HomeMy WebLinkAbout2003 Stormwater Peer Review SPR WITHDRAWN EGGLESTa'V EAWR®1VMENTAL
July 18, 2003
Mr. Scott Masse, Chairman
North Andover Conservation Commission
27 Charles Street --
North Andover, MA 01845 -
RE: Stormwater Management Review
Foster Farm School Site
Dear Mr. Masse and Commission Members:
Per your request, I have conducted an initial technical review of the Notice of Intent
(NOI) submittal package for the proposed Foster Farm School Site in North Andover,
with respect to storm-water nmagement. The materials I have received and reviewed to
date include the following:
■ Notice of Intent, Foster Farm Elementary School, prepared for DiNisco Design
Partnership by Schofield Brothers of New England and dated 6/26103.
Design Plans entitled "Foster Farm Elementary School" by DiNisco Design
Partnership et al. Plans are dated 4/21/03 (Sheets 1.0.1 _ 1.0.4), 6/25/03 (Sheets
1.1.4 -- 1.I.10 and 1.2.0 -1.2.2), 6/26/03 (Sheets 1.3.1 - 1.3.7) and 6/3/03 (Sheet
3.3.1).
■ Attachment 3, Stormwater Management Report for the Proposed Foster Farm
Elementary School Project, prepared for DiNisco Design Partnership by Schofield
Brothers of New England and dated 6/26/03. -
Attachment 6, Site Soils and Geotechnical Information, Foster Farm School Site,
including "Soil Test Report" by Schofield Brothers of New England and dated
1/28/03 and "Geotechnical Engineering Report" by Weber Engineering
Associates, LLC and dated 2/5/03.
I also met with Conservation Administrator Julic Parrino and representatives of Schofield
Brothers and Moriece & Gary on April 14, 2003 to discuss the project and conduct a
walkover of the project site.
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
55 OLD COACHROAD SUDBURYMA 01776 T-rLIFAV978.943.9262
Foster Farn, SCilaal, Techlli'-I! Rey icy'.
July 1 S. 2003
Management Policy. My comments are outlined below. In anticipation of my being on
vacation from July 19`" to August 101s', I have also reviewed these comments by
telephone with Fred King of Schofield Brothers Engineering_
Overall Storinwater Management Approach and System Design
The project site is part of the 77-acre Foster Farm parcel located between Foster Street
and Boxford Street and owned by the Town of North Andover. The proposed project, a
public elementary school for 800 students and 100 staff with associated parking, access
driveways and landscaped grounds, encompasses approximately 14 acres of the larger
parcel. The site is currently undeveloped and consists of a peninsula of upland extending
northerly from Boxford Road, with wetland resources to the west, north and east. There is
presently very little surface runoff from the site; flow to the wetlands is largely via
subsurface flow.
Under existing conditions, drainage from the project site flows in several directions, as
depicted on the Applicant's Existing Conditions Watershed Map. The southern portion of
the site drains to a culvert under Boxford Road, although very little surface flow actually
reaches the culvert except in extreme storm events. Drainage from the westerly portion of
the site is to a vernal pool located in the low area between the project site and the parking
area for the adjacent town athletic fields. The vernal pool discharges toward Mosquito
Brook via an intermittent stream to the north. Drainage from the northern and eastern
upland portions of the site flows to vegetated wetlands associated with Mosquito Brook.
Under proposed conditions, runoff from the paved portions of the project site would be
collected, treated and infiltrated onsite and roof runoff would be directly infiltrated. As
Proposed, runoff from the entrance driveway, and paved areas in the southern portion of
the site would be collected in a conventional storm drain system with deep sump
catchbasins and directed through an in-line Storrnceptor treatment unit, then distributed
between two planted rock filter beds for additional water quality treatment. Overflow
from the filter beds would flow overland into two drywells for .subsurface infiltration.
With a proposed outlet control to allow only flows in excess of the 100-year storm event
into the culvert under Boxford Street, the entire low area where the filter beds and
drywells are located would serve as a secondary retention/infiltration basin.
