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HomeMy WebLinkAbout20171491 Ver 1_waiver_request_triple_20171120DATE: October 6, 2017 SUBJECT: Project: 45342.1.FR14 (W-5212N) Gaston County Addition of passing lanes and intersection improvements along NC 279 from SR 1438 (St. Mark's Church Road) to Tandem Drive Request for Waiver This is a request for a waiver from the culvert burial requirement for the proposed downstream extension of the existing 3@ 9' x 9' RCBC on Little Beaverdam Creek at -L- Sta. 42+19. If you have any questions, please contact Stephen Morgan at 919-707-6739. la.) The requirement is question is Corps Regional Conditions for All Nationwide Permits, section 3.6) Safe Passage Requirements for Culvert Placement. The specific requirement states: "Culverts greater than 48 inches in diameter will be buried at least one foot below the bed of the stream. Culverts 48 inches in diameter or less shall be buried or placed on the stream bed as practicable and appropriate to maintain aquatic passage, and every effort shall be made to maintain the existing channel slope. The bottom of the culvert must be placed at a depth below the natural stream bottom to provide for passage during drought or low flow conditions. " lb.) A waiver for this requirement is being requested. 1 c.) This project is funded thru the safety program due to the 295 total crashes (five fatals) occurring along the project limits within a five year period. Total project funding is determined using a benefit/cost ratio. The inverts of the existing culvert are not buried below the stream bed. Based on visual inspection, the existing culvert appears to provide adequate flow for aquatic passage through the low flow (East) barrel. The site was visited in November 2016 in what would be considered drought conditions (See Attachment 1) with approximately 0.5' of silt and 0.2' of water observed in the low flow barrel. The shallowness of the slope on the existing culvert (0.3%) is the greatest contributor to this silt accumulation and allows adequate flow depth for aquatic passage in the culvert. The existing culvert shows no signs of being "perched" above the stream bed and has no pools forming or scour at the inlet or outlet. For both options discussed below, no extension is proposed on the upstream end of the culvert. Option 1(The preferred option) is to extend the existing culvert at the same size (3 @ 9' x 9') and slope, (See Attachment 3). This option would be simpler and cheaper to construct when compared to the 3@ 9' x 10' extension. An in-kind extension would also provide the same silt accumulation and normal depth for fish passage as both the existing condition and the 3@ 9' x 10' extension option. Although there is no sign of scour at the inlet and outlet of the existing culvert, this option includes the addition of Class `I' Rip-Rap to prevent any possible future scour. Option 2 is to extend the existing 3@ 9' x 9' RCBC with proposed 3@ 9' x 10' RCBC downstream (See Attachment 2). The invert of the extension would be buried 1' below the stream bed with a 1' sill at the outlet. The structural component of constructing a drop in the culvert floor will be accomplished by attaching a vertical wall section to the end of the existing culvert. Additional surcharge load from the fill on the extension may cause slight settlement near the end of extension and create rotational stresses on the culvert floor at the connection point. The vertices between the wall and the culvert floors are the weak points in the design and are subject to failure due to these stresses. Note that the top of the sill would be even with the invert of the existing culvert, and this would not cause the existing culvert to accumulate any additional silt as per the intent of the burial requirement. The burial of the extension will not increase the structure's ability for aquatic passage however an increased cost of $90,000 would occur due to design costs and labor due an approximate three extra weeks of construction when compared to extending the existing culvert in-kind. 1 d.) Attachments Attachment 1 3@ 9' x 9' Culvert Photos fro�i� November 2016 Site Visit '�' , �� . �� � � � , � . � �y` � .. t ��N T w �.