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Home My WebLink AboutWPO202000058 Calculations 2021-03-05 (2)SHIMP ENGINEERING, P.C. Design Focused Engineering December23,2020 APPROVED by the Albemarle County John Anderson, PE Community Development Department County of Albemarle Date 03/05/2021 File W P02 0 2 0 0 0 0 5 8 Department of Community Development 401 McIntire Road, North Wing Charlottesville, Virginia 22902 RE: WPO 2020-OOXXX Albemarle Business Campus SWM Calculation Packet A Revision of WPO 2018-00084 Royal Fern Phase 2 VSMP Dear John, Enclosed is the stormwater calculation packet for Albemarle Business Campus (ABC). This is development will occupy a site that has an area of 8.55 acres. The disturbed area is 9.10 acres. The proposed stormwater design meets the runoff requirements set forth in 9VAC25-870-95 as the Part IIC regulations are applicable to the project per 9VAC25-870-47. This VSMP plan is intended to revise the approved plans for Royal Fern Phase 2, also prepared by Shimp Engineering. In effect, the strategies for ABC and Royal Fern Phase 2 are the same. All onsite runoff is routed to the existing pond built as part of the site plan for Sterling University Housing (SDP 2001000129). The narrative from Royal Fem Phase 2 is copied below, but to achieve approval, all that remains to be shown is that the impervious area / Curve Number for the ABC plan remains lower than what the pond was designed for, and that the runoff to the offsite storm pipes remains lower than the original design capacity for those pipes. Both of these conditions are still met with the ABC development. COPIED FROM WPO2018-84: After research of the approved site plan for Sterling University Housing (SDP 200100129), it was determined that the downstream sewer system empties into the wet pond on the Cavalier Crossing Apartments property (TMP 76-54AI ). This pond was designed to manage and treat runoff from a fully -built Sparrow Hill Phase 2 [Now ABC] site, listed in the approved calculation packet for SDP 2001000129 as the "western residue of the Heischman property". The pond was designed under VA Part IIC regulations and constructed before 2014 while these regulations were the current VSMP ordinance. Per 9VAC25-870-47 (B), "Land -disturbing activities that obtain an initial state permit or commence land disturbance prior to July 1, 2014, shall be conducted in accordance with the Part H C (9 VAC25-8 70-93 et seq) technical criteria of this chapter. Such projects shall remain subject to the Part H C technical criteria for two additional state permit cycles." Only one permit cycle has completed since the pond was approved, so Part IIC regulations are applicable to the Royal Fern Phase 2 project. Since Phase 2 [Now ABC] falls under Part IIC regulations, and since the pond to which all site runoff shall drain was designed for a fully developed site, the Royal Fern Phase 2 project [Now ABC] must fall within the