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HomeMy WebLinkAboutWPO201700037 Calculations 2020-08-08BROOKHILL STORMWATER MANAGEMENT CALCULATIONS PACKET (Town Center) Date of Calculations MARCH 16, 2020 Date of Approved Plans & Calculations AUGUST 41 2020 KKp�' H O opt O� SCOTT S' Lrc. No 03Oj�oL1MS Y '�s,g�y%�o `"ONA L E!1G I:1C1W_1Cf4oil YAq COLLINS-NGINEERING 200 GARRETT STREET, SUITE K CHARLOTTESVILLE, VA 22902 434.293.3719 PH 434.293.2813 FX www.collins-engineering.com APPROVED by the Albemarle County Community Development Department Date 08/07/2020 File WP0201700037—amendment2 SCS TR-55 Calculations Soils Mapping (Source: NRCS Web Soil Survey Online Database) Soils Table (Source: NRCS Web Soil Survey Online Database) Albemarle County, Virginia (VA003) Map Unit Map Unit Name Percent of Symbol AOI 27B Elioak loam, 2 35.5% to 7 percent slopes 27C Elioak loam, 7 27.0% to 15 percent slopes 27D Elioak loam, 2.6% 15 to 25 percent slopes 34C Glenelg loam, 24.9% 7 to 15 percent slopes 34D Glenelg loam, 9.9% 15 to 25 percent slopes Totals for Area of 100.00i0 Interest Note: The soils' properties within this development are predominantly composed of hydrologic group type B. Pre -Construction Aerial Photography This aerial photograph was taken prior to recent disturbances (shown on the previous page). This photograph is used soley for the purposes of establishing the pre -construction land covers (i.e. for the pre -development CN values) for Subareas A & B. Subareas A & B are approximated to be 25% woods and 75% grass in the pre -development state for this section of Brookhill. This is a more conservative approach/land cover when compared to using the exposed earth shown on the previous page. Precipitation Data (Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14 Point Precipitation Frequency Estimates) wdC I Map � p Terrain 3 cg, e 4 G I �J I� POINT PRECIPITATION FREQUENCY (PF) ESTIMATES WITH 90%CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION NOAA Atlas 14, Volume 2, Version 3 PDS-based precipitation frequency estimates with 90% confidence intervals (in inches)' Average rewirerlre Interval (yeah) Duration 1 z s 1a zs so Igo zoo soo long F o0000-0 z4.M 3.00 3.62 4.63 (2.69-3.37) (3.25d.08) (4.14520) 5.47 6.72 7.79 (4.886.14) (5.95-7.51) (6.86-8.69) 8.97 10.3 12.2 133 p.839.99) (8.87-11.4) (70.4-13.6) (71.615.4) O ry b N O N O 00 b b Z m m w 0 w w w O 2 w c 0 o m 2 _ w c o � w w Y m a m c u y c e O E z c o . o m o i0 d j M M 0 jp OJ b DJ ei j > tp 'I IG ei m u u � a m °p y n m o m o m 3 m o z o " u x u O ^' O w O ri ri ^ r o o ai vi O a o o ry ti M o o ry m m� o a 0 a 4 ¢ a o .rv-i ."-i uiCi o ui 'i v ri Ci Z (J m ei N m �O N m ei N m tD N O] ei N m tD N O] ei N m tD N a+ O C '- w m W O 6 d C O r O_ C it n 0 E c 0 o a 0 01 n � a o c c o c o c o c o w E c O .2 o ff, o o , ' m e `aCC caO `CC caO aC caO aC cOc O - U - -mu u T a T a T0a a 0 0 o o o 0 0> a ,. O m O O O O O O 00 y .Z o c c o c c o c c o c c u w c a w c m w c a w c a E E 0 o O O E33 E -93E33 �333 .m O x o >a L y a n c n m m m m m ¢ E n E 3 C O _ m O N m w c a v a 01 m y m y n o o o o 1O c v w a i O o` .ao.. 0 K U.S. Department of Agriculture Natural Resources