The URL can be used to link to this page

Home My WebLink AboutWPO201400064 Calculations 2020-04-09HYDRAULIC/HYDROLOGIC COMPUTATIONS March 10, 2020 LOCATION: Rivanna District, Albemarle County, Virginia OWNER: Hyland Ridge Property Owners Association, Inc DEVELOPER: Virginia Land Co. PREPARED BY: Roudabush, Gale & Associates 172 South Pantops Drive Charlottesville, VA 22911 P: 434.977.0205 Contents: 1. Project SWM Summary 2. Hydrographs APPROVED by the Albemarle County Community Development Department Date 4/8/20 File WP0201400064 EpLTy Q� r 0 DUSTIN E. GREENE a Lic. No. 55833 Project SWM Summary Stormwater Management Facility #1 was designed as an extended detention basin that provides water quality treatment by detaining the required water quality volume for a period of a 30-hour drawdown. SWM #1 was also designed to attenuate the post -developed peak flow from the 2yr and 10yr storm events. The facility was built as a retention basin and it is the purpose of these calculations to return the facility back to its original design and purpose using as -built conditions in the field. The critical storm duration was used to verify the greatest volume of storage with respect to the allowable release rate. SMW #1 storm duration factors for the 2yr, 10yr and 100yr storms were 72, 81 and 117 minutes. The design post -developed time of concentration for 20.5 acre drainage area was 9 minutes. The storm duration factor for the Hydraflow Hydrograph Extension are 8 (72/9), 9 (81/9) and 13 (117/9) for the 2yr, 10yr and 100 yr storm events. SWM #1 Postdeveloped Drainage Area: A = 20.5 Acres Weighted C = 0.46 Tc = 9 min WQV required = 33,156 cf WQV provided = 43,072 cf OK SWM #1 Stage Storage: Stage Elevation Contour Incremental Total _ Area Storage Storage (ft) (ft) (sgft) (tuft) — (tuft) 0.00 472.03 2,629 0.000 0.000 0.50 472.50 2.767 1,349 1,349 0.51 472.51 6,908 46.8 1,396 1.00 473.00 7,251 3,468 4,864 2.00 474.00 7,956 7,600 12,464 4.00 476.00 9,443 17,376 29,840 5.33 477.33 10,465 13,232 43,072 6.00 478.00 11.033 7,200 50,272 8.00 480.00 13,266 24262 74,534 10.00 482.00 16,572 29,774 104,308 The contractor has identified a spring at the bottom of the facility. In order to create a detention facility with a bottom that does not need continual maintenance and be a burden on the HOA the designer is proposing to add #3 gravel stone from elevation 472 to 472.5'. The water will be able to flow out of the facility and will not need continual maintenance. Please see that the routing information provided gives 40% of the actual contour area that exists in the field so that the facility could be routed properly. SWM#1 Predeveloped Flows: 2yr = 6.25 cfs > 2.21 cfs OK 10yr = 8.11 cfs > 5.49 cfs OK 100yr elevation = 2.04' freeboard (no emergency spillway required), Proposed 100yr wse = 480.50', There is enough width on the top of the dam to increase the dam height to 482.5' so that no emergency spillway is necessary. Hydrograph Report Hydraflow Hydrographs Extension for AutodeskO Civil 31DO 2019 by Autodesk, Inc. v2020 Tuesday, 03 110 / 2020 Hyd. No. 14 Postdeveloped Inflow SWM#1 Hydrograph type = Mod. Rational Peak discharge = 12.36 cfs Storm frequency = 2 yrs Time to peak = 9 min Time interval = 1 min Hyd. volume = 53,378 cuft Drainage area = 20.500 ac Runoff coeff. = 0.46 Intensity = 1.310 in/hr Tc by User = 9.00 min OF Curve = ALBEMARLE IDF.IDF Storm duration = 8.0 x Tc Target Q =n/a Est. Req'd Storage =n/a Q (cfs) 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 0 10 20 Hyd No. 14 Postdeveloped Inflow SWM#1 Hyd. No. 14 -- 2 Year 30 40 50 60 70 80 Q (cfs) 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 90 Time (min) Hydrograph report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020 Tuesday, 03 / 10 12020 Hyd. No. 14 Postdeveloped Inflow SWM#1 Hydrograph type = Mod. Rational Peak discharge = 16.25 cfs Storm frequency = 10 yrs Time to peak = 9 min Time interval = 1 min Hyd. volume = 78,996 cuft Drainage area = 20.500 ac Runoff coeff. = 0.46 Intensity = 1.724 in/hr Tc by User = 9.00 min OF Curve = ALBEMARLE IDF.IDF Storm duration = 9.0 x Tc Target Q =n/a Est. Req'd Storage =n/a Postdeveloped Inflow SWM#1 Hyd. No. 14 -- 10 Year 15.00 12.00 -Me • 11 i 11 Q (cfs) 18.00 15.00 12.00 • 11 430 0.00 -' I 3 0.00 0 10 20 30 40 50 60 70 80 90 — Hyd No. 14 Time (min) Hydrograph Report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020 