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Home My WebLink AboutFDP201900006 Calculations 2020-12-30HEC RAS Analysis for Pleasant Green Proposed Crossing Crozet, VA Prepared for Collins Engineering April 1, 2019 Revised: June 6, 2019 BACKGROUND The proposed project is located along the northeast side of Powells Creek between Jarmans Gap Road and Cling Lane in Crozet, Virginia. The proposed development, known as Pleasant Green, includes an entrance road that crosses the regulatory floodplain (FEMA Zone AE) of Powells Creek from Orchard Drive in the vicinity of FEMA cross section "G" (FEMA Map: 51003CO229D). A hydraulic analysis was conducted to assess the impacts of this proposed stream crossing on the Base Flood Elevations upstream of the crossing. A prior hydraulic analysis of this reach was completed in May 2017 by Dominion Engineering for the then proposed West Glen Subdivision - which included a similar crossing in the vicinity of FEMA cross section "I". HYDROLOGY Peak flows for Powells Creek have been estimated by FEMA and are presented within Table 3 of Flood Insurance Study #51003CV000C, for Albemarle County, VA. No further hydrologic calculations were conducted for this analysis. HYDRAULICS A HEC-RAS (version 5.0.6) hydraulic model was developed, for both existing and proposed conditions, to assess impacts to the (100-year) base flood elevations and floodplain extents for Powells Creek immediately upstream of the proposed Pleasant Green crossing. Dominion Engineering's previous existing condition ("effective") model was used as the basis for this new model. The following model components were used in this analysis. Cross Sections A terrain model built with updated topographic survey data provided by Dominion Engineering (2017) was used to provide elevation data for the HEC RAS model. FEMA cross section data was revised accordingly for cross sections F — M. For this analysis, cross section G was replaced with 6 new sections (G1, G2, G3, G4, G5, G6) to adequately model the proposed crossing. Manning's Roughness (n) values were set at 0.08 within the channel and 0.06 for overbank conditions, as referenced in Table 4 of FEMA Flood Insurance Study #51003CV000C. Culverts Three (3) 8'H x 10'W concrete box culverts were modeled for proposed conditions. Culverts were countersunk 6-8" and modeled with a natural bottom and roughness value of 0.03. Channel roughness values immediately upstream and downstream of the culverts were also set at 0.03 to account for proposed inlet and outlet protection. Culverts were modeled as Inlet Controlled. Ineffective flow areas were set upstream and downstream of the proposed crossing, in accordance with HEC RAS guidance. Flows A steady flow model using a subcritical flow regime was used for this analysis. Flow data was unchanged from Dominion Engineering's model. Flow values are as follows: Reach RS 10 YR 50 YR 100 YR FLOODWAY 500 YR POWELLS CREEK 4182.66 750 1200 770 1230 1370 1370 1400 1400 1410 1410 1430 1430 1450 1450 1460 1460 1480 1480 1510 1510 2010 2040 POWELLS CREEK 3384.41 POWELLS CREEK 2959.02 780 1240 2050 POWELLS CREEK 2559.86 790 1260 2070 POWELLS CREEK 1979.71 800 1280 2090 POWELLS CREEK 1651.13 810 1290 2100 POWELLS CREEK 908.45 820 1320 2130 POWELLS CREEK 202.38 840 1340 2160 RESULTS The results of this model estimate a maximum rise of 2.59' in 100-year flood elevations immediately upstream of the proposed crossing. The model also indicates a small rise downstream of the proposed crossing — due to a modeled hydraulic jump as flows rapidly decrease in velocity from geometry expansion and high downstream channel roughness (0.08). Impacts to the extents of the 100-year floodplain are displayed on the design plans. HEC RAS output table, flood profiles, and cross sections have been provided in Appendix A of this memo. APPENDIX A Cross Section River Sta Profile Plan Q Total W.S. Elev W.S.E. Rise Vel Chnl Top Width (cfs) (ft) (ft) NO (ft) M 4182.66 100 YR Proposed 1370 769.02 0.00 7.76 166.87 M 4182.66 100 YR Existing 1370 769.02 7.76 166.87 L4 3932.07 100 YR Proposed 1370 765.97 0.00 2.05 206.54 L4 3932.07 100 YR Existing 1370 765.97 2.05 206.54 L3 3891.62 100 YR Proposed 1370 765.68 0.00 4.57 166.44 L3 3891.62 100 YR Existing 1370 765.68 4.57 166.44 EX. CULVERT - CLING LANE 3861.37 Culvert L2 3831.12 100 YR Proposed 1370 761.23 0.00 7.69 136.93 L2 3831.12 100 YR Existing 1370 761.23 7.69 136.93 Ll 3791.09 100 YR Proposed 1370 759.93 0.00 5.7 164.91 Ll 3791.09 100 YR Existing 1370 759.93 5.7 164.89 L 3384.41 100 YR Proposed 1400 747.97 0.00 7.96 118.84 L 3384.41 100 YR Existing 1400 747.97 7.95 118.84 K 2959.02 