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Home My WebLink AboutSWP272730 DODDS ENGI INC. 7 STORM DRAINAGE REPORT FOR WHISPERING PINES RENTON, WASHINGTON PREPARED BY: Gina R. Brooks APPROVED BY: Edgar T. Jones, P.E. �,�.�oF ti••. DATE: July, 1998 ,h DEI PROJECT NO.: 98098 ••H 2035 : ,STE _. `S7 N AL � 'q 1 Planning•Engineering•Surveying EXPIRES _4.. 4205-148th Avenue NE Suite 200 a> Bellevue,Washington 98007 Tel.425-885-7877 Fox.425-885-7963 E Mail.ofce@DElonline.com WHISPERING PINES TABLE OF CONTENTS Section I: Project Overview Section II: Off-site Analysis Section III: Detention Analysis and Design Section IV: Conveyance System Analysis and Design Section V: Basin and Community Plan Areas Section VI: Erosion/Sedimentation Control Design i SECTION I: PROJECT OVERVIEW: The Whispering Pines Plat is a proposed single family development located between 2612 and 2616 Jones Avenue NE in Renton, Washington in the Northwest Quartcr of the Northeast Quarter of Section 5, Township 23 North, Range 5 East W.M. The property will be accessed from Jones Avenue NE in a right of way dedication or NE 26th Place located between 2612 and 2616 Jones Avenue NE. The subject property is approximately 2.79 acres in size with 2.19 acres to be developed. The remaining 0.60 acre of the property is wetland and buffer. The property is bounded by Jones Avenue NE on the west, residental property on the south, catagory 2 wetlands to the cast, and residential property to the north. The site consists of four stuctures; a shop, a shed, and two horse barns. The remaining poriton of the site is covered by lawn near the 2612 house, pasture grasses, a few scattered trees, and a wetland at the northwest corner of the property. The site is divided into two drainage basins, east and west, which are separated by a ridge that runs in a general north-south direction through the property. Drainage in the west basin is conveyed southeast and then east to Jones Avenue NE via sheet flow. Slopes in the west basin generally vary from 2% to 8% except for a small portion north of the existing asphalt access road which has a slope of 40%. Drainage in the cast basin is conveyed northwest to the wetlands in the northwest corner of the property via shcet(low. Slopes in the east basin generally vary from 2% to 18% except along the south property line where slopes are 66% and into the wetland where slopes vary from 15% to 66%. The project proposes to develop the existing site into 11 single family lots. As a result, more than 5,000 square feet of new impervious surface will be added. The runoff from the site will be infiltrated and/or detained on site and released at a controlled rate, based on the ERC mitigation measures for this proposal. The boundaries of the cast and west basins will be altered slightly to promote consistent drainage through the developed property but the basins will remain equal in size after development as they were before development. The developed volume of stormwater runoff will be equal to the existing volume of stormwater runoff for each basin. .5 •3Ay 2131NId2� D czi m J z Z 0 0 --� PARK AVE. N. _ U JONES AVE. N.E. z m ABERDEEN AVE. N.E. EDMONDS AVE. N.E. SECTION II: OFFSITE ANALYSIS : Upstream Tributary Area The site receives some offsitc runoff via sheet flow from the adjacent lots in the Sunset Hills development. The roof drains in the Sunset Hills development arc either infiltrated into the soil or connected directly to the Sunset Hills stormdrainage system; therefore, only drainage from the lawns will flow onto the property. Downstream Analysis A downstream analysis was conducted by the O.L.D.S. Group in their Preliminary Engineering Design Report, July 1997. On July 14, 1998, a site visit was conducted to verify consistency. See attached offsite analysis copied from the O.L.D.S Group report. on the following pages. A few discrepancies were noted in the report and arc described below. West Basin: The downstream analysis at distance 600 feet to 700 feet describes a ditch. The ditch also contains a 12-foot long 8" concrete culvert that runs under a driveway. In the same ditch, drainage is intercepted by an 8-foot long 12" CMP that directs drainage into the catchbasin described. At distances of 700 feet to 750 feet, the pipe is described as an 8" but was verified to be a 12". At distances of 750 feet to 970 feet, the pipe that goes from the first Type II CB travels in southwest direction rather than west to the second Type 11 CB at the interesection of Jones Avenue NE.and NE 24th Street. The line is an 18" diameter that is 42 feet long. After the second Type II CB, drainage travels in an 18" diameter storni line for 177 feet on NE 24th Street where it intersects another Type II CB. From there the drainage travels in a 30" concrete storm line rather than an 18" PVC for 30 feet before discharging into the unnamed creek discribed in the O.L.D.S Group report.. At the time of the field investigation, no erosion