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Home My WebLink AboutWPO201100071 Correspondence 2012-04-23 NP Engineering April 23, 2012 County of Albemarle Department of Community Development 401 McIntire Road Charlottesville, VA 22901 Re: ESC Plan comments Restore'n Station (WPO-2011-00071) Mr. Phil Custer: I am writing to address the Erosion and Sediment Control Plan comments provided in your letter dated March 2, 2012 for the Restore'n Station site plan. I have addressed each comment individually and listed the comment and intended response for clarity. 1. It appears that the ESC Plans were all printed at 30 scale, not 40. If this is correct, please replace all references to 40 scale with 30 scale. RESPONSE: The plans are 30 scale and all reference to 40 scale have been removed. 2. On all three phases, please clearly show and label the limits of disturbance boundary. RESPONSE: The disturbance boundary has been clearly identified. (Rev. 1)In phases 1 and 2, extend the limits of disturbance around the outlet protection below the sediment basin. In phase 2, show the limits into the VDOT ROW. In phase 3, show limits of disturbance. REV 1 RESPONSE: Items above have been addressed in this plan revision. 3. Please show all proposed grading on Phase 2 and Phase 3. It is difficult to evaluate the adequacy of these phases without proposed contours. Additional comments may be necessary because the plan was not able to be fully evaluated. RESPONSE: Phase 2 and 3 grading have been shown as well as the items to be completed in each phase. (Rev. 1)In phase 3,please show the proposed grade lines for the fill required to provide the proper cover over the detention facility and the final proposed grading in the area of the sediment basin after it is removed. An adequate channel will need to be provided in the area previous disturbed by the sediment basin. REV I RESPONSE: Items above have been addressed in this plan revision. 4. Related to the previous comment, the sediment basin and sediment trap should be sized for the largest watershed each facility will likely experience as the site is filled in. For instance, the site will likely be tipped towards the sediment basin, since the stormwater 680 IVY FARM DRIVE•CHARLOTTESVILLE,VIRGINIA 22901 •PHONE:(434)531-7387• Restore'n Station-WPO C.,...,nents-EC Plan (WPO-2011-00071) Page 2 April 23, 2012 facility is located in the vicinity, so its drainage area will increase as the earthwork progresses. RESPONSE: The sediment basin has been designed for a total of 4.68 acres (sum of the 3.83 acres in drainage area I and 0.85 acres in drainage area 2). Revised sediment basin calculation sheets are attached. (Rev. 1) The sediment trap drainage area is larger than the watershed shown in the plan. Free Town Lane does not divert water away from this trap as the map implies. Since the last submittal, structure 7B was added which indicates a larger watershed to the sediment basin than designed Please account for this acreage in the calculations. Also, when the design of the sediment basin was moded, an error was made with regard to the embankment. According to the calculations, the top of embankment must be 691, but the plan is graded to show an embankment top of 690. Please correct. An embankment width of at least 6ft is required REV 1 RESPONSE: Items above have been addressed in this plan revision. The silt trap near Free Town lane has been moved north. Structure 7B area has been added- recalculating the area, due to more defined drainage areas, has resulted in the same area to the sediment basin as 4.68 acres. The drainage map has been modified to show this. In addition, the sediment basin was modified to show a top of embankment elevation of 681, and a top width of 6'. 