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02785 - Technical Information Report - Revised Storm Drainage Calculations
DODDS ENGI INC. STORM DRAINAGE CALCULATIONS for ASHBURN CONDOMINIUMS RENTON,WASHINGTON O�Pa w J Prepared For: Burnstead Construction Co. t�, : �o. Prepared By : Brennan P. Taylor,P.E. Approved By: Edgar T. Jones,P.E. . — , � DEI Project No.: 94172 0:'�v March 1998 �' • ....•'' �� °`"'� �2�� ��'{ ' l 3. �,,�g ����� � Planning•Engineering•Surveying 4205-148thAvenue NE Suite 200 Bellevue,Washington 98007 _� 7�'>� Te1.425-885-7877 Fax.425-885-7963 � � E Mail.o�ce@DElonline.com TABLE OF CONTENTS I. PROJECT OVERVIEW II. OFF-SITE �.'�.ALYSIS IIL EXISTING SITE CO�TDITIONS IV. DEVELOPED SITE CONDITIONS , V. DETENTION DESIGI�T VI. CO��EYANCE CALCU�L,ATIONS i i I. PROJECT OVERVIEW ' The proposed project of Talbot Road is situated on approximately 26.6 acres in south Renton, of which approximately 22 acres will be developed. The site is located just east of Highway 167 and west of Talbot Road, approximately 1500 feet south of S.VV. 43rd Street. The proposed project includes 240 units in 53 separate townhouse and condominium buildings. Access to the site will be from Talbot road, with a private street system through the development. The east half of the site is moderately sloped to the west at 3% to 10%. This area is presently developed with several residences, outbuildings and pasture area. The western portion of the proposed developed area continues to slope to the west at 8%to 30%. Much of this area is covered with second growth forest. The far west portion of the site will not be developed due to steeper slopes and wetlands. The King County Soils Survey map indicates that the soils onsite are classified as `Alderwood', hydrologic group C. Drainage onsite will be collected and conveyed to a detention vault in the southwest corner of the site. All roofdrains will be tightlined to the conveyance system and all road runoff will be collected. The detention facility will be designed to the 1990 King County Surface Water Manual criteria for release rates and water quality treatment. Additionally, the detention vault will be designed to more strict detention criteria as required by Mitigation Measure No. 3 in the MDNS for this project. The detention vault will discharge to a biofiltration swale prior to release into the existing wetland. This drainage analysis is based on SBUH methodology and utilizes the WaterWorks hydrology program to generate and manipulate hydrographs. � � � � � � !!! 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N f� ��; f O �. 1 � „ �TMp � ' � N - a'" - � > � r �:�� � VICINITY MAP � �� � n 18 �e, n�n n " � ` �,�17❑-� � 54 TM � ' }- n° �,�< �, DEI PROJECT NO 94172 IWAK � ! 2)15T P� �H OI ` JF � �n SE � 1 � ' � SE �p ST � � r � 1 , �` � x � � � cr s a g f 4a.r � �'e h � � 9 ■ � t n SEE �16 �P II. OFF-SITE ANALYSIS UPSTREAM: There are two parcels west of Talbot Road South which are tributary to the proposed project. The north property is 1.69 acres in size and consists of two separate building structures and a small pond. The south property is 0.74 acres in size and consists of one building structure. The remaining portion of both properties is covered with grass. Drainage flows to the west onto the proposed development and will be intercepted by the onsite drainage system and conveyed through the detention system. An offsite drainage basin of approximately 36.79 acres, contributes runoff to the roadside ditch on the east side of Talbot Road South. A portion of the flows from the basin are intercepted in a 12"CPEP which travels under Talbot Road South and discharges into a catch basin. The catch basin is located approximately 180 feet south of the northeast property corner of the proposed site. The existing catch basin has a solid lid and consists of the 12" CPEP described above along with a 4"PVC outlet pipe with a 4" down-turned elbow. Flow out of the catch basin is restricted by a 4"PVC which travels west and discharges into a small decorative pond on the property. Drainage in the 12" CPEP eventually backs up due to the restriction of the 4" PVC. Drainage not intercepted by the 12"CPEP or drainage that has backflowed back into the ditch along Talbot Road South, flows south in the ditch where it is intercepted by a 18" CPEP. The 18" CPEP which travels under Talbot Road South is located approximately 670 feet south of the northeast property corner of the proposed site. The 18" CPEP appears to have been recently installed or improved. The culvert discharges into a well defined ditch, which flows to the west through the southerly portion of the property, and then continues through the site to a ravine on the south side of the property. At the time , of our field visit on February 26, 1998, there was a steady flow of water in the ditch and � culvert. The proposed onsite drainage design will include intercepting this basin's runoff as it enters the site, conveying it through the project, and discharging it directly into the ravine, bypassing the detention system. DOWNSTREAM: Runoff from the site sheet flows to the west where it enters one of two onsite ravines, continuing to the west. The ravines are approximately 600 feet apart and both discharge into the wetland at the west property line. The flows eventually merge within the wetland and continue west for approximately 100 feet to Highway 167. Three culverts cross the Highway in the vicinity of the wetland; one 36" CMP culvert approximately 800 feet to � the south, one 30" CMP culvert approximately 400 feet north, and one 24" concrete near ' the site. While we have assumed that the northern culvert is the main discharge route, it � is difficult to determine far sure due to the flat terrain. During our site visit in February of 1996, the 24"culvert was dry and appeazs to be at a slightly higher elevation, which tends to indicate that it may be a secondary discharge for the wetland. It is safe to say that during periods of high flows, the wetland will fill up and overflow into all three of these culverts. 1 � - i I j l I ` rl / ' / � / I �,' �o,.,� f �� / � .�11� _�J�../ j , I � 1 , � ` x �X � .��- - � r � , f /, / � �. 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OFF-SITE ANALYSIS (cont.) The 30" CIVIP crosses SR167 to the west and enters a well defined channel, approximately 8 feet wide and 8 feet deep. The channel flows west for 1100 feet to the East Valley highway, where a 6 foot X 4 foot concrete box culvert crosses to the west. The channel then tums to the north where it continues for over 1000 feet where it enters industrial developments. The system ultimately reaches the Green River The 36" CMP south of the site, crosses SR 167 and continues to the west along the north side of South 55th Street in a 6 foot X 6 foot channel. It flows for approximately 600 feet and crosses SSth to the south in a 36" concrete culvert. The channel flows south and west for 500 feet to a pair of 48" concrete culverts. These culverts discharge into a large stream, designated as Drainage District#1 on the Kroll Maps. It flows to the northwest for 800 feet to the East Valley highway, where a stream gauging station is located. The stream crosses under the highway through a bridge and continues west. The 24" concrete culvert crosses under SR 167 to the west into an industrial development. A discharge point was not located during field investigations, but this area originally drained to a wetland with discharges both north and south, it appeazs, towards the drainage paths described above. The developed drainage system must also follow one or both of these routes. � - � i � �fS !