Inflow Design Flood Control System Plan

Transcription

1 Inflow Design Flood Control System Plan Scrubber Ponds Lewis and Clark Station Prepared for Montana-Dakota Utilities Co. October 2016 Paul T. Swenson :54:29-05'00' 234 West Century Avenue Bismarck, ND

2 Inflow Design Flood Control System Plan October 2016 Contents 1.0 Introduction Description of Lewis and Clark Station CCR Units Inflow Flood Control Plan Model Inputs Model Results Conclusion... 5 P:\Bismarck\26 MT\41\ L&C Scrubber Ash Ponds CCR Compliance\WorkFiles\Task II-4A Run-on Run-off Inflow Flood\IFC Plan\Inflow Flood Control Plan Rev docx i

3 List of Tables Table 1 Scrubber Ponds Design Storm Event Rainfall Data... 3 Table 2 Modeled Scrubber Ponds Watershed Run-off... 4 Table 3 Scrubber Ponds Hydrologic Modeling Results... 4 List of Figures Figure 1 Figure 2 Figure 3 Site Plan Site Topography Scrubber Ponds Inflow Design Flood Control System Plan List of Appendices Appendix A Scrubber Pond Hydrology Appendix A-1 Scrubber Pond Hydrologic Model Appendix A-2 TP-40: 100-Year, 24-Hour Rainfall ii

4 Certification I hereby certify that this Inflow Design Flood Control System Plan at the Lewis & Clark Station, Sidney, Montana, meet the requirements of the Coal Combustion Residual Rule 40 CFR 257 Subp. D, Hydrologic and hydraulic capacity requirements for CCR surface impoundments. Revision Date Summary of Revisions 0 October 17, 2016 Initial Plan Paul T. Swenson Barr Engineering Co. MT Registration Number 12805PE Dated this 14th day of October, 2016 iii

5 1.0 Introduction Montana-Dakota Utilities Co. (MDU) operates the Lewis & Clark Station (Lewis & Clark), near Sidney, Montana, a coal-fired steam-electric generating plant that produces coal combustion residuals (CCR). CCR management is subject to the requirements of 40 CFR 257 Subpart D, Standards for Disposal of Coal Combustion Residuals in Landfills and Surface Impoundments (CCR Rule). The CCR Rule, Hydrologic and hydraulic capacity requirements for CCR surface impoundments, establishes design criteria for management of surface water inflow into CCR surface impoundments. The Scrubber Ponds that receive flue gas desulfurization solids and other CCR materials at Lewis & Clark Station are defined by the CCR Rule to be existing surface impoundments, and therefore a plan is required for these impoundments to satisfy the rule. 1

6 2.0 Description of Lewis and Clark Station CCR Units CCR from plant operations is slurried to the Scrubber Ponds. Each of the Scrubber Ponds has an area of approximately 1.66 acres. A berm has been constructed within each of the Scrubber Ponds to separate each into a CCR solids settling cell with an area of approximately 1.48 acres, and an effluent polishing cell with an area of approximately 0.68 acres. Water from the effluent polishing cells is returned back to the plant for reuse. See Figures 1 and 2 for the site layout and locations of the Scrubber Ponds. 2

