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Home My WebLink AboutSW6220802_Response To Comments_20230217 (2)x0�� 0 ,. , �Qpf ESSjO•.�i .�. ECLS. A `Veteran -Owned' Con any." Q 9' GLOBAL Inc. - 14580 — 19 North Mckinley Street Coats, N.C. 27521 Champs Convenience Store (Stormwater # SW6220802) '' J Summary Letter of Responses to NCDEMLR Review Comments dated 11/1/2022 This letter is to serve as a summary letter of our review responses to the above referenced review comments. We are submitting 1) Two sets of sheets that were revised with revision date of 11-10-2022. 2) This summary letter with revised calculations. 3) Revised sheets form the application as per your request. 4) Revised sheets from the FZ supplement as per your request. 5) Revised calculations using Hydro-cadd software to show routing of stormwater through the SCM and the areas that will bypass the SCM pre -development And post -development Conditions). Our responses are as follows: First we address the pretreatment requirement for the SCM by providing a 2 foot deep sump in the two inlet structures at the SCM ( inlet structure # 26 and # 30). Having met the requirement for providing pretreament for the SCM all of the Stormtech chambers should be able to be used for volume storage and infiltration area. Next we determine the maximum design volume for the SCM ( Stormtech infiltration chambers) as follows: Using equation #2 from the MDC C-1 Infiltration system guidance document we the maximum design volume using a revised and enlarged SCM surface area of 4,577 SF. We also are using a factor of safety of 2, a K value of 0.6 in/hr, and a Time of 72 hours. Solving for maximum volume using these values we get a volume of 8,238.6 CF. We however are using a smaller design volume based on using the volume of a 1 inch runoff from the drainage area of the SCM as follows: DV= 2 Acres X ( 43,560 SF/AC) X ( 1"/ 12 inches per foot) = 7,260 CF. This smaller volume DV will result in a larger factor of safety of 2.27 as follows: FS = 4577SF ( 0.6 in/hr)( 72 hours) / 7,260 CF ( 12) = 2.27, 910-897-3257 OFFICE WWW.ECLSGLOBAL.COM 910-897-2329 FAX ECLS. A `Veteran —Owned' Company GLOBAL Inc. The design volume of 7,260 CF equates to an elevation of 330.37 within our revised and enlarged stormtech chamber system ( see Stormtech MC-700 cumulative storage volume chart on sheet 11 of 16 of project plans). We have shown a 2 inch diameter outlet orifice with invert equal to elevation 330.37 to isolate the design volume below this elevation of 330.37 feet. We have shown a 4 foot wide outlet weir at elevation 334.00 in order to maximize the total storage volume and to minimize the post development runoff volume after routing through the underground storage. We have analyzed the 10 year pre -development runoff for the entire site versus the 10 year post- development runoff from the combination of the post-dev. Areas that bypass the SCM plus the past-dev. of the drainage area that is routed through the SCM as follows. 10 year- predevelopment runoff ( entire site) = 0.515 Acre feet. 10 year- post development runoff ( areas that bypass the SCM) = 0.226 Acre feet. 10 year- post development runoff ( area that is routed through the SCM) = 0.254 Acre feet Note: Total 10 year post-- development runoff ( 0.226AF +0.254 AF) equals 0.480 Acre feet. Since 0.480 Acre feet is less than the 0.515 Acre feet, the post development runoff volume will be less than the pre -development runoff volume for the 10 year storm event. Also the post development Q10 peak flow rate leaving the entire site can be seen to be 3.81 cfs ( 1.55cfs routed through SCM plus 2.26 cfs bypassing SCM). The pre - development Q10 peak flow rate leaving the entire site can be seen to be 5.14 cfs. Since 3.81 cfs is less than 5.14 cfs the post development Q10 flow rate is less than the pre - development Q10 peak flow rate. We have included 1 hard copy of the Hydro-cadd calculations which verify these quantities. 910-897-3257 OFFICE WWW.ECLSGLOBAL.COM 910-897-2329 FAX ELLS. GLOBAL Inc. A `Veteran —Owned' Company We are also including 2 sets of hard copy revised plans which have a latest revision date of 11-10-2022 which reflect the responses to the review comments and the enlarged underground storage system. We are also including 1 hard copy of sheets from the application and the EZ supplement showing the revised values as per the review comments requests. We have addressed each of the review items as follows: 1) We have increased the area and volume of the underground storage chambers. We have also increased the factor of safety to 2.27 for the design volume to be infiltrated as per the calculations at the beginning of this letter. We have provided hydro-cadd calculations to show the post -development runoff volume for the entire site is less than the pre -development runoff volume for the entire site for the 10 year storm event. 