� J {{{/// I \I J� � i .! I ' � ' � ] 1�r y �" �{ :, u I �� � � �/ . .. ' i •~,� '+ .n� ��; �w ;.•% . ,',:._. � ' /L ,� /�+j/ J �'�� f �� � � � � , ' ;; � ; ,,'�i — l. .y'� :�;' ' � —�l .?i i Gi�t����.�; � r , ►�/ �/i� �' �• � 0 I � / 2 p� ' i � 1 �.�,-'i , ��� � � � . ! � /�' � 1 �7 . 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'��;.����'_Z.%r� .:•1� :,t�, C DEI PROJECT NO 94172 • g.�g co�u- e u ll f ��,,.�.... .�.�.,».,, • l-YE�.fS �2� � - e . III. EXISTING SITE CONDITIONS: As described previously, the eastern portion of the site is partially developed with several buildings and driveways. This area is also cleared of trees and is mainly pasture. Proceeding to the west, the site is forested w�ith second grow-th cover. Forested wetlands extend along the western properly line. Precipitation Amounts: 6-month/24-hour=0.67 inches (P,/3, King County SWM) 6-month/24-hour= 1.28 inches (0.64*P,, DOE, used for«etland recharge) 2-year/24-hour= 2.00 inches 10-year/24-hour=2.90 inches 100-year/24-hour= 3.90 inches � Existine Onsite Basin Input: Hydrologic Soils Group = C Total Area=20.20 acres Impervious Area=0.10 acres a� CN= 98 (roofs) Pervious Area=20.10 acres @ CNa�.g= 83.01 (10.10 ac. pasture @ CN= 85, 10.00 ac. second �ro«��th forest C�, CN = 81) Time of Concentration =43.98 minutes RESULTS (Peak Flow Rates): 2-year/24-hour= 1.46 cfs 10-year/24-hour=3.53 cfs 100-year/24-hour=6.18 cfs • � � � • � � ���. ..��• � ��1 � . �"��,A�.. � �� t � �`=�r ��� � � `. , �` � �►� � �! .��x■�� �.. ��' � ,�.,- _ ��.���i�,� ,� ,- � ���1 �- � .�, ��r.�a - � � � = _li���" � w �� � ` `�- �,�•�,�� ��wc: �.r�+�� �-� , , ��� f �� e . , . .����t • . �.-- \� .. w// c � ��^ ��il.�..�� �6- �r * A� _.� � _ � � �— �v �� �� -�I�l�� � ����, ,'�'O��la��� �,,, s �, .�.., �� ..,.� ��, �� � ��:,. � ������'``�� �� �,,, 'A1�� �� l� a �, .:,;-_ , ��j j�,�� _ � � . � E '� � S �//1 . 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"•�AmC • Pu /A�C I;J• Am9/'•• ���• �� � • �o / � � � Wo �PY e u AgC 9 •• AmC• ! � Os � Br , � Na No � +2 Pk � � � � �n• •��•� .- �, � 31 N s a aq a • �' AmC � � � u r .n• ,� "'' AmC. •• ° o �aai V� � ' , AkF AgB• •� • - •• •• raile• ' + ,U�� � � • F�OInC � � AgC i � �� q . , � ' � , • , � • � q . R� �" � , s�:• •• Z N�'�.�y . Os , AkF �AgC�\ • � • • � EvC �-5- -_ ___- -- Re _B��a,ao -- ,- �•\--�I .�gC • ��= ~�� .- F� _-Sk -- � Re �'rr�'� . AkF t� • � • �•1 � • � � �•e I. • ABC •• Os � � ; i �� . .'(�j • AgD AmC � , •• � � ��r � �Ur � � � � . .r� �: . :�,y � �� . AgD : N. � � �.� � Pu Sm AkF • • ' :�- � .� ' Ilr � • • . � TALBOT ROAD 13 �`''��� . eMoS z� ;,��;� . � �` � B� .• 5 �.N� A&C�;��. ' � I�NG COUNTY k N i:R 1\•£L SO�S S V 1\�i:JY l�lAl J t! p5 + • AR6 � � I I�l� • a d � � � "` _`_ �'�1i'�•• +'� DEI PROJECT NO 94172 ;_ ` 'l/ f .�.;�� .�_-�. Pu'_ _-_ `,_ _ µ� ; �' � , I—I, ', �: ---�I_-sh ,I� � . ,� �o_ �/ 'l�, M `' 1� � I � ••� = � '�="-- • . _ ..l' i � � , .� . .. �. _ :iqr � u � u _.. n t�� uo ___�_�_ 5/7/99 10 : 37 : 7 am Dodds Engineers, Incorporated page 1 TALBOT ROAD DEI JOB NO 94172 �'�_S T?� ST,J!v!N'' Si3vri N1E1Hu�OLGuY TOTAL AREA. . . . . . . : 20 . 20 Acres BASEFLOWS : O . G.: �__ RAINFALL TYPE. . . . : TYPElA PERV _ ._ PRECIPITATION. . . . : 2 . 00 inches AREA. . : 20 . 10 Acres 0 . 10 � TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 83 . 01 98 . 00 TC. . . . . 43 . 98 min _� . a� ABSTRACTION COEFF : 0 .20 PEAK RATE : 1 .46 cfs VOL: 1 . 18 Ac-ft TIME : _ BASIN ID: EXO10 NAME: EXISTING BASIN lOYR EVEP� SBUH METHODOLOGY TOTAL AREA. . . . . . . : 20 .20 Acres BASEFLOWS : O . C � ,.� . RAINFALL TYPE. . . . : TYPElA PERV =. ._ aRECIPITATION. . . . : 2 . 90 inches AREA. . : 20 . 10 Acres 0 . �: :___ TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 83 . 01 98 . 00 TC. . . . : 43 . 98 min 43 . 98 m��= ABSTRACTION COEFF: 0 . 20 PEAK R.ATE: 3 . 53 cfs VOL: 2 . 31 Ac-ft TIME : 49C � . - BASIN ID: EX100 NAME: EXISTING BASIN 100YR EVENT SBUH METHODOLOGY TOTAL AREA. . . . . . . : 20 .20 Acres BASEFLOWS : 0 . 00 _ _ RAINFALL TYPE. . . . : TYPElA PERV PRECIPITATION. . . . : 3 . 90 inches AREA. . : 20 . 10 Acres O . iO :. TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 83 . 01 98 . 00 _ � . . . . . _� . � � --�_=- .� . � � ...-_- ABSTRACTION COEFF : 0 .2�� PEA� �_A'?'E : 5 . 18 cfS VC� : _ . � ..�-_� _ �. .� . -�;;: ,.�_. III. EXISTING SITE CONDITIONS (cootinued) L�STREAM OFF-SITE CONDITIONS: As described previously, there are three basin areas that drain from tributary upstream areas onto the proposed site. Two of the basins are located on the west side of Talbot Road South and are surrounded by,but not part of the proposed project. As described above these basins are known as the north property which is 1.69 acres and the south property v��hich is 0.74 acres. These basins will be referred to in the calculations as "Offsite Basin X" and "Offsite Basin W"respectively. The third basin is the basin located east of Talbot Road South and contributes runoff to the roadside ditch on the east side of Talbot Road South. For calculation purposes, the third basin will be divided into two sub-basins. The first sub-basin will be referred to as "Offsite Basin Y". This sub-basin naturally drains into the 12" CPEP described previously. The second sub-basin will be referred to as "Offsite Basin Z". This sub- basin naturally drains into the 18"CPEP described previously. Offsite Basin W Input: Total Area= 0.74 acres Impervious Area= 0.21 acres C CN = 98 Pervious Area=0.53 acres @ CN= 86 (grass) Time of Concentration=31.49 minutes RESULTS(Peak Flow Rates): 2-year/24-hour= 0.12 cfs 10-year/24-hour=0.22 cfs 100-yeaz/24-hour=0.34 cfs Offsite Basin X Input: Total Area= 1.69 acres Impervious Area=0.30 acres @ CNa�.g= 98.33 (0.25 ac. impervious surfaces @ CN= 98, 0.05 ac. pond C. CN= 100) Pervious Area= 1.39 acres @ CN= 86 (grass) Time of Concentration=35.48 minutes RESULTS(Peak Flow Rates): 2-year/24-hour=0.24 cfs 10-year/24-hour=0.45 cfs 100-year/24-hour=0.71 cfs 5/7/99 10 : 40 :4 am Dodds Engineers, Incorporated page 1 TALBOT ROAD DEI JOB NO 94172 BAS IN SUn^,MARY BASIN ID : OW002 NAME: OFFSITE TRIB BASIN W 2YR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 0 . 74 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 00 inches AREA. . : 0 . 53 Acres 0 . 21 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00 TC. . . . : 31 . 49 min 31 .49 min ABSTRACTION COEFF: 0 .20 TcReach - Sheet L: 69 . 00 ns : 0 . 0110 p2yr: 2 . 00 s : 0 . 0556 TcReach - Sheet L: 216 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 0685 impTcReach - Sheet L: 69 . 00 ns : 0 . 0110 p2yr: 2 . 00 s : 0 . 05 impTcReach - Sheet L: 216 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 06 PEAK RATE : 0 . 12 cfs VOL: 0 . 07 Ac-ft TIME: 4 � `� - - ��. BASIN ID: OWO10 NAME: OFFSITE TRIB BASIN W 10Y=. SBUH METHODOLOGY TOTAL AREA. . . . . . . : 0 . 74 Acres BASEFLOWS : O . C� ct� RAINFALL TYPE. . . . : TYPElA PERV i'�:r PRECIPITATION. . . . : 2 . 90 inches AREA. . : 0 . 53 Acres 0 . 21 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00 TC . . . . . 3� . 49 m_r_ 3"� . 49 r-1�r_ ABSTRACTION COEFF: 0 . 2C TcReach - Sheet L: 69 . 00 ns : 0 . C110 p2yr : 2 . 00 s : 0 . 0555 TcReach - Sheet L: 216 . 00 ns : 0 . 4000 p2yr: 2 . 00 s : 0 . 0685 impTcReach - Sheet L: 69 . 00 ns : 0 . 0110 p2yr: 2 . 00 s : 0 . 0556 impTcReach - Sheet L: 216 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 0685 PEAK RATE: 0 . 22 cfs VOL: 0 . 12 Ac-ft TIME: 490 min BASIN ID: OW100 NAME: OFFSITE TRIB BASIN W 100YR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 0 . 74 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 0 . 53 Acres 0 . 