Conservation Service TR 55 Worksheet 3: Time of Concentration (TJ or Travel Time (T,) Project: Brookhill Location: Village Center Check One: Present X Developed X Check One: Tc X T, Segment ID: Sheet Flow: (Applicable to T. only) 1 Surface description (Table 3-1) 2 Manning's roughness coeff., n (Table 3-1 3 Flow length, L (total L < 100) (ft) 4 Two-year 24-hour rainfall, Pz (in.) 5 Land slope, s (ft/ft) 6 Compute Tr= [0.007(n*L)0.e) / Pz .5 S A Designed By: FGM, PE Checked By: SRC, PE Through subarea n/a FL-ENG-21A 06/04 Date: 3/16/2020 Date: 3/16/2020 DA A DA A DA B DA B (Pre-Dev.) (Post-Dev.) (Pre-Dev.) (Post-Dev.) Dense Dense Dense Dense Grass Grass Grass Grass 0.24 0.24 0.24 0.24 100 100 100 60 3.62 3.62 3.62 3.62 0.015 0.22 0.015 0.23 0.25 0.09 0.25 0.06 Shallow Concentrated Flow: 7 Surface description (paved or unpaved) Unpaved Unpaved 8 Flow Length, L (ft) 950 160 9 Watercourse slope, s (ft/ft) 0.011 0.013 10 Average velocity, V (Figure 3-1) (ft/s) 1.7 1.8 11 T,= L / 3600*V 0.16 0.02 Channel Flow: 12 Cross sectional flow area, a (ft) 13 Wetted perimeter, Pw (ft) 14 Hydraulic radius, r = a/Pw (ft) 15 Channel Slope, s (ft/ft) 16 Manning's Roughness Coeff, n 17 V= [ 1.49r2/3s0 s ) / n 18 Flow length, L (ft) 19T, L/3600*V 20 Watershed or subarea Tor T, (Add T, in steps 6, 11 and 19) Note: The shoretest SCS Tc is 0.10 hrs. c c .-, 0 E 0 E 0 c c of o �, o`. o0.E. w 3 v s LL v ° w w y a w E—y E C y C y C C q C r c '-'a z c '-'a r a H u E m e ua o f `o E n E 0 p d a V y w in L w - W mL. o.00 o.00 o.o0 0.00 0.41 1 0.10 1 0.27 1 0.10 U.S. Department of Agriculture Natural Resources Conservation Service TR 55 Worksheet 4: Graphical Peak Discharge Method Project: Brookhill Designed By: FGM, PE Date: 3/16/2020 Location: Village Center Checked By: SRC, PE Date: 3/16/2020 Check One: Present X DevelcpedX 1. Data Drainage Area Description Drainage Area Description Drainage Area Description Drainage Area Description DA A (Pre-Dev.) DA A (Post-Dev.) DA B (Pre-Dev.) DAB (Post-Dev.) Drainage Area (Am) in miles 0.0254 0.0266 0.0113 0.0100 Runoffcurve number CN= 60 86 60 90 Time of concentration (Tc)= 0.41 0.10 0.27 0.10 Rainfall distribution type= II II II II Pond and swamp areas spread throughout the watershed= 0 0 0 0 2. Frequency -years 1 2 10 1 2 10 1 2 10 1 2 10 3. Rainfall, P (24 hour)- inches 3.00 3.62 5.47 3.00 3.62 5.47 3.00 3.62 5.47 3.00 3.62 5.47 4. Initial Abstraction, la -inches 1.36 1.36 1.36 0.33 0.33 0.33 1.36 1.36 1.36 0.23 0.23 0.23 5. Compute la/P 0.45 0.38 0.25 0.11 0.09 0.06 0.45 0.38 0.25 0.08 0.06 0.04 6. Unit peak discharge, Qu-am/in 350 425 525 975 1000 1000 425 525 650 1000 10DO 1000 7. Runoff, Q from Worksheet 2-inches 0.32 0.56 1.55 1.66 2.20 3.91 0.32 0.56 1.55 1.95 2.52 4.29 S. Pond and Swamp adjustment factor, Fp 1 1 1 1 1 1 1 1 1 1 1 1 9. Peak Discharge, Qp- cfs where Qp=Qu Am Q Fp 2.83 6.07 20.62 Routed Through The SWM Facility 1.52 3.32 11.31 Routed Through The SWM Facility WATERSHED SUMMARY FOR SUBAREA A CN Area, 1-year Flow, 2-year Flow, 10-year Flow, ac. cfs cts cis DAA(Pre-Dev.) DAA(Post-Dev) 60 86 16.25 17.04 2.83 0.34 6.07 2.17 20.62 54.09 WATERSHED SUMMARY FOR SUBAREA B Area, 1-year Flow, 2-year Flow, 10-year Flow, CN