or sedimentation problem areas were present. East Basin: The downstream route for the cast basin to the Canyon Oaks development was consistent with the field investigation. The text of the downstream analysis though refers to points D , Ey, and F which are not labled on the attached downstream map. The text though fully explains the downstream system so that these points arc not necessary to follow the downstream route. . During the field investigation, a few items were noted that should be added to the report. After Point "C", the stream goes into a 25-foot long 24" diameter concrete pipe which goes under a driveway that at one time was NE 28th Street. The stream then drains into another ornamental pond which is located behind 2808 Kennewick Place NE. The pond ends at a log dam which has a "water wheel" in the center to allow water through. The • stream continues to the 24" CMP pipe which travels under NE 29th Court as described in the O.L.D.S. Group report. The drainage could not be confirmed to enter the Canyon Oaks development. Since the Canyon Oaks development is private property, no plans were available at the City of Renton to confirm the downstream route described in the O.L.D.S. Group report going into the Canyon Oaks development. As described in the O.L.D.S. Group report there are no signs of erosion or sedimentation of the downstream system. As described in the Report, drainage at the 24" culvert at NE 27th Street does accumulate some depth but the headwall has ample height to prevent any problems. Mike Dotson at the City of Renton was contacted to find out if there has been any documented drainage complaints for either of the downstream systems described above. Mike confirmed on July 17, 1998 that no drainage problems have been documented. Conclusions Based on our field investigation and research with the City of Renton, there is no evidence of existing capacity problems, overtopping, or siltation. Since this project proposes to detain runoff for the cast basin and developed runoff for the wcst basin will be negligible, it's development poses very little risk to the downstream system. IOFFSITE ANALYSIS • DOWNSTREAM ANALYSIS: S East Basin - The east basin downstream analysis was performed by Lance Y.K. Gyotoku, P.E. and is included as Appendix Exhibit II. The review by the O.L.D.S. Group found the report to complete and consistent with our observations. West Basin - Storm waters from the west 0.68 acres of the site currently sheet flow from the lawn and pasture, and driveway areas westward onto Jones Avenue Northeast. From there the storm water flows: 0 to 240' South along the street east curb and shoulder of Jones Avenue to a catch basin/inlet in the shoulder. 240' to 600' South from the catch basin/inlet in an 8"concrete pipe to an outlet in a ditch on east margin of Jones Avenue. 600' to 700' South in a V deep by 1.25' wide ditch that flows down to a catch basin/inlet off the shoulder of the intersection of Jones Avenue and Northeast 24 Street. 700' to 750' West from the catch basin/inlet in an 8"PVC pipe to a Type II drainage control structure on the west curb line of Jones Avenue north of Northeast 24'h St. 750' to 970' West from the drainage control structure fifty feet to another Type II locked structure. Continuing west for another one hundred sixty feet to another Type II locked structure from which an 18"PVC pipe discharges to the north. 970' to 1,560' North in an unnamed creek, which measured 2.5' to 4' on the bottom, and 4' to 6' across the top of a 1.5'to 3' deep channel. 1,560' Creek channel enters a large bore pipe for conveyance west under I-405. See next page for flow routing map and stationing. ®oI�o�o�o GROURD 7 WEST BASIN OFFSITE ROUTING MAP I '6013) o �� r ios 75 ly%• �K ? 17 `" � �' ry f t-rs Ito— rc ra i i �t �Iti - --------------------------- I I, N.E. 2 27.9 •i HT ; [ 278 B.l I11 I 71 r 7t o , o• � a r, � 5 2 'S o IntIj 282 Ell 295 'FLOW �FA-rfq.j = Z 1 }� '• 2 ].S v io rr,.� isrr . A' ZLb t 24D10 (C) r Lu �] Z� r c .t.. I err.; t ••T.c.• I U� 1 96 /S6 G. 1 N E 24 TH EXHIBIT II Y �1 �!t N In• k!. ,{r v 1� �i 11;1, X. 1 i 1 . °�{y ! •1�•"vy°�•I�`, .a} .a;t� ;., '+ rl:•1? xyk'y q,•.l ,(. .+yip aA�rYy`t'.�#�l4•Jj � ih'{ •.t� �t-��Y�'iyK,r, ty.5r s ? r ,. ,la. �k y,`ia. q b '�r a r.. ( J•.'r 5 t.,ry � }}}4 v �. �het� •1 , � �. '�s„ f •�x'i�t,�;y,q�,t.1�gly,�,�,� dry.... , y3...frKy .f�4,s +' �t`i� i� ff �.r + . rY•i ��W �:: ,.. .{r'4'(i!t r.i j''x y �ls h •„Yk' "rnr* + !�rrW' ibrMr/jt t�Y'el+iirari`�M�n{r .r�y� , v3r.t�+ +a�r�� yr� .( riA �`" .�,�>.,t; r,>R � � 7 4 !'�,' �..,.:.• �� 1 ^.fr 1_q,. ,,,..:. 'r - ��. r .�a y,• ,�✓,�) � -.,.t,. k 9".p�1 j ,'. �( 'ki r 1:� F75q j y vi�,tCi .,� �4... ..q .1,, t ,��',�i' � ��t ti" N. • j. 