21 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00 TC. . . . : 31 .49 min 31 . 49 min ABSTRACTION COEFF: 0 .20 TcReach - Sheet L: 69 . 00 ns : 0 . 0110 p2yr: 2 . 00 s : 0 . 0556 TcReach - Sheet L: 216 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 0685 impTcReach - Sheet L: 69 . 00 ns : 0 . 0110 p2yr: 2 . 00 s : 0 . 0556 impTcReach - Sheet L : 216 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 0685 PEAK RATE : 0 . 34 cfs VOL: 0 . 17 Ac-ft TIME : 490 min 5/7/99 10 : 40 : 31 am Dodds Engir.eers, Incorporated page 1 TALBOT ROAD DEI JOB NO 94172 --------------------------------------------------------------------- --------------------------------------------------------------------- BASIN SUMMARY BASIN ID: OX002 NAME: OFFSITE TRIB BASIN X 2YR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 69 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 00 inches AREA. . : 1 . 39 Acres 0 . 30 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 33 TC. . . . : 35 . 48 min 35 . 48 mi� ABSTRACTION COEFF: 0 .20 TcReach - Sheet L: 280 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 0804 impTcReach - Sheet L: 280 . 00 ns : 0 . 4000 p2yr: 2 . 00 s : 0 . 0804 PEAK RATE : 0 .24 cfs VOL: 0 . 14 Ac-ft TIME : 490 min BASIN ID: OXO10 NAME : OFFSITE TRIB BASIN X lOYR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 69 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 90 inches AREA. . : 1 . 39 Acres 0 . 30 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 33 TC. . . . : 35 . 48 min 35 . 48 min ABSTRACTION COEFF: 0 . 20 TcReach - Sheet L: 280 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 0804 impTcReach - Sheet L: 280 . 00 ns : 0 . 4000 p2yr: 2 . 00 s : 0 . 0804 PEAK RATE: 0 .45 cfs VOL: 0 .25 Ac-ft TIME: 490 min BASIN ID: OX100 NAME: OFFSITE TRIB BASIN X 100YR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 69 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 1 . 39 Acres 0 . 30 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 33 TC . . . . : 35 . 48 min 35 . 48 min ABSTRACTION COEFF : 0 .20 TcReach - Sheet L: 280 . 00 ns : 0 . 4000 p2yr : 2 . G0 s : 0 . 0804 impTcReach - Sheet L: 280 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 0804 PEAK RATE : 0 . 71 cfs VOL: 0 . 38 Ac-ft TIME : 490 min III. EXISTING SITE CONDITIONS (continued) Offsite Basin Y Input: Total Area= 5.27 acres Impervious Area= 0.0 acres Pervious Area= 5.27 acres @ CNa�,e= 84.17 (1.09 ac. second growth forest @ CN= 81, 4.18 ac. pasture @ CN= 85) Time of Concentration=45.76 minutes Peak Flow Rates: (Note: These flows are unrestricted) 6-month/24-hour=0.11 cfs (DOE 6 month storm) 2-year/24-hour=0.43 cfs 10-year/24-hour=0.98 cfs 100-year/24-hour= 1.68 cfs Flows Coming On-site from Offsite Basin Y: Flows from Offsite Basin Y first enter a ditch before entering the 12"pipe that crosses under Talbot Road S to the existing catch basin described previously. Upon entering the � catch basin, flow is restricted by the 4"PVC pipe and down-turned elbow exiting the ?, ' structure. The amount of flow that tributary to the Ashburn Condominium site from Offsite Basin Y at this crossing can be deternuned by calculating the flow through the 4" II PVC pipe and down-turned elbow. The flow through the elbow is the limiting restriction in the system. The flow through it can be calculated by using the orifice equation. The head working on the 4"orifice (elbow) is calculated by finding the flow depth in the ditch on the east side of Talbot Road and then adding the flow depth to the difference in invert elevations from the inlet into the 12"pipe to the 4" outlet in the existing catch basin. This overall head will be calculated for the 6-month, 2-year, 10-year, and 100-year storm events so that we can obtain the total flow rates tributary to the site for these separate events. Calculation ofDepth ofFlow in Ditch: The ditch has the following properties: Depth= 1.5 feet Side Slopes = l:l Bottom Width= 1 foot n=0.03 (earth bottom and rubble sides) ` Slope=0.9% '�, 5/7/99 2 : 28 :23 pm Dodds Engineers, Incorporated page 1 TALBOT ROAD DEI JOB NO 94172 �1iJ -:� ,.i:' . v��l�o .v�ii°:: . .,_ _ 5_T� �tC�� �riJii� _ b:._ SBUH METHODOLOGY TOTAL AREA. . . . . . . : 5 .27 Acres BASEFLOWS : O . G RAINFALL TYPE. . . . : TYPElA PERV PRECIPITATION. . . . : 1 .28 inches AREA. . : 5 .27 Acres 0 . 00 : TIME INTERVAL. . . . : 10 . 00 mi� CN. . . . . 34 . 17 O . 00 ABSTRACTION COEFF: 0 .2 = � TcReach - Sheet L: 30C . TcReach - Shallow L: 55 . OG ks : � . GO s : 0 . 09�� TcReach - Shallow L: 360 . 00 ks : 5 . 00 s : 0 . 375C TcReach - Channel L: 610 . 00 kc : 5 . 00 s : 0 . l0EF ?EAK RATE : 0 . 11 cfs VO:�: G . 13 Ac-ft TI:��:� : _ _ � �.-__ 5/7/99 2 : 28 : 42 pm Dodds Er.gineers, Incorporated page 1 TALBOT ROAD DEI JOB NO 94172 BASiN SUMMARY BASIN ID: OY002 NAME : OFFSITE TRIB BASIN Y 2YR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 5 . 27 Acres BASEFLOWS : 0 . 00 c�s RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 00 inches AREA. . : 5 . 27 Acres 0 . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 84 . 17 0 . 00 TC . . . . . 45 . 76 min 0 . 00 min � ABSTRACTION COEFF: 0 . 20 I TcReach - Sheet L: 300 . 00 r.s : 0 . 4000 p2yr: 2 . 00 s : 0 . 0833 ' TcReach - Shallow L: 55 . 00 ks : 5 . 00 s : 0 . C909 ' TcReach - Shallow L: 360 . 00 ks : 5 . 00 s : 0 . 3750 ' TcReach - Channel L: 610 . 00 kc : 5 . 00 s : 0 . 1066 PEAK RATE: 0 .43 cfs VOL: 0 . 33 Ac-ft TIME : 490 min BASIN ID: OY010 NAME : OFFSITE TRIB BASIN Y lOYR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 5 . 27 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 90 inches AREA. . : 5 . 27 Acres 0 . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 84 . 17 0 . 00 TC. . . . : 45 . 76 min 0 . 00 min ABSTRACTION COEFF: 0 . 20 TcReach - Sheet L: 300 . 00 ns : 0 . 4000 p2yr: 2 . 00 s : 0 . 0833 TcReach - Shallow L: 55 . 00 ks : 5 . 00 s : 0 . 0909 TcReach - Shallow L: 360 . 00 ks : 5 . 00 s : 0 . 3750 TcReach - Channel L: 610 . 00 kc : 5 . 00 s : 0 . 1066 PEAK RATE : 0 . 98 cfs VOL: 0 . 64 Ac-ft TIME : 490 min BASIN ID: OY100 NAME: OFFSITE TRIB BASIN Y 100YR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 5 .27 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 5 . 27 Acres 0 . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 84 . 17 0 . 00 TC. . . . . 45 . 76 min 0 . 00 min ABSTRACTION COEFF: 0 . 20 TcReach - Sheet L: 300 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 0833 TcReach - Shallow L: 55 . 00 ks : 5 . 00 s : 0 . 0909 TcReach - Shallow L: 360 . 00 ks : 5 . 00 s : 0 . 3750 TcReach - Channel L: 610 . 00 kc : 5 . 00 s : 0 . 1066 PEAK RATE: 1 . 68 cfs VOL: 1 . 01 Ac-ft TIME : 490 min DITCI-� FLQ� ��TALYSIS DITCH DATA: BOTT011-1 WIDTH (FT) = 1.00 JOB NA1��IE: TALBOT ROAD LEFT SIDE SLOPE (L:1) 1.00 JOB #: 94172 RIGHT SIDE SLOPE (R:l 1.00 OPERATOR: G.R. BROOKS BOTTOM SLOPE (FT/FT 0.0090 DATE: 3i3/99 ?�ZANNING'S n = 0.030 FLOW FLO�'�' WETTED HYDRAULIC DITCH FLO�'�' DEPTH AREA PERINZETER RADIUS CAPACITY VELOCITY (FEET) (SQ FT) (FEET) (FEET) (CFS) (FPS) 0.00 0.0000 0.0000 0.0000 0.000 0.000 0.05 0.0525 1.1414 0.0460 0.032 0.603 0.10 0.1100 1.2828 0.0857 0.101 0.914 _ _ _ _ _ _ _ _ _ _ __ _ _ _ __ _ _ _ _ __ o.�� a���1 �.3��:� a.���x o:��s o:��s 0.15 0.1725 1.4243 0.1211 0.198 1.150 0.20 0.2400 1.5657 0.1533 0.323 1.346 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ ! 0�24 i 0'.2�76 ' ' ',1.6�8$ 4'.1'��3 : 0.4�1 1".�8� !' I 0.25 0.3125 1.7071 0.1831 0.473 1.515 0.30 0.3900 1.8485 0.2110 0.649 1.665 0.35 0.4725 1.9899 0.2374 0.851 1.802 _. _ _ __ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ ' �.38I ; 0:5�4� :�.(1748 ; ����7 ; O:�gS 1:8?