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Home My WebLink About20151229 Ver 2_Stormwater Info_20170922'PTI PIEDMONT TRIAD AIRPORT AUTHORITY September 19, 2017 Ms. Sue Homewood Department of Environmental Quality Division of Water Resources, Winston Salem Regional Office 450 W. Hanes Mill Rd, Suite 300 Winston Salem, NC 27105 Subject: HAECO Parking Lot Addition and Apron Redevelopment at PTIA 401 Water Quality Certification DWR#15-1229, USACE Action ID. No. SAW -2016-00920 Dear Ms. Homewood: This letter is to confirm update of the subject Certification recently discussed with Priyanka Prakash of BRPH regarding an increase of impervious area, proposed as part of the HAECO Facility Improvement project at PTIA. This update includes the addition of a 100 -space employee parking area, as well as a potential future apron redevelopment and building expansion of the adjacent Hangar 5. See the attached depictions of the proposed revisions. The proposed bio -retention area was approved to accommodate 53.57 acres of impervious area and this capacity is listed in the conditions of the certification approval. The following table reflects the permitted impervious areas, and the proposed update, including all previous modifications to date: Location Impervious Cover (acres) Status Proposed Hangar 5.06 No change. Proposed Apron 5.11 Small area added, see update below. Proposed Fire Access Roads 0.32 No change. Proposed Sidewalk 0.09 To be removed, see update below. Existing HAECO Site to the East 11.92 No change. Sub -Basin 60 0.42 No change. Sub -Basin 70 9.22 No change. Sub -Basin 80 14.09 No change. Sub -Basin 90 7.34 No change. ERMITTED TOTAL 53.57 Areas included in original permit. Sidewalk (above) removed -0.09 Sidewalk removed, replaced with grass. Additional apron infill 1 0.13 IConcrete apron area added, new impervious. MODIFICATION 1 TOTAL (6/22/17) 53.61 Results in an increase of BUA less than 0.1% New Parking Lot 0.90 Replaces pervious area with new employee parking. Apron Redevelopment/Building Expansion 0.25 Project largely consists of existing impervious apron to be replaced with in-kind surfacing, but includes 0.25 ac of current grass area to be replaced with new concrete apron. MODIFICATION 2 TOTAL (8/15/17) 54.76 Increase in BUA determined to be within allowable impervious to bio -retention area. 1000ATed Johnson Parkway • Greensboro, North Carolina 27409 • Phone: 336.665.5600 • Fax: 336,665,1425 SPT/ PIEDMONT TRIAD AIRPORT AUTHORITY The revised impervious area to be directed to the bio -retention area is 54.67 acres, which reflects an increase of approximately 2.3% beyond the original total treatment capacity of the measure. The original design for the bio - retention area included treatment of 22.51 acres of new impervious area requiring treatment, as well as an additional 31.06 acres of existing impervious area that did not require treatment. Furthermore, the NCDEQ DEMLR Stormwater Permitting section has determined that the bio -retention area calculations were based on conservative design assumptions, and that this increase is within the capacity of the current bio -retention area per current NCDEQ design standards (see attached email from Mike Randall). Therefore, the existing bio -retention area has been deemed adequate for the proposed improvements. If you need any other information to process this request, please contact Priyanka Prakash at 321-751-3095. Sincerely, PIEDMONT TRIAD AIRPORT AUTHORITY DJ.lex Rosser, P.E. ky�, 2,A., Deputy Executive Director Enclosures: • Design Drawings, August 15th, 2017, "HAECO — Parking Lot". • Email from NCDEQ Stormwater (Mike Randall), dated June 19th, 2017, "RE: PTIA/HAECO Hangar Stormwater Modification". • Exhibit, "HAECO Americas —Parking Lot and Apron Redevelopment Projects, Impervious Impacts". • Excerpts from "401 Water Quality Certification Report", rev. March 2016. cc: David Bailey, USACE Priyanka Prakesh, BRPH Richard Darling, MICHAEL BAKER RE: PTIA / HAECO Hangar Stormwater Modification Randall, Mike <mike.randall@ncdenr.gov> Mon 6/19/2017 7:48 PM To:Priyanka Prakash <pprakash@brph.com>; Riddle, Rick L <rick.riddle@ncdenr.gov>; cc:Homewood, Sue <sue.homewood@ncdenr.gov>; Priya ka, I did some quick calculations. In January 2017, NC revised our rules. The original design is based on a more