5. Silt fence is frequently proposed perpendicular to contour lines in this plan which is not allowed per state guidelines. In these instances,water will not travel through the fence but will be redirected downhill, like a diversion. Please replace the silt fence on the east side of the property line with a diversion from the northeast corner to the sediment trap. On the west end of the property, please simply remove the silt fence. If a visual boundary is desired by the applicant, I recommend a type of safety fence. RESPONSE: Revisions to the silt fence have been made as noted above. (Rev. 1) Comment has been addressed, though the first step in the construction sequence was not modified accordingly. It seems step 3 can refer to silt fence. Similarly, the construction sequence still refers to raintanks, which have been eliminated with this application. The contech structures should be installed after the site has been stabilized, in phase 3. REV 1 RESPONSE: Items above have been addressed in this plan revision. Raintank references have been removed. Contech structures (Cistern and System 2) will be installed in Phase 3. 6. Please remove the silt fence from directly below the outlet of both settling facilities. Silt fence cannot receive concentrated flow. RESPONSE: Silt fence has been removed from the plan as noted above. 7. During the review of the preliminary site plan, the Chief of Current Development, Bill Fritz, approved the disturbance of a 20ft wide section of the undisturbed buffer to the south. Please show the area of the buffer that was approved for disturbance. It appears that the area shown on this submittal does match the area approved by Bill. If the area proposed for disturbance is, in fact, different, another waiver of this disturbance may need to be processed. RESPONSE: The undisturbed buffer is shown per prior approval. No revisions are necessary. (Rev. 1)In this ESC application, the center of the 2011 of allowable disturbed buffer is 99ft from the 680 IVY FARM DRIVE•CHARLOTrESVILLE,VIRGINIA 22901 •PHONE:(434)531-7387• Restore'n Station-WPO C.,....nents-EC Plan (WPO-2011-00071) Page 3 April 23, 2012 property corner. In the approved preliminary plan, the center of the 20ft buffer is 74 5ft from the property corner. The disturbed area of this buffer was moved However, Bill Fritz has determined that this change does not negatively affect the previous approval and the revised location is acceptable 8. The embankment of the sediment basin ties into existing grade within the undisturbed buffer (see where the 676 contour line would be). Please modify the plan to make sure no disturbance to the undisturbed buffer occurs, other than the area previously authorized. RESPONSE: The sediment basin has been shifted north to prevent disturbance to the buffer area. 9. In plan view, please draw the sediment basin bottom as 671. RESPONSE: Sediment basin bottom has been noted as 671 on the plan as noted above. (Rev. 1) Comment has not been addressed. Please draw the 671 contour line in the sediment basin and modem the bottom dimensions to meet this contour line. Update the sediment basin bottom dimension reference in the embankment detail accordingly. REV 1 RESPONSE: Items above have been addressed in this plan revision. The bottom elevation is 671. The 671 contour line is shown as well as bottom dimensions. 10. Please show Permanent Seeding (PS) symbols throughout the plan where necessary. RESPONSE: Permanent seeding symbols have been added to the plan as noted above. 1 1. Please include the county's paved wash rack detail in this sheet set. This detail can be found in the county's design manual on page 28 of 35. RESPONSE: Paved wash rack detail has been added to the plan as noted above. 12. The county has updated its standard ESC Note set. Please use the latest version found in the county's design manual. RESPONSE: The ESC note set has been updated on the plan as noted above. 13. An analysis of the downstream channel per Minimum Standard 19 was not provided with the ESC plan. Please refer to page 7 (of 35) of the County's Design Manual and the Virginia Erosion and Sediment Control Handbook for what the county expects for an analysis of downstream channels. Per state law, this analysis must be performed using the 24-hour storm. RESPONSE: The calculations for the MS-19 are included in the SWM plan and calculations. (Rev. 1)Comment has not been addressed. A downstream channel analysis is required in any situation where the velocity,peak rate, or volume of runoff is increased in a development. Normal detention does not eliminate the requirement for the applicant to check downstream channels because volume is increased unless a significant amount of infiltration is provided. If an analysis cannot be performed, the applicant must proceed as if an inadequate channel is present and provide over-detention meeting State Code Section 10.1-561. REV 1 RESPONSE: This item is addressed through the calculations of the SWM plan. It is the intent to "over-detain" to meet the requirements. 