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HomeMy WebLinkAboutWPO201700029 Plan - VSMP 2019-12-16 PROJECT MANAGEMENT sHimp CIVIL ENGINEERING LAND PLANNING ENGINEERING C December 16th, 2019 Frank Pohl County Engineer Albemarle County Department of Community Development 401 McIntire Road Charlottesville, VA 22902 Regarding: Hillbrook Subdivision—VSMP Exception Request—WPO 201500033 Dear Mr. Pohl, Pursuant to section 9VAC25-870-122 (Exceptions) of the state code and section 17-408 (Exception from the requirements of the VSMP) of the county code, we request that the VSMP authority grant an exception to a portion of the provisions of part IIB of the regulations for the Hillbrook Subdivision VSMP. The Hillbrook Subdivision meets all criteria of the SWM regulations (9VAC25-870-66.B.3.a) with the exception of the requirements for channel protection, or the one year storm calculation which is required to be reduced by a small amount due to the requirements of the energy balance equation. All other criteria for water quality and for the 10 year flow rates are met. The project has a very small pre-development area which drains towards Old Brook Road. Due to the combination of the low overall flow rate from a one year storm and the requirements of the County ordinances and VDOT regulations for slope and entrance design it was not possible to design a system which met the energy balance equation by capturing onsite runoff without capturing offsite runoff. The area that could not be captured from the new road and graded entrance slopes by-pass the detention systems and would exceed the allowable flow for the one year storm from the site. In an attempt to achieve the required reduction the design sought to capture offsite water from the road into the detention system to over-detain that runoff in an effort to meet the requirements. This design proved impossible to achieve in the field for at least two reasons: One being that the offsite flow from the road did not flow across the road as indicated by surveys that were performed. This limits the amount of available offsite capture. Secondly utility conflicts and field adjustments authorized by VDOT inspectors to meet their criteria for the entrance did not yield a construction which captures what offsite water was available to the site. A major electric line which is not buried deep enough per VDOT standards was found to be present along the edge of the road after construction began that drove many of the field adjustments. II UTILITY CONFLICT: DEPTH TO TOP OF ELECTRIC SURVEY ERROR: CONDUIT FROM ASBUILT GRADE SURVEY SHOWS SUPERELEVATION _ / LDBR / � 1 \ I \,...... lo+00-\ / C/ ( 1 --. ...,.. ------ ,,, • ....... ---. .__. ......--- -..... ..._:--- ir„. ,.. / / — — _ _ _ J I 5"y \\ _ _ J I I , UTILITY CONFLICT: ` `` .� / �__- -�— — — — — — — — — \ I / TEST BORE LOCATIONS b __ _ C\ ) \\ I i d o y 0 0 \ �� \\� \ AS-BUILT CURB �D \ AND 9U j \\ ii 1 \\ / SIDEWALK SHOWN 0 \ I 1 I \\ ( I I II II h ll lbrook Ct . 