conservative approach than currently allowed by NC stormwater rules. The new NC Stormwater Rules, based on research at NC State, are less conservative. The approved design had a safety factor built in and was based on some assumptions, rather than actual data. As it turns out, adding 1.25 acres of BUA to the 53 acre of BUA, approximately .023 added BUA, will not compromise the current approved design. The design as previously approved is adequate for the proposed added BUA. I prefer the stormwater runoff from the added 1.25 acres of BUA be directed into the proposed bio -cell rather than bypassing the bio cell as we discussed earlier. Mike Randall Stormwater Permitting Program NC Division of Energy, Mineral and Land Resources (DEMLR) Land Quality Section Stormwater Permitting 1612 Mail Service Center - ------------------------------------------------------------- Raleigh, NC 27699-1612 Office: 919-807-6374 Cell: 919-389-7801 Email correspondence to and from this address may be subject to public records laws ■ L3 Blue Ridge Mountains Greens 'bo'r8_�E' O Americas" r North Carclifla Charlotte a Harker. -island South Carolirtal fp i666�77f' � 4 - Y E *> �Y I 7 �h r. a q ORA, No HAEGO AMERICAS - PARKING LUT AN RFDFVF OPMFNT PROJFC.TS- IMPFR J APRON J10US IMPA ori► 401 WATER QUALITY CERTIFICATION REPORT HAECO FACILITY IMPROVEMENTS PROJECT Submitted on Behalf of.. Piedmont -Triad International Airport Prepared by: WK Dickson & Co., Inc.g ` A. ��, ,�o 1� 720 Corporate Center Drive . P°<•','°QUQ°��;' Raleigh, North Carolina 27607 r � • 4 November 2015 K Revised March 2016 �._ _ _•° LIL u``s Modifications for HAECO Parking Lot Addition and Apron Redevelopment, 8/15/2017 1-14 Section 1: Introduction 1.1 Project Description This report supports the design of the stormwater control measures (SCMs) needed to develop the HAECO Facility Improvements project at the Piedmont Triad International Airport in compliance with the North Carolina Department of Environmental Quality (NCDEQ) regulatory requirements for new development at an airport. A 0.8 -acre high flow rate bioretention pond is being proposed to meet the regulatory water quality requirements for NCDEQ. This bioretention pond was designed to infiltrate runoff generated from the 1St inch of rainfall at a relatively high rate to satisfy the water quality requirements outlined in Session Law 2012-200. As shown in the concept plans included in Appendix A, the airport is proposing a 15.9 -acre site development project including the construction of the following: ♦ 5.06 acres of new impervious area associated with the proposed HAECO hangar; ♦ 5.51 acres of new impervious area associated with the proposed HAECO apron; ♦ 0.32 acres of new impervious area associated with the proposed HAECO fire lanes flanking the proposed hangar; ♦ 0.09 acres of new impervious area associated with the proposed HAECO sidewalks; ♦ Removal of an existing fire suppression pond; ♦ Removal of an existing 1.1 -acre wet pond being used for detention and water quality; and ♦ Construction of a new 0.8 -acre high flow rate bioretention pond that will result in infiltration of the water quality rainfall event. In addition to providing treatment for the proposed new impervious areas associated with the HAECO Facility Improvements project, the SCM will replace the treatment being provided by an existing wet pond located on the eastern side of the site. This existing wet pond has a contributing drainage area of 15.29 acres with 13.95 acres of impervious cover. In total, the proposed SCM will need to provide treatment for 22.51 acres of impervious cover as shown in the following table: Table 1: Summary of Area Required for Treatment Location Impervious Cover (acres) Proposed Hangar 5.06 Proposed Apron 5.11 Proposed Access Route 0.32 Removed 6122117 Existing HAECO Site (Wet Pond) 11.92 TOTAL = 22.51 acres Apron Infill - 6122117 0.13 ac New Parking Lot - 8115117 0.90 ac Apron RedevelopmentlBuilding Expansion - 8115117 0.25 ac REVISED TOTAL (8115117) = 23.70 ac HAECO Facility Improvements Project Page 1-1 Stormwater Report Section 1: Introduction As outlined in this report, the proposed SCM will provide water quality treatment for a total of 53.57 acres of impervious cover which exceeds the minimum required for treatment (22.5 acres). As a result, the airport