14. Please provide the cross-sections and calculations (2-year velocity and 10-year capacity) for the new VDOT ditch. Please specify any liner, if necessary. RESPONSE: North arrows have been added to the site plan. 680 IVY FARM DRIVE•CHARLOTTESVILLE,VIRGINIA 22901 •PHONE:(434)531-7387• Restore'n Station-WPO C _ ments-EC Plan (WPO-201 1-00071) Page 4 April 23, 2012 (Rev. 1)Comment has not been addressed. The response to this comment in the applicant's letter refers to the placement of north arrows on the plan. REV 1 RESPONSE: Items above have been addressed in this plan revision. Ditch calculations are included in the plan. 15. To receive a bond estimate, please provide a completed Bond Estimate Request Form to the County Engineer after all comments have been addressed. All owners of properties disturbed with this application must sign this document and be party to the bond unless all offsite easements are recorded. The easements must be written in a manner that allows the county and its assigns to the construction site. RESPONSE: The bond estimate request is attached as noted above. (Rev. 1) WPO bond amounts will be calculated after plan approval. REV 1 RESPONSE: Ok. I trust that the above comments are adequately addressed for final site plan and WPO approval. Should you have questions regarding the revisions, please call me at (434) 531-7387. Sincerely, at erkins, P. . 680 IVY FARM DRIVE•CHARLO'iTESVILLE,VIRGINIA 22901 •PHONE:(434)531-7387• Pipes.txt BasinFlow printout INPUT: Basin: 13 Contour Areas Elevation(ft) Area(sf) Computed Vol . (cy) 671.60 1513.60 0.0 672.10 942.00 22.5 672.60 743.00 38.1 673.10 619.00 50.7 673.60 540.00 61.4 674.10 495.00 71.0 674.60 481.00 80.0 675.10 495.00 89.1 675.60 540.00 98.6 676.10 619.00 109.4 676.60 743.00 122.0 677.10 942.00 137.5 677.60 1513.60 160.1 5 Storage Pipes Storage pipe 0 name: one diameter (in) 72.000 length (ft) 84.000 invert (ft) 671.600 angle 0.000 volume (cy) 87.965 Storage pipe 1 name: two diameter (in) 72.000 length (ft) 84.000 invert (ft) 671.600 angle 0.000 volume (cy) 87.965 Storage pipe 2 name: three diameter (in) 72.000 length (ft) 84.000 invert (ft) 671.600 angle 0.000 volume (cy) 87.965 Storage pipe 3 name: four diameter (in) 72.000 length (ft) 84.000 invert (ft) 671.600 angle 0.000 volume (cy) 87.965 Storage pipe 4 name: five diameter (in) 72.000 length (ft) 84.000 invert (ft) 671.600 angle 0.000 volume (cy) 87.965 Page 1 Pipes.txt Start_Elevation(ft) 671.60 Vol . (cy) 0.00 4 Outlet Structures Outlet structure 0 Orifice name: lower area (sf) 0.136 diameter or depth (in) 5.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 671.600 multiple 1 discharge into riser outlet structure 1 Orifice name: upper area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 674.000 multiple 1 discharge into riser Outlet structure 2 Culvert name: name multiple 1 discharge out of riser D (in) 15.000 h (in) 0.000 Length (ft) 160.000 Slope 0.050 Manning's n 0.025 Inlet coeff. Ke 0. 500 Equation constant set 0 Invert (ft) 671.600 Outlet structure 3 weir name: name length (ft) 1.500 side angle 0.000 coefficient 3.000 invert (ft) 676.600 multiple 1 discharge into riser 4 Inflow Hydrographs Hydro9raph 0 Modified_Rational name: post dev captured C 0.850 Area (acres) 1.750 IDF file: a2 time of concentration (min) 6.00 receding factor 1.670 time increment 3.00 time limit (min) 200.00 fudge factor 1.00 storm: crit. duration (79 iterations) volume (cy) 306.70 Page 2 Pipes.txt peak flow (cfs) 2.223 intensity (in/hr) 1.494 time to peak (min) 6.000 duration of peak (min) 54.08 routed true Hydrograph 1 Modified_Rational name: post dev captured C 0.850 Area (acres) 1.750 IDF file: a10 time of concentration (min) 6.00 receding factor 1.670 time increment 5.00 time limit (min) 200.00 fudge factor 1.00 storm: crit. duration (250 iterations) volume (cy) 339.52 peak flow (cfs) 4.767 intensity (in/hr) 3.205 time to