7 3.0 Inflow Design Flood Control System Plan This section describes the hydrologic modeling and plan development for the Inflow Design Flood Control System Plan for the Scrubber Ponds. CCR Rule (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 as specified in paragraphs (a)(1) and (2) of this section. Paragraph (a)(3) defines the storm event that the inflow design flood control system must accommodate. The Scrubber Ponds were previously determined to have a low hazard potential classification (Hazard Potential Classification, Lewis and Clark Station, Barr Engineering Co., August 2016). Under the rule, surface impoundments with a low hazard potential classification must have an inflow design flood control system that will accommodate a 100-year flood (design flood event). For the Scrubber Ponds, this design requirement was determined to be represented by run-off from the 100-year, 24-hour rainstorm. The Scrubber Ponds have no tributary area outside of the limits of their containment dikes. Therefore, the only inflow flood flowage that the system must accommodate is direct precipitation Model Inputs Hydrologic modeling of the site for the Inflow Design Flood Control System Plan was performed with HydroCAD software. The input data and results of the modeling are described in this section. Detailed model inputs and results are provided in Appendix B-1. Rainfall data for the site was acquired from the National Oceanic and Atmospheric Administration (NOAA), Technical Paper No. 40 (TP-40). The 100-year, 24-hour map from TP-40, showing the depth of rainfall in the vicinity of Lewis & Clark Station is provided in Appendix B-2. The 100-year, 24-hour rainfall depth identified for the Lewis & Clark Station location is shown in Table 1. Table 1 Scrubber Ponds Design Storm Event Rainfall Data Storm Event Rainfall 100 year, 24-hour 4.3 inches The Scrubber Ponds are nearly identical in area and storage capacity, so one hydrologic model was completed, and it is considered representative of conditions in each Scrubber Pond. The drainage area was evaluated as two subwatershed areas, as shown on Figure 3. Scrubber Pond subwatershed 1 includes the CCR solids settling cell. The cell was assumed to be at maximum CCR solids storage capacity for the model. Since CCR solids are slurried to the Scrubber Ponds, and because the slurry pipe spigot is located on the northeast side of the Scrubber Ponds, the CCR solids form a delta that slopes toward the effluent polishing cell of the Scrubber Pond. Surface water drains through a 12-inch-diameter PVC pipe installed in 3

8 the separation berm, and then accumulates in the effluent polishing pond until it is pumped back to the generating plant. Scrubber Pond subwatershed 2 consists of the effluent polishing cell. Since either Scrubber Pond may be in use and nearing capacity at any given time, the model was used to determine what the maximum operating elevation of the water in the effluent polishing cell can be while still providing a freeboard of approximately one foot after the design flood event Model Results Modeled run-off for the design storm event is summarized in Table 2. Table 2 Modeled Scrubber Ponds Watershed Run-off Scrubber Pond Subwatershed Area Run-off Volume* Subwatershed 1 (CCR solids settling cell) Subwatershed 2 (effluent polishing cell) 1.48 acres 0.35 acre-feet 0.68 acres 0.19 acre-feet * Run-off volumes rounded to the nearest hundredth acre-foot The hydrologic modeling results of the Scrubber Pond area are summarized in Table 3. Table 3 Scrubber Ponds Hydrologic Modeling Results Effluent Polishing Cell Condition Maximum normal operating level 100-year, 24-hour Minimum perimeter dike Storage Area Maximum Water Elevation Elevation feet Elevation feet Elevation feet The total run-off volume for the design event can be stored in the effluent polishing cell, assuming that the Scrubber Pond is maintained with a maximum normal operating level as identified in the Table 3. Storage of the design flood event will result in freeboard of approximately one foot. 4

9 4.0 Conclusion As demonstrated by the topographic map shown on Figure 2, potential run-on around the Scrubber Ponds water drains away from the Scrubber Ponds. The hydrologic model and the analysis presented in this document demonstrate that the Scrubber Ponds are adequately designed to properly manage the 100-year, 24-hour inflow flood. 5

10 Figures

11 COUNTY 350 Sewage Lagoon West Scrubber Pond TSP East Scrubber Pond Barr Footer: ArcGIS 10.4, :49 File: I:\Projects\26\41\1007\Maps\Reports\Task_II_4A\Figure01 Site Plan.mxd User: MRQ HIGHWAY 23 Lewis & Clark Station Imagery Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community Richland County Sidney Lewis & Clark Station Montana!(!( North Dakota!(!( Lewis & Clark Station Temporary CCR Storage Pad!;N Feet Yellowstone River Lewis & Clark Station Montana-Dakota Utilities Richland County, Montana FIGURE 1 - SITE PLAN

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14 Appendix A Scrubber Pond Hydrology