30 0 30 60 90 As-Built Exhibit SIIIMP ENGINEERING, P.C. ■ • • ■ ■ ENGINEERING LAND ANNING PROJECT MANAGEMENT 1"-30' Rcvised • 12 17 20 19 012 E HIGH ST PHONE: (434) 227-3140 CHARLOTTESVILLE. VA 22902 JUSTINOSHIMP-ENGINEERINO.COM , 4441* , ,, s, , •, , t, ,' ' ; , •• ir g'" -. s •7.1„.\`i. \ S' 4 A , ,,, , ; , , ,,14:-.• \:1-4,-.....v.- ,,,,._ ---.'.:11,1 ,- t ' ' Y - - — '--- q,e, •'' . ''''.,•.f.w.' 44' A 6 - Vat=,. ilit ' • 4a f•., , .,:04,.... ' 'A.,''' 1. 4 A e• ... 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't. � d r ,,,yx," ,„, V Sh ,i :y r-;r. r''?;- S �s<' S ,vs,..� i �i,: �,�.t`�,[.� s t -'rf� �.ivx ay. ^'. ,i-¢¢ ��� '"�, � ' "�� r �W{� a, � -,d � -„ h't�.'"``"�+i�t,`: .,e aR 4� t#,.+tiw. .k`y+ •I -! +. n +"'! ,fie ?-,. `4�, -4 t4,-* e' fat' rt -t r, `Nf'Y3 ik cv; nay ,'it,. ,±. s , . � ,r:i��� £a ��:. : ,,."',-z. Y.; h��;�` v.as✓l.�i� ��, e 9 �3 ;,~° #�tt Cs �. t L,yt + iy .,y. :t� :d"�., d eb J"� ... r . � � },! � 6� < � .. :� saSY��,.�. ,- ,.. a ,.. )"� - n �", +�'t, , � a�.� .c, �1?.� ti '��� sHimp PROJECTCIVIL MANAGEMENTENGINEERING LAND PLANNING ENGINEERINGa , , ,II �y, • . _, iliec,,, , ".., „, December 17th, 2019o` r�C U fc. r. G Frank Pohl No 4s18 County Engineer ..,-A /Z!l7ll9 Albemarle County �,<, ��,_ Department of Community Development t�4 Regarding: WPO 201500033 Hillbrook Subdivision SWM As-Built Analysis Dear Mr. Pohl, Please find attached a summary of the As-Built SWM Analysis for the Hillbrook Subdivision. An error in analysis of the detention drainage area has been found. The originally proposed detention drainage area included approximately 0.31 acres of both on-site and off-site area that is not actually being detained. The attached documents provide an analysis of the "As-Built" conditions for the site and for the additional included drainage areas. The energy balance equation has been reanalyzed at a new POA, located at the existing channel at the outlet of the STM X2 storm structure. This POA includes the additional areas shown on the attached Pre and Post Dev SWM Total DA Maps. The Post Development DA Map for on-site has been revised to describe the as-built drainage areas being either detained or not detained. The TOC and HydroCAD models have been revised to reflect the as-built drainage areas, showing both on-site and off-site areas not being detained. Please see the attached Energy Balance Equation Summary sheet for a summary of the flows, including percentage of increase/decrease from predevelopment. Also included are the pre and post development HydroCAD models along with the TOC summary. While the energy balance equation was not met with the as-built condition, the post-development flows show that the receiving channel will have minimal increased risk of flooding or erosion due to the negligible increase in runoff to the channel. For this reason, we believe that the increase in runoff generated by the as-built conditions will be within an acceptable range for the watershed in my judgement as a professional engineer; and that the runoff will have no negative effect on the SWM channels in the area. If you have any questions please do not hesitate to contact me at michael@shimp-engineering.com or you may contact Justin Shimp at Justin@shimp-engineering.com or by phone at 434-227-5140. 