HomeMy WebLinkAbout1996-10-18 2009 Stormwater Review _lar)uai•y 14, 2009
Nei-Eli Andover Conservation Commission
1600 Osgood Street Building 20, Suite 2-3
No'th Andover, '�M 01845
Attn: Jcnnifor Hughes
RE, Stormwater Ivlanagerni-,n( Review
1.eiriniack Cc114cge Detention Pond As-Built
Deur R-nnifer and ['ommissk)n Memhers:
Per your request, 1 have condaded a technical review of the stonnwater detention Bond
system tha€ xN.js constructed behind the Derr-iimack College Facilities Administration
Building mid Garagc, in order to asses_ its functionality and ccmflianee with the origh3aI
[_gofer of Cond (00Q issued under Dl3P File 42.42-834_ My review included the
original project plans {revised through 1 1 } and drainage report, the 1016108 as-built
Plan, and the as-built detention basin analysis prepared by UK FBgineeraag Associates,
fnc., including the 118/09 revisions. 1 allo conducted a site inspection of the pond with
Diehard Kaminski on November 19, 2008, Based on my review, 1 have the following
1. The detention hasip as constructed is of smaller dimension and prnuicles
approximat0y 40 percent loss slorage volume than the originally approved hashl.
In addition, two ofthe outlet orifices are hi her and larger than were called for in
the original design, and the, third cutlet is higher than was originally specified,
Nevertheless, the rnode,aiiig analysis of the pond demonsUates that the freak
discharge rates horn the as-built puBd for the 10- and 100-year storms are still
lower than tinder pre-dtwtdopnicnt conditions, and [he peak rate for the -year
storin is only slightly (9 percent) higher than the pradevelopment rate, While 1
suspect that storm events smaller than those modeled may also discharge at
slightly NgN-r rates than under pre-devOopment conditions, the down-gradient
wetlands have at this point stabilized and any awmpt to mitigate this increase
may in Not have a dcleteHous impact an the wetlands. 1€ i,�; thevAbre my
reconi mcndation that uo modifications Io [he pond capacity or the nutlets Uri,-
warranted,
2. Since the smaller pond provides less fteeboard during high Pow conditions. 1 do
concur with DK Fn&ecring's recomrneridatiou that a 10-1bot ernergency
spillway he added to the pond, with an invert of239.4.
3, The ctitlet sEructuve needs to be fitted with an oil and gas trap, as specified on tfYc
originally-appi roved Plan..
ii-i,t= :' `> 5'�li�i. 'l:i; }I i'. P31'i�l\
4. Tt was evident dwing our site visit that little, if any, maintenance has been done
on the burin sim� it was installed, and tlla( snow plowing of the parking lot has
bf,= directed toward the basin, such that a small berm of accumulated sand and
solids has been built tip between portions orthe edge of pavement and the basin.
This iiihibits street flow off the parking lot and results in gullying of the basin
mere the runoIT is able to get thmugb. hi addition, the bottom of the basin has
sediment accumulation hi areas to a level above the lowest outlei inverE. It i my
re 3mmeWation that the entire bashi be cleaned out to remove accumulated
sediments and to restore it to its original design invert o f 2 37.0,
5. Finally, an Operations & Maintenance (O&M) Phtn needs to be prepared and
implemented to ensure the ongoing operability of the basin. The G&M plan
should include regular hispections, rno ing and sedimt-m removal, and should
address ,now management on the site so that snow stockpiles and accumulated
sediments do not UiLibit sheet molt:
T appi iate the opportunity to assist the North Andover Conservation Commission with
the review oITthis project, and hope that this information is suitaW for your needs. Please
feel free to contact me if you or the, applicants have any questions regarding the issues
addressed herein.
Sinc t rely,
EGoLr3-slyw FNvjRoN L:NTAL
Lisa D. Egglcston, PR
: Richard Km-ninski, R.F. Kaminski & AssociaEe-
Dan bravos, DK Engineering & Associates