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Home My WebLink AboutWPO201700029 Correspondence 2020-03-06 SHIMP ENGINEERING, P.C. Design Focused Engineering 7Y\41\ March 6th, 2020 Frank Pohl, PE, CFM - County Engineer Albemarle County Department of Community Development 401 McIntire Road Charlottesville, VA 22902 Regarding: Hillbrook Subdivision—VSMP Exception Request&As-Built Rev. 2 Transmittal Dear Mr. Pohl, Please find attached one (1) copy of the revised VSMP Exception Request &As-Built package for the Hillbrook Subdivision. Attachments: • Comment Response Letter • VSMP Exception Request Letter • SWM As-Built Calculation Packet • As-Built VSMP Plan If you have any questions please email me at justin@skimp-engineering.com or you may call our office at 434- 227-5140. Best Regards, Michael Chandler ct: i+' ,.;,_. Shimp Engineering, P.C. pule is $A, SHIMP ENGINEERING, P.C. Design Focused Engineering March 6th, 2020 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. 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 excessive rock cut required field adjustments to the entrance that 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. The discovery of excessive rock in the original location of the underground detention system drove further field adjustments, including the relocation of the underground detention system to within the Hillbrook Ct. Right-of-way. • Our revised calculations demonstrate the following pre and post development flows from the site at the point at which the man-made system discharges to the natural channel. Further, we have proposed reducing the asbuilt 5" 1-yr orifice located in Str 3 to 2". The calculations show this will provide a reduction of the 2-yr flow from predevelopment. Both the As-Built and As-Built Amended results are shown below. Q (1 year) Q (2 year) Q (10 year) Pre Dev 5.56 CFS 8.22 CFS 16.83 CFS As-Built 5.78 CFS 8.40 CFS 16.93 CFS As-Built Amended 5.67 CFS 8.18 CFS 17.66 CFS In Summary: • The exception is the minimum necessary as all other criteria of the regulations are met. • Detention as provided on the site maintains post development flows to nearly identical pre- development levels, preserving the intent of the regulations. • Flow will be further reduced by modifying the 1-yr orifice inside structure 3 from 5" to 2". • There are no privileges granted to the developer, upon detailed review it appears that meeting the criteria would likely not have been possible based on the unique factors of this development that were outside of the control of the developer. • Utility conflicts, and field adjustments made and approved by prior VDOT inspectors were outside of the control of the developer and were in no way self-imposed. The result of the calculations demonstrate that no adverse impacts are expected to any downstream waters or properties as a result of this exception. The 2-yr flowrate to the existing channel will be reduced and both the 1 and 10 year flows have negligible change. The total volume to the POA is increased by only 0.014 acre-feet from predevelopment conditions. It was noted before construction of the Hillbrook subdivision that the existing channel receiving the flows was already showing signs of erosion. The current channel condition does not appear to show signs that erosion has increased since that time. Considering the reduction of the 2-yr flows, there is no concern regarding excessive erosion of the channel. Considering the minor increase in the 10-yr flow and total volume of flow, there is no concern regarding flooding downstream or overflow of the channel. If you have any questions please feel free to contact me via e-mail at Justin@shimp-engineering.com or by telephone at 434-227-5140. 1' n Shimp, P.E. p imp Engineering, P.C. Page 12 SHIMP ENGINEERING, P.C. Design Focused Engineering March 6th, 2020 John Anderson Albemarle County Department of Community Development Regarding: Hillbrook Subdivision —WPO201500033 As-Built Comment Response Dear Mr. Anderson, Thank you for your review of the Hillbrook Subdivision VSMP As-Built Plan and Waiver Request. We have revised the As-Built Plan and SWM As-Built Analysis Packet per your most recent comments dated February 24th, 2020. Please find below a detailed response to each of your comments. 1. Excess rock: Applicant furnished general statements from 2016 correspondence (3/15/16 Don Franco letter) re. export, but not rock, specifically, in location interfering with installation of initially-approved detention along Old Brook Drive. a. (Also please recall reference to a 'VDOT directive'; no further mention of VDOT. Recommend we seek written verification of VDOT role, since Applicant raised topic of VDOT directive. If nothing existing in writing, recommend Applicant state this, in writing.) An invoice from Earthworks Excavating, Inc. has been provided showing the rental and operation of a rock hammer from 3/24/2016—3/29/2016. Written verification from the VDOT representative cannot be acquired due to the representative no longer working for VDOT and is not able to be reached. 2. Device parameters (slope, L: Asbuilt b. routings) in one or more instances: mismatch between As-built and routings. For example: pond 3 vault, 182.9' 15" pipe (Asbuilt slope 0.86%; routing: 0.75%; 15.7' 15" pipe (Asbuilt slope: 0.32%; routing: 0.48%) The mismatches between the routing calculations and the Asbuilt plans have been corrected. All other dimensions have been verified for accuracy between plans and routing. 3. 1/16/20 waiver request RV developed was 0.518, with Q1_yrmax/allowable = 2.99 cfs. 2/5/20 response to waiver request comments: RV dev is now 0.359 with no change to % imperviousness, or in Area of contributing DA (developed condition). a. Qiyrmax increases to 4.30 cfs. Recommend request explanation/justification about what happened with the math. The RV at the POA was obtained incorrectly in the 1/16/20 waiver request. The values were added up among the drainage areas, however due to the nature of the system at vault 3, drainage area 2 is split between the two detention systems. Therefore, the appropriate RV developed was taken directly from the POA (node 10). If you have any questions please email me at justin@shimp-engineering.com or you may call our office at 434- 227-5140. Best R gards, Michael Chandler Shimp Engineering, P.C. ti2Ear $works Excavating, Inc. INVOICE P.O. Box 7372 Charlottesville, VA 22906-7372 DATE INVOICE# 3/28/2016 9916 BILL TO Hillbrook Jess Achenbach-434-242-4802 Red Dirt Developments , PO Box 645 Charlottesville,VA 22902 T h R M S DUE DATE LOCATION Net 5 4/2/2016 HILLBROOK QUANTITY j DESCRIPTION RATE AMOUNT Cleanng 1,000.00 1,000 00 Erosion Control 1,500.00 1,500 00 • Excavation&Grading 8,000.00 8,000.00 Utilities-Water 10,000.00 10,,000.00 Utilities-Storm Drain 2,500.00 2,500 00 62 CHANGE ORDER-Load 62 loads of fill dirt for Shimp at S 15 pci 15 00 930.00 load CHANGE ORDER-Furnish and operate rental rock hammer @ 13,000.00 13,000.00 $325 per hour with 40 hour contract minimum-this includes work from 3/24/16-through 4:30pm on 3/29/16 **Contract rate was$350 per hour for a different machine than was available,discount reflects machine actually in use. CHANGE ORDER-Load and haul/dispose of 106 loads of dirt 8,283.00 8,283.00 Sales Tax 5.30% 0.00 Total S45.213.00 A 2%PER MONTH FINANCE CHARGE WILL BE ADDED TO ALL PAST DUE AMOUNTS.