In the area tributary to the vernal pool, the proposed plan calls for pavement runoff to be
collected in a single deep sump catchbasin at the western edge of the parking area and
conveyed through a Stormceptor treatment unit into a pair of drywells located beneath the
parking area. Overflow from the drywells would discharge to a planted rock filter bed,
which in turn would overflow to the vegetated slope upgradient of the vernal pool. Roof
runoff in this drainage area would be directly infiltrated through another pair of drywells
below the parking area, with overflow to the vegetated slope. Soils in this area are
reported to be highly permeable; hence overflow from the drywells would occur only
during large, infrequent storm events.
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13(,bocd. Technical hnical P'e
}l4\' 15. 20C,
Roof drainage from the northeastern portion of the building roof would be discharged
directly to an infiltration system comprised of three 12-foot diameter drywells. Overflow
from the drywell system would be discharged toward the wetlands associated with
Mosquito Brook.
The northwesterly portion of the project site is characterized by less permeable soils and
a shallower depth to groundwater. As proposed, runoff from the paved surfaces in this
area would be collected in deep sump catchbasins, conveyed through a Stormceptor
treatment unit, and discharged to a subsurface Cultec chamber infiltration system. Roof
drainage would be directly infiltrated through a separate Cultec chamber infiltration
system. Overflow from both Cultec systems would be to a surface detentionfinfiltration
basin which, in extreme storm conditions, would overflow toward the wetlands
associated with Mosquito Brook.
My comments on the overall stormwater management approach and drainage system
design are outlined below:
• The proposed stormwater management approach for this project is generally
consistent with existing drainage patterns and sheets or exceeds the treatment,
recharge and flow attenuation objectives of the standards.
As proposed, both the rate and volume of surface runoff would approximate existing
conditions on a sub-watersled basis. It appears that there may be some redirection of
groundwater flow among sub-drainage areas under post-development conditions,
which could impact downgradient resources. I suggest that the Applicant provide a
summary table (siinilar to Tables 1 and 2 summarizing runoff flows) that lists the
approximate pre- and post-development groundwater volumes on a sub-watershed
basis (or as discharged to the three downgradient. receptors).
Runoff from paved areas of the site will undergo a high level of treatment through the
proposed combination of deep sump catch basins, Stormceptor treatment units,
planted rock filters, and infiltration BMPs. The proposed segregation and direct
infiltration of roof runoff from the site is appropriate and serves to n-raxinuze the
treatment effectiveness of the pavement BMPs. I do recommend that roof downspouts
be screened or otherwise protected to prevent leaf litter and debris from getting into
the infiltration systems.
1 have some concerns regarding the performance of the planted rock filters as
described in firrther detail below. I must emphasize, however, that the stormwater
management BMPs proposed for this project meet or exceed the water quality
treatment objectives of the stormwater policy without the planted rock filters, hence
tlse effectiveness of their treatment may not be pertinent.
In the southernmost drainage system. (including the entrance driveway), the
infiltration BMPs are located downgradient of the planted rock filter beds, thus all of
the runoff flow passes through the filter bed(s). In order for any flow to reach the
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Foster 1 -it-ill School_ -Technical Review
311y 1 S. 2003
drywells, both filter beds would have to be totally submerged. Based on the
HydroCAD modeling, levels in the filter beds would only reach this elevation in
storms of 10-year frequency or larger, however the modeling does not accurately
nts since it assumes exfitration through the drywells
represent the smaller storm eve
throughout each runoff event. Flow to the drywells may actually occur on a more
frequent basis than is represented by the model.
• Due to the shallow depth of the rock filters, the distribution pipes to the filters are less
than a foot below the ground surface, making them potentially subject to frost and
icing conditions. Given this fact, and the possibility of icing over the surface of the
rock filters, I would recommend that a subsurface overflow line to the downstream
drywells be provided to prevent now froze backing up in the event that flow through
the filters is restricted.
• It is also not clear from the design plans how the distribution of flow between the two
rock filter beds in this area would be accomplished.
• The Applicant has provided a hydrologic budget for flow to the vernal pool, which
was used in the determination of the roof area to be recharged within the drainage
area. While the overall approach is valid, the calculation of post-development
groundwater recharge assumes that 95 percent of the rainfall over the impervious
surface on the site is converted to runoff, and that 100 percent of this runoff would be
recharged. This factor does not adequately account for evaporation loss from the
paved/rooftop surfaces, which is indicated on Figure 1.1 (page 2-15 of the
calculations) to be about 30 percent of the total volume.
