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HomeMy WebLinkAboutNC0023965_plan of action_19790628 CH2M :: H ILL engineers p nners economists SCIentISt_s 28 June 1979 Mr. Forest Westfall N. C. Division of Environmental Management 512 North Salisbury Archdale Building, 8th Floor Raleigh, N. C. 27611 RE: City of Wilmington Outfall Dear Mr. Westfall: Attached is a copy of a letter and a preliminary design memo I sent to Mr. Charles Wakild earlier this month. I would appreciate receiving your opinion on the effects of our pro- posal discharge point on water quality in the Cape Fear and Northeast Cape Fear Rivers. We would like to complete our design and furnish plans and specifications to your agency for review by August 1. There- fore , an early response to my request would be appreciated. Sincerely, /J� wCLL, Glenn W. Dukes , P .E. Regional Manager jtc Attachment cc: Mr. Ward Andrews , City of Wilmington Columbia Office Dutch Center, 810 Dutch Sq. Blvd., Columbia, South Carolina 29210 803/798-4511 I June 21 , 1979 MEMORANDUM TO: Forrest Westall Modeling and Allocations Unit FROM: Charles Wakild, Regional Supervisor Wilmington Regional Office SUBJECT: Proposed Effluent Outfall y of Wilmington Northside WWTP New Hanover County Please find attached a copy of a preliminary design memorandum and cover letter from Mr. Glenn Dukes of CH2M Hill regarding subject project. It appears that the point of effluent discharge for the Northside Plant originally approved in the New Hanover County 201 Facilities Plan has been changed, due to the reason outlined in Mr. Duke's letter. He has indicated that it would be very expensive, perhaps cost prohibitive, to construct the outfall line within the limestone bedrock underneath the navigation channel of the Cape Fear River. This would involve blasting through approximately ten (10) feet of rock, due to the depth of the channel and the buffer zone between channel bottom and the pipeline required by the Corps of Engineers. For some unknown reason, this was not foreseen by the engineer during the Plan's development. It is requested that the original wasteload allocation be verified, if possible, for this discharge point. It is our opinion that secondary treatment will be adequate to protect water quality, based upon a review of allocations received for other discharges in this vicinity. However, if it appears the allocation will change, please notify me as to what extent prior to sending the allocation to Mr. Dukes. Vernon Harris has a copy of .the Facilities Plan, and has indicated he will provide any help he can on this situation. Please give Joey Hill or myself a call if you have any questions. CW/JH/sh Attachment cc: Vernon Harris A. C. Turnage Wilmington Regional Office , a . . Lj . •'�' 1 l.fir � � ��/ [�` . MISS, 7w 7. WT � `Ili l�•"s'•'��'"� ,•� ��i �.���9! �)��['^'-�� -r !'.�xas. !I i� ��ir �.. ' Ali 10. MCI mus"Lf,"PlIKAM 11 t��, a' fig■g■ ���� � �I � J :� �'p�f1:r,71{r���. i' rRi 11� �u�''�a t.� 1- �,� '� 'i ,ns.� —�` Hof TUS wo MWAM i i '� ``11 .►ate `�` Illi , f � 11��, �_1�_�1 } � 4 ��NFV5. amffl MINOR �M ;�� �L'VI/1M_� T}s 4l�fi,`ip�r`^w�i' •• .' J ��. r ti..117� �j. ��� ter.' ��• rF���.S-1 I - - CH2M ON engineers planners eCOnorllsts scientists WILMINCTON ®EN1 5 June 1979 Mr. Charles Wakild Regional Supervisor for Environmental Management N. C. Department of Natural Resources and Community Development 3143 Wrightsville Avenue Wilmington, N. C. 28403 RE: City of Wilmington Outfall CA84000. 91 Dear Mr. Wakild: Attached is a Preliminary Design Memorandum on the point of discharge for the City of Wilmington' s Northside WWTP Outfall. We have selected a discharge point on the East bank of the Cape Fear River immediately upstream from the Cape Fear Technical Institute dock in about 35 feet of water. At this site there is not sufficient distance between the navigation channel and the toe of the bank to construct a multiport diffuser perpendicular to the river. Therefore, we propose to terminate the outfall pipe with 3 nozzles. The nozzles will break up the discharge into 3 high velocity jets for blending with the river. We are furnishing this information to you at this time for your information. Should your agency have any comments or suggestions regarding our proposed method and location for introducing Northside WWTP effluent into the river we would appreciate hearing them by 15 June 1979 . Sincerely, Glenn W. Dukes, P.E. Regional Manager j at Attachment cc: Mr. Ward Andrews Mr. Bill Hoffman Mr. Bob Stroud Columbia Office Dutch Center,810 Dutch Sq. Blvd., Columbia, South Carolina 29210 803/798-4511 in, a�Hu PRELIMINARY DESIGN MEMORANDUM TO: Design Team FROM: Glenn Dukes DATE: June 1, 1979 SUBJECT: Northside WWTP Outfall Discharge Location Introduction The effluent from the City of Wilmington' s Northside -Waste Water Treatment Plant presently discharges into Smith Creek, a small tidal tributary to the Northeast Cape Fear River. The purpose of this project is