� ;: ;; 0.40 0.5600 2.1314 0.2627 1.080 1.928 0.45 0.6525 2.2728 0.2871 1.334 2.045 0.50 0.7500 2.4142 0.3107 1.617 2.15� _ _ _ ' 0.5� ; 0.7701 ' 12.4�4�5 0:31�3 !! 1:�'�6 '2.177 '' 0.55 0.8525 2.5556 0.3336 1.927 2.260 0.60 0.9600 2.6971 0.3559 2.266 2.360 0.65 1.0725 2.8385 0.3778 2.634 2.456 0.70 1.1900 2.9799 0.3993 3.032 2.548 0.75 1.3125 3.1213 0.4205 3.462 2.638 0.80 1.4400 3.2627 0.4413 3.923 2.724 0.90 1.7100 3.5456 0.4823 4.942 2.890 1.00 2.0000 3.8284 0.5224 6.096 3.048 ��'7/99 DODDS ENGINEERS, INC. PAGE 1 II' III. EXISTING SITE CONDITIONS (continued) The depth of flow in the ditch is calculated using a Ditch Flow Analysis spreadsheet attached. Results of Flow Depth: , Storm Flow De th 6-month 0.11' 2- ear 0.24' 10- ear 0.38' 100- ear 0.51' The HW elevation is therefore equal to Flo"� Depth + Elevation Difference in the 12" pipe. The12" CPEP has the following properties: Diameter= 12" Slope= 0.054 Length= 35 feet Manning's n=0.012 The difference is elevation of the 12"pipe is the slope times the length which is (0.054)*(35) = 1.90 feet. Therefore the head working on the 4" PVC is: Storm Flow Depth � Elevation Head on Di erence 4"ori rce 6-month 0.11' 1.90' 2.01' 2- ear 0.24' 1.90' 2.14' 10- ear 0.38' 1.90' 2.28' 100- ear 0.51' 1.90' 2.41' 4" Orifice Calculation: The amount of flow through the 4"onfice can be calculated using the orifice equation: Q= CA(2gh)°s Where:C = 0.62 h=head on 4" orifice (from table above) A=�r� , r= 2„ g = 32.2 III. EXISTING SITE CONDITIONS (continued) ' Solving for Q we can obtain the maximum flow through the 4" elbow for the different storm events. The following table shows that the 4"pipe accepts all of Basin Y's flows for the 6 month and 2 year storm events. The flows to the site are restricted for the 10 and 100 year events. The flow that cannot pass the 4" elbow continues south in the ditch along Talbot Road and is tributary to the 18"culvert that receives flow from Basin Z. Storm Basin Y (4"of-i icej restricted? (to 18" culvert) 6-month 0.11 cfs 0.62 cfs No 0.0 ' 2- ear 0.43 cfs 0.64 cfs No 0.0 10- ear 0.98 cfs 0.66 cfs Yes 0.32 cfs '� 100-vear 1.68 cfs 0.67 cfs Yes 1.01 cfs ' In summary, Peak Flow Rates Tributary to Ashburn Condos (Note: These flows are resultant flows leaving 4"PVC and entering site from Offsite Basin Y): 6-month/24-hour=0.11 cfs 2-year/24-hour=0.43 cfs 10-year/24-hour=0.66 cfs 100-year/24-hour=0.67 cfs Offsite Basin Z Input: Total Area= 31.52 acres Impervious Area=0.0 acres Pervious Area= 31.52 acres @ CNa„g= 84.93 (0.56 ac. second growth forest @ CN= 81, 30.96 ac. pasture @ CN= 85) Time of Concentration=42.87 minutes RESULTS: Peak Flow Rates (Note: These flows due not account for additional flow coming from Offsite Basin Y due to 4"PVC restriction): 6-month/24-hour=0.80 cfs 2-year/24-hour=2.87 cfs 10-year/24-hour= 6.36 cfs 100-year/24-hour= 10.72 cfs The peak flow rates coming onto the site through the 18" culvert will be the sum of the flows from Offsite Basin Z plus those flows that backed up into the ditch due to the 4" PVC elbow restriction in Offsite Subbasin Y. III. EXISTING SITE CONDITIONS (continued) Storm Q from Offsite Q from Offsite Q total ��� Basin Z with Basin Y through 18" no Additional culvert Flotiv ' 6-month 0.80 cfs 0.0 0.80 cfs 2- ear 2.87 cfs 0.0 2.87 cfs 10- ear 6.36 cfs 0.32 cfs 6.68 cfs 100- ear 10.72 cfs 1.01 cfs 11.73 cfs In summary, Peak Flow Rates tributary to Ashburn Condominium site via 18 " culvert: 6-month/24-hour=0.80 cfs 2-yeaz/24-hour=2.87cfs 10-year/24-hour=6.68 cfs 100-year/24-hour= 11.73 cfs 5/7/99 2 : 35 :42 pm Dodds Engineers, Incorporated page 1 TALBOT ROAD DEI JOB NC 94172 BASIN SUMMARY BASIN ID: OZ006 NAME: OFFSITE TRIB BASIN Z 6M0 SBUH METHODOLOGY TOTAL AREA. . . . . . . : 31 . 52 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 1 . 28 inches AREA. . : 31 . 52 Acres 0 . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 84 . 93 0 . 00 TC. . . . . 42 . 87 min 0 . 00 min ABSTRACTION COEFF: 0 . 20 TcReach - Sheet L: 300 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 1167 TcReach - Shallow L: 250 . 00 ks : 5 . 00 s : 0 .4200 TcReach - Shallow L: 700 . 00 ks : 5 . 00 s : 0 . 1286 TcReach - Channel L: 480 . 00 kc :20 . 00 s : 0 . 0208 PEAK RATE : 0 . 80 cfs VOL: 0 . 83 Ac-ft TIME : 54a mir. 5/7/99 2 : 35 : 34 pm Dodds Engineers, Incorporated page 1 TALBOT ROAD DEI JOB NO 94172 BASIN SUMMARY BASIN ID: OZ002 NAME : OFFSITE TRIB BASIN Z 2YR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 31 . 52 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 00 inches AREA. . : 31 . 52 Acres a . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 84 . 93 0 . 00 � TC. . . . : 42 . 87 min 0 . 00 min i ABSTRACTION COEFF: 0 .20 TcReach - Sheet L: 300 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 1167 ' TcReach - Shallow L: 250 . 00 ks : 5 . 00 s : 0 . 4200 TcReach - Shallow L: 700 . 00 ks : 5 . 00 s : 0 . 1286 TcReach - Channel L: 480 . 00 kc :20 . 00 s : 0 . 0208 � PEAK RATE : 2 . 87 cfs VOL: 2 . 08 Ac-ft TIME : 490 min BASIN ID: OZO10 NAME: OFFSITE TRIB BASIN Z lOYR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 31 . 52 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 90 inches AREA. . : 31 . 52 Acres 0 . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 84 . 93 0 . 00 TC. . . . : 42 . 87 min 0 . 00 min ABSTRACTION COEFF: 0 .20 TcReach - Sheet L: 300 . 00 ns : 0 .4000 p2yr: 2 . 00 s : 0 . 1167 TcReach - Shallow L: 250 . 00 ks : 5 . 00 s : 0 .4200 TcReach - Shallow L: 700 . 00 ks : 5 . 00 s : 0 . 1286 TcReach - Channel L: 480 . 00 kc :20 . 00 s : 0 . 0208 PEAK RATE : 6 . 36 cfs VOL: 3 . 94 Ac-ft TIME : 490 min BASIN ID: OZ100 NAME: OFFSITE TRIB BASIN Z 100YR SBUH METHODOLOGY TOTAL AREA. . . . . . . : 31 . 52 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 31 . 52 Acres 0 . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 84 . 93 0 . 00 TC. . . . : 42 . 87 min 0 . 00 min ABSTR.ACTION COEFF: 0 . 20 TcReach - Sheet L: 300 . 00 ns : 0 . 4000 p2yr: 2 . 00 s : 0 . 1167 TcReach - Shallow L: 250 . 00 ks : 5 . 00 s : 0 .4200 TcReach - Shallow L: 700 . 00 ks : 5 . 00 s : 0 . 1286 TcReach - Channel L: 480 . 00 kc : 20 . 00 s : 0 . 0208 PEAK RATE : 10 . 72 cfs VOL: 6 . 21 Ac-ft TIME: 490 min IV. DEVELOPED SITE COI�DITIO'�S As described previously, all runoff from roof drains, streets, driveways and the majority of the yard areas will be intercepted by the proposed storm drainage system and conveyed to the detention pond. Additionally, the drainage from Offsite Basin X will flo�v through the detention pond. Developed Basin Input: Total Area=20.2 acres Impervious Area=9.75 acres @ CI�' = 98 (impervious surfaces) Pervious Area= 10.45 acres @ CN= 86 (grass) Time of Concentration=21.48 minutes RESULTS: Peak Flow Rates: 2-year/24-hour=4.65 cfs 10-year/24-hour= 7.76 cfs 100-year/24-hour= 11.36 cfs ! � , r � ., - _ - - -- -� - - - - _ _ _ . = - - - - � -- N D8'96�ca. W pne � . � _- _.. . ( ~ � �a�\� Q ��_v` � /� ri� .. ' �,�+— • � _ . 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' r -- I �. ,.;r.-�or�T.r.; � �, ' .y�,ee _ -''S= ; _ . tH _ . : f' .1'• - �i � \ --��-i/,�'. "�' `�"x i j��I� T�--=�- Q—�--�-'-� , + j , DEI PROJECT NO 94172 s 1: B8'=<•C6' W . __, I y � � / � - � --._ � ' -- � . '/, . � 5/7/99 2 : 26 : 26 pm Dodds Eng�neers , Incorporated page � TA�BOT ROAD DEI JOB NO 94172 BASIN SUMMARY BASIN ID: DE002 NAME : DEVELOPED BASIN 2YR EVENT SBUH METHODOLOGY TOTAL AREA. . . . . . . : 20 .20 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 00 inches AREA. . : 10 .45 Acres 9 . 75 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00 TC. . . . : 21 . 48 min 21 . 48 min ABSTRACTZON COEFF: 0 . 20 PEAK RATE : 4 . 65 cfs VOL: 2 . 