ac. cfs cfs cis DAB(Pre-Dev.) 60 7.20 1.52 3.32 11.31 DAB(Post-Dev.) 90 6.41 0.20 1.04 11.18 Overall Acreage Summary Pre -Development Post -Development Area,ac. Area, ac. DA A 16.25 17.04 DA B 7.20 6.41 23.45 23.45 Channel & Flood Protection Compliance Verifications 9 VAC 25-870-66 (Point of Interest #1) Section B: Channel Protection "Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet the criteria in subdivision 1, 2 or 3 of this subsection..." Section B.1: Manmade stormwater conveyance systems "When stormwater from a development is discharged to a manmade stormwater conveyance system, following the land -disturbing activity, either..." a. or b. shall be met: Section B.1.b. "The peak discharge requirements for concentrated stormwater flow to natural stormwater conveyance systems in subdivision 3 of this subsection shall be met." Section B.3 "... the maximum peak flow rate from the one-year 24-hour storm following the land -disturbing activity shall be calculated either.." a. or b." Section B.3.a. In accordance with the following methodology: Q Allowed < I.F. * [ (Q Pre -Developed * RV Pre -Developed) / RV Developed ] Q Allowed < 0.43 cfs Where: QPre-Developed= 2.83cfs (From Worksheet 4 Watershed Summary) CN Pre -Developed- 60 (From Worksheet 4 Watershed Summary) 5 Pre -Developed = 6.81 (From SCS TR-55 eq. 2-4) RV Pre -Developed= 0.32 In. (From SCS TR-55 eq. 2-3) CN Post -Developed= 86 (From Worksheet 4 Watershed Summary) S Post -Developed = 1.63 (From SCS TR-55 eq. 2-4) RV Post -Developed = 1.66 in. (From SCS TR-55 eq. 2-3) I.F. = 0.80 (Factor of Safety) Q Developed = 0.34 cfs Section B.4 Limits of Analysis The analysis terminates at Approved 'Wet Pond 4' since subsection B.3.a is met. 9 VAC 25-870-66 (Point of Interest #1) Section C: Flood Protection "Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet one of the following criteria as demonstrated by use of acceptable hydrologic and hydraulic methodologies:" Section C.2: "Concentrated stormwater flow to stormwater conveyance systems that currently experience localized flooding [downstream floodplain] during the 10-year 24-hour storm event: The point of discharge either" must meet a. or b. Section C.2.a: "Confines the postdevelopment peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system [Wet Pond] to avoid the localized flooding. Detention of stormwater..." was incorporated. Section C.3: Limits of Analysis "Stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to a point where:" a., b. or c. is met. Section C.3.c The limits of analysis ends when "The stormwater conveyance system enters a mapped floodplain or other flood -prone area..." 9 VAC 25-870-66 (Point of Interest #2) Section B: Channel Protection "Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet the criteria in subdivision 1, 2 or 3 of this subsection..." Section B.1: Manmade stormwater conveyance systems "When stormwater from a development is discharged to a manmade stormwater conveyance system, following the land -disturbing activity, either..." a. or b. shall be met: Section B.1.b. "The peak discharge requirements for concentrated stormwater flow to natural stormwater conveyance systems in subdivision 3 