��d .. � �� r i���f � �Y+ ..> J:._ ...�..Y~`°� "Y . ;!.^P . �. :. ,. �k. .� .. R +. • � ..�, � . � � �ZI� ;� �1 � 3�� :.�� <•�YL 4�'F . ;;�. �Y � — = � � - � '' "` . 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I � O i'i �i ' �i i I I - I APPRO . W.S. ELEV. = 8 6 i i � � � �--..—_.__._ �--- ---.�_._ . _ _ _ _ .._..._._ .._ ..---.. _.__._. _. � SCALES: ii � i �i � i _ � I' � I -(l0 T7 ��Tb)- _:. � � i � �„=50' HORIZ. , -- � II II - , �„ =10' VERT. � _ � � , � i ... l l � � CHANNEL IMPROVEMENTS UPSTREAM - --�_ �_r _ , , : _.T__ . . - - --. _ . . _ . - � � r � � �c . . _ . . . _ , - -. _. . STREAMBANK STABILIZATION ; ` w .�. . ,-� - - I ( Not to Scale) - _ ` \ � . - - O I E I �� - _ __ _ _ � _ _ � I_ � v _ ._ . . _ � -- --Q� . __._.- � _._ _ �- ---._. _�. _ _ _._. � cia55 �i� _- 1 �, � ,. � . _ -.I Natural � ' - P- P ! _ __ - � _ _ \ i ` I �Ok , . 0 �ounTc — i Gr�ou d � - i _ _ _ . bo ,�� o \ _ I . FLO DPLAIN BENC - - I 3 ! � � ^.� . . - --� _ . • _ ,,4 :.. : , _ -- \ ,., . � ` Stream " , MATTING (TY .j � u- B� — -� ,. � - -1 I I � 1 � � II Geotextile . . . � LINED ITH COIR FIBE l i i I I � I 2 !� , TREA/v16ANK. S A:BILIZATION: : _ T � �_ 1 0 I I I � Fabnc � I ��,._ ' r i I-, i I� i I I I i i j � i I _.. _.._ �I . -�v I '. i I I i ,y � - GEOTEXTILE ABRIC (TYP.} - -- -- , ; _ . _. EST. 30 CY EXCAVATION 1 � ' -- _ T= 1.5 Ft. EST. 20 TONS CLASS 'I' RIP-RAP j . I� � i u- 1 � 1 EST.40 SY GEOTEXTILE FABRIC �_ � . _ . _ . _. UTTLE EAVERD CREEK S I 3�I b�J S�, , _ 1 _ _ _. � i Lii .. �� ` � �, r T ----� - --�. : � . ' UT � - ---AV - --- _ _ _ �. I � ----�� - _ �. __ _ -_ ' \ I __ --- `—'' I L i E,�p __ _ _ -I -- -- -. _ _ . . . ! . _ _.�_ 1 I— _� - _ I . � . ' , ` _ i ! � --- � i � � � z �• EE f\�-. � � : CHANNEL IMPROVEMENTS DOWNSTREAM -- � � � � � � II CLASS I RIP- 1 'I r Z �� -- � (� -- - � L�i' `' �o 0 o i I i � - �%i _ isF��Eii- -��1.F i \ I = Iv v � � --. . __ ._.. _ - � i I -- i � II ( Not to Scale) - - I I - \ -- , � }1'N" %�� - - - -. I I I i --- I I I I i � -- FABRI�(T P.- \� - u. � i � i , -- _I_� _�_ _� � _:_ � . � --I --- -- --- � � � . Class 'P Lined w/CoBeF ber Class 'I' _- I I I I I --- �_ ' P I I I�I I� ii i I 1 Ri Ra MaHing Rip-Rap i I I I' I �� f I I I I 1 I I I I 1 I � f 1 � ��- -- --y � � P- P 02 � i I - - - - . �i 1 � i 1 _ -- . � - i -- I � ' —.- � I I I 1 I I �\ iA I �----- �b F O Grou�n H T Nat�ral __ � (� 6 I I i I I I ;�� 1� �Ground I I I I I I � � _ , I � - - --. � . _. ti - - - - - � - � r I I V- - J� ---- - � � I �— Sire Geotextile I I I � - - - 1 � I ---_. _ � Geotextile Bed Fabric _ � 1 � � I I I I .. �., .�. . �. -- - - - - -- . . _ _ � .. . , . i , , , , . , . . . � - Fabric I ' I I I I I I � - - - -- EST. 50 CY EXCAVATION i i i i I II I,- o - I'. - -- i i i I - -- i i i T= 1.5 Ft. EST. 30 TONS CLASS 'I' RIP-RAP i �' I I O�►- i I -- I' I I I I I EST. 60 SY GEOTEXTILE FABRIC - I I I I i -_'_' .. 4��'i'� �. � �I � � � i i � I I o I-_ I I H= 2.0 Ft. EST.50 SY COIR FIBER MATTING ._ I — __ � __ ---_ - -- -- I , -_ - - - - I , ---- ---- ---- ---- ---- - - i i i � i i i i i � i i iii _ _ _ -_ _ - = iu�d _--- ---- — --- -- - -- �- � - --- --- - i ' SCALE: I I I I ' i I i i i i I I i i i. -_. __ --_ _--_ __--_ _. ___ _- __I_i -_ _ i i i i I : 1" = 50' HORIZ.y � �� � � � � � � � I I I I I I � �i I I I _ I I Ili , O I li � i I i i i li II II i i ! il il I!I 1! . :II _ _ 1 e.) The project letting is tentatively scheduled for Spring 2018. The waiver is requested for this specific culvert extension only and will not be used for justification on other projects. 1 f.) The in-kind extension (3 @ 9' x 9') provides equal silt accumulation and normal depth flow for aquatic passage when compared to the buried extension alternate (3 @ 9' x 10') and the existing culvert. This is supported by silt accumulation and normal flow in the existing culvert observed at the site, which is due to its shallow slope (0.3%). 1 g.) An Activity Hazard Analysis is not applicable for this waiver request. lh.) No additional requirements or standards address the requirement in question.