18 Ac-ft TIME : 480 m;n BASIN ID: DE010 NAME : DEVELOPED BASIN lOYR EVENT SBUH METHODOLOGY TOTAL AREA. . . . . . . : 20 . 20 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 90 inches AREA. . : 10 .45 Acres 9 . 75 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00 TC. . . . : 21 . 48 min 21 . 48 min ABSTRACTION COEFF: 0 . 20 PEAK RATE : 7 . 76 cfs VOL: 3 . 54 Ac-ft TIME : 480 min BASIN ID: DE100 NAME: DEVELOPED BASIN 100YR EVENT SBUH METHODOLOGY TOTAL AREA. . . . . . . : 20 . 20 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 10 . 45 Acres 9 . 75 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00 TC. . . . : 21 .48 min 21 . 48 min ABSTRP_CTION COEFF: 0 .20 PEAK RATE : 11 . 36 cfs VOL: 5 . 12 Ac-ft TIME : 480 min IV. DEVELOPED SITE CONDITIONS (CONT.) The developed site will consist of a bypass system that will intercept flows from Offsite Basins Y and Z. In the existing condition, these offsite basins flow through the site and discharge into the ravines on the western portion of the site. The bypass system will I ', � maintain this existing drainage pattern. The other offsite basins either sheetflow onsite i " and will be included in detention calculations or sheetflow into wetlands which are , drained into a bypass system. ' Offsite Basin Y is tributary to the project site via concentrated flow in a ditch. The flows from the ditch will be picked up in a tightlined storm drainage system with a flow splitter. The flow splitter will be designed such that the Offsite Basin Y 6-month storm will be metered into Wetland H. This will be done in an effort to provide a base flow to the wetland. All flows above the 6 month storm will be routed through a bypass system which discharges to Wetlands E and F. Offsite Basin Z (and Offsite Basin Y overflow) is tributary to the site via an existing 18" culvert. The 18 inch culvert will be intercepted into a proposed tightline storm drainage system and flow splitter. The flow splitter will be designed such that the Offsite Basin Z 6-month storm will be metered into Wetland E. This will be done in an effort to provide a base flow to the wetland. All flows above the 6 month storm will be routed through the site in a bypass system which discharges to Wetlands C in the southeastern portion of the development. All of the flows that enter Wetlands E and F will not be tributary to the on site detention system and therefore will not be included in the detention system calculations. All of the flows that enter Wetland H will be tributary to the detention system and will be included in the detention system calculations. Offsite Basin X, which is adjacent to the site, primarily sheet flows into Wetland `H' in the exiting condition and will continue to do so in the developed condition in an effort to maintain flow into Wetland `H' Offsite Basin W, which is adjacent to the site,primarily sheet flows into Wetland `E' in the exiting condition and will continue to do so in the developed condition in an effort to maintain flow into VVetland `E'. IV. DEVELOPED SITE CONDITIONS (CONT.) Flow� Splitter Calculations: Offsite Basin Y Splittef-Desigri: The flow splitter for Offsite Basin Y«�ill be designed such that there will be 0.11 cfs of flow diverted to Wetland H and the remainder of the flows will be directed to Wetland E via a bypass system. The 0.11 cfs discharge to Wetland H ��ill be via a sheet flo«� spreader. The sheet flow spreader is a gravel filled trench with a pipe at the bottom. The water will pond up in the trench and overflow onto the surface. A level grade board will be set on the top edge of the trench, along the length of the trench, to ensure that the sheet flow onto the surface is well controlled and evenly distributed. The flow splitting will be accomplished by installing a standpipe with an orifice that will pass 0.11 cfs for a particular head elevation above the grade board elevation. The water in the flow splitter catch basin will pond up to that elevation. At that same elevation an ' overflow standpipe will be set to take all additional flo��-s to �'l'etland E. The orifice equation is: Q=CA(2gh)os Where:C = 0.62 h=head elevation (distance above grade board elevation) A=�r' _ g=32.2 Q=0.11 cfs (6 month storm from basin Y) We will assume the orifice size is 2 %z inches. Therefore A= .0341 sq ft Solving for h... h= (Q/Ca),/2g = 0.42 ft 3 � �GX�s-r�r�c� c��E bV�r=�o�n!�=Lt'V _ � ��► ' � — — — — — — Grz�o� C�AK� �c.�,Y d`s'� ►q o� -r o i-�.o v����.��2 t�,P°'`'S — � � 4 2�/2 6iztF�c,t ( �w.� 5���-�-�Ect. �3 ?.�t� l—L IV. DEVELOPED SITE CONDITIONS (CONT.) Offsite Basin Z Splitter Design: The flow splitter for Offsite Basin Z will be designed such that there will be 1-foot of head acting on an orifice during the 2-year storm. The orifice equation used is: Q=CA(2gh)os Where:C =0.62 h= 1' A=�r� Q (6-month) =0.80 cfs � A=0.16 sf or r=2.7", dia= 5.4" To find the elevation difference benveen the bypass invert and the invert of the pipe going to the wetland, the depth of the 2-year storm needs to be determined. The bypass pipe leaving the flow splitter has the following properties: Diameter= 15" Slope= 0.0400 The flow in the pipe when flowing full is calculated using Manning's Equation: QF = 1.49/n (ARz�3svz� QF= 14.03 cfs The amount of flow going to the bypass system for the 2-year storm will be the 2-year storm less the amount going to the wetland (Basin Z 6-month storm= 0.80). QBYPnss—2•8� cfs—0.80 cfs =2.07 cfs The depth of flow is deternuned using a"Hydraulic elements graph for circular sewers" attached. QBrn4ss�QF=0.15 � d/D= 0.30 � d=0.30*1.25' = 0.38' , The elevation difference between the bypass invert and the invert to the wetland should II be the head acting on the orifice less the depth of water in the bypass pipe. Elevation Difference= 1' —0.38' = 0.62' IV. DEVELOPED SITE CONDITIONS (CONT.) The flow into the wetland for the 10-year and 100-year will be determined by finding the depth of flow in the bypass pipe by assuming that the flow in the bypass pipe is the total flow less the targeted 6-month flow. The orifice equation is then used to calculate the flow going to the wetland using the depth to calculate the head working on the orifice. If the flow to the wetland is different from the assumed outflow then the flow in the bypass pipe is recalculated and a new depth of flow is determined. The flow into the wetland is iteratively derived when the depth of flow is the same as the previously calculated depth of flow. 10-YEAR STORM: Qs�Ass —6.68 cfs—0.80 cfs= 5.88 cfs The depth of flow is determined using a"Hydraulic elements graph for circular sewers" attached. QsYrnss� QF —0.42 � d/D = 0.52 � d = 0.52*1.25' = 0.65' H= elevation difference +depth of flow H= 0.62' + 0.65' = 1.27' Q= CA(2gh)°5 =0.62*0.16*(2.7/12)'`(2g*1.27)°s = 0.92 cfs MORE THAN 0.80 cfs QBYrAss—6.68 cfs—0.92 cfs= 5.76 cfs Qs�-�..�ss� QF=0.41 � d/D=0.52 OKAY (same as above) therefore QB,.P,�SS = 5.76 cfs 100-YEAR STORM: QsYrnss = 11.73 cfs—0.80 cfs= 10.93 cfs The depth of flow is determined using a"Hydraulic elements graph for circular sewers" attached. QB�.4ss� QF=0.78 � d/D = 0.74 � d= 0.74*1.25' = 0.93' H = elevation difference+depth of flow H= 0.62' +0.93' = 1.55' Q = CA(2gh)°�5 =0.62*0.16*(2.7/12)2(2g*1.55)°' = 0.99 cfs MORE THAN 0.80 cfs Qs�rnss= 11.73 cfs—0.99 cfs= 10.74 cfs QB,�Ass�QF —0.82 � d/D = 0.77 OKAY (same as above) therefore QB,:P,ass = 10.74 cfs I'RliCTIUN OF SANITaRY, STOIi�I SI:�VLRS � IiYDRAi1LIC5 OF SEWERS S7 �crease as the diameter of the pipe incrcases. This vei��of f and ^ �ced, the srnoother the pi��e. Thus, from a hydraulic lo.o i.z i.a i.e i.a z.o z.z ti.a 2e 2.s 3.o a.z s.a 3.e ce of using somewhat srnuller vulues of n for large . E�C 13 gener�lly not correct. (A similar deduction is 09 n,/rariabla with depth \� �\ (' 1'f'���e(�.