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Home My WebLink About10/28/2016 - Eggleston Environmental - - 1250 OSGOOD STREET uuuuuu Vumum"'It u uumm IlVuluul ��n uuuuuu Ipuum I VUV,uu o IlVuull puu°lu m Iluuum Vuuuo "'It u uuuuu glululu ll� g gi �I""IlllllluomuuuIIII "IIIIIIIIIuI �u dull °°°u*wavrrrrraaumulllp111111A111))�� October 28, 2016 North Andover Planning Board 1600 Osgood Street North Andover, MA 01845 Attn: Rebecca Oldham, Staff Planner RE: Stormwater Review Princeton at North Andover, 1210 Osgood Street Dear Ms. Oldham and Board Members: In follow-up to my September 28, 2016 review comments on the above-referenced project and our September 30, 2016 meeting with the Applicant's engineer, I have received and reviewed the October 20, 2016 response letter, revised Stormwater Report and Site Plans (34 Sheets) submitted by Hancock Associates. As discussed at our meeting, Hancock has completed additional soil testing to verify that the soils on the site are not consistent with the NRCS mapped soils and are HSG B sandy loams. The runoff calculations have been revised accordingly and some of the subsurface detention and infiltration facilities have been enlarged to provide the necessary flow mitigation. The design revisions also include some rerouting of the roof runoff to the proposed recharge structures, and the addition of proprietary media filters in lieu of the previously proposed hydrodynamic separators to treat the runoff from the commercial areas of the site. A number of my previous comments have been satisfactorily addressed by this revised submittal; however, I do still have several concerns with the design as currently proposed. My specific comments are outlined below: 1. In response to my September 28th Comment #3 the overflow outlet from infiltration system 21 B has been redirected to discharge toward Wetland Series 3, providing some improvement to the balancing of flows between wetlands. Based on the summary provided on Page 6 of the revised Stormwater Report though, there would still be an increase in the total volume of runoff discharged to analysis point C, and a corresponding decrease in the discharge to analysis point B and the offsite 3-series wetland. It is unclear whether the decrease in surface flow would impact the hydrologic regime of this apparently isolated wetland or whether it would be mitigated by an increase in recharge, but this is something that the Conservation Commission may want to consider in its review of the project. Hancock has indicated that the offsite drainage from analysis point C is free-flowing, in which case it should not cause flooding. �� II��I II�� IIIII�. I��„ IIIIY"°°' I III �„ ��I��I�I �'� I IP' �� , ��,��I� �w � � u III III VII p� �� II °�I � ilVi Ilpl�� IIII II III IIIII (III' III IIII iiir IIII iiii : iiii�� IIII iiiir IIII IIII IIII Ills II I PV t i.i .II l,1 s IIIIIIIII .t.-( 1 dl!1 Ili �r 1QIItV .l.�" 1, 1 ,I 2. In the response to my previous Comment #3 Hancock requests that the Board waive the requirement for additional survey to provide offsite topography within 50 feet of the property boundary. I want to clarify that the required information should be available from the Town's GIS system and should not entail any additional survey work. It may, however, provide additional clarification as to the path of runoff leaving the site and any hydraulic connections between the wetlands. 3. The locations of the soil tests conducted in October 2016 are shown on the Stormwater Plan, but it is difficult to see the existing topography at some of the test locations. I'd like to request that the additional soils tests also be shown on the Existing Conditions Plan. 4. Sheet 27 of the 10/20/16 plan set lists the ESHGW elevation at System #10 as about 142 based on Test Pits SW-2 and T-X-1, and the bottom of the system 2.25 feet above that assumed ESHGW elevation at 144.25. Per my previous comments, the log for Test Pit SW-2, located in the middle of the proposed infiltration system toward the northern end, indicates redox at 36 inches below grade. Based on the Existing Conditions plan the existing grade at SW-2 is about el. 147 putting the ESHGW elevation beneath the system at 144, two feet higher than the elevation used in the current design. System #10 still does not meet the separation requirements. 5. Additional documentation is needed to demonstrate that the Baysaver filters are a suitable means of treating the runoff from a land use with higher potential pollutant loading of oil and grease. Baysaver's design manual says that the filters are designed to remove fine sediments, heavy metals, and phosphorus from stormwater runoff but says nothing about removing oil & grease. 6. With the proposed location of the Baysaver filters on the downgradient side of the ADS closed pipe detention systems It is likely that some sediment is going to settle out within the detention system before it flows through the filter. This is fine as a pretreatment measure, but there is currently no way to clean out the ADS systems and maintain the system capacity. 