'f• I I I • DOWNSTREAM ANALYSIS THE PRELIMINARY PLAT OF WHISPERING PINES • May 6, 1995 G� o (AG O �Ir c� "ZC y r 0 30i25 Ss��NAL ECG EXPIRES 3/3/0(* PREPARED BY: �ancc Y.K. Gyotoku, P.E. 1G i The Plat or Whispering Pines Downstream Malvus _ st f1�1 KENNYDAL svN '1 N S�1TN n �-1 F cow N z TN ST I 11TH PL d �� 1...:.. � • NE N 51 ,—* E _ , ` h in ton i I ' 1ST I � • 5 NE 9y V � � Z NE 12111 IT 11srN IOAr LAmcV $ 117rN $T 1 lOt1K Re NT \V /' Z ST N > = R E N f O N N IfTN 51 Zy r�1 a,N S 121"0$T K`"�"'".•�" `S sarrlut:p, n"r �7 ST [ '1Z $T iN ST a en on $ Aww z N ,1rN Figure 1: Vicinity Map Not to Scale - 17 ' The Plat of Whispering Pines Downstream Analysis Proiect Overview The site is located on the east side of Jones Avenue NE between NE 28th Street and NE 24th Street and is composed of approximately 2.7 acres. The proposed development will consist of 11 single-family residential lots, with a cul-de-sac public road and detention system. Site Conditions The site is consists of mainly cleared land with a 2 story barn. There is a high point near the westerly third of the site that slopes to the West at 6%. To the East the grade falls generally east Iat a slope of 6.5% until the defined bank near the northeast corner at which point the grade drops 14 feet in roughly 25 feet. The site would be classified as.pasture land with very poor vegetation coverage. The area of the Northeast of the bank is undeveloped and would be considered young second growth. Existing Drainage The site is located within the Cedar River Drainage Basin, and the East Lake Washington Sub- Basin. Runoff from this site presently sheetflows the East and to the West. There are no drainage conveyance structures, swales, or ditches onsite to channelize flow. The site probably infiltrates rainfall from smaller storms, and sheetflows the larger storm events. Proposed Drainage The proposed plat will provide an underground drainage system that will collect runoff and direct it to the detention tank located in the cul-de-sac. This tank will outflow into a grass-lined biofiltration system and ultimately discharge to the low point of the site (the northeast corner). The vault and biofiltration system will be sized using the current City of Renton design standards. Downstream Analysis The proposed storm system will discharge into the low point of the site located at the bottom of the bank near the northeast corner. The runoff enters a wetland-type area that is very heavily vegetated. A slight ditch is present in this area and runs in a northerly direction for roughly 210 feet at a very gradual, flat slope. (Please refer to Figure 2, Point "A".) This ditch has bottom width that varies from 2 to 5 feet with a depth that also varies from 0.5 to 1.0 feet. The area surrounding this ditch is very low and wooded. There is no evidence of erosion or very heavy flows. This drainage course then intersects NE 27th Street where it enters a 24" CMP culvert. (Point "B") This culvert is partially blocked with debris. The headwall created by NE 27th Street roughly extends 3 feet above the culvert. This culvert crosses NE 27th Street in a northwesterly direction where it terminates in a manhole. This manhole discharges into a heavily landscaped ornamental pond/stream system located on private property. (Point "C") The pond and stream are located approximately 4 feet vertically and 20 feet horizontally from the property owner's homes. This drainage course continues to wind thru private property in a northerly direction for about 300 feet. The drainage course then intersects NE 29th Court. The drainage course now has a section with a bottom width of 1.5 feet, and vertical 2 foot high rockery-type side walls. From these rockery walls, the grade extends up at a 2:1 slope to an additional depth of 7 feet. These side slopes are covered with mowed grass in very good condition. (Point "D") The channel enters a 24" CMP pipe and crosses the street. The 24" CMP inlet seemed to be in good working order. (Point "E") From this pipe the drainage system enters the Canyon Oaks Townhome Development. The outfall is located in the back of the buildings, roughly 150 feet from the 24" CMP inlet. It is not known exactly where the conveyance pipes are on this property. 1S rThe Plat or Whisocring Pines Downstream Analysis It is believed that the drainage course discharges from a 36" CMP pipe which daylights from a . steep slope. (Point "F") The runoff discharges into an energy-dissipation pool approximately 5 feet in diameter. (Point "G") The runoff then overflows from the pool into a very defined, heavily vegetated and wooded channel that is sloped at roughly 2.5:1 and runs northeasterly toward May Creek. This area was not easily accessible for further observation due to the slope. There seemed to be no evidence throughout this downstream course of g any erosion or drainage problems. It is assumed that the 24" culvert at NE 27th Street does backup and accumulate some water depth due to the debris at the inlet. Due to the ample height of the headwall and surrounding area, this does not seem to be a problem. This proposed development will not have a significant impact on the downstream properties