15 Appendix A-1 Scrubber Pond Hydrologic Model

16 4S Scrubber Pond Watershed 5R 12" PVC Pipe 2S 3P Polishing Pond Watershed Depression Subcat Reach Pond Link Routing Diagram for MDU impoundment runoff Prepared by {enter your company name here}, Printed 10/7/2016 HydroCAD s/n HydroCAD Software Solutions LLC

17 MDU impoundment runoff Prepared by {enter your company name here} HydroCAD s/n HydroCAD Software Solutions LLC Printed 10/7/2016 Page 2 Area (acres) CN Area Listing (selected nodes) Description (subcatchment-numbers) Row crops, straight row, Poor, HSG C (2S, 4S) pond area (2S) TOTAL AREA

18 MDU impoundment runoff Type II 24-hr 100yr-24hr Rainfall=4.30" Prepared by {enter your company name here} Printed 10/7/2016 HydroCAD s/n HydroCAD Software Solutions LLC Page 3 Summary for Subcatchment 2S: Polishing Pond Watershed Runoff = hrs, Volume= af, Depth> 3.29" Runoff by SCS TR-20 method, UH=SCS, Time Span= hrs, dt= 0.05 hrs Type II 24-hr 100yr-24hr Rainfall=4.30" Area (sf) CN Description 17, Row crops, straight row, Poor, HSG C * 12, pond area 29, Weighted Average 17, % Pervious Area 12, % Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 10.0 Direct Entry, Subcatchment 2S: Polishing Pond Watershed Hydrograph 3.39 cfs Runoff Flow (cfs) Runoff=3.39 cfs Type II 24-hr 100yr-24hr Rainfall=4.30" Runoff Area=29,772 sf Runoff Volume=0.187 af Runoff Depth>3.29" Tc=10.0 min CN= Time (hours)

19 MDU impoundment runoff Type II 24-hr 100yr-24hr Rainfall=4.30" Prepared by {enter your company name here} Printed 10/7/2016 HydroCAD s/n HydroCAD Software Solutions LLC Page 4 Summary for Subcatchment 4S: Scrubber Pond Watershed Runoff = hrs, Volume= af, Depth> 2.81" Runoff by SCS TR-20 method, UH=SCS, Time Span= hrs, dt= 0.05 hrs Type II 24-hr 100yr-24hr Rainfall=4.30" Area (sf) CN Description 64, Row crops, straight row, Poor, HSG C 64, % Pervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 10.0 Direct Entry, Subcatchment 4S: Scrubber Pond Watershed Hydrograph cfs Runoff Flow (cfs) Runoff=6.54 cfs Type II 24-hr 100yr-24hr Rainfall=4.30" Runoff Area=64,257 sf Runoff Volume=0.345 af Runoff Depth>2.81" Tc=10.0 min CN= Time (hours)

20 MDU impoundment runoff Type II 24-hr 100yr-24hr Rainfall=4.30" Prepared by {enter your company name here} Printed 10/7/2016 HydroCAD s/n HydroCAD Software Solutions LLC Page 5 Summary for Reach 5R: 12" PVC Pipe Inflow Area = ac, 0.00% Impervious, Inflow Depth > 2.81" for 100yr-24hr event Inflow = hrs, Volume= af Outflow = hrs, Volume= af, Atten= 0%, Lag= 0.0 min Routing by Stor-Ind+Trans method, Time Span= hrs, dt= 0.05 hrs Max. Velocity= fps, Min. Travel Time= 0.0 min Avg. Velocity = 3.75 fps, Avg. Travel Time= 0.1 min Peak Storage= hrs Average Depth at Peak Storage= 0.70' Bank-Full Depth= 1.00' Flow Area= 0.8 sf, Capacity= 7.80 cfs 12.0" Round Pipe n= PVC, smooth interior Length= 18.0' Slope= '/' Inlet Invert= 1,928.09', Outlet Invert= 1,927.58' Reach 5R: 12" PVC Pipe Hydrograph Flow (cfs) cfs Inflow Area=1.475 ac Inflow=6.54 cfs Outflow=6.53 cfs Avg. Flow Depth=0.70' Max Vel=11.11 fps 12.0" Round Pipe n=0.010 L=18.0' S= '/' Capacity=7.80 cfs Inflow Outflow Time (hours)

