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HomeMy WebLinkAbout2005-11-15 Stormwater Review SPR Oct 11 05 01 : 32p Eggleston Environmental U'18'4q JZJ4t;P4 All EGGLEST01V EN VIRONMENTAL NGFJH ANDOVE-fk October 7, 2005 North Andover Conservation Commission 4U0()sgoou otleet North Andover, MA 01845 Attn: Alison McKay RE Stormwater Management Review Proposed Retail &Walgreen's Pharmacy Dear Alison and Commission Members: Per your request, I have conducted an initial technical review of the Notice of Intent (NOI) submittal package for the proposed retail and Walgreen's Pharmacy development on Route 114, with respect to stormwater management. The materials I have received and reviewed to date include the following: ■ Drainage Report,Proposed Retail & Walgreen's Pharmacy, Salem Turnpike (Rte. 114) at Winthrop Ave, North Andover MA prepared for Mark Investments by Bohler Engineering and dated August 4, 2005 with revisions through September 15, 2005. ■ Site Development Plans for WalgreenstRetail Store, Salem Turnpike and Waverly Road, North Andover MA, Sheets I through I I of 11, 1 of 1, Al.I and A1.2, prepared by Bohler Engineering, P.C., dated 8/4/05 and revised through 9/13/05. My primary focus in this initial review is on the overall stormwater management approach and design concepts used in the project, as well as its compliance with the stormwater management and flood storage standards (Section IV) of the North Andover Wetlands Bylaw and with the Massachusetts Wetlands Protection Act and Stormwater Management Policy. My comments are outlined below. onrall S:Lormwater Mgag&ement A roach d System Design The project site, located at the corner of Waverly Road and Route 117 in North Andover, is approximately 5 acres in size and is currently occupied by to single family homes, their associated driveways, outbuildings and lawns, and some wooded areas. The triangular shaped parcel slopes from east to west, with a total elevation drop of about 35 feet and an average slope of approximately 8 percent- Surface (and presumably r.ub- S),Irface) drainage from the site hence flows in a westerly direction toward Waverly Road. It appears that runoff from the northern portion of the site(3,65 acres) is conveyed to the municipal storm drain in Route '114, and the ru)-toff fl-on, the :;oudiera portion -.,)F llhc flows through the drain in Waverly Road to the wetland system located just South of the property- -55 OLD CoAcHROAD SriDBU,RyAM 01776 TgL1'p.4x978.443.9262 Oct 11 05 01 : 02p Egg t eston tnv x r•onmenza 1 r oYr��coc r-- 'N'-dareen"'. 'echilical C\JCA\ Se}Member T. 200 The proposed project is a retail development comprised of a Walgreen's pharmacy and a second retail building, two access driveways off of Waverly Road and one off Route 114, a 158-space parking lot, and associated landscaping and drainage structures. As proposed, the stormwater runoff from all of the impervious areas except the two access drives off Waverly Road would be conveyed through a closed drainage system to one of two stormwater detention basins located next to Waverly Road. The basins are intended to attenuate peak flow rates from the development, such that the peak runoff rates discharged from the site do not exceed pre-development rates. Water quality treatment would be provided through a combination of pavement sweeping, deep sump hooded catchbasins, and Vorteehnics treatment units before being discharged to the detention basins. The detention basins are also proposed to store and infiltrate the required recharge volume from the site. My comments on the overall stormwater management approach and drainage system design are outlined below: 1. The drainage system design provides no segregation of the relatively clean rooftop runoff from pavement runoff on the site. Combining the two and routing all the flow through the treatment structures reduces the overall effectiveness of the treatment provided and precludes the opportunity for direct recharge of roof runoff. 2. As is addressed iri further detail below, the project as proposed provides inadequate groundwater recharge and concentrates the recharge at the furthest downgradient portion of the property. 3. There is no indication of the seasonal high groundwater elevations on the site. The extensive subdrain system included in the design is presumably intended to intercept and direct groundwater flow, yet seems inconsistent with the functioning of the detention ponds for detention storage and groundwater recharge. My specific comments on the project as they relate to the project's compliance with the nine North Andover and State Stormwater Standards follow. Stand ard #1. Unt Bated to ter Standard #1 prohibits any new discharges of untreated stormwater to wetland resource areas of :he Town of North Andover, , hereby treated stormwater is defined to the stormwater that meets the requirements in Standards 2 through 9. As currently proposed, all of the runoff flows from the developed areas of the project site - with the exception of two �,ntrance drivcvvays will aild"Cgo trt�al l,;rit thr<.,-og 5 -,nc -if t,vo �I�rter,$7ics stormwater treatment systems prior to discharge, with pretreatment provided by deep Oct 11 05 01 : 3"'2P LCgleston tnvlronmrir�,�a %val"reen`s, Tcclhni:0 XC%i�4� September 7. 