with respect to erosion or capacity. Further research of city and county records will be necessary to analyze the remaining downstream drainage system. The Plat ofWhispering Pines Downstream Xnalysis I —Y Js4 zo i-'0 ' A4A Y 31,9 �- I 1 J6B CREE K 58� a 57 ' soy 3�s, 200 —�-- R I I p 165e1 I I II O S E o �-• ----- 1502% ------- I t Z 55 ' 54 P A R K a F s • S `��- yµ i S` W`-`i an N I 4.3 all • \ 1,1 v '�J $ rlJr 4 /• FORM R JZ ZIV 1106CP 51 I o 'J9��9r �sl� 53 . • e� t ?i b t I N I •'� e E S =A _ E _2i 10 9 8 Z 7 �a+•S}�$ _._.:� •,�,�/i6tj L • k83:-( 13 7o I to • 65 Z � 60 /05 1 78 Iy74.8Z, ?,,(A). b��, ° 273 ^'I ?s Ll 1259 v� ✓I ' % � 1 17 180 1 — /913 11 1 1 J., y r l lV E. 279 o , 1 2�]Q / v • 11 / N 1 N 55 / I r /00 1 65 1 /oo , N 2 5,7 �Zs 1 ! I 6 • ~ O I � 1 1 � 1 rl 1"� 617 H Sat ^ I 2 5 5 i I lee rn rob 9 loaf. 3oa �••' t x 8 2 Z5 0 ti ae7 ZB) 1 I ...•rrna •- '�O '1 i '17/11 41 10 1C w �dn1 R Figure 2: Downstream Analysis Exhibit Scale 1"=200' 20 SECTION III: DETENTION ANALYSIS & DESIGN: • Storm water runoff for the site was modeled using WatcrWorks hydrology software and the Santa Barbara Urban Hydrograph Methodology. Development of the site took into consideration the subbasin boundary. The site was graded to allow the drainage areas in each subbasin to remain consistent in size. A combination of infiltration trenches for roof drains and detention will be used to control increased runoff from development. A vault will be used for detention of excess stornwater. Dead storage in the vault will be added to allow for water quality treatment of the stormwatcr. Soils type on-site are Indianola per the King County SCS Map which is hydrologic l;roup A. The precipitation rates used for this analysis areas follows: •6-month, 24-hour storm -> 1.28 in. *2-year, 24-hour storm -> 2.00 in. •10-year, 24-hour storm 2.90 in. 925-year, 24-hour store 3.40 in. •100-year, 24-hour storm -> 3.90 in. All roof drains will be infiltrated at the Hearing Examiner's request since site conditions arc favorable. A geotechnical report was completed for this site with attached infiltration • rates. The geotechnical report was written by Earth Consultants, Inc. on May 22, 1997. The worst infiltration rate on site is 20 in/hr which will be used as a conservative infiltration rate for design of the roof drain trench systems. A conservative roof area of 2500 square feet will be used for design. Stone water runoff from the roof was modeled using WaterWorks hydrology software and the Santa Barbara Urban Hydrograph Methodology. Peak flow from a 100-year, 24-hour storm event from the roof will be routed through the roof drain trench. A trial and error solution was derived by assuming a length of 2-foot wide trench and calculating the infiltration rate through the bottom of the trench. Since this calculation does not take into account side wall infiltration, the calculated infiltration rate is conservative. Storage in the trench was assumed to be equal to the volume of the 6" perforated pipe plus the volume of the remaining portion of the trench with 30% voids. 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I••.,� ' r `y r•'y� '• S I ri li r•f�a rCA/1 a�rT�'r.'r,�,4bYr r ,�, v/r ' .+ �. , ,•' ` / .'.t, ��. rt, .A• Irr�'l •1��' '— '�t .•r•rw!`w.��~ � •• ,r � . .. -=J---�� -r`�,"� y f ,t�� i6};�. ;J C • OL ^ '�'r'� ri . t'•'.nrr r • RdC '� 1 M r y � .�j t AkF • L.II� a... k � . 1 Ur •� t m0 - 1a^; � 1�A' , :S t� �. 1 •G -t 1 ." `�.Jr .'I• `':�., VI .�A i\� �f .i"I'F4\'-,J ,�... •v •� '''�t� +N��a , r� ,_1 a w '�Y,i►�. �: �� r d" ,.1.�ry;, �• � `IA�t� I. 'T r� �I'� •�.�' r ;1 , - '.x I' �t:�(..t• ` v'i• Jye, j . +'I �,L: uu �/,�illf: Uri -ter'\�J •r,�',r •• .� �" .1tI - r• �:J17 1', � a 11DY t. .• a Pry,' . t .� •�c a L!1. R(�,S'v+rr.I nor .. �.: �► •.t .j"(•''�' •I e-• IL'f-Li _ )P.1.1h11=:_{L •}. i �I•IJ.• eiler� iI- a 1 1l q` — ,. w.` . Jr r 1 r' �f� + ;I[ srMd__�(�� ,��'OM ►:�I :: 'i e. 'a a• ♦ Ss • _• a .I.1!+Y.I�'�'C. (( ��RJ• ',• lowY1fi�••III. ", �; i' � .:r. ' -�! _-��II I�; _: •' IL/))) II ItreY,7 `•Ir.1 ••IG•:II=r��' ... tins sheet t I) MIC .r - 1c4 --�:- SOILS MAP PEN r0N l,7 Ml. 10, WHISPERING PINES Scale I:zn oau D E I #98098 The maximum storage for the trench is: • Storage in 6" pipe: 7u/4(0.5)2 * 30 = 5.89 cf Storage in gravel portion of trench: (2*2*30 - 5.89)*30% = 34.23 cf Total maximum storage: 5.89 + 34.23 =40.12 cf The infiltration rate through the bottom of the trench is: 20 111/hr * ft/121n * hr/3600s * 50 ft * 2 ft = 0.028 cfs The maximum stage the roof drain runoff will reach is 1.38 feet above the bottom of the trench. Note, in the Waterworks prinouts, the bottom of the trench is at elevation 100. A 2-foot deep trench will allow over 0.5 feet of freeboard. • 7/15/98 8 :37 : 13 am Dodds Engineers, Incorporated page 1 DEI JOB N0. 98098 WHISPERING PINES • BASIN SUMMARY ------______ BASIN ID: ROOF100 NAME : ROOF INFILTRATION DESIGN STORM SBUH METHODOLOGY TOTAL AREA. . . . . . . ; 0 . 