to relocate the point of discharge into the Cape Fear River Estuary. The location of the discharge point chosen .in the 201 plan is in the vicinity of the confluence of the Cape Fear and Northeast Cape Fear Rivers. The object of this preliminary design memorandum is to present a summary of .the work accomplished to date on selecting the type discharge into the Cape Fear River. Location There -are two possible locations available for the diffuser at the terminus of the pipeline route selected in the 201 study. The two locations are shown on Figure 1. Point .Peter Site - This site is between the Northeast Cape Fear Navigation Channel and the Cape Fear River Channel approximately 400 feet downstream of Point Peter at a depth of about 25 feet. East Bank Site - This site is on the east bank of the Cape Fear River, near the Cape Fear Technical Institute Docks. It would be located at a depth of about 35 feet, just inside of the pier line. Location of a diffuser in the navigation channel is not acceptable because of the potential for damage during dredging. Also damage may occur to the diffusers by ships dragging anchors. Preliminary Design Memorandum Page 2 1 June 1979 Receiving Water The information on the receiving water is presented below. Tides - The average tide range at the outfall sites is about 4. 2 feet. The tides are semi-diurnal (two highs and two lows per day) . The extremes are from about 1. 5 to 5. 2 mean low water datum. Storm surges and floods will increase the level above this. For example, the estimated water level for various floods are: - Recurrance Interval Water Level (MSL Datum) 10 years 6.5 feet 50 years 9 . 8 feet 100 years 1.1. 0 feet 500 years 13. 5 feet Currents - From the "Tidal Current Tables 1979 Atlantic Coast of North America" National Oceanic and Atmospheric Administration, the minimum current (average) are 1. 7 knots on flood -tide and are 1. 5 knots on ebb tide. The current will vary from near zero during slack water to a maximum current about 3 hours after high or low tide. The currents are generally parallel to the center line of the river. Flow - The range of flow in the two rivers vary greatly from flood to drought conditions. There are no gaging_ stations near. Wilmington. The following information is estimates from gaging. stations upstream and prorated to Wilmington on a discharge per square mile basis . Northeast Cape Cape Fear Fear Upstream Gaging Station Near Chinquapin ' Near Tarheel Drainage Area at Station 600 sq. mi. 4, 810 sq. mi. Drainage Area at Wilmington 1, 740 sq. mi. 7,060 sq. mi. Avg. Discharge at Station 700 cfs 4 , 800 cfs Estimated Avg. Discharge at 2,000 cfs 7,000 cfs Wilmington 1954 Minimum Daily Discharge 5. 3 cfs 208 cfs at Station 1954 Minimum Estimated Discharge 15 cfs 290 cfs at Wilmington Preliminary Design Memorandum Page 3 1 June 1979 Salinity and Stratification - Data from various sources indicate that generally the estuary is well mixed with very low salinity. During period of low fresh water inflow, high tide or heavy flood conditions, it is ex- pected that the salinity will increase, and/or a salt water wedge will be found. Effluent Flows Maximum 11, 000 gpm 16 mgd 24. 7 cfs Average 5,500 gpm 8 mgd 12 . 4 cfs Minimum 1,500 gpm 2 . 2 mgd 3. 4 cfs Preliminary Design Concepts The concepts of a diffuser at the two possible locations are discussed below. Point Peter Site - . The diffuser located between the two river channels about 400 feet down stream below Point Peter would be a multiport diffuser. The outfall pipe would be constructed by trenching across the navigation channel and terminating in a diffuser. Some of the problems associated with a diffuser in this location are: 1. Depth of Trench: The Corps of Engineers has requested authority to maintain a 35 foot deep channel in this area with. a 3-foot over depth. _ This requires the top of the :pip.e .to be at least 6 feet below the 3-foot over- depth which would place the top of the pipe at 6-feet below 38 feet or 44 feet below MLW. 2. Limestone: The trench will be in limestone rock, probably requiring drilling and blasting. 3. Profile: Outfall pipes are generally constructed on a decending grade without high or low spots. To con- form to the practice will require that the diffuser be constructed several feet below the mud line, and the effluent brought up with long risers. Locating the diffuser below Point Peter would distribute effluent to both the Northeast Cape Fear- and Cape Fear River channels. As will be pointed out in later discussion, there does not seem to be any benefit due to long-term cummulative dilution and dis- tribution of the effluent or rate of flushing from the estuary. However, the construction cost associated with the Point Peter location would be from $200 , 000 to $300, 000 greater than locating the point of discharge on the East bank. Preliminary Design Memorandum Page 4 1 June 1979 East Bank Site - At this site there does not appear to be sufficient distance. between