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IMP. * W W W ' W W W W C .-.-- . + W \^\ .Alit i W W W W W W W W \wirommilli? ,0V .W W W �W W W W W W SIMPLE W W W W .� O OFFSITE NOT $, ,� » ' W� W ._ W W W W !Y W W W W W W DISCONNECTION DETAINED (1 ) � W . . . - _ _ _ i� `� 7 ORIG IAL SHEET — \ 0.16 AC. \ \ /00,440, W W W W -------''' 0 FLOW AREA SWM DIVIDE INE MN MO 0. 4 AC. IMP. \ ,, W W W \ : 0.246 C. TOTAL STORM DIVIDE womm ., .1 16A W ToC=1 .6 MIN. SIMPLE \ 1. fd - Qi=0.1 CFS DISCONNECTION — 40 0 40 80 120 RV=0. 16 AF REVISED: 11-6-2019 DIVIDE 1 ' Scale: 1"=40' 1 Hillbrook Pre-Development Time of Concentration Calculations DA 2 Onsite 091 ft overland flow 11 0%slope 0 35 C—value 8 4 min Seelye Chort 311 ft s c flow—woods 5 8%slope 4 8 fps velocity I 1 min NEH Figure 15-4 158 ft swale 12 ft height 1 1 min Kirpich Chart 10 6 min DA 2 Offsite 080 ft overland flow 7 5%slope 0 35 C—value 9 4 min Seelye Chart 244 ft s c flow—woods 5 3%slope 4 5 fps velocity 0 9 min NEH Figure 15-4 55 ft swale 8 ft height 0 2 min Kirpich Chart 10 5 min STM X2A 153 ft overland flow 5 3%slope 0 35 C—value 11 5 min Seelye Chart 298 ft s c flow—woods 8 0%slope 5 6 fps velocity 0 9 min NEH Figure 15-4 242 ft swale 20 ft height 0 9 min Kirpich Chart 13 3 min SIN X3 162 ft overland flow 13 6%slope 0 35 C—value 10 2 min Seelye Chart 38 ft s c flow—woods 4 0%slope 4 0 fps velocity 0 2 min NEH Figure 15-4 97 ft swale 12 ft height 0 5 min Kirpich Chart 109 min Hillbrook Post-Development Time of Concentration Calculations SIN X2A 153 ft overland flow 5 3%slope 0 35 C—value 11 5 min Seelye Chart 298 ft s c flow—woods 8 0%slope 5 6 fps velocity 0 9 min NEH Figure 15-4 242 ft swale 20 ft height 0 9 min Kirpich Chart 13 3 min STM X3 162 ft overland flow 13 6%slope 0 35 C—value 10 2 min Seelye Chart 38 ft s c flow—woods 4 0%slope 4 0 fps velocity 0 2 min NEH Figure 15-4 97 ft swale 12 ft height 0 5 min Kirpich Chart 109 min Onsite Not Detained (1) 100 ft overland flow 8 0%slope 0 35 C—value 9 6 min Seelye Chart 175 ft s c flow—woods 8 0%slope 1 3 fps velocity 2 2 min NEH Figure 15-4 284 ft swale 15 ft height 1 7 min Kirpich Chart 13 5 min Offsite Not Detained (1) 045 ft overland flow 0 5%slope 0 35 C—value 3 0 min Seelye Chart 220 ft s c flow—woods 5 5%slope 1 3 fps velocity 2 8 min NEH Figure 15-4 284 ft swale 15 ft height 1 7 min Kirpich Chart 7 5 min Offsite Undetained (2) 070 ft overland flow 3 0%slope 0 35 C—value I 0 min Seelye Chart 205 ft s c flow—woods 5 0%slope 1 0 fps velocity 3 4 min NEH Figure 15-4 284 ft swale 12 ft height 1 7 min Kirpich Chart 6 I min Onsite Undetained (2) 052 ft overland flow 40 0%slope 0 35 C—value 5 0 min Seelye Chart 90 ft s c flow—woods 6 6%slope 5 0 fps velocity 0 3 min NEH Figure 15-4 284 ft swale 12 ft height 1 7 min Kirpich Chart 7 0 min Onsite Detained 050 ft overland flow 16 0%slope 0 35 C—value 9 5 min Seelye Chart 18 ft s c flow—woods 1 0%slope 1 0 fps velocity 0 3 min NEH Figure 15-4 300 ft swale 15 ft height 2 2 min Kirpich Chart 12 0 min Energy Balance Equation