associates Inc. Attachment

16 Chesapeake Energy Center - Bottom Ash Pond HEC-HMS Input Tables January 2018 CEC Existing Sediment Basin Riser Weir 1 elevation Weir 1 length Orifice 1 area Weir discharge coefficient Orifice discharge coefficient Culvert Outlet from UD culvert: Inv in 4 Inv out -2.1 Length 129 Dia 21 inches (24" SDR-17 HDPE) Slope Tw El 2 Elevation Weir 1 Orifice 1 Min Culvert Rating Existing Riser (Open for Clarity) Orifice Crest EL 8.78 Barrel Invert 4.0 :";td<. v,,w ' # c='t,_!aa,--p11~ CJ> f.' )i~t: Golder Associates Inc. Attachment

17 Established in 1960, Golder Associates is a global, employee-owned Africa organization that helps clients find sustainable solutions to the challenges of Asia finite resources, energy and water supply and management, waste Australasia Europe + management, urbanization, and climate change. We provide a wide range of North America independent consulting, design, and construction services in our specialist South America areas of earth, environment, and energy. By building strong relationships and meeting the needs of clients, our people have created one of the most trusted solutions@golder.com professional services organizations in the world. Golder Associates Inc W. Laburnum Avenue, Suite 200 Richmond, VA USA Tel: (804) Fax: (804) tagolder \ZPAssoclates Engineering Earth's Development, Preserving Earth's Integrity Golder, Golder Associates and the GA globe design are trademarks of Golder Associates Corporation