is formally requesting water quality treatment credits offset a site development project in the future with up to 19.91 acres of impervious surfa The following table summarizes the water quality treatment credits being requested:23.70 ac 8/15/2017 Table 2: Summary of Water Quality Treatment Credits Description Impervious Area (acres) Required Area for Treatment 22.51— - - - - - - - - - Provided Area for Treatment 53.57 DIFFERENCE = 31.06 acres Also provided in this report is an evaluation of downstream flooding resulting from the proposed site changes. The analysis showed that the proposed project will cause increases to peak flows downstream but will not flood insurable structures, roads, or cause damage to existing property or the existing Harris Teeter detention pond. HAECO Facility Improvements Project Page 1-2 Stormwater Report 23.70 ac 811512017 29.87 ac 811512017 Section 3: Water Quality Compliance 3.1 Overview To satisfy the water quality requirements outlined in Session Law 2012-200, a proposed 0.8 -acre high flow rate bioretention pond is being proposed. Session Law 2012-200 requires runoff generated from the 1St inch of rainfall for a development project shall be infiltrated into the ground. There are no specific requirements to remove total suspended solids (TSS), nitrogen, or phosphorus. In addition, there are no requirements to detain the 1 -year or any other storm event to at or below pre -project conditions. As shown in this report, the proposed high flow rate bioretention pond exceeds the minimum infiltration requirements set forth in Session Law 2012-200. 3.1.1 Proposed Impervious Areas The separately attached construction plans and concept plan provided in Appendix A show the proposed pond, new and existing impervious areas, location of flow splitters and overall site layout. The following table summarizes the proposed impervious areas associated with the HAECO Facility Improvements project: Table 11: Minimum Area of Impervious Cover Required for Treatment Location Impervious Cover (acres) Proposed Hangar 5.06 Proposed Apron 5.11 Proposed Fire Access Roads 0.32 rroposeZIMMMIM 6.09 Existing HAECO Site to the East 11.92 Apron Infill - 6122117 New Parking Lot - 8115117 Apron Redevelopment/Building Expansion - 8115117 0.13 ac 0.90 ac 0.25 ac Sub -Basin 60 REVISED TOTAL (8115117) = 23.70 ac Because the existing fire suppression pond is being abandoned as part of this project, the proposed SCM will need to be designed to accept runoff from the system currently going to the existing fire suppression pond. The stormwater runoff generated in sub - basins 60, 70 and 80 will be redirected into the proposed SCM. Appendix C highlights the areas that will drain to the pond along with a breakdown for the impervious area contributed from each sub -basin. As a result, an additional 31.06 acres of impervious area will be infiltrated in the proposed SCM as shown in the following table: Table 12: Proposed Impervious Cover to SCM Location Impervious Cover (acres) Pro osed Hangar 5.06 Proposed Apron 5.11 Proposed Fire Access Roads 0.32 Proposed Side Walk 0.09 +0.13 +0.9( Existing HAECO Site to the East 11.92 +0.2,1 Sub -Basin 60 0.42 Sub -Basin 70 9.22 Sub -Basin 80 14.09 Sub -Basin 90 7.34 TOTAL = 53.57 acres HAECO Facility Improvements Project Page 3-1 Stormwater Report Removed 6122117 ac (apron infill) ) ac (new parking) i (new apron) 54.76 ac 8115/2017 Section 3: Water Quality Compliance In total, the proposed SCM will have a contributing drainage area of 64.67 acres with 53.57 acres of impervious cover. 3.1.2 High Flow Rate Bioretention Pond Design Criteria The State BMP Manual does not specifically have a set of design guidelines for a high flow bioretention pond so the following guidelines were used in the design of the proposed high flow bioretention pond: ♦ Infiltrate 100% of the runoff generated from the 1St inch of rainfall; ♦ Side slopes shall be no steeper than 3(H):1(V); ♦ SCM shall be located in a recorded drainage easement; ♦ A bypass or internal overflow is required for bypassing storm flows in excess of the design flow; ♦ Media permeability shall be between 6 and 10 inches per hour with a targeted detention time of 10 to 15 hours for infiltrating the water quality volume; ♦ Ponding depth for the water quality event shall be limited to 4.0 