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Home My WebLink AboutSW3220302_Response To Comments_20220601Comment Response Letter for: Denver Defense Lincoln County NC Prepared by: HensonFoley Landscape Architecture I Civil Engineering HensonFoley 121 Gilead Road Huntersville, NC 28078 704-875-1615 p kennedy@hensonfoley.com May 24t" 2022 Please correct the following: - General MDC 4 — Please provide calculations showing that the outlet of the SCM (existing outfall to remain) will be stable during the 10-year design storm. Response: Calc provided showing min. apron size. Note added to plan to ensure existing is compliant - Off -site component of the drainage area to the Sand Filter — Per 15A NCAC 02H .1003(3)(b), any off -site areas that are not bypassed around the SCM are required to be accounted for at their full build -out capacity. Since the off -site portions of the drainage area are not owned by the permittee, the full build out potential of the off -site area must either be limited by a legal agreement between the property owners (limiting the amount of BUA and drainage area that the off -site property owner can direct toward the permittee's SCM) or by assuming that the off - site area is 100% BUA (it's full build -out potential). It is recommended to bypass these off -site areas around SCMs (as to avoid having to deal with stormwater runoff that the permittee has no control over), but since this is such a small amount of off -site drainage area, it might be easier to just account for it at 100% BUA (The SCM size/design should not need to be modified to account for this extra area). Response: Offsite treated as 100% BUA for simplification. - Application, Section IV, 10: o On -site Drainage Area — Per the information provided in other parts of the submission, the on -site portion of the drainage area to the SCM is 117,333 sf. Response: Drainage area number updated. Drainage area changed slightly. o Off -site Drainage Area — Per the information provided in other parts of the submission, the off -site portion of the drainage area to the SCM is 2,269 sf. Response: Drainage area number updated. Drainage area changed slightly. o % Impervious Area —The percent impervious area for the drainage area to the SCM would be calculated as the total BUA within the drainage area divided by the total drainage area. Response: Updated. o On -site Buildings/Lots —This item is equal to the amount of new BUA added within the drainage area to the SCM that corresponds to a building or roof area. It should match the "Roof" item under Line 12 of the Drainage Area 1 Column on the Drainage Areas Page of the Supplement-EZ Form. Response: Updated. o On -site Parking —This item is equal to the amount of new BUA added within the drainage area to the SCM that corresponds to parking area. It should match the "Parking" item under Line 12 of the Drainage Area 1 Column on the Drainage Areas Page of the Supplement-EZ Form. Response: Updated. o On -site Sidewalks —This item is equal to the amount of new BUA added within the drainage area to the SCM that corresponds to sidewalk area. It should match the "Sidewalk" item under Line 12 of the Drainage Area 1 Column on the Drainage Areas Page of the Supplement-EZ Form. Response: Updated. o Other On -site —This item is equal to the amount of new BUA added within the drainage area to the SCM that does not correspond to one of the other listed BUA types. It should match the "Other" item under Line 12 of the Drainage Area 1 Column on the Drainage Areas Page of the Supplement-EZ Form. Response: Updated. o Future —This item is equal to the amount of new BUA that will be added at some future time within the drainage area to the SCM. It should match the "Future" item under Line 12 of the Drainage Area 1 Column on the Drainage Areas Page of the Supplement-EZ Form. Response: Updated. o Off -site —This item is equal to the amount of BUA that is located within the off -site portion of the drainage area to the SCM. It should match the "Offsite BUA" listed on Line 11 of the Drainage Area 1 Column on the Drainage Areas Page of the Supplement-EZ Form. Response: Updated. o Existing BUA —This item is equal to the amount of existing BUA that will remain within the drainage area to the SCM. It should match the "Existing BUA that will Remain" listed on Line 15 of the Drainage Area 1 Column on the Drainage Areas Page of the Supplement-EZ Form. Response: Updated. o Total - This item is equal to the total amount of BUA that will exist within the drainage area to the SCM once all construction is complete (final condition). It should match the "Total BUA" listed on Line 8 of the Drainage Area 1 Column on the Drainage Areas Page of the Supplement-EZ Form. Response: Updated. - Supplement-EZ Form: o Cover Page: ■ Line 7 — Please include the required vegetated setback width from on -site surface waters as required per 15A NCAC 02H .1017(10). It is noted that there are no surface waters located on -site, however, this value should still be reported. Response: Updated. o Drainage Areas Page: ■ General: • Line 16 — Since this site does not have an existing post -construction stormwater permit, there should not be any "already permitted" BUA listed (The total amount of existing BUA can be inferred by adding together Lines 15 & 17). Response: Updated. ■ Entire Site Column: • Lines 5 & 6 — For a project like this the "drainage area" for the entire site is equal to the project area. As such, these values would be 195,759 sf. • Line 7 — Similar to the above comment, for a project like this, there is no off -site component to the "drainage area" (project area). Response: Updated. ■ Drainage Area 1 Column: • Please see the earlier comments for a description of the items of Lines 8,11,12,&15 Response: Updated. • Line 10 —This item is the sum of all of the BUA listed under Line 12 for the Drainage Area 1 Column. Response: Updated. • Line 17 —This item is the amount of existing BUA that will be removed from the drainage area to the SCM. NOTE: You may wish to make a table similar to the one provided on plan sheet EX01, but instead of the entire site, the bounding area would be the (this is not required). If you are having trouble with theses BUA tables, let me know, I can help you out some. Response: Updated. o Sand Filter Page: ■ Line 3 —This item askes if the SCM is sized to treat all surfaces draining to it at build -out (corresponds to General MDC 1). It appears as though this SCM is adequately sized for its drainage area. Response: Updated. Response: Updated. Response: Updated. ■ Line 18 — Please update this item per the provided SHWT report. ■ Lines 23 & 30 —These items should show the volume that can be stored in the sediment chamber and sand chamber respectively at the temporary pool elevation (843.5'). The sum of these items should be equal to the value shown on the stage -storage table at this elevation and the volumes of the sediment chamber and sand chamber should be approximately the same (Sand Filter MDC 2). ■ Line 25 — Please indicate this value on the plans. Response: shown on plans Response: Updated. ■ Lines 27 & 28 — For this configuration, the depth of the sediment chamber is the distance from the temporary pool elevation (843.5') to the bottom of the sediment chamber (Line 25). ■ Line 29 —The plans indicate that this value 843.5. Please revise either as needed. Response: Updated. ■ Lines 33 & 34 — For this configuration, the depth of the sand chamber is the distance from the temporary pool elevation (843.5') to the top of the sand layer (bottom of the sand chamber) (Line 31). Response: Updated. ■ Line 35 — Per the plans, the lowest bypass elevation for the sand filter is the 6" orifice with an invert of 843.5. Please revise. Response: Updated. ■ Line 36 — As noted above, the lowest bypass device is an orifice. Response: Updated. Line 39 —This item refers to the depth of the sand layer (not the entire sand and gravel layer) above the underdrain. Per the plans, this value is 18". Response: Updated. NOTE: You are only required to treat the net increase in BUA for this project. If you like, you can cache the 17,360 sf of overtreated BUA (53,453 sf required to be treated, 70,813 sf actually treated) as "future" BUA that can be used for future improvements to the site. If you are doing this, please indicate the amount of future BUA in Section IV, 10 of the Application and the Supplement-EZ Form. NOTE: if the off -site portion of the drainage area is assumed to be 100% BUA, it does not count toward the overtreated BUA amount. Response: We decided to take advantage of this. 17,360 sf of future bua shown on application and supplement — ez form.