Tuesday, 03 / 10 / 2020 Hyd. No. 14 Postdeveloped Inflow SWM#1 Hydrograph type = Mod. Rational Peak discharge = 20.42 cfs Storm frequency = 100 yrs Time to peak = 9 min Time interval = 1 min Hyd. volume = 143,369 cuft Drainage area = 20.500 ac Runoff coeff. = 0.46 Intensity = 2.166 in/hr Tc by User = 9.00 min OF Curve = ALBEMARLE IDF.IDF Storm duration = 13.0 x Tc Target Q =n/a Est. Req'd Storage =n/a Postdeveloped Inflow SWM#1 Q (cfs) Hyd. No. 14 -- 100 Year Q (cfs) 21.00 18.00 15.00 12.00 9.00 6.00 3.00 0 00 21.00 18.00 15.00 12.00 9.00 6.00 3.00 0 00 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time (min) Hyd No. 14 Hydrograph Report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020 Hyd. No. 18 As -Built routed SWM #1 Hydrograph type = Reservoir Peak discharge Storm frequency = 2 yrs Time to peak Time interval = 1 min Hyd. volume Inflow hyd. No. = 14 - Postdeveloped Inflow SWLUt. Elevation Reservoir name = As -Built Detention SWM#1 Max. Storage Storage Indication method used Q (cfs) 14.00 12.00 10.00 WO M 4.00 r M 0.00 -M 0 As -Built routed SWM #1 Hyd. No. 18 -- 2 Year 5 10 15 20 Hyd No. 18 Hyd No. 14 Tuesday, 03 / 1012020 = 2.205 cfs = 1.32 hrs = 50,695 cuft = 478.07 ft = 51,074 cuft 25 30 35 40 45 Total storage used = 51,074 cuft Q (cfs 14.00 12.00 10.00 4.00 2.00 0.00 50 Time (hrs) Pond Report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020 Tuesday, 03 / 10 12020 Pond No. 12 - As -Built Detention SWM#1 Pond Data Contours -User-defined contour areas. Conic method used for volume calculation. Begining Elevation = 472.03 ft Stage / Storage Table Stage (ft) Elevation (ft) Contour area (sgft) Incr. Storage (cult) Total storage (cuft) 0.00 472.03 2,629 0 0 0.50 472.50 2,767 1,349 1,349 0.51 472.51 6,908 47 1,396 1 00 473.00 7,251 3,468 4,864 200 474.00 7,956 7,600 12,464 4.00 47600 9,443 17,376 29,840 5.33 477.33 10,465 13,232 43,072 6.00 478.00 11,033 7,200 50,272 8.00 480.00 13,266 24,262 74,534 10.00 482.00 16,572 29,774 104,308 Culvert / Orifice Structures Weir Structures [A] [B] [C] [PrfRsr] [A] [B] [C] [D] Rise (in) = 24.00 2.50 12.00 0.00 Crest Len (ft) = 12.57 Inactive 0.00 0.00 Span (in) = 24.00 2.50 12.00 000 Crest El. (ft) = 480.03 480.50 0.00 0.00 No. Barrels = 1 1 1 0 Weir Coeff. = 3.33 3.33 3.33 3.33 Invert El. (ft) = 472.03 472.03 477.33 0.00 Weir Type = 1 Ciplti --- --- Length (ft) = 100.61 0.00 0.00 0.00 Multi -Stage = Yes No No No Slope (%) = 0.67 0.00 0.00 n/a N-Value = .013 .013 .013 n/a Orifice Coeff. = 0.60 060 0.60 060 Exfil.(in/hr) = 0.000 (by Contour) Multi -Stage = n/a No No No TW Elev. (ft) = 0.00 Stage (ft) 10.00 8.00 6.00 4.00 2.00 0.00 0.00 5.00 Total Q 10.00 Note: Culvert/Orifice outflows are analyzed under inlet (ic) and outlet (oc) control. Weir risers checked for orifice conditions (ic) and submergence (s) Stage / Discharge Elev (ft) 482.03 480.03 478.03 476.03 474.03 472.03 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 Discharge (cfs) Hydrograph Report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020 Hyd. No. 18 As -Built routed SWM #1 Hydrograph type = Reservoir Peak discharge Storm frequency = 10 yrs Time to peak Time interval = 1 min Hyd. volume Inflow hyd. No. = 14 - Postdeveloped Inflow SWlklbk. Elevation Reservoir name = As -Built Detention SWM#1 Max. Storage Storage Indication method used Tuesday, 03 / 10 / 2020 = 5.489 cfs = 1.45 hrs = 75,635 cuft = 479.61 ft = 69,766 cuft As -Built routed SWM #1 Q (cfs) Hyd. No. 18 -- 10 Year Q (cfs 18.00 18.00 15.00 15.00 12.00 12.00 9.00 9.00 6.00 - 6.00 3.00 3.00 0.00 0.00 0 5 10 15 20 25 30 35 40 45 50 Time (hrs) Hyd No. 18 Hyd No. 14 Total storage used = 69,766 cult Hlydrograph Report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc v2020 Hyd. No. 18 As -Built routed SWM #1 Hydrograph type = Reservoir Peak discharge Storm frequency = 100 yrs Time to peak Time interval = 1 min Hyd. volume Inflow hyd. No. = 14 - Postdeveloped Inflow SWt. Elevation Reservoir name = As -Built Detention SWM#1 Max. Storage Storage Indication method used As -Built routed SWM #1 Tuesday, 03 / 10 / 2020 = 20.41 cfs = 1.95 hrs = 139,524 cuft = 480.50 ft = 81,992 cuft Q (cfs) Hyd. No. 18 -- 100 Year Q (cfs) 21.00 21.00 18.00 - 18.00 15.00 15.00 12.00 12.00 9.00 9.00 6.00 6.00 3.00 3.00 0.00 0.00 0 4 8 12 16 20 24 28 32 36 40 Time (hrs) Hyd No. 18 Hyd No. 14 Total storage used = 81,992 cult