2) We have accounted for the post -development condition of the off -site area by assuming that this area will be 100% impervious ( worst case condition). We are unable to divert this off -site flow due to the fact that we are using this area as a septic drain field area ( which can not be disturbed). 3) We are specifying that a 2 foot sump be installed at both of the inlet structures into the SCM ( structures # 26 and #30) to serve as pretreatment for the SCM. 4) We have provided for the design volume to be above the bottom of the storage chambers system ( elevation 328.00). We have provided a 2 inch diameter orifice with invert of 330.37 which provides a design volume of 7,260 CF ( based on 1 inch runoff from the 2 acre drainage area flowing to the SCM. This provides a factor of safety of 2.27 ( greater than 2.0 minimum) due to a larger footprint area under a larger storage volume of chambers ( 4,577CF). We also are using the design K-factor of 0.6 in/hr and a maximum drawdown time of 72 hours. We used equation 2 from MDCS to determine these values. 5) We have used the drainage area flowing to the SCM ( 2 acres) to determine the design volume of 7,260CF. 910-897-3257 OFFICE WWW.ECLSGLOBAL.COM 910-897-2329 FAX 5CLSA `Veteran —Owned' Company GLOBAL Inc. --------�^mm-_ 6) We have submitted this project to the Harnett County Development Review Board and their requirement is only that we provide them a copy of the permit obtained from NCDEMLR ( which we will do upon receiving the permit). 7a) We have addressed this item ( IV-10) as follows: We are providing a revised copy of Section IV-10 of the stormwater application with the values filled in that relate to the area flowing to the SCM only. 7b) We have noted in item IV-11 as follows: Offsite drainage area was determined from the GIS topo. map. We have assumed this post --development condition is 100% impervious ( see attached GIS topo. map). 8ai) We have shown a value of 50 feet for this item ( we have also answered "yes" for line #8 on cover sheet for EZ supplement document). 8bi) We have shown the value of 7,260 CF for this item on Drainage Area sheet of the EZ supplement document ( see calculations at first of this letter). Note: We are providing a completely revised supplement coversheet, drainage area sheet, and infiltration system sheet due to the fact that several items have changed due to revisions. 8ci) We have shown 7,260 CF for this item ( using only the area flowing to the SCM and 1 inch runoff). 8cii) We have answered "yes" to this item. As discussed previously all the areas flowing to the SCM are designed at 100% buildout conditions, however there are some areas of the entire site that bypass the SCM due to site grades. The areas of the entire site that bypass the SCM have an impervious percentage of 15.92% ( 0.25 Acres paved parking, HSG B and 1.32 Acres grass,75% grass cover HSG B — 1.57 Acres total). 8ciii) We have answered "yes" to this question. 910-897-3257 OFFICE WWW.ECLSGLOBAL.COM 910-897-2329 FAX ECLS. GLOBAL Inc. A 'Veteran -Owned' Company 8civ) We have answered "other" to this item ( we have specified a 2 foot deep sump in the bottom of structures #26 and #30 which are inlet structures into the SCM). 8cv) We have answered 2.27 for this item. See calculations at the beginning of this letter). 8cvi) We have answered 330.37 for lines #30 and #31. This is the elevation of the invert of the 2 inch diameter orifice through the weir wall within the outlet structure. All water below 330.37 will be infiltrated and all water above 330.37 will be routed through the SCM. 8cvii) We have answered 4,577 CF for items # 32 and #33. This is due to the addition of storage chambers and increasing the storage foot print area. We have either answered items # 34 through # 43 or we have written NA and given an explanation why the question does not apply to our design. 8cviii) We have answered 7,260CF for this item. Line #44 is now same as line #20 on drainage area sheet EZ supplement). Note: We have revised the answer in item #46 to note that 2 each 2 foot deep sumps have now been designated as pretreatment for the SCM. This concludes our summary of responses to the review comments dated 11/1/2022. 910-897-3257 OFFICE WWW.ECLSGLOBAL.COM 910-897-2329 FAX NOT FOR LEGAL USE . Gi