21 MDU impoundment runoff Type II 24-hr 100yr-24hr Rainfall=4.30" Prepared by {enter your company name here} Printed 10/7/2016 HydroCAD s/n HydroCAD Software Solutions LLC Page 6 Stage-Area-Storage for Reach 5R: 12" PVC Pipe Elevation (feet) End-Area (sq-ft) Storage (cubic-feet) 1, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Elevation (feet) End-Area (sq-ft) Storage (cubic-feet) 1, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

22 MDU impoundment runoff Type II 24-hr 100yr-24hr Rainfall=4.30" Prepared by {enter your company name here} Printed 10/7/2016 HydroCAD s/n HydroCAD Software Solutions LLC Page 7 Summary for Pond 3P: Depression Inflow Area = ac, 13.47% Impervious, Inflow Depth > 2.96" for 100yr-24hr event Inflow = hrs, Volume= af Outflow = hrs, Volume= af, Atten= 100%, Lag= 0.0 min Routing by Stor-Ind method, Time Span= hrs, dt= 0.05 hrs Starting Elev= 1,925.50' Surf.Area= 0 sf Storage= 39,474 cf Peak Elev= hrs Surf.Area= 0 sf Storage= 62,668 cf (23,194 cf above start) Flood Elev= 1,928.10' Surf.Area= 0 sf Storage= 81,146 cf (41,672 cf above start) Plug-Flow detention time= (not calculated: initial storage excedes outflow) Center-of-Mass det. time= (not calculated: no outflow) Volume Invert Avail.Storage Storage Description #1 1,920.45' 99,711 cf Custom Stage Data Listed below Elevation Cum.Store (feet) (cubic-feet) 1, , , ,402 1, ,340 1, ,222 1, ,805 1, ,143 1, ,533 1, ,083 1, ,711

23 MDU impoundment runoff Type II 24-hr 100yr-24hr Rainfall=4.30" Prepared by {enter your company name here} Printed 10/7/2016 HydroCAD s/n HydroCAD Software Solutions LLC Page 8 Flow (cfs) Pond 3P: Depression Hydrograph 9.92 cfs Inflow Area=2.159 ac Inflow=9.92 cfs Peak Elev=1,927.06' Storage=62,668 cf Inflow Time (hours)

24 MDU impoundment runoff Type II 24-hr 100yr-24hr Rainfall=4.30" Prepared by {enter your company name here} Printed 10/7/2016 HydroCAD s/n HydroCAD Software Solutions LLC Page 9 Stage-Area-Storage for Pond 3P: Depression Elevation (feet) Storage (cubic-feet) 1, , , , , , , , , ,013 1, ,331 1, ,650 1, ,968 1, ,287 1, ,606 1, ,924 1, ,243 1, ,799 1, ,593 1, ,387 1, ,180 1, ,974 1, ,768 1, ,562 1, ,356 1, ,149 1, ,943 1, ,834 1, ,822 1, ,811 1, ,799 1, ,787 1, ,775 1, ,763 1, ,752 1, ,740 1, ,728 1, ,801 1, ,959 1, ,118 1, ,276 1, ,434 1, ,593 1, ,751 1, ,909 1, ,068 1, ,226 1, ,472 1, ,806 1, ,140 1, ,473 1, ,807 1, ,141 Elevation (feet) Storage (cubic-feet) 1, ,475 1, ,809 1, ,142 1, ,476 1, ,912 1, ,452 1, ,991 1, ,530 1, ,069 1, ,607 1, ,147 1, ,686 1, ,225 1, ,764 1, ,410 1, ,166 1, ,921 1, ,676 1, ,431 1, ,185 1, ,941 1, ,696 1, ,451 1, ,206 1, ,114 1, ,177 1, ,240 1, ,303 1, ,366 1, ,428 1, ,491 1, ,554 1, ,617 1, ,680

25 Appendix A-2 TP-40: 100-Year, 24-Hour Rainfall

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