the navigation channel and the toe of the slope to construct a multiport diffuser perpendicular to the river flow. A multiport diffuser constructed parallel to the river flow does not distribute the effluent efficiently because the current will sweep the plume together. One scheme is to terminate the out- fall pipe with 3 nozzles as shown on Figures 2 and 3. This has the advantage of breaking up the discharge into 3 high velocity jets. There will be some over-lap of the plume when the currents are strong, however, with the fairly high current the jet should move: horizontally for a substantial distance before blending with the river current. Advantages -of this type of outfall/diffuser system are: 1. Eliminate the expensive crossing of the navigation channel. 2. Less vunerable to damage by dredging, etc. 3. Easy access to -repair: 4. Discharges into deep water thus providing good di- lution if there is a density difference. The obvious disadvantages to this scheme .is -that effluent discharing on the flood tide will be swept up the North- east Cape Fear Channel. On the ebb tide the effluent will move downstream and be partially distributed across the river. Based on our present knowledge we recommend that the east bank diffuser be constructed. The following section will present a detailed discussion of the dilution process which should take place. Discussion of Initial Dilution, Short-Term and Long-Term Distribution Initial Dilution - The energy which produces the initial dilution from a nozzle is the turbulent mixing caused by the velocity of the jet, and the density difference which causes the plume to form and rise, continuing to entrain diluting water. Preliminary Design Memorandum Page 5 1 June 1979 The "salinity data, indicates that the river is generally well mixed and it' s salinity is low. When this is the condition there is no buoyant plume formed. Based on this assumption, average dilution for various port diameter at various distances downstream can be estimated. For the proposed 11" diameter ports the average dilution is predicted to be approximately 10 to 1 at a distance approximately 20 to 25 feet from the nozzle, and 20 to 1 about 40 to 50 feet from the nozzle. The jet will spread as a cone with the dia- meter increasing at about 2 units for each 5 units down- stream. Thus at 40 feet downstream the diameter of the jet may be, about 16 feet. When there is a density difference, the plume will form and rise. This will produce additional dilution. The height of rise will depend on the stratification. If there is a salt water wedge then the effluent may become trapped and not rise to the surface. Short-Term and. Long-Term Distribution - Little informa- tion is available for making a prediction of the short- term distribution. For this case, short-term is con- sidered to be over 2 or 3 tidal cycles. As the effluent leaves the East Bank site with the current, the receiving water has already been used for diluting previously discharged effluent. It will travel with the current and spread across the river by the turbulance and mixing . characteristic. On the flood tide all of this effluent is expected to be drawn into the Northeast Cape Fear Channel. Based on the USGS Water Supply Paper 1873-E, the distribution across the channel can be expected to be quite rapid. On the ebb tide the flow will go past the outfall and receive additional effluent. Depending on how the two rivers mix, the effluent will spread out across the estuary. The next flood tide will now carry part of the effluent up into the Cape Fear River. After two or three tidal cycles it is anticipated that the effluent will be fairly well mixed in both channels . The long-term dilution is dependent on the fresh water inflow. Thus for the long-term average flow in the two rivers, estimated to be about 9 ,000 cfs, the dilution would be extensive if the average outfall discharge was 15 cfs. At this rate with complete mixing, the average dilution would be 600 to 1. Preliminary Design Memorandum Page 6 1 June 1979 The critical time is when there are drought conditions. If the fresh water inflow during an extended drought was only 250 cfs, the long-term dilution would be 15 to 1. During drought conditions the fresh water inflow appears to be much less for the Northeast Cape Fear River than for the Cape Fear River. Thus during this condition the concentration will be considerably higher in the North- east Cape Fear River. qe•16 �5 PT. -PETER 3 -r�'.�--- ,t POI T I �r , pf BAR �I VER N PETER D�'TAASF- to TLC- Its tf � `L• rSN SO' �9�Dy �� 96• 9 qO' .:.� 0 9 4 ► O 8 +���t �' 6' '�� Ors t'1� -'� .� 9b• - � 1 '� � 9b 6 lb, h 9 0 9 fib. 9$. 'b. g 6 6 ;o �b•* °� 9 9�• 910 9E'9 ti ��ti a�rO A j '1'4 ° '�, 96• b-91g6o9 b6• °�"• 6 - 9 e •��` a�� 9 y�o' _9�D• y69 �y6 9b0 96, gg a 9 96 '1 e�ro• ss; 9lb �9 °�6 '69 grOYy':�A E A S 1 �'�" N A L g�6��9'g BANK i9lifilS/'R �b $z �6• SITE C.F T.I. 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