� ----n./constant / 1 , —Independ�nt of n,/ � � �sc that tests of � l2-in. (30.5-crri) pipe, with a D/k � 0e _ ___ —' _ ' j :e an n v�,ltte of 0.010; then, the v&Ill@ f01' fl GO-lll. Darey-Weisbacb �� � Frittion fador,( i � tl�e same k vulue (Equatio►i 20) would be n= . o� __ � . � i an n �•alue is cleveloped for large pipe, it may be b , �� ' � � ❑e ��alue for �mall pipes, since they may be subject �0.6 � ' � oiscnerge, �� , Il�llel]CC5. A Q � � Flydraulie � � ti ndlus,R i�uilar channels of non-circular cross section roughly �o.e uay- be reactied if 4R is suUstituted for the pipe g ManninQ�:„ ; a , 0 o.a C 111 tllflf� O❑ the one hand, f will decrease with con- � Q _ ��� � � ` Velxity,V /% i� other hand n generally must increase. This results o.3 , . e �tanning coefficient., assumed a� a constant, does , � � Af6�� � '� �II of thc vnriables and conditions of flow. 0�2 � � �� . � � . iit and Iiyc:raulic-Elements Graphs o.l � — , _ �on�iuit flnd oPen-channel fiow the hydraulic radius o / ` �'� X " 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 - friction formulns Wlt�] SStlBfactory accuracy even Hydraullceleme�ts y � '� �nd R esperimental data «�ere de��eloped from circular v�' e�' A�' R� �r roughnesses of the t}�pe often found iii pipe work r FIGURE 24.-Hydraulic-elemente graph for circular sewera. ����nerete surfaces), pi��e resistance diagracns of the ?11 may be used for estimating f for open channels. • et al. (22). Graphs for sewers of other than circular cross sectiom m�y isl�-. 4R should be used in place of D in computing be developed by the same general method. nnd relative roughness. It shoiild be realized that 1�1ost of the hydraulic-elements grAphs in common use have been pre- nc�tiit changcs the frictional resistance from that pared on the assumption that the llanning n does not change with the ��1�- chart. Chow (2) has discussed the shape effect depth of flow for the particular conduit shape. Nonetheless, many ex- ioii:il resistance. perimenters have observed a variat•ion of n with depth of flow. The es- I�;innel shnpe on Manning's n tiave not been defined periments of Wilcos (23) and of YArnell and Woodward (24) show that aft�ects f it must also affect n. the value of n for a pipe flowing partly full is greater than for the full �h oi flow on and n ha��e not been defined com- � 1 pipe; and the average n values for 824 experiments are as indicated by ttie efiect of clepth of flo«• or the height of veoetal cover ,, curve through the points marked by circles in Figure 24. A simila,r curve h�ient has been investigated in grass-lined channels for the llarcy-Weisbach fruction factor f also is shown in the same figure. aLo for naturr�l channels and flood plains (2). A . The relation betn-een the two friction coefficients is ��ith deptl� of flow �cithout an apparent change in l�a; been observed in circulftr conduits and other i n R � f '� n`_CR�I f�l . . . . . . . . . . . . . . . . . . . . . .21 partly fu]]. This vari�tion is due to the effects of i / / which is similar to Equation 19. �t� graph may be used for the �olution of problems The points in Figure 24 marked by triangles and x's were estimated �cl flow in closed conduits. It also is useful to � from the messurements made by Johnson (25) in large I,ouisville, Ky., �f �-ariation of n with d�pt,h of flow. I'igure 24 is a I sewers flowing partly full. Since individual vulues of j/f� in the esperi- , or s of � ir c� �ctic t un me► Wi �nd �rne i W �rd i wi fror. IV. DEVELOPED SITE CONDITIONS (CONT.) FLOW SPLITTER RESULTS: Total Flow Coming Flow Going To Flow Going Directly to Onsite Wetland B ass S stem Basin Y 2- ear 0.43 cfs 0.11 cfs 0.32 cfs Basin Y 10- ear 0.66 cfs 0.11 cfs 0.55 cfs Basin Y 100- ear 0.67 cfs 0.11 cfs 0.56 cfs Basin Z 2- ear 2.87 cfs 0.80 cfs 2.07 cfs Basin Z 10- ear 6.68 cfs 0.92 cfs 5.76 cfs Basin Z 100- ear 11.73 cfs 0.99 cfs 10.74 cfs i , � , T V. DETEI�TION DESIGN The detention criteria used for this analysis is from the 1990 King County Surface Water Manual (SWM). The resulting detention volume must then be increased by 30 % as required by Mitigation Measure No. 3 in the MDNS for this project. Water quality for this project will be accomplished via a "wetvault"or"dead" storage in the detention vault. This is required by Special Requirement#5 of the 1990 King County SWM. The detention vault will also discharge to a bioswale for additional water quality treatment Since offsite tributary basins are involved in this analysis, the design release rates must be adjusted to allow the runoff from those flows from offsite basins that will not be bypassed to pass through the detention system without being detained. This simply involves adding the appropriate offsite flows to the existing basin release hydrographs. The required peak release rates are as follows: •Developed 2-year release � Existing Onsite 2-year+ Offsite Basin"X"2-year+Flow Diverted to Wetland `H' 2-year� 1.81 cfs •Developed 10-year release � Existing Onsite 10-year+ Offsite Basin"X" 10-year+Flow Diverted to Wetland `H' 10-year� 4.09 cfs � •Developed 100-year release � Existing Onsite 100-year+ Offsite Basin"X" 100-year+Flow Diverted to Wetland `H' 100-year�7.00 cfs The flows entering the detention system from the developed site are: •Developed Onsite 2-year+ Offsite Basin"X"2-year+Flow Diverted to Wetland `H' 2-year� 5.00 cfs •Developed Onsite 10-year+Offsite Basin "X" 10-year+Flow Diverted to Wetland `H' 10-year� 8.32 cfs •Developed Onsite 100-year+ OffsiteBasin"X" 100-year+Flow Diverted to Wetland `H' 100-year�12.18 cfs The detention in ut h dro a hs are the result of combinin the develo ed onsite P Y � P g P hydrographs to the offsite hydrographs for each storm event. A dummy hydrograph equal to a constant value of 0.11 cfs was also created to accomplish the desired input. The final detention vault is to be 200' long X 40'wide. Therefore we will begin our modeling��ith a detention vault that is 30% smaller(153.85' long X 40' wide) so that our final vault will be 200' X 40'. The results of the routing and sizing produces a detention vault that is 200' long X 40' wide X 7.24' deep (live storage). See attached Watenvorks printouts and calculations. 5/7/99 2 : 26 : 27 �n: Dod�s Engi:�eers , �r_ccrporatea page 3 TALBOT ROAD DEI JOB NO 94172 STAGE STO�AGE TABLE RECTANGULAR VAULT ID No. VAULT Description: DETENTION VAULT Length: . -�5.3:. 85 ft . Width : 40 . 00 ft . voids : 1 . 000 _ �. �: . STAGE <---STORr1GE----> STAGn <----S.C�RA�E----> STAGE <----STOZ�GE----> �_'A3E <----Si'O:tt'�.GE----> � (ft; ---cf--- --Ac-Ft- ;ftj --cf-- --A�-Ft- (fti ---cf--- --Ac-Ft- ;ft) ---cf--- --Ac-Ft- 50.00 0.0000 0.0000 53.60 22154 0.5086 57.20 44308 1.0172 60.80 664'0l 1.5257 50.10 615.38 0.0141 53.70 22769 0.5227 57.30 44923 1.0313 60.90 67077 1.5399 � 50.20 1231 0.0283 53.80 23385 0.5368 57.40 45538 1.0454 61.00 67692 1.5540 50.30 1846 0.0424 53.90 24000 0.5510 57.50 46154 1.0595 61.10 68308 1.5681 50.40 2462 0.0565 54.00 24615 0.5651 57.60 46769 1.0737 61.20 68923 1.582? 