06 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPEIA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 0 . 00 Acres 0 . 06 Acres TIME INTERVAL . . . . : 10 . 00 min CN. . . . : 0 . 00 98 . 00 ABSTRACTION COEFF: 0 . 20 TC. . . . : 0 . 00 min 10 . 00 min PEAK RATE : 0 . 04 cfs VOL: 0 . 02 Ac-ft TIME : 480 min • • 7/15/98 11 : 5 :34 am Dodds Engineers, Incorporated DEI JOB NO . 98098 1 - WHISPERING PINES page • STAGE STORAGE TABLE CUSTOM STORAGE ID No. ROOFSTOR Description: ROOF TRENCH STORAGE (30o VOID) STAGE <----STORAGE----> STAGE <----STORAGE----> STAGE <----STORAGE----> STAGE <----STORAGE----> (ft) ---Cf--- --AC-Ft- (ft) ---cf--- --AC-Ft- (ft) ---Cf--- --Ac-Ft- (ft) ---cf--- --AC-Ft- 100.00 0.0000 0.0000 100.60 12.036 0.0003 101.20 24.072 0.0006 101.80 36.108 0.0008 100.10 2.0060 0.0000 100.70 14.042 0.0003 101.30 26.078 0.0006 101.90 38.114 0.0009 100.20 4.0120 0.0001 100.80 16.048 0.0004 101.40 28.084 0.0006 102.00 40.120 0.0009 100.30 6.0180 0.0001 100.90 18.054 0.0004 101.50 30.090 0.0007 102.00 40.120 0.0009 100.40 8.0240 0.0002 101.00 20.060 0.0005 101.60 32.096 0.0007 100.50 10.030 0.0002 101.10 22.066 0.0005 101.70 34.102 0.0008 • • 7/15/98 11 :5 : 34 am Dodds Engineers, Incorporated page 2 DEI JOB N0 . 98098 WHISPERING PINES STAGE DISCHARGE TABLE DISCHARGE LIST ID No. ROOF Description: ROOF DRAIN TRENCH INFIL RATE STAGE <--DISCHARGE---> STAGE <--DISCHARGE---> STAGE <--DISCHARGE---> STAGE <--DISCHARGE---> (ft) ---CfS-- ------- (ft) ---CfS-- ------- (ft) ---CfS-- ------ 100.00 0.0280 100.60 0.0280 101.20 0.0280 101.80 0.0280 100.10 0.0280 100.70 0.0280 101.30 0.0280 101.90 0.0280 100.20 0.0280 100.80 0.0280 101.40 0.0280 102.00 0.0280 100.30 0.0280 100.90 0.0280 101.50 0.0280 102.00 0.0280 100.40 0.0280 101.00 0.0280 101.60 0.0280 100.50 0.0280 101.10 0.0280 101.70 0.0280 7/15/98 11 : 5 : 36 am Dodds Engineers, Incorporated page 3 DEI JOB N0. 98098 WHISPERING PINES LEVEL POOL TABLE SUMMARY MATCH INFLOW -STO- -DIS- <-PEAK-> OUTFLOW STORAGE <-- ----DESCRIPTION---------> (Cfs) (Cfs) --id- --id- <-STAGE> id (Cfs) VOL (cf) ROOF DRAIN 0.03 0.04 ROOFSTOR ROOF 101.38 21 0.03 27.69 Cf I I F;RE Y9RA%- j 33439C2'8t j 4 3343902180 FYI 3I 3C. �J 30 FIRcr 60�333 I DRI vE wA w I 30-� ' 3' 39C2845 ^ ,..,.Q I _ _x SS uH \ 7 a+ Z I 3 I 26' F.R 32 LA L U L u 258.0 24- I. - Ir v1 LL 21' IE _ C 06 ( 16 2� 19. 402. 1 32' FIR \ •I ASPH 2 570RT MOOD o FRAME HOUSE �I J' `N 26t6 ,:ONES AVE N.E "V1 11 7 o 119 0 334390: SANITARY SEWER MH/ 3 FiR - IE=242.91 DRIVEWAY QI �,R 2000' 1 ° 10' FIR 3 ♦ 31.57' I 7�--_ 2�g 1 N all 32'1J' rI Y4.RE FEN[[ \ O 7 ch 0i • 6'0 250i SPHALT� \ �� P EAST ASIN ° m 2 U TIUTY 1 i0 COTiONKJOC UTILITY POLE 'U�� '' , uH ?� --' A$1HAL7 ^ A1N-�INK FENCE t6_ 7(Nw) 260 SANITARY SEWER 1.IH/ � �111 ' \ \ LINE I! I IE 243_ 3 �i 31.2W \ a 3343902840 of o WAtr ONC 39.6' o g WEST BASIN ••YY ?' `x , V" DRIVEWAY �' o I^ = g < 24' IE 251.16 (E) 10e' a ua a' IE 251 08 f d j a 30 I zs s.v' CL m � En5n1:0 2- a �— yyy�� ci µ SINOLE STORY HOUSE 1? ^ STORY SHC10 4390. U I 2612 JONES AVE. N.E. $_ j Z 278 cr we v ^c = w^. — 280 278OVIER ?) P 2a8 33439 638 \ OFFSITE 33 \ 23 22 I 21 I 2^ 19 439C3641 \ o � � 1 i 5AN77aRr SEWER MH/ I I II —\ � I .,._. ...-. 'I j 33�3903539 � I � �I 3343902803 cA-c aasN� ti 2 D4 FIRE Hr nR N 7 WATER VAL N EXISTING CONDITIONS 111=50' BASIN MAP WHISPERING PINES DEI #98098 it SAN Sf"JII SA N SF%j- '1AN ';fV1fR NES AVE� —F'25 N.E. &41 r cp"p \�Vj)Wile WA III 25 0' ... ....... 24 0, ?.3' Lc)) r r N W33'45 E N Q=45'F 94.40' m cn cn z lej .4., CONC PAO sit zi urn. VAI) DI im C/) 0 -n -Ti m 81 .6 )(A cr ..... .. onoUl 00 22107 It I V 1a 14 The detention criteria used for this analysis is from the Examiner's Review Committee. • The required peak release rates are as follows: *Developed 2-year, 24-hr peak release --> 50% Existing 2-year, 24-hr peak release *Developed 10-year, 24-hr peak release —> 50% Existing 10-year, 24-hr peak release *Developed 100-year, 24-hr peak release —> Existing 100-year, 24-hr peak release A combination detention/water quality vault will be used for developed runoff for the east basin. Discharge from the vault will be split. Storms less the release rate for the 10-year, 24-hour storm will be routed to the wetlands on site via a sheet flow spreader. Routing some runoff to the wetlands will ensure recharge of the wetland. Storms exceeding the release rate for the 10-year, 24-hour storm will be discharged into the proposed stormmain system that will run through the east portion of the property. The volume of the vault will be determined using the criteria above and then increased 30% as specified by the Hearing Examiner. The amount of runoff received by the vault will include lawn areas from the proposed lots, driveways, road, and sidewalks. As discussed previously, drainage from the roof areas of the proposed houses will be infiltrated. The amount of new impervious area for the developed site was derived from Table 3.5.2B of the King County Surface Water