Summary Shimp Engineering,P.C. 11/6/2019 24 Hr 1 year Storm: POA 1 q,peds I F '(Qy,r,A,No RVpreDeedoped)/RVde,doped Required 2 57 Qdeoe+opeds(Qtore.,ed•RVro,e„eel)/Rvd,,,doped Reduction 46% Post Row Decr'cfss'dBy %Decreased Drainage Area 4 477 Ac Qp,e-developed = S 56, Ql ,-,e,reioe,d - 5 71' Qlor,-developed = INCREASED -0 15 -2 7% RVp,e-,,,,,doped = I 0 3481 Max Qd„,pe, = 2.99 cfs Q2poso-developed = 8.24 Q2predeveioped = INCREASED -0 02 -0 2 RVD,,,dop,d 05181 Q10poa-drooped = 16.39 Q10p„-de,emped = REDUCED 044 26% Post Development Runoff Peak Flow=5 71 cfs,meets 9VAC25-870-66-B(3) Post Development Q10=16 39 cfs Hillbrook Subdivision Pre-Development HydroCAD Report <16S> X A 17S /18s X2 X3 4 Po k\ D.A.2 Offsite D.A.2 Onsite / Drainage Diagram for Hillbrook Pre-Development ASBUILT REV 'Subcat' Reach Pon Ink 9 9 P / Prepared by Shimp Engineering, Printed 11/7/2019 HydroCADO 9.10 s/n 07054 ©2010 HydroCAD Software Solutions LLC Pre-Development Hil!brook Pre-Development ASBUILT REV Type II 24-hr lyr Rainfall=3.01" Prepared by Shimp Engineering Printed 11/7/2019 HydroCAD®9.10 s/n 07054 ©2010 HydroCAD Software Solutions LLC Page 2 Time span=0.00-24.00 hrs, dt=0.01 hrs, 2401 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method Subcatchment2:D.A.2 Onsite Runoff Area=0.568 ac 0.00%Impervious Runoff Depth>0.37" Tc=10.6 min CN=61 Runoff=0.21 cfs 0.017 af Subcatchment3:D.A.2 Offsite Runoff Area=0.510 ac 64.31%Impervious Runoff Depth>1.59" Tc=10.5 min CN=85 Runoff=1.23 cfs 0.068 af Subcatchment 16S:X2A Runoff Area=1.550 ac 34.58%Impervious Runoff Depth>0.91" Tc=13.3 min CN=74 Runoff=1.86 cfs 0.118 of Subcatchment 17S:X2 Runoff Area=0.190 ac 81.05%Impervious Runoff Depth>2.08" Tc=5.0 min CN=91 Runoff=0.70 cfs 0.033 af Subcatchment18S:X3 Runoff Area=1.390 ac 37.05%Impervious Runoff Depth>0.96" Tc=10.9 min CN=75 Runoff=1 95 cfs 0.112 of Link 4:POA Inflow=5 56 cfs 0.347 af Primary=5 56 cfs 0.347 af Total Runoff Area=4.208 ac Runoff Volume=0.347 af Average Runoff Depth=0.99" 63.57%Pervious=2.675 ac 36.43%Impervious=1.533 ac Pre-Development Hil!brook Pre-Development ASBUILT REV Type 1124-hr 2yr Rainfall=3.65" Prepared by Shimp Engineering Printed 11/7/2019 HydroCAD®9.10 s/n 07054 ©2010 HydroCAD Software Solutions LLC Page 5 Time span=0.00-24.00 hrs, dt=0.01 hrs, 2401 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method Subcatchment2:D.A.2 Onsite Runoff Area=0.568 ac 0.00%Impervious Runoff Depth>0.64" Tc=10.6 min CN=61 Runoff=0.46 cfs 0.030 af Subcatchment 3:D.A.2 Offsite Runoff Area=0.510 ac 64.31%Impervious Runoff Depth>2.14" Tc=10.5 min CN=85 Runoff=1.65 cfs 0.091 af Subcatchment 16S:X2A Runoff Area=1.550 ac 34.58%Impervious Runoff Depth>1.34" Tc=13.3 min CN=74 Runoff=2.81 cfs 0.173 af Subcatchment 17S:X2 Runoff Area=0.190 ac 81.05%Impervious Runoff Depth>2.68" Tc=5.0 min CN=91 Runoff=0.89 cfs 0.042 af Subcatchment 18S:X3 Runoff Area=1.390 ac 37.05%Impervious Runoff Depth>1.40" Tc=10.9 min CN=75 Runoff=2.90 cfs 0.163 af Link 4:POA Inflow=8.22 cfs 0.499 af Pnmary=8 22 cfs 0.499 af Total Runoff Area=4.208 ac Runoff Volume=0.499 af Average Runoff Depth=1.42" 