50.50 3077 0.0706 54.10 25231 0.5792 57.70 47385 1.0878 61.30 69538 1.5964 50.60 3692 0.0848 54.20 25846 0.5933 57.80 48000 1.1019 61.4D 70154 1.6105 50.70 4308 0.0989 54.30 26462 0.6075 57.90 48615 1.1161 61.50 70769 1.6245 50.80 4923 0.1130 54.40 27077 0.6216 58.00 49231 1.1302 61.60 71385 1.6365 50.90 5538 0.1271 54.50 27692 0.6357 58.10 49846 1.1443 61.70 72000 1.6529 51.00 6154 0.1413 54.60 28308 C.6499 58.20 50461 1.1584 61.80 72615 1.6670 51.10 6769 0.1554 54.70 28923 0.6640 58.30 51077 1.1726 61.90 73231 1.6811 5:.20 7385 0.1695 54.80 29538 C.6761 58.44 51692 1.1867 62.00 73846 1.6953 51.30 8000 O.1B37 54.90 30154 0.6922 58.50 52308 1.2006 62.10 74401 1.7094 51.40 8615 0.1978 SS.OD 30769 D.7064 58.60 52923 1.2149 62.20 7507'I 1.7235 51.50 9231 0.2119 55.i0 31385 0.7205 58.70 53538 1.2291 62.30 75692 1.7377 , 51.60 9846 D.2260 55.20 320D0 0.7346 58.80 54154 1.2432 62.40 76308 1.7516 51.70 10462 D.2402 55.30 32615 0.7467 58.90 54769 1.2573 62.50 76923 1.7659 51.80 11077 0.2543 55.40 33231 0.7629 59.00 55385 1.2715 62.60 77538 1.7800 51.90 11692 0.2684 55.50 33846 0.7770 59.10 56000 1.2856 62.70 78154 1.7942 52.00 12308 0.2825 55.60 34462 0.7911 59.20 56615 1.2997 62.80 78769 i.8083 52.10 12923 0.2967 55.70 35077 0.8053 59.30 57231 1.3138 62.90 79385 1.8229 52.20 13538 0.3108 55.80 35692 D.8194 59.40 57646 1.3280 63.00 80000 1.8365 52.30 14154 0.3249 55.90 36308 �.8335 59.50 58461 1.3421 63.10 60615 1.8507 52.40 14769 0.3391 56.00 36923 0.8476 59.60 59077 1.3562 63.2D Bi231 1.8648 52.50 15385 0.3532 56.10 37538 0.8618 59.70 59692 1.3703 63.30 81846 1.87c�y 52.60 16000 0.3673 56.20 38154 0.8759 59.80 60308 1.3645 63.40 82461 1.8931 52.70 16615 0.3814 56.30 36769 0.8900 59.90 60923 1.3966 63.50 83077 1.9072 52.80 17231 0.3956 56.40 39365 0.9041 60.00 61538 1.4127 63.60 83692 1.9213 52.90 17846 0.4097 56.50 40000 0.9183 6D.10 62154 1.4269 63.70 84308 1.9354 53.00 18462 0.4238 56.60 40615 0.9324 60.2G 62769 1.4410 63.80 84923 1.9490 53.10 19077 0.4379 56.70 41231 0.9465 60.30 63385 1.4551 63.90 85538 1.963? 53.20 19692 0.4521 56.80 41846 0.9607 60.40 64000 1.4692 64.00 86154 1.977E 53.30 20306 0.4662 56.90 42461 0.9748 60.50 64615 1.4834 c4.00 86154 1.977c 53.40 20923 C.4803 57.80 43077 0.9889 60.60 65231 1.4975 >;.50 21�38 C.4945 57.1: 43E52 1.CC3�� 50.70 6584c _.5:_0 5/7/99 2 : 26 : 27 pm Dodds Engineers, Incorporated page � TALBOT ROAD DEI JOB NO 94172 --------------------------------------------------------------------- --------------------------------------------------------------------- STAGE DISCHARGE TABLE MULTIPLE ORIFICE ID No. REST Description: RESTRICTOR Outlet Elev: 50 . 00 Elev: 48 . 00 ft Orifice Diameter: 5 . 9297 in. Elev: 53 . 70 ft Orifice 2 Diameter: 7 .2891 in. Elev: 55 . 50 ft Orifice 3 Diameter: 6 . 9375 in. STAGE <--D:SCi�rlRGE---> STh3S <--D_S��FiAP.GE---> STA33 <--D_SCHF.P,�E---> ST�GE <--DISCHr`,RGE---> (ft) ---cfs-- ----- (fti ---cfs-- ---- (fti ---cfs-- ------- (ft; ---cfs-- ________________________________________________________________________________________________________ 50.00 0.0000 52.1C 1.3827 54.20 2.9750 56.30 5.8860 � 50.10 0.3017 52.2D 1.4153 54.30 3.0954 56.40 6.��22_ 50.20 C.4267 52.30 1.4471 54.40 3.2078 So.50 6.15:3 50.30 0.5226 52.40 1.4782 54.50 3.3137 56.60 6.2704 50.40 0.6035 52.50 1.5087 54.60 3.4143 56.70 6.39'8 50.50 0.6747 52.60 1.5386 54.70 3.5104 56.80 6.5159 50.60 0.7391 52.70 1.5679 54.80 3.6027 56.90 6.6?C5 50.70 0.7963 52.80 1.5966 54.90 3.6916 57.00 6.743; � 50.8C 0.8534 52.90 1.6249 55.00 3.7775 57.10 6.8531 SC.90 0.9052 53.00 1.6527 55.10 3.8608 57.20 6.95Cc 51.00 0.9542 53.10 1.6800 55.20 3.9417 57.30 7.Oo=; 51.10 1.0007 53.20 1.7069 55.30 4.0204 57.40 7.:6;s 51.20 1.0452 53.30 1.7333 55.40 4.0972 57.50 7.27�8 51.30 1.0879 53.40 i.7594 55.50 4.1721 57.60 7.3,== 51.40 1.1290 53.50 1.7851 55.60 4.6584 57.70 7.40?: 51.50 1.1686 53.60 1.8104 55.70 4.9012 57.60 7.So=C 51.60 1.2069 53.70 1.8354 SS.80 5.1027 57.90 7.Ec��'_ 51.70 1.2441 53.80 2.3160 55.90 5.2822 56.00 7.7�37 51.9Q 1.2802 53.90 2.5291 56.00 5.4474 51.90 1.3152 54.00 2.6980 56.10 5.6019 52.C�0 1.3494 5�2.10 2.6439 56.20 5.7483 5/7/99 2 : 26 : 26 pm �odds �ngi�eers, �rcorpo�ated paae 2 TALBOT ROAD DEI JOB NO 94172 --------------------------------------------------------------------- --------------------------------------------------------------------- HYDROGRAPH SUMMARY PEAK TIME VOLJME HYD RUNOFF OF OF Contrib NUM RATE PEAK HYDRO Area cfs min. cf\AcFt Acres --------------------------------------------- --------------------------------------------- 1 4 . 650 480 95004 cf 20 .20 2 7 . 756 480 154278 cf 20 .20 3 11 . 362 480 222887 cf 20 .20 4 0 . 110 110 9339 cf 20 .20 5 0 . 352 490 15594 cf 21 . 89 6 0 . 564 490 20244 cf 21 . 89 7 0 . 820 490 25764 cf 21 . 89 8 4 . 993 480 110598 cf 42 . 09 9 8 . 308 480 174522 cf 42 . 09 10 12 . 168 480 248652 cf 42 . 09 17 1 . 808 670 110598 cf 42 . 09 18 4 . 087 560 174522 cf 42 . 09 19 7 . 000 540 248652 cf 42 . 09 7 II 5/ /99 2 :41 : 31 pm Dodds Englneers, Incorporated page 1 TALBOT ROAD DEI JOB NO 94172 --------------------------------------------------------------------- --------------------------------------------------------------------- LEVEL POOL TABLE SUMMARY '�=."��H I'�F��i: -.._..- -D=�- <-FcP,^:-> 0_i=r�.Cn cT��rts,;�g I <------�--�ESCRIFTICi7--—-----> ;c_si (cfsi --'_d- --ic- <-STAGE> _d (c=s; VO:, icf; '. 2�iR . . . ....................... i.81 4.99 VAULT REST 53.59 i7 1.81 22102.80 cf :C'z3 .. ........ ............. .. 4.09 8.31 VAL'LT REST 55.39 16 4.09 33146.55 cf _��Y� . . . . . . . .. . . . . . .. .... . . .. 7.00 12.:7 VAULT REST 57.24 19 7.00 44539.38 cf REPo.�5E1JT$ I53.84-�� X40' X 7•Z4'�O�_P T�t+�s ��Tt'NTic�l VoLUnn� MUsT 7�NtN r3E ��.tc2,EqSEp �`� �O°�o F�o(2„ Tt1E 'F 1�1q1� �/AJ L'e S�Z�. l�� t�}i t.t. t h1 G�Z.��L T�+� l.t`ti!� Frl ��y 30'�o To �GN E �"�1 E 'rN►S � = l �3• � x I• 3 = 2Do ' r-��l A�. �IAJ L"j' S�"Z,t., ; ZOU � �C Q-O ��C �,Z�}� ���V� sTv�R.R�� ..� . •�� MSMY������� �������N�tl00 NB���[�/��NNt/ :.. a��a�adou � • v�i��r�uu ����oe�o����n ... ������1�H�� �i��a�iiuu •• , , . . , NiI��B�Oflfl „ ��/1/��l�i11� � . � : , ��d//� ■111! .. //�ill�l��qA� .��' ����� .. �d� ������ �� ������� � .. �d rY• ///lII/L`J�LiBVNANdp Y��////�/��O i��v v o O O p �// L/11��9NOpNNNqd :• /�II�/Y/I'�rS��iO��M�1M�■ J�vI1// IA�'�!Y=�t�t�B��O�� i�rii�'it�a���as�nw��nu ��� aiirir���m��a�ao��n�� � �WIf��N�d�/ �1fI///� ����p�1/�q - ���Ip�����Yd NY����d�����H e�� i�d.��///��v�nU��11 � . �����iW�/�/O���������11 1 . �����'�v� ��inuniiiiii � ��� . �������� _..�� . , �J, �a � �� :g ��a � r � �BH��■ i�� ■��� S � � r'��� �o �� ��� �■� ■�� , � /1�/./���i��/�//.�'� ��.��� ��a ,���Yi�/J/�If! �� i�N YM��tl�� ,..s,/;r:���.:���r��,�:-�--�vood�::: �/����//��%l�,���ii�����iv�fpf�U //!l��A/� • ■�5����N��B�1� s/r/�/JLIA,�6■ ��-q�aa��n 1•!I'J/Y/J�/�B� ����vvOdBA ��ti�i"�✓i��i����iiinuunri �����������An�����u��11�1�111 I,� 1� �� . �SG������ � � ���=� ��� �i�.� ��� —����;��� �► �:s�..� � �����.� . . . . . . . .: . . . . . . . : . .. . � � . . . 11 � i V. DETENTION DESIGN (continued) li SLTMM:ARY: ' Design Peak Peak Release Rate Input Rate Release Rate 2-year 1.81 cfs 4.99 cfs 1.81 cfs 10-vear 4.09 cfs 8.31 cfs 4.09 cfs 100-vear 7.00 cfs 12.17 cfs 7.00 cfs V. DETENTION DESIGN (continued) w'ETVAULT CALCULATIONS Special Requirement#5 of the King County SWM requires this project to have dead storage in the detention vault equal to the "6 month" storm event for the developed site. The "6 month" storm event as defined by the 1990 King County SWM is a storm whose precipitation is 1/3 of the 2year. P,/3 = 0.67 inches The volume of the corresponding hydrograph is 19,096 cu. ft. (see attached). The vault size is already determined to be 200' X 40'. The required depth of dead storage is: Depth= 19096/(200' X 40') =2.4 ft Therefore the final vault dimensions are 200' X 40' X 9.64'. BIOSWALE CALCULATIONS The vault will discharge into a biofiltration swale for water quality treatment prior to discharge into the wetlands. The bioswale will be designed according to the King County Manual using a design water depth of 8 inches for wetland vegetation(see attached spreadsheet). 