of this subsection shall be met." Section B.3 "... the maximum peak flow rate from the one-year 24-hour storm following the land -disturbing activity shall be calculated either.." a. or b." Section B.3.a. In accordance with the following methodology: Q Allowed < I.F. * [ (Q Pre -Developed * RV Pre -Developed) / RV Developed ] Q Allowed < 0.20 cfs Where: QPre-Developed= 1.52cfs (From Worksheet 4 Watershed Summary) CN Pre -Developed- 60 (From Worksheet 4 Watershed Summary) 5 Pre -Developed = 6.81 (From SCS TR-55 eq. 2-4) RV Pre -Developed— 0.32 in. (From SCS TR-55 eq. 2-3) CN Post -Developed= 90 (From Worksheet 4 Watershed Summary) S Post -Developed = 1.16 (From SCS TR-55 eq. 2-4) RV Post -Developed = 1.95 in. (From SCS TR-55 eq. 2-3) I.F. = 0.80 (Factor of Safety) Q Developed = 0.20 cfs Section B.4 Limits of Analysis The analysis terminates at Wet Pond B since subsection B.3.a is met. 9 VAC 25-870-66 (Point of Interest #2) Section C: Flood Protection "Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet one of the following criteria as demonstrated by use of acceptable hydrologic and hydraulic methodologies:" Section C.2: "Concentrated stormwater flow to stormwater conveyance systems that currently experience localized flooding [downstream floodplain] during the 10-year 24-hour storm event: The point of discharge either" must meet a. or b. Section C.2.b: The proposed Wet Pond B "Releases a postdevelopment peak flow rate for the 10-year 24- hour storm event that is less than the predevelopment peak flow rate from the 10-year 24- hour storm event. Downstream stormwater conveyance systems do not require any additional analysis to show compliance with flood protection criteria if this option is utilized." Section C.3: Limits of Analysis The limits of analysis ends with Wet Pond B since subdivsion 2b is met. 'Wet Pond 4' Routing Calculations This plan only proposes one change to the approved 'Wet Pond 4'. This plan proposes to modify the approved riser structure's two SWM orifices, which are currently plugged for erosion & sediment control / construction purposes. The approved 6" low -flow and 9" mid -flow orifices are being replaced with new 3" low -flow and 8" mid -flow orifices. The following routing calculations otherwise simply show the Approved 'Wet Pond 4' can adequately handle Block 5 and the future Town Center conceptually shown. BH Town Center- Wet Pond 4 Routings BasinFlow printout INPUT: Basin: BH Town Center- Wet Pond 4 Routings 9 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 358.00 3690.00 0.0 360.00 5715.00 345.6 362.00 8315.00 862.2 364.00 11483.00 1592.3 365.00 14332.00 2069.4 366.00 18294.00 2672.1 368.00 29044.00 4410.1 370.00 33823.00 6736.3 372.00 38733.00 9421.5 Start—Elevation(ft) 5 Outlet Structures Outlet structure 0 Orifice 366.00 Vol.(cy) 2672.13 name: APPROVED Barrel area (sf) 19.635 diameter or depth (in) 60.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 363.300 multiple 1 discharge out of riser Outlet structure 1 Orifice name: MODIFIED Low -Flow Orifice area (sf) 0.049 diameter or depth (in) 3.