63.57%Pervious=2.675 ac 36.43%Impervious=1.533 ac Pre-Development Hil!brook Pre-Development ASBUILT REV Type II 24-hr 10yr Rainfall=5.51" Prepared by Shimp Engineering Printed 11I7I2019 HydroCAD®9.10 sin 07054 ©2010 HydroCAD Software Solutions LLC Page 8 Time span=0.00-24.00 hrs, dt=0.01 hrs, 2401 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method Subcatchment2:D.A.2 Onsite Runoff Area=0.568 ac 0.00%Impervious Runoff Depth>1.68" Tc=10.6 min CN=61 Runoff=1.40 cfs 0.079 of Subcatchment3:D.A.2 Offsite Runoff Area=0.510 ac 64.31%Impervious Runoff Depth>3.83" Tc=10.5 min CN=85 Runoff=2.88 cfs 0.163 of Subcatchment 16S:X2A Runoff Area=1.550 ac 34.58%Impervious Runoff Depth>2.77" Tc=13.3 min CN=74 Runoff=5.91 cfs 0.357 of Subcatchment 17S:X2 Runoff Area=0.190 ac 81.05%Impervious Runoff Depth>4.47" Tc=5.0 min CN=91 Runoff=1.44 cfs 0.071 of Subcatchment 18S:X3 Runoff Area=1.390 ac 37.05%Impervious Runoff Depth>2.86" Tc=10.9 min CN=75 Runoff=5.95 cfs 0.331 of Link 4: POA Inflow=16.83 cfs 1.002 of Primary=16.83 cfs 1.002 of Total Runoff Area=4.208 ac Runoff Volume=1.002 of Average Runoff Depth=2.86" 63.57%Pervious=2.675 ac 36.43%Impervious=1.533 ac Hillbrook Subdivision Post-Development HydroCAD Report o <16S> 0 X2A X2 $ ) X30 Onsite Undetalned D.A.B(Ne Sheetflow) 0 Or' nal Sheetflow Area 9 D 10 Offsite Undetained OA 11 '4) e a— _©--0 a Flow at Open Space Detention Vault Detention Offsite Not Detained(1) (A) Aiff" len l'oji4-4 Onsite Not Detained(1) Onsite Detained(1) DA A (subc 1 peach on• MU Drainage Diagram for Hillbrook Post-Development Amendment As-Built 8-15-19(4500 used)(3in outlet)10.31-19 !l Prepared by Shimp Engineering, P.C., Printed 11/7/2019 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Post-Dev As-Built Hillbrook Post-Development Amendment As-Built 8-15-19 ( Type II 24-hr 1yr Rainfall=3.01" Prepared by Shimp Engineering, P.C. Printed 11/7/2019 HydroCAD®9.10 sin 07054 ©2011 HydroCAD Software Solutions LLC Page 2 Time span=0.00-24.00 hrs, dt=0.01 hrs, 2401 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method Subcatchment 1: D.A. B (New Sheetflow) Runoff Area=0.240 ac 0.00% Impervious Runoff Depth>0.91" Tc=11.6 min CN=74 Runoff=0.31 cfs 0.018 af Subcatchment 2: D.A.A Runoff Area=0.990 ac 0.00% Impervious Runoff Depth>1.07" Tc=10.0 min CN=77 Runoff=1.63 cfs 0.089 af Pond 3: Vault Peak Elev=496.64' Storage=9 cf Inflow=1.63 cfs 0.089 af Primary=1.63 cfs 0.089 af Secondary=0.00 cfs 0.000 af Outflow=1.63 cfs 0.089 af Pond 4: Detention Peak EIev=491.40' Storage=1,692 cf Inflow=1.63 cfs 0.089 af Outflow=0.13 cfs 0.086 af Subcatchment 5: Onsite Not Detained (1) Runoff Area=0.150 ac 0.00% Impervious Runoff Depth>0.37" Tc=13.5 min CN=61 Runoff=0.05 cfs 0.005 af Subcatchment 6: Offsite Not Detained (1) Runoff Area=0.160 ac 87.50% Impervious Runoff Depth>2.26" Tc=7.5 min CN=93 Runoff=0.58 cfs 0.030 of Pond 7: Detention Peak Elev=490.57' Storage=48 cf Inflow=0.22 cfs 0.014 af Outflow=0.16 cfs 0.014 af Subcatchment 8: Onsite Undetained Runoff Area=0.169 ac 31.95% Impervious Runoff Depth>0.86" Tc=7.0 min CN=73 Runoff=0.25 cfs 0.012 af Subcatchment 