original design assumptions for the pond to achieve SWM compliance. Royal Fern Phase 2 [Now ABC] has less onsite impervious area than the assumed drainage area used for the pond design, so nutrient treatment and downstream natural channel requirements are satisfied. Royal Fern Phase 2 /Now ABC] outlets runoff into the SDP 2001000129 storm sewer at rates that do not exceed the capacity of the system, so manmade channel requirements are satisfied. SWM Comparison: SDP 2001000129 Pond/Sewer Design Limits Vs. ABCAchieved Conditions Design Assumption Achieved Condition Land Cover 63% Impervious (total pond 47°/u Impervious (total pond contributing Drainage Area) contributing Drainage Area Original Inlet El 10 yr runoff: 10 yr runoff: (ABC Str. A2) 51.11 cfs 50.30 cfs Original Inlet Cl 10 yr runoff: 10 yr runoff: (ABC Str. B2) 45.58 cfs 40.17 cfs If you have any questions please do not hesitate to contact me at: keane@shimp-en ing eerieg com or by phone at 434-299-9843. Contents: Post-Dev Runoff Calculations: Post-Dev Drainage Spreadsheet Post-Dev Structure Drainage Map Post-Dev Pond Drainage Map - Aerial Post-Dev Pond Drainage Map — Topo Post-Dev Onsite Land Cover Map VDOT LD-229 Culvert Capacity ALBEMARLE BUSINESS CAMPUS Inlet Drainage Area Summary C for Disturbed Soil during Construction 0.6 Impervious C 0.9 Pervious C 0.3 Str. Type Number Total Area Impervious (ft) Turf (ft) C Area (ac) DI-3C A2 337,600 202,560 135,040 0.66 7.75 DI-313 B5 385,070 192,535 192,535 0.60 8.84 (Offsite DA) DI-3C B2 113,200 79,240 33,960 0.72 2.60 III �'__-- , i111 li 1 {' ice' _�-_-��__� � �•� ___ _________ -- = ='�' �' '' �• - - ; I IiI I I �� II11' nI 1 �� ilil A r �• � '/ i/ 1�_I, 1,1 � �I aso-� ' r W - -� I 11 _ _____ -i _so- -- �•p` — 0" o- 1 �� •� -� ` �� - --aa° I 1 ill li �� ,A� 1 /� �� - _ ��'�� -- � - - - -_ - a "-� - - / ,♦ /, , _ -� a ,�- ! i 1 / `1 Il lill l` i i � II �� __ ' o � -_ � ,%/n•� � J --_ i//'I � r � 1� Ili 'i'i i L_ it 1 1 i 1 . __ __ ______ __ _ _ •.,__ _,,� i _ -,-� - . - _ III 11 �1A�--_ �•,_ _ ____ ___�. i� '� 1 I 1� it__ _ �� a5 �• x �- 1 - - - 4— Vlb, SP- 1 - 1 �I 1 •�� .i "aa°� ----- -- - — -k =-__ T61 r/ SITE DRI AINA 0 VIA/OFFSITE 5T T-T 6- �{ - 1'=80'Sca \ ROYALFERN POST-DEV INLET DRAINAGE MAP 80 160 240 IMAGERY PROVIDED BY BING MAPS, ACCESSED APRIL 24, 2019. 02019 ROADS, BUILDINGS, & DRIVEWAY LOCATIONS PROVIDED BY ALBEMARLE COUNTY GIS, ACCESSED APRIL 24, 2019. JSINESS CAMPUS AERIAL PHOTOGRAPHY - POND DRAINAGE MAP LEGEND Building: Road: Driveway: New Imp: Pond DA: Pond: Stream: -- 11 1 11 .11 •11 N m N M P O AC �c 0 LD-229 Storm Drain Design Computations Albemarle Business Campus To Catch. Runoff Increment Accum. Total Total 10-yr Total 10-yr Up Down Pipe Invert Pipe Pipe Velocity Flow time ;JS-2 Pi tructure Area Coef AC AC TOC Intensity Flow Invert Invert Length Slope Diameter Capacity Increment (ac) (min) (in/hr) (cfs) Elev. Elev. (ft) % (in) (cfs) (ftfs) (min) 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 A8 A7 0.00 8.63 0.00 #DIV/01 18 8.0 0.00 A7 A6 0.00 8.63 0.00 #DIV/01 18 8.0 0.00 A6 A5 0.00 8.63 2.20 #DIV/01 18 8.0 0.00 A5 A4 0.00 8.63 0.00 #DIV/01 18 8.0 0.00 A4 A3 0.00 8.63 0.00 #DIV/01 18 8.0 0.00 A3 A2 0.00 8.63 0.00 #DIV/01 24 8.0 0.00 C6 C5 0.00 8.63 0.00 #DIV/01 15 8.0 0.00 C5A C5 0.00 8.63 0.00 #DIV/01 15 8.0 0.00 C5 C4 0.00 8.63 0.00 #DIV/0! 