20,Or sump catchbasins and pavement sweeping. The adequacy of the treatment provided is addressed under Standard 44. Stand #2. Post-D v to ment Feak i char a Rates The North Andover regulations require that post-development peak discharge rates for the 1, 10 and 100-year storm events do not exceed pre-development peak discharge rates. The state standards require this of the 2 and 10-year 24-hour storm events, with evaluation to determine that the 100-year event does not cause increased flooding impacts offsite. The North Andover regulations also call for a sub-watershed delineation of the site, including the peak flo-w rate, time of peak flow and the volume of runoff at each discharge point, to be used as a basis for the hydrologic analyses. The drainage report utcludes a hydrologic analysis based on modeling conducted using the HydroCAD 7.0 model for the 2, 10 and 100-year design storms, which indicate that the peak rate of runoff from the site will not exceed pre-development rates. I have the following comments on the hydrologic analysis: 1. The analysis does not include the comparison of pre-and post-development peak runoff flows from the 1--year storm event required by the local bylaw, nor does it address impacts to the timing of peak: flow and the volume of runoff discharged to the two municipal drainage systems. 2. Based on the HydroCAD modeling presented, the total volume of flow discharged from the site in a two-year storm event would increase by about 100 percent to the northern (Route 114)drainage system, and by about 200 percent to the southern (Waverly Road) drainage system. The increases associated with the 100-year storm event are proportionately less, at 76 percent to the north system and 7 percent to the south system(as modeled, a portion of the flow through the southern site drainage system is discharged through the northern outfall under 100-yr storm conditions), but not insignificant. The potential impacts of these increases on downstream resources and properties need to be addressed. 1 will point out that the HydroCAD analysis from which these numbers were derived does not include any exftltration from the detention areas. Without accounting for groundwater recharge, the projected volume of runoff from the site will necessarily increase. 3. Under post-development 100-year storm conditions, the peak water level in detention basins(99.83 feet in basin 1 and 89,80 feet in basin 2)exceeds the level of the southernmost site access driveway, hence the excess now would spill down the driveway onto Waverly Road, There is no storage in the basins above elevation 99.0 as they are currently designed. 4. Test pit data documenting the maximum seasonal groundwater elevation in the vicinity of the detention basins are needed in accordance with the bylaw, and the Oct 11 05 Ul : Jep tgglesron r_nvlr-uruPtvilkaa storage capacity calculations for each basin should be based on the volume of active storage above this level. 5. Storage capacity in the detention basins below the outlet control inverts should not be included hi the model unless it can be demonstrated that this portion of the basin will be emptied (e.g. through exfiltration) within 48 to 72 hours after a SIUI'ifi iGJc:l�i. Stan d d#3. Rech a to oundwater Standard #3 requires that the annual groundwater recharge from the post-development site should approximate the annual recharge from the pre-development site,based on pre- development soil conditions. 1. Based on the calculations provided in the Drainage Report, the proposed impervious area on the site is 122,217 square feet, and the required recharge volume is 4,073 cubic feet. This calculation is more conservative than it needs to be, since the recharge factor for type B soils is 0.25 inches, not the 0.40 inches used in the calculation. The basis for the net impervious area calculation should be provided, e.g. the total new roof area, total pavement area, and existing impervious area. Assuming the area remains at 122,217 square feet, the required recharge volume would be about 2,546 cubic feet. 2. The Massachusetts Stormwater Standards specify that the recharge from the post- development site needs to approximate the recharge from the pre-development site on an annual basis. The recharge calculations submitted, based on a single I- year storm event, do not demonstrate compliance with this standard. The capture volume in the two detention systems proposed (approx. I,219 of total)appears to fall well short of the recharge volume required, even when the lower recharge factor is applied. 3, Per the attached table from DEP's October 1999 draft Recharge Calculation WoMook,the design infiltration rate for the fine sandy loam mapped on this site should be between 0.52 and 1.02 inches per hour, not 2 inches per hour as used in the calculations. In addition, the calculated exfiltration rate should be over the bottom area of each pond only. 4. The suitability of these locations for infiltration needs to be demonstrated through soil testing to determine high groundwater le=eels and soil percolation rates as requireA by the bylaw. A two foot separation between the bottom of the basins and high groundwater is needed. tandat'd #4. 