9: Offsite Undetained Runoff Area=0.328 ac 63.41% Impervious Runoff Depth>1.52" Tc=6.1 min CN=84 Runoff=0.89 cfs 0.042 of Link 10: POA Inflow=5.71 cfs 0.359 af Primary=5.71 cfs 0.359 af Link 11: Flow at Open Space Inflow=0.53 cfs 0.121 af Primary=0.53 cfs 0.121 af Subcatchment 12S: Original Sheetflow Area Runoff Area=0.800 ac 0.00% Impervious Runoff Depth>0.25" Tc=11.6 min CN=57 Runoff=0.13 cfs 0.016 of Subcatchment 14S: Onsite Detained (1) Runoff Area=0.190 ac 31.58% Impervious Runoff Depth>0.86" Tc=12.0 min CN=73 Runoff=0.22 cfs 0.014 af Subcatchment 15S: X2A Runoff Area=1.550 ac 34.58% Impervious Runoff Depth>0.91" Tc=13.3 min CN=74 Runoff=1.86 cfs 0.118 af Subcatchment 16S: X2 Runoff Area=0.190 ac 81.05% Impervious Runoff Depth>2.08" Tc=5.0 min CN=91 Runoff=0.70 cfs 0.033 of Subcatchment 17S: X3 Runoff Area=1.330 ac 38.72% Impervious Runoff Depth>0.96" Tc=10.9 min CN=75 Runoff=1.87 cfs 0.107 af Post-Dev As-Built Hil!brook Post-Development Amendment As-Built 8-15-19 ( Type II 24-hr 1 yr Rainfall=3.01" Prepared by Shimp Engineering, P.C. Printed 11/7/2019 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Page 3 Total Runoff Area = 6.097 ac Runoff Volume= 0.483 of Average Runoff Depth =0.95" 72.66% Pervious=4.430 ac 27.34%Impervious= 1.667 ac Post-Dev As-Built Hillbrook Post-Development Amendment As-Built 8-15-19 ( Type II 24-hr 2yr Rainfall=3.65" Prepared by Shimp Engineering, P.C. Printed 11/7/2019 HydroCAD®9.10 sin 07054 ©2011 HydroCAD Software Solutions LLC Page 20 Time span=0.00-24.00 hrs, dt=0.01 hrs, 2401 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method Subcatchment 1: D.A. B (New Sheetflow) Runoff Area=0.240 ac 0.00% Impervious Runoff Depth>1.34" Tc=11 6 min CN=74 Runoff=0.46 cfs 0.027 af Subcatchment 2: D.A.A Runoff Area=0.990 ac 0.00% Impervious Runoff Depth>1.54" Tc=10.0 min CN=77 Runoff=2.35 cfs 0.127 af Pond 3: Vault Peak EIev=497.08' Storage=16 cf Inflow=2.35 cfs 0.127 af Primary=2.35 cfs 0.127 af Secondary=0.00 cfs 0.000 of Outflow=2.35 cfs 0.127 of Pond 4: Detention Peak Elev=492.42' Storage=2,673 cf Inflow=2.35 cfs 0.127 af Outflow=0.14 cfs 0.122 af Subcatchment 5: Onsite Not Detained (1) Runoff Area=0.150 ac 0.00% Impervious Runoff Depth>0.64" Tc=13.5 min CN=61 Runoff=0.11 cfs 0.008 of Subcatchment 6: Offsite Not Detained (1) Runoff Area=0.160 ac 87.50% Impervious Runoff Depth>2.88" Tc=7 5 min CN=93 Runoff=0.73 cfs 0.038 of Pond 7: Detention Peak Elev=490.80' Storage=111 cf Inflow=0.34 cfs 0.020 af Outflow=0.20 cfs 0.020 af Subcatchment 8: Onsite Undetained Runoff Area=0.169 ac 31.95% Impervious Runoff Depth>1.28" Tc=7.0 min CN=73 Runoff=0.37 cfs 0.018 of Subcatchment 9: Offsite Undetained Runoff Area=0.328 ac 63.41% Impervious Runoff Depth>2.06" Tc=6.1 min CN=84 Runoff=1.19 cfs 0.056 of Link 10: POA Inflow=8.24 cfs 0.512 of Primary=8.24 cfs 0.512 of Link 11: Flow at Open Space Inflow=0.96 cfs 0.181 of Primary=0.96 cfs 0.181 of Subcatchment 12S: Original Sheetflow Area Runoff Area=0.800 ac 0.00% Impervious Runoff Depth>0.47" Tc=11.6 min CN=57 Runoff=0.38 cfs 0.031 of Subcatchment 14S: Onsite Detained (1) Runoff Area=0.190 