Design Manual (KCSWDM). The amount of impervious area draining into the vault will be adjusted for roof area that will be • infiltrated. The amount of impervious area that will be infiltrated is 2500 SF*11 houses = 27,500 SF or 0.63 acres. According to Table 3.5.2B of KCSWDM, the percentage of impervious area for the site is 48 % for 11 DU/2.19 AC = 5.0 DU/AC. The east basin will consist of 8 houses. The 8 houses will be infiltrated eliminating impervious runoff from (8 houses)(2500 SF) = 0.46 AC. The amount of impervious area flowing into the east basin vault will be (1.54 AC)(48%) - 0.46 AC = 0.28 AC. The amount of pervious area flowing into the vault will be (1.54 AC - 0.28 AC - 0.46 AC) _ 0.80 AC. The east basin receives runoff from 0.35 acres as described above. Runoff generated from this area will flow through the detention system undetained. The allowable release rates from the detention system will therefore be increased by the peak runoff coming from the offsite area. The impervious and pervious areas for the east basin is described below: Stormwater peak flows for the 2-year, 10-year, and 100-year storms are attached on the following pages. • EXISTING EAST BASIN Total Area= 1.54 acres GROUND COVER AREA acre CN Pasture/Grass 1.51 65 Impervious 0.03 98 Time of concentration 17.40 min. OFFSITE INTO EAST Total Area= 0.35 acres BASIN GROUND COVER AREA acre CN Lawn 0.35 68 Time of concentration Assumed 10.00 min. DEVELOPED EAST BASIN Total Area= 1.54 acres GROUND COVER AREA acre CN Lawn 0.80 68 Impervious 0.28 98 Roof Area infiltrated 0.46 Time of concentration Assumed 10.00 min. • • 7/21/98 9 : 33 :28 am Dodds Engineers, Incorporated page 1 ---WHISPERING PINES • DEI JOB NO. 98098 ------------------ BASIN SUMMARY BASIN ID : DEVE002 NAME : 2-YR STORM DEV EAST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 08 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 00 inches AREA. . : 0 . 80 Acres 0 . 28 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 68 . 00 98 . 00 ABSTRACTION COEFF: 0 . 20 TC. . . . : 10 . 00 min 10 . 00 min PEAK RATE : 0 . 11 cfs VOL: 0 . 05 Ac-ft TIME : 480 min BASIN ID: DEVE010 NAME : 10-YR STORM DEV EAST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 08 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 90 inches AREA. . : 0 . 80 Acres 0 .28 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 68 . 00 98 .00 ABSTRACTION COEFF: 0 .20 TC. . . . : 10 . 00 min 10 . 00 min PEAK RATE: 0 . 20 cfs VOL: 0 . 10 Ac-ft TIME : 480 min BASIN ID: DEVE100 NAME : 100-YR STORM DEV EAST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 08 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 0 . 80 Acres 0 .28 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 68 . 00 98 . 00 ABSTRACTION COEFF: 0 . 20 TC. . . . : 10 . 00 min 10 . 00 min PEAK RATE: 0 . 35 cfs VOL: 0 . 16 Ac-ft TIME : 480 min BASIN ID: EXE002 NAME : 2-YR STORM EXIST EAST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 54 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 00 inches AREA. . : 1 . 51 Acres 0 . 03 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 65 . 00 98 . 00 ABSTRACTION COEFF: 0 . 20 TC. . . . : 17 .40 min 17 .40 min PEAK RATE : 0 . 02 cfs VOL: 0 . 02 Ac-ft TIME: 1120 min 7/21/98 9 : 33 : 28 am Dodds Engineers, Incorporated page 2 WHISPERING PINES DEI JOB NO. 98098 BASIN SUMMARY BASIN ID: EXE010 NAME : 10-YR STORM EXIST EAST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 .54 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPEIA PERV IMP PRECIPITATION. . . . : 2 . 90 inches AREA. . : 1 . 51 Acres 0 . 03 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 65 . 00 98 . 00 TC. . . . . 17 .40 min 17 .40 min ABSTRACTION COEFF: 0 .20 PEAK RATE : 0 . 06 cfs VOL: 0 . 06 Ac-ft TIME : 960 min BASIN ID: EXE100 NAME : 100-YR STORM EXIST EAST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 54 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPEIA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 1 . 51 Acres 0 . 03 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 65 . 00 98 . 00 TC. . . . . 17 .40 min 17 .40 min ABSTRACTION COEFF: 0 .20 PEAK RATE : 0 . 18 cfs VOL : 0 . 13 Ac-ft TIME : 490 min 7/23/98 11 : 3 : 20 am Dodds Engineers, Incorporated page 1 WHISPERING PINES DEI JOB NO. 98098 ------------------- BASIN SUMMARY BASIN ID: OSE002 NAME : 2-YR STORM OFFSITE SBUH METHODOLOGY TOTAL AREA. . . . . . . : 0 . 35 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 00 inches AREA. . : 0 . 35 Acres 0 . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 68 . 00 0 . 00 ABSTRACTION COEFF: 0 .20 TC. . . . : 10 . 00 min 0 . 00 min PEAK RATE : 0 . 01 cfs VOL: 0 . 01 Ac-ft TIME : 1120 min BASIN ID: OSE010 NAME : 10-YR STORM OFFSITE SBUH METHODOLOGY TOTAL AREA. . . . . . . : 0 . 35 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 90 inches AREA. . : 0 . 35 Acres 0 . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 68 . 00 0 . 00 ABSTRACTION COEFF: 0 .20 TC . . . . : 10 . 