The area of roof being recharged toward the vernal pool is significantly greater than
the calculations indicate is required to approximate existing conditions. This would
result in an increase in groundwater flow (hence total flow) to the vernal pool, which
could impact the hydrologic regime of this resource area. While it is certainly
preferable to err on the side of providing more rather than less recharge iii this area,
the total volume of flow to the resource area should not be significantly altered.
Again, a tabulation of groundwater flow to each resource area is needed.
• Based on the grading of the parking lot along the south side of the school building,
there is potential for ponding of surface runoff in the vicinity of the handicapped
parking space. This area should either be regraded to prevent ponding, or a second
catchbasin added.
• Unlike the planted rock filter beds serving the southerly portion of the site, which are
upgradient of the infiltration BMPS and would therefore receive discharge from most
runoff events, the planted rock filter to the southwest of the school is downgradient of
the infiltration BMPs. Although there may be some direct runoff from the adjacent
slope, the HydroCAD modeling indicates that the only flow through the distribution
system of the rock filter bed would be from storms of 10-year frequency or larger,
when the infiltration capacity of the drywell system is exceeded. There is relatively
Foster yarn, Sc11ot)i, Technical Rey j(w
July IS. 2003
little water duality benefit to be gained from additional filtration of runoff from such a
rrra in providing it. I would also assume that
large storm, although there is also no ha
some form of supplemental 'irrigation would need to be provided to sustain vegetative
growth in the system.
■ Catchbasin ##12 is shown as being the inlet structure for the single planted rock filter
southwest of the building. The structure has a grated cover and only one invert is
shown. It is not clear whether there should be a second outlet going to the distribution
system, or whether the overflow line from the drywells is intended to discharge
through the surface grate.
■ The Stormceptor units for this project are being used in an in-line configuration, as is
appropriate since the Stormceptors have a bypass capability. Any "approved equal"
should also be equipped with a bypass chamber, or should be relocated to an off-line
configuration to prevent washout.
■ 1 recommend relocating the influent lines to the multiple-drywell infiltration systems
(one recharging parking lot drainage southwest of the building and one for roof
drainage from the northeastern portion of the building) so that they do not connect to
the sae unit as the effluent (overflow) line. This will facilitate flow distribution
among the drywells and prevent short-circuiting of the system.
■ Although extensive soil testing has been conducted on the site, none of the test holes
were conducted in the currently proposed infiltration areas. I suggest that the
Corn nission request confirmatory test pits or borings in the proposed infiltration
areas prior to construction to ensure that the soil conditions are suitable and
consistent with the design parameters.
■ The work to be done at the intermittent stream crossing of the cart path, including
water main installation and stone culvert replacement, is shown on Sheet 1.3.3,
Sediment and Erosion Control Details. Since the flow through this culvert plays a
part in controlling flow out of the Vernal Pool, it should be noted that the culvert
replacement needs to replicate the existing flow capacity.
■ The design plans should designate clearly which drywells are to have grated or
beehive covers and which are to have solid covers.
■ Locations of double-grated catchbasins should be clearly marked on the plan. The
units should be equipped with separate grates to facilitate access and maintenance.
■ Inspection ports should be provided on the Cultee infiltration systems.
My specific comments on the project as they relate to the project's compliance with the
nine North Andover and State Stormwater Standards follow. Please note that there is an
inconsistency in the numbering of the standards between Section IV of the Bylaw and the
Stormwater Management Form included in Appendix F of the Bylaw, although the
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Foster Farm Sc11001_ Tech("ca1 Plevie\t
Jtily 1 S.. 2003
content is consistent. I have opted to use the numbering system used in the latter, since it
is also consistent with that of the State Stormwater Policy.
Standard #1. Untreated Storrnwater
harges of untreated stormwater to wetland resource
Standard #1 prohibits any new disc
areas of the Town of North Andover, whereby treated stormwater is defined to the
stormwater that meets the requirements in Standards 2 through 9. As proposed, all the
runoff Bows from the paved areas of the project site would undergo treatment through a
system of deep sump catchbasins, Stormceptor treatment units and onsite infiltration
BMPs. Planted rock filters are also provided for additional polishing of the runoff. Clean
roof runoff would be directly infiltrated onsite. No new discharges of untreated
stormwater are proposed.