100 YR Proposedl 1410 737.63 1 0.00 6.31 1 119.09 K 2959.02 100 YR Existing 1410 737.63 6.31 119.08 1 2559.86 100 YR Proposed 1430 727.79 0.00 7.35 153.27 1 2559.86 100 YR Existing 1430 727.79 7.35 153.27 1 1979.71 100 YR Proposed 1450 714.42 0.00 6.5 102.6 1 1979.71 100 YR Existing 1450 714.42 6.5 102.6 H 1651.13 100 YR Proposed 1460 708.4 0.00 5.92 130.07 H 1651.13 100 YR Existing 1460 708.4 5.92 130.08 G6 1159.5 100 YR Proposed 1460 695.93 0.00 7.26 194.41 G6 1159.5 100 YR Existing 1460 695.93 7.26 194.41 G5 1057.41 100 YR I Proposedl 1460 694.87 1 1.61 6.07 1 315.68 G5 1057.41 100 YR Existing 1460 693.26 4.52 229.04 G4 1021.3 100 YR Proposed 1460 694.82 2.59 4.92 310.2 G4 1021.3 100 YR Existing 1460 692.23 4.56 189.38 PROPOSED CROSSING 966.8 Culvert G3 908.45 100 YR Proposed 1480 690.91 0.25 10.12 1 229.3 G3 908.45 100 YR Existing 1480 690.66 2.92 219.1 G2 835.86 100 YR Proposed 1480 690.45 0.61 5.18 251.09 G2 835.86 100 YR Existing 1480 689.84 4.23 238.2 Gl 562.24 100 YR Proposed 1480 683.57 0.00 6.62 225.94 Gl 562.24 1 100 YR Existing 1480 683.57 6.67 226.08 F 202.38 100 YR I Proposedl 1510 1 678.4 1 0.00 1 4.65 1 154.37 F 202.38 1 100YR I Existing 1 1510 1 678.4 1 1 4.65 1 154.37 Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 DWELLS CREEK POWELLS CREEK S 780 Legend WS 100 YR - Existlng WS 100 YR -Proposed POWELLS CREEK S ,4182.61 932.0-, D384.4f q 2959.02 N �559.84 nP F 1979.71] ,1,1651.11 1159A D08.41 B35.8� 562.24 202.34 Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION M .06 _1- .08 _�' .06 � 771 Legend WS 100 YR -Existing WS 100 YR -Proposed Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION L4 .06 ) .08 i( .06 774 Legend WS 100 YR -Existing WS 100 YR -Proposed Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION L3 .06 J( .08 fi .06 772 Legend WS 100 YR -Existing WS 100 YR -Proposed Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 EXISTING CULVERT AT CLING LANE .06 )i( .08)i( .06 774 Legend WS 100 YR -Existing Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 EXISTING CULVERT AT CLING LANE .06 .08 )i( -06 '� 774 Legend WS 100 YR -Existing Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION L2 .06 .08 )i( .06 '� 772 Legend WS 100 YR -Existing WS 100 YR -Proposed Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION L1 .06 )i(.08 .06' 772 Legend WS 100 YR -Proposed WS 100 YR -Existing Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION L .06 )i(.08 _L .06 _ 765 760 755 c c 0 Ta N W 750 MEN 745 740 0 50 100 150 200 250 Station (tt) Legend WS 100 YR -Existing WS 100 YR -Proposed Ground • Bank Sta Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION K .06 'i' .08 )i( .06� 750 Legend WS 100 YR -Proposed Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION J .06 'i" .08�06� 750 Legend WS 100 YR -Existing WS 100 YR -Proposed Ground Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION I .06 .08) i( .06 740 735 730 725 c c 0 Ta N W 720 715 710 705 0 100 200 300 400 Station ($) Legend WS 100 YR -Proposed WS 100 YR -Existing Ground • Bank Sta Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION H .06 'I' .08fi .06 ,I 740 Legend WS 100 YR -Existing WS 100 YR -Proposed Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION G6 .06 _L .08)i .06� 708 Legend WS 100 YR -Existing WS 100 YR -Proposed Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION G5 .06 _I_ .03)i( .06 _ 715 710 705 c c m 700 a' w 695 690 685 0 100 200 300 400 500 600 Station ($) Legend WS 100 YR -Proposed WS 100 YR -Existing Ground • Ineff • Bank Sta Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION G4 .06 _I( .03)1_ .06 _ 715 710 705 c c m 700 1 w 695 690 685 0 100 200 300 400 500 660 Station ($) Legend WS 100 YR -Proposed WS 100 YR -Existing Ground • Ineff • Bank Sta Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 PROPOSED CROSSING .06 _I( .03)1_ .06 _ 715 Legend WS 100 YR -Proposed Ground Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 PROPOSED CROSSING .06 _I' .03 _' .06 710 Legend WS 100 YR -Proposed Ground • IneR Bank Sta Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION G3 .06 .03 .06 ' 705 700 c c m 695 w 690 685 0 100 200 300 400 500 600 Station ($) Legend WS 100 YR -Proposed WS 100 YR -Existing Ground • Ineff • Bank Sta Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION G2 .06 'I' .08fi -06 _ 702 Legend WS 100 YR -Proposed Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION G1 .06 'i" .08�.06� 694 Legend WS 100 YR -Existing WS 100 YR -Proposed Pleasant Green Plan: 1) Existing 3/27/2019 2) Proposed 3/28/2019 CROSS SECTION F .06 .08 .06 700 695 Legend WS 100 YR -Existing WS 100 YR -Proposed Ground Bank Sta