5/7/99 2 : 37 : 18 pm Dodds Engineers , Incor�orated page 1 TALBOT ROAD DEI JOB NO 94172 --------------------------------------------------------------------- --------------------------------------------------------------------- BAS�N SUMMARY BASIN ID: DE6M0 NAME : DEVELOPED BASIN 6M0 (CITY) SBUH METHODOLOGY TOTAL AREA. . . . . . . : . 20 .20 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP PRECIPITATION. . . . : 0 . 67 inches AREA. . : 10 . 45 Acres 9 . 75 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00 TC. . . . : 21 . 48 min 21 . 48 min ABSTRACTION COEFF : 0 .20 _ PEAK RATE : 0 . 88 cfs VOL: 0 . 44 Ac-ft TIME : 480 m�n U�lA'rt� Q�A�tr/ V O l.V i►�t E V��o = ta, o9� cF Size Biofiltration Swale Note: All values in boxes are entered and this computer spreadsheet calculates the rest Flow (Q2) = 1.81 cfs slope (s) = 2.00% (bottom slope for for biofiltration design) depth (y) = 0.67 ft. (Table 4.6.4A) Mannin�s n 0.35 for biofiltration design Sides (z) = 3 (3 to 1 side slope preferred) Required bottom width (b) calculation Q = (1.49/n) x A x R^.66 x S^.5, (mannings equation) A = (b+zy)y (Fig. 4.3.7E common sections) R = A/(b+2y(1+z^2)^.5 (Fig. 4.3.7E common sections) , Q= (1.49/n) X ��b+zY)Y� X �b+zY)Y/b+2Y�1+z^2)^.5]^.66 x S^.5) I, All variables in the above equation are given except "b" "b" is solved for using a computer spreadsheet function to obtain II a trail and error solution of b 5.00 feet, for a desired length of 200 feei check Q = 1.8105 cfs, OK same as given Find bottom width required if actual length is 180 ft. (Minimum length is 50 feet.) Find water surface area of 200 foot long biofiltration swale as required. Pequired��ater Surface area = (2zy+b) x 200 feet = 1805 SF Actual bottom�ridth required = (Des. Surf. Area)/(length (actual)) -2zy = 6.01 feet Find depth required for conveyance of 100-y�ear storm peak floti� after developement. Flow (Q100) 12.18 cfs slope (s) 2.00% (actual) ' bottom�n�idt 6.01 ft. (actual as calculated) Nlannings n 0.027 (high"n" due to very shallow flow) Sides (z) 3 (3 to 1 side slope preferred) Flow�Depth (y) calculation Q = (1.49/n) x A x R^.66 x S�(mannnings equations) � I A = (b+zy)y {Fig. 4.3.7E common sections) R = A/(b+2y(1+z^2)^.5 (Fig. 4.3.7E common sections) Q= (1.49/n)X ��+zY)Y� X �b+zY)Y/b+2Y�1+z^2)^.5]^.66 x S^.5) All variables in the above equation are given except"y" "y" is solved for using a computer spreadsheet function to obtain a trail and error solution of y 0.423 feet, for a desired length of 200 feei check Q = 12.1309 cfs,OK same as given r '�_� � �I Check velocity at 100-year flow rate,use y = 0.423 as calculated. ' V=Q/A -Q/b+�Y) =3.938 fps «< 5 fps limit OK The velocity of the 2-yr flow rate will be less than the maximum 1.5fps since the 100-yr flowrate is les Size Biofiltration Swale Note: �, All values in boxes are entered and this computer spreadsheet calculates the rest �, Flow(Q2)= 1.08 cfs slope(s)= 2.00% (bottom slope for for biofiltration design) depth{y)= 0.67 ft. (Table 4.6.4A) Mannings n 0.3� for biofiltration design , Sides (z)= 3 (3 to 1 side slope preferred) �, Required bottom width(b) calculation � Q= (1.49/n)x A x R^.66 x S^.5, (mannings equation) ' A= (b+zy)y (Fig. 43.7E common sections) R= A/(b+2y(l+z^2)^.5 (Fig. 4.3.7E common sections) Q= (1.49/n)x[(b+zy)Y] x [b+zy)Y/b+2y(l+z^2)^.5]^.66 x S^.5) All variables in the above equation are given except "b" "b" is solved for using a computer spreadsheet function to obtain a trail and error solution of b= 4.10 feet, for a desired length of 200 feet. check Q= 1.5399 cfs, OK same as given ' Find bottom width required if actual length is 180 ft. (Minimum length is 50 feet.) Find water surface area of 200 foot long biofiltration swale as required. Required water Surface area=(2zy+b) x 200 feet = 1623 SF Actual bottom�vidth required=(Des. Surf. Area)/(length (actual))- 2zy _ �.00 feet Find depth required for conveyance of 100-yeaz storm peak flow after developement. Flow (Q 100} 12.36 cfs slope(s) 2.00% (actual) bottom width 5.00 ft. (actual as calculated) Mannings n 0.027 (high "n"due to very shallow flow) Sides(z) 3 (3 to 1 side slope prefened) , Flow Depth(y)calculation Q=(1.49/n)x A x R^.66 x S^.5 (mannnings equations) A= (b+ry)y (Fig. 4.3.7E common sections) R= A/(b+2y(l+z^2)^.5 (Fig. 4.3.7E common sections) Q=(1.49/n)x[(b+zy)Y] x[b+zy)y/b+2y(l+z^2)^.5]^.66 x S^.5) All variables in the above equation aze given except "y" "y" is solved for using a computer spreadsheet function to obtain a trail and error solution of y= 0.289 feet, for a desired length of 200 feet. check Q= 5.2741 cfs, OK same as given Check velocity at 100-year flow rate,use y= 0.289 as calculated. V=Q/A =Q/b+2zY) =3.112 fps «< 5 fps limit OK The velocity of the 2-yr flow rate will be less than the maximum 1.Sfps since the 100-yr flowrate is less than �fF r � VI. CONVEYANCE CALCULATIONS The proposed conveyance system consists of a pipe/catch basin network that was designed in accordance with Chapter 4 of the 1990 King County Surface Water Design Manual. The conveyance analysis uses the rational method for calculation of flows. A conveyance spreadsheet was produced for both the 25-year and 100-year storms. The attached spreadsheet shows that the system as designed has adequate capacity to convey the 100-vear storm event. The attached spreadsheets analyze conveyance for the on-site drainage system that is tributary to the detention vault. There are three additional bypass systems on the site: 1. Intake 58A to Outfa1156A This system receives runoff from Offsite Basin Y as identified in Section III of this report and discharges the high flows to Wetland H. The 100yr flow from Offsite Basin Y is 1.68 cfs. The pipes in this system that have the least amount of flowing full capacity are 12"pipes at a slope of s= 0.006. The flowing full capacity of a 12"pipe @ s = 0.006 is 3.0 cfs. Therefore, this system is adequately sized for the 100yr storm event. 2. CB �5 to Outfall 54B This system receives the input from Offsite Basin Y as discussed above (1.68cfs for the 100yr event). This system also receives runoff from Offsite Basin W as identified in Section III of this report (0.34 cfs for the 100yr event). This system also receives the low flow, 6 month storm from the Offsite Basin Z flow splitter (0.99cfs for the 100yr event). This system also receive the runoff from the rainfall that falls on wetlands E and F (0.46cfs for the 100yr event). The total 100yr flow from all of these basins is approximately 3.47 cfs. The pipe in this system that has the least amount of flowing full capacity is a 15"pipe at a slope of s= 0.0166. The flowing full capacity of a 15"pipe @ s=0.0166 is 9.04 cfs. Therefore, this system is adequately sized for the 100yr storm event. 3. CB 63 to Outfall 59A This system receives the high flows from the Offsite Basin Z flow splitter as identified in Section IV of this report. The 100yr flow from the flow splitter is 10.74 cfs. The pipe in this system that has the least amount of flowing full capacity is an 18"pipe at a slope of s=0.0100. The flowing full capacity of an 18"pipe @ s=0.0100 is 11.4 cfs. Therefore, this system is adequately sized for the 100yr storm event. .::y�1�.1�..{Y���1�.�':�;{��\��.{��:y7��1X�:;}l�j���}I�.\ �: L(H:A'I�IUN: KINCCUUN7Y(ftEN7'ON) =�-lIRIiA1NFALL: lA INl'IIES OB NAME: ASdBUNN CONDOMINIUMS JOB NUMBER: 9tl72 �REPARED BY: BPT DESIGN STORM: t5 YF:AH '��11YCµtMW1A1,. �;'�t1jN0�1�,: I�I�F,Rj'IPIA �77M¢UF�.',M7f�tALI': I�N�BVjANY;; PPL :: tIPB 'f[t[ �y.*rop,':�'Tp.�vpi .�RIPF.CA3AS:11'YS11M1tMAR.Y,:;: �IPY6K�N�±�' . .::�p%�'::::' .t:4K�hUk�?! .lNc,. 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