7. The proposed plans call for the outlets from the ADS systems to the filters to be about 1.5 feet above the bottom of the system, presumably because they need a minimum 1.5 driving head to function properly. However, the inverts of the "bypass" outlets are all lower than that, ranging from 0 to 0.9 ft above the bottom invert of the ADS systems. As a result the entire water quality volume, and in some cases the entire 2-year event, would bypass the filters completely and not be treated. 8. In closed detention systems #11 and #3 6 both outlets would be above the bottom of the ADS pipes, thus some flow would be trapped with no way to drain out between storms. c. .III ,��I'.. "s �I��'II�I �h, jl il ii�II',yV r � 1II�/��r it li I�" �lii��M III III.11 m�,��"�I�� ,..� ��W��.IIIIIIIIIIII I.�.� � III�..� .�.��mi � ..�. IIII c,, I ... ail uN �I�.1..6 9. The inverts of infiltration systems #10 and #3 5 did not get revised in the model, although these systems were raised a foot or more on the revised plans. The outlet inverts did get revised, which means that the model is overestimating the storage that would occur prior to discharge. 10. System #21 is modeled as having a 24-in outlet orifice at 135.5; the plan lists the invert as 135.3. 11. The manifold invert at IS-21 is more than a foot higher than the overflow outlet at DMH-23, hence excess flow to the Isolator Row would back up and bypass the system altogether instead of being distributed to the remaining infiltration chambers. 12. Infiltration system #21 B is identified on the plan as having an Isolator Row, but no Isolator Row is shown and one should not be necessary since only roof drainage would be discharged to the system. The invert of the header row should therefore be commensurate with the inlet inverts and below the low flow orifice on the outlet structure to better distribute flow within the system. 13. The invert given for the roof drain outlet on Building E is below the bottom of Infiltration System #31. 1 also recommend connecting the roof drainage to the infiltration system directly, rather than routing it through the Isolator Row. 14. The Outlet Structure details on Sheets 22 and 23 should be revised to show the 15-in manifold as an inlet to the OCS on the other side of the weir. 15. The plans are missing an inlet structure detail for the MC-4500/SC-310 systems, and inlet and outlet structure details for the ADS closed systems. 16. Sheet 18 calls for a HDS-3 (Model 1200) treatment unit on the drainage line to closed system #3 8. I believe this is an oversight since a different form of treatment is now being proposed. 17. The Stormtech detail on Sheet 28 should be identified as #3 9, not#3 9B. 18. Several of the proposed ADS closed systems would be at or below seasonal high groundwater and may need additional cover or alternative measures to prevent buoyancy. 19. The estimated annual O&M costs do not appear to have been revised to reflect the substitution of the BayFilter systems for the hydrodynamic separators previously proposed. It should be confirmed that the current estimate is still valid, and that it includes periodic replacement of the filter cartridges. c .III (" � 0- ..JI11I II',yli K 01II1/r ` it ��I illi''Q lm��M III III 16 20. The O&M Plan calls for replacing the BayFilter cartridges at or above the level of the 4-in collector pipes to the manifold, design components that are not included in the proposed design. Assuming the design is revised to ensure that the entire water quality volume is captured and directed through the filters, the threshold for cartridge replacement should be when that volume is not draining down within 24-48 hours. An estimated replacement frequency based on design assumptions should also be provided. 21. Per my previous comments on the O&M Plan: ■ The Long Term Pollution Prevention Plan should address deicing and snow management. The snow storage locations on the revised Layout and Materials Plan do not appear to be adequate for the site. ■ If the commercial and residential portions of the site are to have different owners separate plans may be required. ■ The deep sump catchbasins should be cleaned a minimum of once per year, not just when sediment thresholds are reached. ■ Subsurface storage/infiltration systems should be inspected at least once a year following a storm of one inch or more to ensure that they are fully drained within 72 hours. If they are not, corrective action must be taken. ■ The plan should also include a maintenance checklist and a simple figure showing the locations of all stormwater BMPs to be maintained as well as designated snow storage locations. Once, I appreciate the opportunity to assist the North Andover Planning Board and Conservation Commission with the review of this project, and hope that this information is suitable for your needs. Please feel free to contact me if you or the applicants have any questions regarding the issues addressed herein. Sincerely, EGGLESTON ENVIRONMENTAL e � .�� Lisa D. Eggleston, P.E. C: Jennifer Hughes, Conservation Coordinator