feet; ♦ Media depth will be 2 feet for each of the two soil media zones of the bioretention pond; ♦ An underdrain shall be located under the soil media to keep the pond dry and prevent groundwater from entering the pond; and ♦ A rip -rap energy dissipater shall be located at the outfall of each pipe entering the pond. 3.1.3 Water Quality Volume (WQV) The volume of runoff generated from the 1St inch of rainfall was calculated using an in- house spreadsheet based on the Schuler Simple Method. This spreadsheet shows the calculated water quality volume along with proposed SCMs stage -storage sizing (see Appendix H). The following table summarizes the minimum required volume along with the provided volume: 23.70 ac Table 13: Calculated Storage Volumes 811512017 Description Impervious Ar (acres) Surface Runoff (ft3) Required Area for Treatment 22.51 81,703 Compensatory Treatment of Sub -basins 60, 70, 80 & 90 31.06 105,057 Total Provided Area for Treatment 53.57 1861761 Net Credit for WQ Treatment 31.06 105,057 29.87 ac As shown in Table 13, the proposed high flow rate bioretention pond will infiltrate an 8/15/2017 additional 105,057 cubic feet of runoff and 31.06 acres of impervious cover more than required. HAECO Facility Improvements Project Page 3-2 Stormwater Report Project: HAECO Facility Improvement @ PTIA, Greensboro, NC Prepared by: DJK Date: March 9, 2016 L 1.61 (Apron Infill) 6/22/17 Appendix B SWMM Input Data 241.31 EXISTING CONDITIONS SUBBASINS SWMM Sub- Basin ID Pervious RCN Area (acres) Area (sq. ft.) Flow Length ft Width (ft.) Elevation Change (ft.) Basin Slope (%) Percent Impervious 10 74 77.1 3356329 3294 1019 21 0.6% 31% 20 74 28.8 1253061 1259 995 11 0.8% 34% 30 74 19.3 842697 1856 454 19 1.0% 40% 40 74 4.1 180429 1027 176 12 1.1% 44% 50 74 3.6 156421 520 301 10 1.9% 21% 60 74 5.2 225908 841 269 14 1.7% 8% 70 74 9.2 401743 1215 331 16 1.3% 100% 80 71 11.2 489458 1095 447 21 1.9% 100% 85 71 6.7 293360 338 868 36 10.7% 24% 90 74 6.5 282480 1359 208 27 2.0% 88% 100 74 19.3 840092 1465 573 9 0.6% 43% 110 74 6.1 263574 496 531 7 1.3% 29% 120 74 16.3 711138 958 742 25 2.6% 92% 130 74 4.5 195877 944 207 35 1 3.7% 1 44% 142 74 9.4 407511 707 576 35 5.0% 26% 145 1 74 12.2 529689 944 561 35 3.7% 2% 150 74 1.9 81743 850 96 5 0.6% 75% 241.31 241.31 70131 (Apron Infill) 6/22/17 t No change (Parking included in original modeling) 8/15/17 45 0 (Apron Infill) 8/15/17 PROPOSED CONDITIONS SUBBASINS SWMM Sub- Basin ID Pervious RCN Area (acres) Area (sq. ft.) Flow Length ft Width (ft.) Elevation Change (ft.) Basin Slope (%) Percent Impervious N 10 74 77.05 3356329 3294 1019 21 0.6% 31% 20 74 28.77 1253061 1259 995 11 0.8% 34% 30 74 19.35 842697 1856 454 19 1.0% 40% 40 74 4.14 180429 1027 176 12 1.1% 44% 50 74 3.59 156421 520 301 10 1.9% 21% 60 74 5.19 225908 841 269 14 1.7% 8% 70 74 9.22 401743 1215 331 16 1.3% 100% 80 71 10.89 474558 1095 433 21 1.9% 100% 90 74 6.5 282480 1359 208 27 2.0% 88% 100 74 19.29 840092 1465 573 9 0.6% 43% 110 74 6.05 263574 496 531 7 1.3% 29% 120A 74 15.00 653246 1487 439 25 1.6% -810. 120B 74 2.43 105911 640 165 21 3.3% 9 ou 130 74 4.50 195877 944 207 35 3.7% 470 142 74 9.36 407511 707 576 35 5.0% 32% 144-A 74 0.84 38104 143 266 0.715 0.5% 100% 144-B 74 0.86 38293 127 302 0.635 0.5% 100% 144-C 74 0.75 32703 137 239 0.685 0.5% 100% 0.88 38306 152 252 0.76 0.5% 100% 144-E 74 1 97 175 126 0.875 0.5% 100% 144-F 7 0.47 20479 102 201 0.51 0.5% 100% 144-G 74 0.54 23709 108 220 0.54 0.5% 100% 4-H 74 :74 7,5931 237 320 1.185 0.5% Q% 146-A 74 4.56 198487 849 234 4.245 0.5% 100% 146-B 4 1.71 74691 406 184 2.03 0.5% 100% 146-C 74 1.53 66657 406 164 2.03 0.5% 100% 14,6-0'74 1.9 86824 450 193 2.25 0.5% 100% 146-E 74 .67 29025 106 274 0.53 0.5% 28% 148 74 1.1 47109 275 171 44 16.16% 10% 150 4 1 1.9 81743 850 96 5 0.59% 75% 241.31 70131 (Apron Infill) 6/22/17 t No change (Parking included in original modeling) 8/15/17 45 0 (Apron Infill) 8/15/17 �� Contributing Drainage Areas and Impervious Cover to SCM - Appendix C300 150 0 300 Feet W DICKSON Piedmont -Triad International Airport %--, community infrastructure consultants HAECO Site Development 1 inch = 300 feet �,l< Proposed Landuse Map - Appendix D � 500 250 0 500 Feet W DICKSON Piedmont -Triad International Airport W , community infrastructure consultants HAECO Site Development 1 inch = 500 feet