00 min 0 . 00 min PEAK RATE : 0 . 02 cfs VOL : 0 . 02 Ac-ft TIME : 490 min BASIN ID: OSE100 NAME : 100-YR STORM OFFSITE SBUH METHODOLOGY TOTAL AREA. . . . . . . : 0 . 35 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 0 . 35 Acres 0 . 00 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 68 . 00 0 . 00 ABSTRACTION COEFF: 4 .20 TC . . . . : 10 . 00 min 0 . 00 min _ PEAK RATE : 0 . 06 cfs VOL: 0 . 03 Ac-ft TIME : 480 min The developed 2-year, 10-year, and 100-year, 24-hour storms were routed through the • east basin vault to confirm adequate sizing. See results on the following pages. Hydrographs were generated and are as specified below: Hydrograph Storm Peak Runoff Rate cfs 1 50% existing 2-year, 24-hour storm + offsite 2- 0.02 year, 24-hour storm 2 50% existing 10-year, 24-hour storm + offsite 10- 0.04 year, 24-hour storm 3 existing 100-year, 24-hour storm + offsite 100- 0.23 year, 24-hour storm 4 developed 2-year, 24-hour storm+ offsite 2-year, 0.11 24-hour storm 5 developed 10-year, 24-hour storm+ offsite 10- 0.22 year, 24-hour storm 6 developed 100-year, 24-hour storm + offsite 100- 0.41 year, 24-hour storm 7 2-year, 24-hour outflow out of vault 0.02 8 10- ear, 24-hour outflow out of vault 0.04 9 100- ear, 24-hour outflow out of vault 0.15 i 7/29/98 3 : 36 :47 pm Dodds Engineers, Incorporated page 1 WHISPERING PINES . DEI JOB NO. 98098 - --------------- HYDROGRAPH SUMMARY PEAK TIME VOLUME HYD RUNOFF OF OF Contrib NUM RATE PEAK HYDRO Area cfs min. cf\AcFt Acres 1 0 . 016 1120 714 cf 1 . 89 2 0 . 042 960 2141 cf 1 . 89 3 0 .229 490 7173 cf 1 . 89 4 0 . 109 480 2615 cf 1 .43 5 0 . 215 480 5116 cf 1 .43 6 0 .411 480 8494 cf 1 .43 7 0 . 016 1460 2065 cf 1 . 43 8 0 . 042 1450 4081 cf 1 .43 9 0 . 152 680 7448 cf 1 .43 7/29/98 3 : 36 :48 pm Dodds Engineers, Incorporated page 2 WHISPERING PINES DEI JOB NO. 98098 ------------------- STAGE STORAGE TABLE RECTANGULAR VAULT ID No . VAULT Description: VAULT FOR EAST BASIN Length: 45 . 00 ft . Width: 10 . 00 ft . voids : 1 . 000 STAGE <----STORAGE----> STAGE <----STORAGE----> STAGE <----STORAGE----> STAGE <----STORAGE----> (ft) ---cf--- --Ac-Ft- (ft) ---cf--- --Ac-Ft- (ft) ---cf--- --Ac-Ft- (ft) ---cf--- --Ac-Ft. 253.98 0.0000 0.0000 256.50 1134 0.0260 259.10 2304 0.0529 261.70 3474 0.0798 254.00 9.0000 0.0002 256.60 1179 0.0271 259.20 2349 0.0539 261.80 3519 0.0808 254.10 54.000 0.0012 256.70 1224 0.0281 259.30 2394 0.0550 261.90 3564 0.0818 254.20 99.000 0.0023 256.80 1269 0.0291 259.40 2439 0.0560 262.00 3609 0.0829 254.30 144.00 0.0033 256.90 1314 0.0302 259.50 2484 0.0570 262.10 3654 0.0839 254.40 189.00 0.0043 257.00 1359 0.0312 259.60 2529 0.0581 262.20 3699 0.0849 254.50 234.00 0.0054 257.10 1404 0.0322 259.70 2574 0.0591 262.30 3744 0.0860 254.60 279.00 0.0064 257.20 1449 0.0333 259.80 2619 0.0601 262.40 3789 0.0870 254.70 324.00 0.0074 257.30 1494 0.0343 259.90 2664 0.0612 262.50 3834 0.0880 254.80 369.00 0.0085 257.40 1539 0.0353 260.00 2709 0.0622 262.60 3879 0.0890 254.90 414.00 0.0095 257.50 1584 0.0364 260.10 2754 0.0632 262.70 3924 0.0901 255.00 459.00 0.0105 257.60 1629 0.0374 260.20 2799 0.0643 262.80 3969 0.0911 • 255.10 504.00 0.0116 257.70 1674 0.0384 260.30 2844 0.0653 262.90 4014 0.0921 255.20 549.00 0.0126 257.80 1719 0.0395 260.40 2889 0,0663 263.00 4059 0.0932 255.30 594.00 0.0136 257.90 1764 0.0405 260.50 2934 0.0674 263.10 4104 0.0942 255.40 639.00 0.0147 258.00 1809 0.0415 260.60 2979 0.0684 263.20 4149 0.0952 255.50 684.00 0.0157 258.10 1854 0.0426 260.70 3024 0.0694 263.30 4194 0.0963 255.60 729.00 0.0167 258.20 1899 0.0436 260.80 3069 0.0705 263.40 4239 0.0973 255.70 774.00 0.0178 258.30 1944 0.0446 260.90 3114 0.0715 263.50 4284 0.0983 255.80 819.00 0.0188 258.40 1989 0.0457 261.00 3159 0.0725 263.60 4329 0.0994 255.90 864.00 0.0198 258.50 2034 0.0467 261.10 3204 0.0736 263.70 4374 0.1004 256.00 909.00 0.0209 258.60 2079 0.0477 261.20 3249 0.0746 263.80 4419 0.1014 256.10 954.00 0.0219 258.70 2124 0.0488 261.30 3294 0.0756 263.90 4464 0.1025 256.20 999.00 0.0229 258.80 2169 0.0498 261.40 3339 0.0767 263.98 4500 0.1033 256.30 1044 0.0240 258.90 2214 0.0508 261.50 3384 0.0777 256.40 1089 0.0250 259.00 2259 0.0519 261.60 3429 0,0787 7/29/98 3 : 36 :48 pm Dodds Engineers, Incorporated page 3 WHISPERING PINES DEI JOB NO. 98098 STAGE DISCHARGE DISCHARGE TABLE MULTIPLE ORIFICE ID No. ORIFICE Description: ORIFICE FOR EAST BASIN VAULT Outlet Elev: 253 . 98 Elev: 251 . 98 ft Orifice Diameter: 0 . 5522 in. Elev: 257 . 88 ft Orifice 2 Diameter: 0 . 6621 in. Elev: 260 . 90 ft Orifice 3 Diameter: 12 . 