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Home My WebLink AboutWAKE-010_Final Report DAM HYDRAULIC CAPACITY ASSESSMENT REPORT ROBERTSON LAKE DAM WAKE-010 WAKE COUNTY, NC PREPARED BY: TYLER MCEWEN, PE 2020 EXECUTIVE SUMMARY A hydraulic capacity analysis was completed for the Robertson Lake Dam in Wake County, North Carolina. The purpose of the study was to analyze the existing potential of the dams for overtopping during predetermined specified rainfall events. The analyses were completed using the United States Army Corps of Engineers developed software Hydrologic Engineering Center – Hydrologic Modeling System (HEC-HMS) in combination with ArcGIS to develop required input parameters. The rainfall events included in the analyses consist of 24-hour durations with return periods of 25 years, 50 years, 100 years, 200 years, 500 years and 1000 years. Results of the analyses indicate that this structure is hydraulically adequate for all of the above rainfall events. The dam maintained a freeboard of 0.3 – 0.8 feet during these events. Data, input parameters and final results can be found on the attached summary sheets and in the appendices of this report as well as in the associated electronic file in which all program files are stored. The elevation-storage calculation does not include the storage below the normal pool elevation for this study. All elevations in this report are based on North American Vertical Datum of 1988 (NAVD88). Drainage Area (Sq. Mi.)= Curve Number = Lag Time (min.)= Rainfall Distribution Methods = 95.1 TYPE 24 HOUR - 50 YEAR EVENT, 6.91 INCHES, MAX W.S. ELEVATION 348.1 (0.7 FT FREEBOARD) 24 HOUR - 25 YEAR EVENT, 6.11 INCHES, MAX W.S. ELEVATION 348.0 (0.8 FT FREEBOARD) SUMMARY OF INPUT PARAMETERS NRCS-NOAA 0.16 77.34 35 NOT TO SCALE 355.0 163.9354.0 348.0 349.0 350.0 42.8 127.4353.0 21.8 203.9 3.6 352.0 0.1 0.2 Emergency SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS ROBERTSON LAKE DAM, WAKE-010 SPILLWAY ANALYSIS PERFORMED BY: TYLER MCEWEN, PE, NCDEQ-DEMLR 24 HOUR - 1000 YEAR EVENT, 10.80 INCHES, MAX W.S. ELEVATION 348.5 (0.3 FT FREEBOARD) 24 HOUR - 500 YEAR EVENT, 9.82 INCHES, MAX W.S. ELEVATION 348.4 (0.4 FT FREEBOARD) 24 HOUR - 200 YEAR EVENT, 8.61 INCHES, MAX W.S. ELEVATION 348.3 (0.5 FT FREEBOARD) TOP OF DAM ELEVATION = 348.8 FT NAVD88 346.0 347.0 24 HOUR - 100 YEAR EVENT, 7.74 INCHES, MAX W.S. ELEVATION 348.2 (0.6 FT FREEBOARD) ELEVATION (FT NAVD88) 347.7Vegetated Trapazoidal Weir Principal ELEVATION 351.0 Vegetated Trapazoidal Weir STORAGE (ACRE-FT)ELEVATION 346.1 STORAGE (ACRE-FT) 67.1 ROBERTSON LAKE DAM WAKE-010 August 25, 2020 APPENDIX A Watershed Boundary Sources: Esri, HERE, Garmin, Intermap, increment P Corp., GEBCO,USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, OrdnanceSurvey, Esri Japan, METI, Esri China (Hong Kong), (c) OpenStreetMapcontributors, and the GIS User Community Robertson Lake Dam - WAKE-010 Watershed Boundar y .Drainage Area: 0.16 Square MilesFlowpath Length: 3,622 Feet Legend Watershed Boundary Longest Flowpath 0 0.1 0.20.05 Miles APPENDIX B Time of Concentration Calculation Dam No:WAKE-010 Date:8/25/2020 Dam Name:Robertson Lake Dam Engineer:TJM Maximum Potential Retention (inches):S =(1000/cn')-10 Time of Concentration (hours):Tc =l0.8*(S+1)0.7 1140*Y0.5 Basin Data:Input Values in yellow Curve Number CN =77.34 from StreamStats S =2.92992 in. Mean Basin Slope Y =2.8 %from StreamStats Longest Flowpath:0.686 mi.from StreamStats Flow Length l =3622.08 ft. Time of Concentration Tc =0.96 Hours Tc =58 mins Lag Time TL =0.58 Hours TL =35 mins APPENDIX C NRCS Curve Number Summary Sheet Change CN according to TR-55 (see tables below) HSG Type A CN B CN C CN D CN Open Water 0.0000 100 0 12203.7366 100 1220373.657 696338.3051 100 69633830.51 0.0000 100 0 Developed, Open Space 0.0000 68 0 97587.7569 79 7709432.796 39087.8142 86 3361552.022 0.0000 89 0 Developed, Low Intensity 0.0000 57 0 81567.3603 72 5872849.939 108579.7797 81 8794962.156 2314.0715 86 199010.1 Developed, Medium Intensity 0.0000 77 0 0.0000 85 0 0.0000 90 0 0.0000 92 0 Developed, High