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HomeMy WebLinkAbout20110695 Ver 1_More Info Received_20111107 `� _ ��`� ;. � � I�tI�At�I Bakw En�M+t�rirp,Me. - �00011p�nc�r 1sA�way . � iu�M 200 I • Gry.NoMCsrocna?1510 ►harw:lf��616tN Fa�c f1f.4a1.6�9D '�. Ms.Tasha McCormick United States Army Corps of Englneers p ����n/�� v p � Ashevllle Re�ulatory Fieid Office ; 151 Patton Avenue,Room 20S N�U ° '� 2011 Asheville,North Carol(n�28801 DENR•INATER L1UN.(TY � WETIANDS AND STORMWATER BRANCH i Dear Ms.McCormick, ' i Thank you for your review of the Skyline Wetland EnMancement ProJect(project). Michaei Baker � Engineertng,tnc.(Baker)and the 7own of Sparta appreciate your comments and will address each of � your concerns in thts document. While the existing wetland at the site does currently provide some water quality b�nefits,the beneflts ' couid be greatly increased by the implementation of the proposed stormwater wetland BMP. The � existing vegetation do@s help to slow storm flow though the site but,as you can see from the photos tn ; the appendix(att�ched),which�rvere taken d�rring a storm�v�nt and are of the rece�ving channel j directly downstream across Grayson Street,the erosive energy of the stormw�ter is stkM very h�gh. The ; hfgh velocity,high energy stormwater current�y causes severe erosion downstream. This aediment is j then carried to an unnamed tributary to Bledsoe Creck wh(ch fiows tnto the little River. The Lower � Bledsoe C��ek Watershed,where the proposed project dr�ins,was ic#entified as the most impaired { a watershed withln the little River VWatershed under a three phased study funded by the North Carolina 3 Ecosystem Enhancement Program(EEP)and the NorEh Carolina Division of Water Quality(DWQ) i � (Biedsoe Creek Local Watershed Pl�n,2�7). The proposed project witl allow the existing wetland are�s � to be expanded and will capture and hoid stormwater runoff for much 4onger than the exlsting wetland, which is on a relatively steep gradient. The pro�ect is designed to capture�nd treat the volume of water from the first one inch of rainfal)within the drainage area of 27.14 acres which is ov�r 33°%tmpervious. The large majority of the runoff originates ofF-site and is a series of open channel and piped stormwater conveyance systems,in addition to surface runoff. The project will treat this stormwater by allowing th�sediment carried by the stormwater to settle out into the proposed wetland celis within the proJect. According to DWQ pollutant removal rates of 85% totai suspended solids,35%total phosphorous,and 40%tTotai nitrogen can be obtained in addition to the removal of inetals,fecal coliform and others. The proposed wetland will removE these pollutants through settling of the total suspended solfds�nd through uptake by the proposed wetland vegetation, a which consists of multiple native wetland species(see attachments for the site planting planj. After the ' treatment of the stormwater,it wiil be released slowiy though water quality orifices. i I I . � I i No ditch has been dug though the existing wetland. The existing swale tMat runs ihough the majority of the wetland was caused by erosive stormwater. This ditch will continue to become more tncised aa heackuts move up valley from the downstream impa(red reach. The hydrology of the existing wetland couid be severel�r impacted if the heackut moves upstream to create an(ncised ditch that would drain the wetland. The heackut in thls situatfon would essentially function the same as a drainage ditch dug through an agr(cu(tural field. The existing rip rap at the bottom end of the site was placed to prevent the headcut from mlgrating up valiey from the cuivert that conveys the stormwvater under the Skyline Telephone Corparation Membership parking lot. I The proposed pro}ect is expected to increase and improve the existing wetland area by converting the existing hydrology and vegetat�n in areas that currently do not function as wettands to wetland hydrology and vegetatton. Even though these areas outside of the existing wetland currently do not have hydric soils,the proposed improvements to the hydrology and vegetation will provide potential for hydric soils to form. The potential net(ncrease in total wetland area is 0.081 acres as shown on the attached map. i Other design alternatives were also considered such as bu}Iding a single or double cell wetland. While ; constructing a single or double cell at the northern end of the wetland cou�d potentially help during ' smaller storm events,the first flush,which is the first one inch of runoff that cont�ins the large rriajority of the poilutants that�re conveyed during a storm event,would not be treatable using onPy on@ or two ce�ls. The existir►g site topography would not allow for single or double cells targe enough to treat the required design votum@ without buiiding targer berms further Impacting the existing wetland and I creating unnecessary risk downstream. Different conflgurations and layouts of the wetland celts were also considered. The proposed layout impacted the least amount of ex(sting wetland. Berm size was also kept to a mir�imum while adhering to stabtlity and safety standards. Specifk deslgn requirements � inc4uded DWQ standards which require,at a minimum,the treatment of the first one inch of runoff. ' Additionai requirements involved constructability,and site constraints such as the existing site � topography. Additionai stormwater controls such as BMIPs are planned further up�n the watershed,but € i implementation of these contrals is still a long way off. The additlonal proposed BMPs In this watershed are stfll(n the planning stages and funding for future destgn and construction has not been obtained. ; I Thank you for reviewing this project. Piease feel free to contact me at(919)459-9048 if you have any ; questions about these responses. We look forward to resolving these issues. �, ; Sincerely, � /, �--��.�.-�--.. ����3��/� ake B ers, EI Michael Baker Englneering,Inc. 8000 Regency Parkway,Suite 200 j Cary, NC 27518 ' Cc: Mr. Dave McHenry Mr. Brian P.Cole � Mr.lan McMillan,NC DWQ Ms.Sue Nomevwood, NC DWQ Mr.Bryan Tompkins,US FWS Mr.Bryan Edwards,Town Manager,Town of Sparta Mr.Kevin Tweedy,P.E.,Mkhael Baker Engineering,Inc. Mr.Scott Hunt,P.E., Michael Baker Englneering, Inc. Attachments: Photo Log,Proposed Project Planting Plan, Potentiai Wetiand Creation Map i i � F � f � f � � � 1 � i I iI ( � � I I � I , , . � � � , �,�. � . ; �� � � �; � _ ;� � Legend � ���`� N �� � : � ,� n �� Potential Wetland Creation 0.18 AC a����� ��� �;` � �: � : _� �. (_] BMP Cell Boundary � � �. � � , ep 0 Parcels �� m � � , � f � 0 Wetland Impacts Area 0.099 AC �'��� �°� � � � , �� ,�� 0 Preliminary Wetland Boundary 0.41 AC ���'�� �� �� � �� �. ��,�: � � � , �, . � �,�� : � , � � , ���:�����;;���° ,"�� =�; ��;'. �„ � �' ���� �� �� ��� � ���� �� � � �'��� ������� F " ���, � a� � `:,��, "}. � � , � ��;� §� �.�� � �� � `�� < . � , N"� a��.��,�* ��Y +� �' �;µ " •�y `�� �_, �r��m §"" a"u��°' �,'�, r�� a"` • � ``'�`'�'�„ � � �� �, t,� ,. . �$� �"�. �aa.` � � � , `5 , ' `;;� � �� .r„ � �.�� ?,"� � a °��At 3 . . `."K �� ,� t-=� ��� �aC� �. 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