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 366.000 multiple 1 discharge into riser Page 1 Outlet structure 2 Orifice BH Town Center- Wet Pond 4 Routings name: MODIFIED Mid -Flow Orifice area (sf) 0.349 diameter or depth (in) 8.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 369.250 multiple 6 discharge into riser Outlet structure 3 Weir name: APPROVED Riser diameter (in) 72.000 side angle 0.000 coefficient 3.300 invert (ft) 369.800 multiple 1 discharge into riser transition at (ft) 1.823 orifice coef. 0.500 orifice area (sf) 28.274 Outlet structure 4 Weir name: Approved APPROVED Emergency Spillway length (ft) 30.000 side angle 0.000 coefficient 3.300 invert (ft) 370.200 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr TR-55 SCS Design Storm Area (acres) 17.040 CN 86.000 Type 2 rainfall, P (in) 3.000 time of conc. (hrs) 0.1000 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 35.946 peak time (hrs) 11.917 volume (cy) 3805.675 Page 2 Hydrograph 1 SCS name: Area (acres) CN Type rainfall, P (in) time of conc. (hrs) time increment (hrs) time limit (hrs) fudge factor routed peak flow (cfs) peak time (hrs) volume (cy) Hydrograph 2 SCS name: Area (acres) CN Type rainfall, P (in) time of conc. (hrs) time increment (hrs) time limit (hrs) fudge factor routed peak flow (cfs) peak time (hrs) volume (cy) Hydrograph 3 SCS name: Area (acres) CN Type rainfall, P (in) time of conc. (hrs) time increment (hrs) time limit (hrs) fudge factor routed peak flow (cfs) peak time (hrs) volume (cy) BH Town Center- Wet Pond 4 Routings 2-yr 24-hr TR-55 SCS Design Storm 17.040 86.000 2 3.620 0.1000 0.0200 30.000 1.00 true 47.675 11.917 5047.342 10-yr 24-hr TR-55 SCS Design Storm 17.040 86.000 2 5.470 0.1000 0.0200 30.000 1.00 true 84.496 11.917 8945.592 100-yr 24-hr TR-55 17.040 86.000 2 8.970 0.1000 0.0200 30.000 1.00 true 157.290 11.917 16652.429 SCS Design Storm Page 3 BH Town Center- Wet Pond 4 Routings OUTPUT: Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1-yr 24-hr TR-55 SCS Design Storm inflow (cfs) 35.778 at 11.92 (hrs) discharge (cfs) 0.344 at 18.50 (hrs) water level (ft) 369.169 at 18.74 (hrs) storage (cy) 5727.177 Hydrograph 1 Routing Summary of Peaks: 2-yr 24-hr TR-55 SCS Design Storm inflow (cfs) 47.451 at 11.92 (hrs) discharge (cfs) 2.165 at 13.14 (hrs) water level (ft) 369.535 at 13.14 (hrs) storage (cy) 6163.090 Hydrograph 2 Routing Summary of Peaks: 10-yr 24-hr TR-55 SCS Design Storm inflow (cfs) 84.100 at 11.92 (hrs) discharge (cfs) 54.087 at 12.02 (hrs) water level (ft) 370.456 at 12.02 (hrs) storage (cy) 7316.336 Hydrograph 3 Routing Summary of Peaks: 100-yr 24-hr TR-55 SCS Design Storm inflow (cfs) 156.553 at 11.92 (hrs) discharge (cfs) 143.265 at 11.96 (hrs) water level (ft) 370.915 at 11.96 (hrs) storage (cy) 7919.961 Wed Mar 04 13:26:37 EST 2020 Page 4 'Wet Pond B' Routing Calculations BH Town Center- Wet Pond B Routings BasinFlow printout INPUT: Basin: BH Town Center- Wet Pond B Routings 8 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 386.00 433.00 0.0 388.00 1387.00 64.1 390.00 3568.00 241.3 391.00 5310.00 404.7 392.00 8605.00 659.9 394.00 13155.00 1459.9 396.00 18066.00 2611.5 398.00 23607.00 4150.3 Start—Elevation(ft) 5 Outlet Structures Outlet structure 0 Orifice 392.00 Vol.