peak (min) 6.000 duration of peak (min) 24.04 routed true Hydrograph 2 Modified_Rational name: post dev captured C 0.850 Area (acres) 1.750 IDF file: a100 time of concentration (min) 6.00 receding factor 1.670 time increment 5.00 time limit (min) 200.00 fudge factor 1.00 storm: crit. duration (125 iterations) volume (cy) 427.86 peak flow (cfs) 7.118 intensity (in/hr) 4.785 time to peak (min) 6.000 duration of peak (min) 19.04 routed true Hydrograph 3 SCs name: name Area (acres) 2.000 CN 93.000 Type 2 rainfall , P (in) 5.600 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) 12.151 peak time (hrs) 11.917 volume (cy) 1286.465 OUTPUT: Routing Method: storage-indication Page 3 Pipes.txt Hydrograph 0 Routing summary of Peaks: post dev captured inflow (cfs) 2.224 at 6.00 (min) discharge (cfs) 0.924 at 66.00 (min) water level (ft) 673.790 at 66.00 (min) storage (cy) 210.391 Hydrograph 1 Routing summary of Peaks: post dev captured inflow (cfs) 4.771 at 10.00 (min) discharge (cfs) 1.27 at 35.00 (min) water level (ft) 674.295 at 35.00 (min) storage (cy) 266.071 Hydrograph 2 Routing summary of Peaks: post dev captured inflow (cfs) 7.123 at 10.00 (min) discharge (cfs) 1.950 at 30.00 (min) water level (ft) 674.945 at 30.00 (min) storage (cy) 338.298 Hydrograph 3 Routing summary of Peaks: name inflow (cfs) 12.094 at 11.92 (hrs) discharge (cfs) 2.697 at 12.12 (hrs) water level (ft) 676.203 at 12.12 (hrs) storage (cy) 473.932 Fri Mar 02 15:41:53 EST 2012 Page 4 raintankl.txt BasinFlow printout INPUT: Basin: 4 Contour Areas Elevation(ft) Area(sf) Computed vol . (cy) 671.60 3974.00 0.0 672.00 3974.00 58.9 674.00 3974.00 353.2 675.80 3974.00 618.2 Start_Elevation(ft) 671.60 Vol . (cy) 0.00 2 Outlet Structures Outlet structure 0 Culvert name: name multiple 1 discharge through dam D (in) 6.000 h (in) 0.000 Length (ft) 50.000 Slope 0.010 Manning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 0 Invert (ft) 672.000 Outlet structure 1 Culvert name: name multiple 1 discharge through dam D (in) 6.000 h (in) 0.000 Length (ft) 50.000 Slope 0.010 Manning's n 0.013 Inlet coeff. Ke 0. 500 Equation constant set 0 Invert (ft) 675.300 3 Inflow Hydrographs Hydro9raph 0 Modif-ied_Rational name: post dev captured C 0.900 Area (acres) 1.490 IDF file: a2 time of concentration (min) 6.00 receding factor 1.670 time increment 1.00 time limit (min) 200.00 fudge factor 1.00 storm: specified duration volume (cy) 298.78 peak flow (cfs) 1.620 intensity (in/hr) 1.208 time to peak (min) 6.000 duration of peak (min) 75.00 routed true Page 1 raintankl.txt Hydrograph 1 Modified_Rational name: post dev captured C 0.900 Area (acres) 1.490 IDF file: a10 time of concentration (min) 6.00 receding factor 1.670 time increment 1.00 time limit (min) 200.00 fudge factor 1.00 storm: specified duration volume (cy) 478.46 peak flow (cfs) 1.905 intensity (in/hr) 1.421 time to peak (min) 6.000 duration of peak (min) 105.00 routed true Hydrograph 2 Modified_Rational name: post dev captured C 0.900 Area (acres) 1.490 IDF file: a100 time of concentration (min) 6.00 receding factor 1.670 time increment 1.00 time limit (min) 300.00 fudge factor 1.00 storm: specified duration volume (cy) 981.02 peak flow (cfs) 2.287 intensity (in/hr) 1.706 time to peak (min) 6.000 duration of peak (min) 185.00 routed true OUTPUT: Routing Method: storage-indication Hydrograph 0 Routing Summary of Peaks: post dev captured inflow (cfs) 1.620 at 6.00 (min) discharge (cfs) 0.749rat 85.00 (min) water level (ft) 673.110 at 86.00 (min) storage (cy) 222.244 Hydrograph 1 Routing Summary of Peaks: post dev captured inflow (cfs) 1.905 at 6.00 (min) discharge (cfs) 0.977 at 116.00 (min) water level (ft) 673.833 at 116.00 (min) storage (cy) 328.667 Hydrograph 2 Routing Summary of Peaks: post dev captured inflow (cfs) 2.287 at 6.00 (min) discharge (cfs) 1.516 at 194.00 (min) water level (ft) 675.462 at 194.00 (min) storage (cy) 568.450 Page 2 raintankl.txt Mon Nov 07 17:36:01 EST 2011 Page 3 z R I _ _ . 071. 6 L 170;-JArec. L 3 W k g6 l 371"( yea. I ,5 2,515.52 )7pq' 6,y R y 2_ `17 2. 6 1.0 2.0 `t.47 I L-6 67,6 77 3 973.1 1 .6 1 ,6 5,11 110 '56 ' I q't-f `ic ( 1q 972),6 G2D I 5.7 16-.7 gb ' ( i)50 710 L114.1 25 . 3 5.v /1-0-1 �6 I 0 -7760y LOC 0 6 0.0 'goti t(rb l