Standard #2_. Post-Development Peak Discharge_.._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 flow 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 applicant has submitted peak runoff calculations based on HydroCAD modeling on a
sub-watershed basis for all of the design storm events required by the standards and
bylaw. T he hydrologic analyses satisfactorily demonstrate that the peak runoff rates from
the site under post-development conditions would not exceed pre-development rates for
all of the design storm events, including the 100-year storm. They also demonstrate that
the total volume of runoff flow discharged to each of the three downgradient wetland
systems will not be altered as a result of the project.
Standard #3. Rechar e to Groundwater
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 soil
types,
The project design makes extensive use of infiltration BMPs to limit the volume of
surface runoff to pre-development conditions, and it is therefore concluded that the total
volume of groundwater recharge is also comparable to existing conditions. While 1 would
concur that this is the case for the total site, it is not clear that it is true on a sub-
watershed basis. The Applicant needs to demonstrate that the total volume of
groundwater recharge discharged toward each of the three downgradient wetland systems
will not be altered as a result of the project.
1=oster Farm School.. Techilic-6 Kevie""
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Standard #4. 800/6 TSS Removal
The stormwater management system proposed for the project would utilize deep sump
catchbasins with outlet hoods followed by treatment through Stormceptor units and
infiltration to reduce total suspended solids (TSS) load from pavement runoff on the site.
Properly maintained, tsforsat leas b80%management
TSS removal setces fort���Standard #4.meet
The
exceed the requirement
planted rock filters would provide additional treatment of the runoff flows.
Standard 115. M er Potential Pollutant Loads
The proposed project does not constitute a land use with higher potential pollutant loads.
Standard #6. Protection of Critical Areas
The project site is not in a critical area as defined by the Stormwater Management Policy.
Standard V. Redevelopment
The proposed project is not a redevelopment project as defined in the Stormwater
Management Standards.
Standard #8. Erosion and Sediment Control
Erosion and control measures to be undertaken during project construction are addressed
in the Narrative & Summary Erosion & Sediment Control Plan included with the NOI
and are shown on Sheets I.3.1, 1.3.2, and 1.3.3 of the design plans. (Note that the title of
Sheet 1.3.2 is mislabeled_ It should be titled Sediment and Erosion Control North). Tile
erosion and sediment control measures for this project include:
W silt fence barriers and haybale & silt fence barriers
■ temporary sediment basins for construction dewatering
■ haybale & silt fence check darns
W storm drain inlet protection
stabilized construction entrance
Several locations are shown on the plan for stockpiling of material during construction
including the area to be used for the northeast retention/infiltration basin and Cultec
chamber system. Stockpiling in this location is not advisable as it could lead to increased
compaction of the soil.
As indicated in the NOI, the project proposed will be subject to NPDES permit
compliance and will require preparation of a construction Stormwater Pollution
Prevention Plan (SWPPP). The Commission should be provided the opportunity to
review and approve the SWPPP when it is developed, including locations for temporary
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Foster Farm School. Technical Review
.lulu 18, 2003
sediment basins, Provision for construction monitoring of the site should also be
established.
Standard 0, O erasion and Maintenance Plan
The Applicant has provided a preliminary O&M Plan for the stormwater management
system, and indicated that a more complete O&M manual for use by Town personnel will
be prepared when the project is constructed. The Commission should be provided the
opportunity for input to the final manual.
Maintenance procedures outlined for the planted rock filters include fertilizing and pest
control. The Commission may want to consider lirn Zing the application of these products,
particularly in the filter bed closest to the vernal pool.
Snow storage areas should be designated on the site plans and a description of the
proposed snow management procedures for the site in accordance with the DEP Bureau
of Resource Protection snow disposal guidelines should be included in the O&M Plan,
Minimum Submittal Requirements__
The NOI application package satisfies the minimum submittal requirements set forth in
the bylaw.
I appreciate the opportunity to assist the North Andover 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
,gddressed herein.
Sincerely,
EGGLES`I-ON ENVIRONMENTAL
, D-� �� � --
Lisa D. Eggleston, P.E.
C! Fred King, Schofield Brothers