(cy) 659.93 name: Barrel area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 386.000 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low -Flow Orifice area (sf) 0.030 diameter or depth (in) 2.330 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 392.000 multiple 1 discharge into riser Page 1 BH Town Center- Wet Pond B Routings Outlet structure 2 Orifice name: Mid -Flow Orifices area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 395.000 multiple 4 discharge into riser Outlet structure 3 Weir name: Riser diameter (in) 24.000 side angle 0.000 coefficient 3.300 invert (ft) 395.750 multiple 1 discharge into riser transition at (ft) 0.608 orifice coef. 0.500 orifice area (sf) 3.142 Outlet structure 4 Weir name: Emergency Spillway length (ft) 100.000 side angle 75.960 coefficient 3.300 invert (ft) 396.750 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr TR-55 SCS Design Storm Area (acres) 6.410 CN 90.000 Type 2 rainfall, P (in) 3.000 time of conc. (hrs) 0.1000 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 16.138 peak time (hrs) 11.917 volume (cy) 1708.549 Page 2 Hydrograph 1 SCS name: Area (acres) CN Type rainfall, P (in) time of conc. (hrs) time increment (hrs) time limit (hrs) fudge factor routed peak flow (cfs) peak time (hrs) volume (cy) Hydrograph 2 SCS name: Area (acres) CN Type rainfall, P (in) time of conc. (hrs) time increment (hrs) time limit (hrs) fudge factor routed peak flow (cfs) peak time (hrs) volume (cy) Hydrograph 3 SCS name: Area (acres) CN Type rainfall, P (in) time of conc. (hrs) time increment (hrs) time limit (hrs) fudge factor routed peak flow (cfs) peak time (hrs) volume (cy) BH Town Center- Wet Pond B Routings 2-yr 24-hr TR-55 SCS Design Storm 6.410 90.000 2 3.620 0.1000 0.0200 30.000 1.00 true 20.826 11.917 2204.861 10-yr 24-hr TR-55 SCS Design Storm 6.410 90.000 2 5.470 0.1000 0.0200 30.000 1.00 true 35.226 11.917 3729.354 100-yr 24-hr TR-55 6.410 90.000 2 8.970 0.1000 0.0200 30.000 1.00 true 63.133 11.917 6683.914 SCS Design Storm Page 3 BH Town Center- Wet Pond B Routings OUTPUT: Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1-yr 24-hr TR-55 SCS Design Storm inflow (cfs) 16.063 at 11.92 (hrs) discharge (cfs) 0.200 at 15.52 (hrs) water level (ft) 394.931 at 17.00 (hrs) storage (cy) 1951.037 Hydrograph 1 Routing Summary of Peaks: 2-yr 24-hr TR-55 SCS Design Storm inflow (cfs) 20.729 at 11.92 (hrs) discharge (cfs) 1.040 at 13.10 (hrs) water level (ft) 395.268 at 13.10 (hrs) storage (cy) 2147.215 Hydrograph 2 Routing Summary of Peaks: 10-yr 24-hr TR-55 SCS Design Storm inflow (cfs) 35.060 at 11.92 (hrs) discharge (cfs) 11.175 at 12.08 (hrs) water level (ft) 396.269 at 12.08 (hrs) storage (cy) 2794.944 Hydrograph 3 Routing Summary of Peaks: 100-yr 24-hr TR-55 SCS Design Storm inflow (cfs) 62.837 at 11.92 (hrs) discharge (cfs) 57.244 at 11.96 (hrs) water level (ft) 396.999 at 11.96 (hrs) storage (cy) 3329.007 Wed Mar 04 13:27:35 EST 2020 Page 4 'Wet Pond B' Drought Tolerance and Riprap Outlet Protection Calculations " CF5 Q o = 16 cFS 4 (OCS-i4w xwl) 4,� 8 OC��_ \.S Mr,x az6 QER ULSC11 1LAYc 3.\$^� , ��o - 6 NaT VOa� CL, Al sToNc "5 P 'ASp' LA LA MIN, r'�IJP+-. 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