30"/u TS movai Standard #4 stipulates that storrnwater management systems be designed to remove 80% of the average annual load of total suspended solids (TSS) Brom the post-development Oct 11 OS 01 : 33p Eggleston Lnvironmentai �fOTTJJGAG ,-- 5l}]ti tlll7�l" 2005 site. It is presumed to be met when suitable BMPs are implemented, sized appropriately to treat the prescribed runoff volume, and properly maintained. The stormwater management system proposed for this project would utilize a combination of deep sump catchbasins with outlet hoods followed by treatment through two Vortechnics Vortsentry stormwater treatment systems operating in parallel to reduce the total suspended solids (TSS) load from pavement runoff on the site. My comments are as follows: 1. Design calculations for the proposed Vortechnics units are needed to demonstrate that they are sized appropriately to achieve the stated pollutant removals from the First 0.5 inches of tributary runoff flow. Model numbers of the units need to be specified. Since roof drainage is not segregated from pavement runoff, the roof area needs to be included in the water quality volume calculation. 2. The Vortechnics units should either be moved to an of#line configuration or equipped with bypass structures to prevent washout during high flow conditions. 3. All catchbasins should be placed in an offline configuration to prevent washout of accumulated pollutants. Several inline catchbasins are currently shown on the plans. 4. The TSS removal calculations include a 10 percent removal rate for pavement sweeping. As you are aware, this credit is discretionary on the part of the Commission. If it is to be granted, I would suggest that the frequency of sweeping proposed in the O&M Plan(biannually,e.g. once every two years)be increased to four tithes per year, and that a vacuum-type sweeper be used. Provided the design/functionality of the other proposed water quality BMPs can be verified,the project as proposed should be able to meet the 80 percent TSS removal requirement without the 10 percent credit for sweeping. 5. The TSS removal calculations should reflect the fact that approximately 10,600 square feet of pavement runoff from the site would undergo no treatment. Standard#S. Higher otential Pollut t Loa s The proposed project is a commercial development with high-intensity use, and therefore constitutes a land use with higher potential pollutant loads as def ned by the Stormwater Management Policy.Provided all pavement runoff undergoes pretreatment,the proposed 13Ws are suitable for use in this application. Impadard #b. Prot e io of i ai Areas The project site is not in a critical area as defined by the Stormwater Management Policy. Oct 11 05 01 : 33P Eggleston tnviranmen4a-L ��'aj��•c?cll`s. t�C11111Ce1� �i',1'!Gl'r September 7,TW Standar #7.Itedevelo meet The proposed project is not a redevelopment project as defined in the Stormwater Management Standards. St d i�8.Er sio11 Sediment C ntrol Erosion and sediment control measures to be undertaken during project construction are identified on Sheet G of 11 of the design plans. his 1. Given the amount of regrading that that thd be e applicant required provide nt cut and fsite, th ll Commission may want torequest calculations. The transport and stockpiling of material on the site should also be addressed. 2. Proposed locations for dewateringlsedimentation facilities should be shown on the plans. 3. Since it entails over an acre of disturbance the proposed project will be subject to NPDES permit compliance and will require preparation of a construction Stormwater P0111.160n Prevention Plan (SWPPP). The Commission should be provided the opportunity to review and approve the SWPPP as it is developed. Provision for construction monitoring of the site should also be established. Standar #9. O eratio d Maintenance Plan The applicant has submitted an 0&M plan outlining measures for maintaining the structural water quality controls O&M on thehe site. The plan identifies site as Mark Investments, Inc. of Newtone current aMAwner and party responsible for 1. The O&M Plan calls for bi-annual sweeping of pavement surfaces (once every two years). At a minimum, the frequency of sweeping should be increased to occur semi-annually (twice per year), preferably in the spring and fall. More frequent sweeping to maintain site cleanliness and prevent litter from entering the drainage system may be warranted. 2. For the first year of operation, all stormwater structures should be inspected following any large storm of 1.5 inches or more to ensure that they are functioning properly. Infiltration structures should be inspected again 72 hours after such storms to make sure that all of the flow has infiltrated. •b�educed to he first year of successful operation, the frequencyinspection 'Molude only the larger storms of several inches or more. 3. Catchbasin maintenance should take into account the presence of oil-trapping floods, so that &e. hoods are not damaged +n the process. Accumulated hydrocarbons must be disposed of separately in accordance with DPP regulations. oct 11 05 01 : 33p Eggleston Environmentea i I Sel�lct�tl7c r 7. 200- 4. Maintenance requirements for the two Vortechnic,treatment d id inn akin accordance O&M plan, with the manufacturer's guidelines should be cleaj y including equipment requirements and disposal of accumulated materials. 5. The note on Sheet 3 regarding snow storage procedures-should be included in the 4&M manual. I appreciate the opportunity to assist the North Andover Conservation Commission with se the review afthis project, and hope that this information is sUes�onsitable for regarding t needs. hePssues feel free to contact me if you or the applicants have any Q addressed herein. Sincerely, EGGLESTON ENVIRONMENTAL Lisa D, Eggleston,P.E.