18 8.0 0.00 C4 C3 0.00 8.63 0.00 #DIV/01 18 8.0 0.00 C3 C2 25.79 #DIV/0! 18 8.0 0.00 C2A C2 0.00 8.63 0.00 #DIV/0! 15 8.0 0.00 C2 C7 0.00 8.63 25.79 #DIV/0! 18 8.0 0.00 C1 A2 0.00 8.63 25.79 #DIV/0! 24 8.0 0.00 A2 Al 7.75 0.66 5.12 5.12 15.00 4.79 50.30 471.25 470.25 59.92 1.670% 36 93.4 10.2 0.10 B3D 133C 0.00 8.63 0.00 #DIV/01 15 93.4 8.0 0.00 B3C B3B 0.00 8.63 25.79 #DIV/0! 15 97.5 8.0 0.00 B3Z 13313 0.00 8.63 0.00 #DIV/01 15 93.4 8.0 0.00 13313 133A 0.00 8.63 0.00 #DIV/01 15 8.0 8.0 0.00 B3Y 133A 0.00 8.63 25.79 #DIV/0! 18 93.4 8.0 0.00 133A B3 0.00 8.63 0.00 #DIV/01 18 13.9 8.0 0.00 B4' B3 8.84 0.60 5.30 5.30 10.00 5.62 29.83 463.00 462.70 31.95 0.94% 15 6.8 4.5 0.12 B3 B2 2.60 0.72 1.87 7.18 10.12 5.60 40.19 462.50 462.00 8.04 6.22% 15 17.5 10.0 0.01 B2 131 0.00 0.00 0.00 7.18 10.13 5.60 40.17 463.00 462.70 31.95 0.94% 15 6.8 4.5 0.12 'From Existing Culvert. Includes offsite drainage area ABC Sediment Basin Riser Buoyancy Calculation Riser Dimensions: D= 48 " H= 10 ' above pond bottom Riser Volume = 125.7 cubic feet Buoyant Force = p*V = 62.4 lb/cf * 125.7 cf Buoyant Force = 7841 Ibs 1.25x BF = 9802 Ibs (factor of safety) Concrete Base Dimensions: ��M D= 25 " Base Volume = 67 cf Base Weight = p*V = 150lb/cf * 54.0 Base Weight= 10035 Ibs Weight> 1.25*Bouvant Force Chapter 8 - Culverts Appendix 8C-1 Inlet Control, Circular Concrete 180 10,000 CHART 10 168 8.000 EXAMPLE (1) (2) (3) 156 6.000 014e i•snx (3.5 reed 6. D•leo n• 6' 149 5,000 5, 4,000 y* 4 nW 6• S. 3Y 3,000 0.0 5' 4. 120 m e.l �.• IOB 2.000 (3) e.4 TJ 'D 1. 4.1 96 1.000 Boo B4 600 500 72 400 N W x 300 z N = BO p 200 O 54 / DOM STREAM W � / so CONDITION: 36" RCP u 42 60 CULVERT W. C N So His ENTRANCE SQUARE c O 40 D SCALE TYPE EDGE ' 36 30 Mo.e' 01 sa•4r• 3• with HEADWALL. Q23=8 CFS a 33 FROM BASIN c 20 Ie) Dreeu and Run 30 euavu (31 Br osv enl Y7 r.epenR4 0 f4 9 8 is eu feeN IYI er 13) Or41ee1 21 5 eer leeerene b •<el• DI, rMn Is 15 4 •., flrele Al W.Iind Ilne Far eee4 D end 0 w•lu, a nnn• e• 3 illunrena. 2 1.0 3. 3. 3. 2.6 3 x 1.5 I.S N OC W r1l x e- o 1.0 e W � c 1.0 .9 .9 9 a .e .7 .T .7 L.5 L- .5 L .5 L-If HEADWATER DEPTH FOR HEADWATER SCALES 283 CONCRETE PIPE CULVERTS BUREAU OF NBLIE ROADS JAM. 1943 REVISED MAY 1964 WITH INLET CONTROL CULVERT INLET IS FOR PRIVATE SYSTEM. TOTAL DEPTH OF WATER=2.6 x 3.0' = 7.8' Source: HDS-5 INVERT AT HEADWALL = 406.0 MAX ELEVATION OF WATER IN 25 YR STORM = 413.8 "RIDGE" ELEVATION ADJACENT TO HEADWALL = 420.0 BASIN OUTFALL CONDITION DOES NOT CAUSE FLOODING IN OUTFALL DEPRESSION AREA AT CULVERT HEADWALL. 1 of 1 VDOT Drainage Manual