HomeMy WebLinkAboutConsultant Review - 1780 OSGOOD STREET 9/8/2011 LJR Engineering
Civil Engineers and Land Surveyors
234 Park Street
North Reading,MA 01864
(978)664-8141 Fax(978)664-8142
September 8, 2011 Email:Iroy @ljrengineering.com
Eggleston Environmental
32 Old Farmington Road, Unit 29
Sudbury,MA 01776
Re: 1780 Osgood Street
North Andover
D.E.P File 242-1528
Attention: Lisa D Eggleston,P.E.
Dear Ms. Eggleston
The following are our responses to your review letter dated August 24, 2011 for the referenced
project. We have prepared our responses in the order and number as listed in your letter.
1.No comment necessary.
2. The plans have been revised to reflect 12 Cultec C-100 units for the garage infiltration system.
Please note,the overall width of the system has been increased to 20ft. which reflects the
installed width of the chambers of 36"instead of the effective width of 32".
Our original submission demonstrated that the smaller subsurface infiltration system for the
runoff from the proposed porch addition will capture and infiltrate the increase volume of
runoff from the 100yr. design storm. However, the subsurface infiltration system for the
garage addition was only designed to contain and infiltrate the increased volume from the
10yr. design storm. Therefore we have provided a stone infiltration trench along the southerly
side of the addition to detain and infiltrate overflow runoff from the subsurface system. The
roof downspouts will overflow into the stone trench in larger storm events.
We have provided revised drainage calculations for the proposed garage addition. The analysis
compares the peak rate of runoff generated from the area of the addition in the existing
condition to post development, since this is the only area for which runoff characteristics will
change. The calculations were also revised to utilize HydroCAD for modeling routing and
storage. We used a revised curve number of 89 for the existing compacted gravel surface
which is still conservatively low for this type of surface. We included exfiltration outflow
from both the subsurface system and the stone trench for which we used the Rawls rate of 1.02
in/hr. for a sandy loam. The results of the analysis are as summarized below for various storm
events which demonstrates that the peak rate of runoff from the site generated by the addition
will not exceed existing conditions upto and including in the 100yr. design storm. Note,
1
September 8, 2011
1780 Osgood Street
North Andover
D.E.P.File 242-1528
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providing peak attenuation as demonstrated provides greater total volumetric storage than only
providing for the difference in total volume of runoff from pre to post development.
Existing Post Development
2yr.(3.2in.) 0.17cfs O.00cfs
10yr.(4.8in.) 0.28cfs 0.03cfs
100yr.(7.Oin.) 0.44cfs 0.38cfs
3. Compensatory flood calculations are enclosed. The Notice of Intent addressed the filling in of
a portion of the edge of the floodplain. The number reported in the NOI was 28.4 cubic yards and
a proposed replication of 30 cubic yards. The form requires the numbers to be in cubic feet. This
fact was brought to the attention of the Conservation Commission at the Public Hearing held
August 24, 2011.
4. The purpose of the crushed stone is to stabilize the soils after excavation of the compensatory
flood storage due to the nature of the site use activities.
5. It is anticipated that there will be no clogging in the infiltration system since they will be
handling clean roof runoff. The filter fabric has been proposed to prevent migration of the fines
from the surrounding into the infiltration system. The surrounding soils are fine loamy sands.
6. We have clarified on the plans that compacted gravel borrow will be placed over the garage
infiltration system to somewhat match the existing material composition. The existing material in
the area of the garage infiltration system is a mixture of crushed bituminous material, gravel,
sand and dirt. The area over the porch infiltration system is called to be loamed and seeded to
match the existing material.
7. An observation port has been added to the infiltration system and a detail provided on the
detail sheet.
8. Since the natural drainage flow and site grading is away from the wetlands we feel the siltation
fence is adequate to protect the resource area.
9. A note has been added to the plan to address this comment.
10. The location of the stockpiled materials is constantly changing since the rear of the lot is used
as a contractor's storage facility. The site drainage pattern is towards the Railroad in the rear of
the property. There is a substantial ditch running along the rear of the property which captures the
runoff from the site. We have called for two check dams to be installed in the ditch, at the
September 8,2011
1780 Osgood Street
North Andover
D.E.P.File 242-1528
Page 3 of 3
direction of the Engineer, to capture any sediment transported in the runoff. On the left side of
the property we have called for"jersey barriers"to be installed to define the limit of storage for
stockpiled materials and direct the runoff to the ditch in the rear. Maintenance of these dams have
been addressed in the revised O and M plan.
We hope we have satisfactorily addressed your review comments. If you have further comments
please do not hesitate to call. We are anxious to close the public hearing at the next Conservation
Meeting to be held on September 14th,2011. We recognize that the time is short for your review
but we would deeply appreciate a favorable reply.
Yours sincerely,
Michael O'Neill P.E.
c: Jennifer Hughes,North Andover Conservation Commission
Enclosures:
Revised Site Plan,Latest Revision 2, dated September 8, 2011
Revised Drainage Calculation
Revised O&M Plan
Floodplain Calculations