Intensity 0.0000 89 0 0.0000 92 0 0.0000 94 0 0.0000 95 0 Barren Land 0.0000 77 0 0.0000 86 0 0.0000 91 0 0.0000 94 0 Deciduous Forest 0 36 0 76723.74633 60 4603424.78 14999.73435 73 1094980.608 76832.32119 79 6069753 Evergreen Forest 0.0000 36 0 551675.3241 60 33100519.44 58801.8974 73 4292538.507 51809.9316 79 4092985 Mixed Forest 0.0000 36 0 15741.9030 60 944514.1791 0.0000 73 0 27476.1921 79 2170619 Shrub/Scrub 0.0000 35 0 0.0000 56 0 154.0625 70 10784.37303 0.0000 77 0 Herbaceuous 0.0000 50 0 283448.6021 71 20124850.75 0.0000 81 0 9716.9902 89 864812.1 Hay/Pasture 0 49 0 938406.9416 69 64750078.97 771879.8578 79 60978508.77 185975.1096 84 15621909 Cultivated Crops 0.0000 71 0 196266.2656 80 15701301.25 53626.6526 87 4665518.775 50685.6738 90 4561711 designed by dan.k Woody Wetlands 0.0000 80 0 0.0000 85 0 0.0000 90 0 0.0000 95 0 Emergent Herbaceuous Wetlands 0.0000 80 0 0.0000 85 0 0.0000 90 0 0.0000 95 0 Weighted CN=77.34 NOTE: Curve Numbers are from SCS TR-55 with the exception of assumed HSG A (CN=60) for Herbaceous,Open Water, Woody Wetlands, and Emergent Herbaceous Wetlands. Hydrologic Condition assumed as "Fair" NOTE: double-check all the land types are being calculated and texts are accurate NOTE: double-check the exported attribute table has the same table format as the "Export_Output" table APPENDIX D Hyetograph Results Used for Analyses 0.0 2.0 4.0 6.0 8.0 10.0 12.0 0 2 4 6 8 10 12 14 16 18 20 22 24Cumulative Inches of RainHours Hyetographs for 24-Hour Rainfall Distributions 25YR 50YR 100YR 200YR 500YR 1000YR APPENDIX E NOAA ATLAS 14 RAINFALL TABLE Point precipitation frequency estimates (inches) NOAA Atlas 14 Volume 2 Version 3 Data type: Precipitation depth Time series type: Partial duration Project area: Ohio River Basin Location name (ESRI Maps): Knightdale, NC, USA Station Name: - Latitude: 35.8250° Longitude: -78.4570° Elevation (USGS): 337.65 ft PRECIPITATION FREQUENCY ESTIMATES by duration for ARI (years):1 2 5 10 25 50 100 200 500 1000 5-min:0.406 0.471 0.537 0.603 0.67 0.722 0.769 0.811 0.858 0.9 10-min:0.648 0.753 0.86 0.965 1.07 1.15 1.22 1.29 1.36 1.42 15-min:0.81 0.947 1.09 1.22 1.35 1.46 1.55 1.62 1.71 1.78 30-min:1.11 1.31 1.55 1.77 2 2.19 2.37 2.53 2.72 2.88 60-min:1.38 1.64 1.98 2.3 2.67 2.97 3.26 3.54 3.9 4.21 2-hr:1.62 1.93 2.36 2.77 3.26 3.69 4.1 4.52 5.07 5.56 3-hr:1.72 2.04 2.51 2.97 3.52 4.02 4.52 5.04 5.74 6.37 6-hr:2.06 2.45 3.01 3.56 4.25 4.87 5.5 6.16 7.05 7.87 12-hr:2.42 2.89 3.56 4.24 5.1 5.88 6.67 7.53 8.71 9.81 24-hr:2.88 3.48 4.39 5.11 6.11 6.91 7.74 8.61 9.82 10.8 2-day:3.33 4.01 5.02 5.82 6.92 7.8 8.71 9.65 11 12 3-day:3.53 4.25 5.29 6.11 7.25 8.16 9.09 10.1 11.4 12.5 4-day:3.73 4.48 5.56 6.41 7.58 8.52 9.48 10.5 11.9 13 7-day:4.33 5.18 6.34 7.26 8.52 9.53 10.6 11.7 13.1 14.3 10-day:4.94 5.88 7.1 8.06 9.35 10.4 11.4 12.5 13.9 15.1 20-day:6.62 7.83 9.29 10.5 12 13.3 14.5 15.8 17.5 18.8 30-day:8.22 9.68 11.3 12.6 14.2 15.5 16.8 18.1 19.8 21.1 45-day:10.5 12.3 14.1 15.5 17.3 18.7 20.1 21.5 23.2 24.6 60-day:12.6 14.7 16.7 18.2 20.2 21.7 23.1 24.5 26.4 27.8 Date/time (GMT): Wed Aug 26 20:09:27 2020 pyRunTime: 0.0111479759216 APPENDIX F Outlet Works Rating Curves Primary Spillway:Vegetated Trapazoidal Weir Emergency Spillway:Vegetated Trapazoidal Weir Top of Dam :348.8 ft NAVD88 Normal Pool :346.1 ft NAVD88 RWS Elevation (ft NAVD88) Primary Spillway Discharge (cfs) Emergency Spillway Discharge (cfs) Total Structure Discharge (cfs) 346.0 0 0 0 346.5 7 0 7 347.0 25 0 25 347.5 59 5 64 348.0 111 25 136 348.5 182 64 246 349.0 276 122 398 349.5 394 201 596 350.0 538 304 842 350.5 709 431 1,141 351.0 909 586 1,495 351.5 1,139 768 1,907 352.0 1,401 980 2,381 352.5 1,696 1,223 2,919 353.0 2,026 1,498 3,524 353.5 2,390 1,807 4,198 354.0 2,792 2,151 4,943 354.5 3,231 2,531 5,763 355.0 3,710 2,949 6,659 SPILLWAY ASSESSMENT Robertyson Lake Dam, WAKE-010 345 346 347 348 349 350 351 352 353 354 355 356 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000RWS Elevation (feet NAVD88)Spillway Discharge (cfs) Spillway Discharge Capacity Primary Spillway:Emergency Spillway:Total Structure