ac 31.58% Impervious Runoff Depth>1.28" Tc=12.0 min CN=73 Runoff=0.34 cfs 0.020 of 1 Subcatchment 15S: X2A Runoff Area=1.550 ac 34.58% Impervious Runoff Depth>1.34" Tc=13.3 min CN=74 Runoff=2.81 cfs 0.173 of Subcatchment 16S: X2 Runoff Area=0.190 ac 81.05% Impervious Runoff Depth>2.68" Tc=5.0 min CN=91 Runoff=0.89 cfs 0.042 of Subcatchment 17S: X3 Runoff Area=1.330 ac 38.72% Impervious Runoff Depth>1.40" Tc=10.9 min CN=75 Runoff=2.78 cfs 0.156 of Post-Dev As-Built Hillbrook Post-Development Amendment As-Built 8-15-19 ( Type 1124-hr 2yr Rainfall=3.65" Prepared by Shimp Engineering, P.C. Printed 11/7/2019 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Page 21 Total Runoff Area = 6.097 ac Runoff Volume= 0.697 of Average Runoff Depth = 1.37" 72.66% Pervious =4.430 ac 27.34%Impervious = 1.667 ac i Post-Dev As-Built Hillbrook Post-Development Amendment As-Built 8-15-19 Type II 24-hr 10yr Rainfall=5.51" Prepared by Shimp Engineering, P.C. Printed 11/7/2019 HydroCAD®9.10 sin 07054 ©2011 HydroCAD Software Solutions LLC Page 38 Time span=0.00-24.00 hrs, dt=0.01 hrs, 2401 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method Subcatchment 1: D.A. B (New Sheetflow) Runoff Area=0.240 ac 0 00% Impervious Runoff Depth>2.77" Tc=11.6 min CN=74 Runoff=0.97 cfs 0.055 af Subcatchment 2: D.A.A Runoff Area=0.990 ac 0.00% Impervious Runoff Depth>3.05" Tc=10.0 min CN=77 Runoff=4.65 cfs 0.251 af Pond 3: Vault Peak Elev=497.93' Storage=29 cf Inflow=4.65 cfs 0.251 af Primary=3.24 cfs 0.238 of Secondary=1.41 cfs 0.014 af Outflow=4.65 cfs 0.251 af Pond 4: Detention Peak EIev=495.01' Storage=4,792 cf Inflow=3.24 cfs 0.238 af Outflow=0.71 cfs 0.191 of Subcatchment 5: Onsite Not Detained (1) Runoff Area=0.150 ac 0.00% Impervious Runoff Depth>1.68" Tc=13.5 min CN=61 Runoff=0 33 cfs 0.021 of Subcatchment 6: Offsite Not Detained (1) Runoff Area=0.160 ac 87.50% Impervious Runoff Depth>4.69" Tc=7.5 min CN=93 Runoff=1.15 cfs 0.063 of Pond 7: Detention Peak EIev=492.21' Storage=692 cf Inflow=2 13 cfs 0.056 af Outflow=1.56 cfs 0.056 af Subcatchment 8: Onsite Undetained Runoff Area=0.169 ac 31.95% Impervious Runoff Depth>2.68" Tc=7.0 min CN=73 Runoff=0.78 cfs 0.038 of Subcatchment 9: Offsite Undetained Runoff Area=0 328 ac 63.41% Impervious Runoff Depth>3.74" Tc=6.1 min CN=84 Runoff-2 11 cfs 0.102 af Link 10: POA Inflow=16.39 cfs 1.025 af Primary=16.39 cfs 1.025 of Link 11: Flow at Open Space Inflow=2.62 cfs 0.338 af Primary=2.62 cfs 0.338 af Subcatchment 12S: Original Sheetflow Area Runoff Area=0.800 ac 0.00% Impervious Runoff Depth>1.38" Tc=11.6 min CN=57 Runoff=1.50 cfs 0.092 of Subcatchment 14S: Onsite Detained (1) Runoff Area=0.190 ac 31.58% Impervious Runoff Depth>2.68" Tc=12.0 min CN=73 Runoff=0.73 cfs 0.042 of Subcatchment 15S: X2A Runoff Area=1.550 ac 34.58% Impervious Runoff Depth>2.77" Tc=13 3 min CN=74 Runoff=5.91 cfs 0.357 af Subcatchment 16S: X2 Runoff Area=0.190 ac 81.05% Impervious Runoff Depth>4.47" Tc=5.0 min CN=91 Runoff=1.44 cfs 0.071 af Subcatchment 17S: X3 Runoff Area=1.330 ac 38.72% Impervious Runoff Depth>2.86" Tc=10.9 min CN=75 Runoff=5.70 cfs 0.317 af 1