0000 in. STAGE <--DISCHARGE---> STAGE <--DISCHARGE---> STAGE <--DISCHARGE---> STAGE <--DISCHARGE---> (ft) ---cfs-- ------- (ft) ---Cfs-- ------- (ft) ---Cfs-- ------- (ft) ---Cfs-- ------- 253.98 0.0000 256.50 0.0131 259.10 0.0319 261.70 3.5414 254.00 0.0012 256.60 0.0134 259.20 0.0326 261.80 3.7539 254.10 0.0029 256.70 0.0136 259.30 0.0333 261.90 3.9549 254.20 0.0039 256.80 0.0139 259.40 0.0339 262.00 4.1460 254.30 0.0047 256.90 0.0141 259.50 0.0346 262.10 4.3287 254.40 0.0054 257.00 0.0144 259.60 0.0352 262.20 4.5040 254.50 0.0060 257.10 0.0146 259.70 0.0358 262.30 4.6726 254.60 0.0065 257.20 0.0149 259.80 0.0365 262.40 4.8353 254.70 0.0070 257.30 0.0151 259.90 0.0370 262.50 4.9927 • 254.80 0.0075 257.40 0.0153 260.00 0.0376 262.60 5.1452 254.90 0.0079 257.50 0.0155 260.10 0.0382 262.70 5.2933 255.00 0.0084 257.60 0.0157 260.20 0.0388 262.80 5.4374 255.10 0.0088 257.70 0.0160 260.30 0.0393 262.90 5.5777 255.20 0.0091 257.80 0.0162 260.40 0.0399 263.00 5.7146 255.30 0.0095 257.90 0.0181 260.50 0.0404 263.10 5.8483 255.40 0.0099 258.00 0.0207 260.60 0.0409 263.20 5.9789 255.50 0.0102 258.10 0.0224 260.70 0.0414 263.30 6.1068 255.60 0.0105 258.20 0.0237 260.80 0.0419 263.40 6.2320 255.70 0.0109 258.30 0.0249 260.90 0.0428 263.50 6.3548 255.80 0.0112 258.40 0.0260 261.00 1.2787 263.60 6.4752 255.90 0.0115 258.50 0.0270 261.10 1.7910 263.70 6.5934 256.00 0.0118 258.60 0.0279 261.20 2.1843 263.80 6.7095 256.10 0.0121 258.70 0.0288 261.30 2.5159 263.90 6.8236 256.20 0.0123 258.80 0.0296 261.40 2.8080 263.98 6.9136 256.30 0.0126 258.90 0.0304 261.50 3.0722 256.40 0.0129 259.00 0.0311 261.60 3.3152 • 7/29/98 3 : 36 : 50 pm Dodds Engineers, Incorporated page 4 WHISPERING PINES • DEI JOB NO. 98098 - ------------ LEVEL POOL TABLE SUMMARY MATCH INFLOW -STO- -DIS- <-PEAK-> OUTFLOW STORAGE <--------DESCRIPTION---------> (cfs) (cfs) --id- --id- <-STAGE> id (cfs) VOL (cf) - - ---------------------------------------------------------------------- 1/2 EX 2 YR+OFFSITE .......... 0.02 0.11 VAULT ORIFICE 257.79 7 0.02 1715.75 cf 1/2 EX 10 YR+OFFSITE ......... 0.04 0.21 VAULT ORIFICE 260.84 8 0.04 3086.05 cf EX 100 YR+OFFSITE . ........... 0.23 0.41 VAULT ORIFICE 260.91 9 0.15 3117.98 cf • The east basin routing results are summarized as follows: EAST BASIN 2 year storm 10 year storm 100 year storm Qp allowable release from site (cfs) 0.02 0.04 0.23 (adjusted 2 yr& 10-yr outflow by 50%) H dro ra hs 1, 2, 3 Qp developed (cfs) 0.11 0.22 0.41 H dro ra hs 4, 5, 6 Qp out of vault (cfs) 0.02 0.04 0.15 H dro ra hs 7, 8, 9 Vault live storage required CF 1716 3086 3118 Vault live storage provided (CF) w/ - - 4053 30% factor of safety To accomodate the 30% increase in volume, the vault's width was increased from 10 feet to 13 feet. • The water quality storm for the east basin is 0.03 ac-ft or 1147 cubic feet. See Basin • Summary for the east basin for volume generated for the developed 6-month, 24-hour storm. The dead storage surface elevation in the vault will therefore be at the starting elevation of the live storage. The depth of the dead storage will be 2 feet. The volume of the dead storage will be equal to 1170 cubic feet which is more than the required volume generated from the water quality storm. • 7/23/98 10 :22 : 17 am Dodds Engineers, Incorporated page 1 WHISPERING PINES DEI JOB NO. 98098 BASIN SUMMARY BASIN ID: WQE006 NAME : WATER QUALITY STORM EAST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 08 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 1 . 28 inches AREA. . : 0 . 80 Acres 0 . 28 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 68 . 00 98 . 00 TC. . . . : 10 . 00 min 10 . 00 min ABSTRACTION COEFF: 0 . 20 PEAK RATE: 0 . 07 cfs VOL: 0 . 03 Ac-ft TIME: 480 min • The west basin will consist of 3 houses. The 3 houses will be infiltrated eliminating impervious runoff from (3 houses)(2500 SF) = 0.17 AC. The amount of impervious area creating runoff in the west basin will be (0.65 AC)(48%) - 0.17 AC = 0.14 AC. The amount of pervious area will be (0.65 AC - 0.14 AC - 0.17 AC) = 0.34 AC. Developed runoff in the west basin will not be detained since the developed runoff will be negligible since the developed peak for the 100-year, 24-hour storm is less than 0.5 cfs above the existing 100-year, 24-hour storm per KCSWDM Core Requirement#3. The impervious and pervious areas for the west basin is described below: EXISTING WEST BASIN Total Area=0.65 acres GROUND COVER AREA acre CN Pasture/Grass 0.57 65 Impervious 0.08 98 Time of concentration 17.61 min. DEVELOPED WEST BASIN Total Area= 0.65 acres GROUND COVER AREA acre CN Lawn 0.34 68 Impervious 0.14 98 Roof Area infiltrated 0.17 Time of concentration Assumed 10.00 min. Developed peak runoff for the 100-year, 24-hour storm in the west basin is 0.17 cfs which is less than the existing runoff of 0.11 cfs plus 0.5 cfs or 0.61 cfs , therefore; developed runoff from the east basin will not be detained since the developed runoff is negligible per KCSWDM Core Requirement#3. See attached basin summary printout on the following page. 7/21/98 9 : 22 : 16 am Dodds Engineers, Incorporated page 1 WHISPERING PINES DEI JOB NO. 98098 BASIN SUMMARY BASIN ID: DEVW100 NAME : 100-YR STORM DEV WEST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 0 .48 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 0 . 34 Acres 0 . 14 Acres TIME INTERVAL . . . . : 10 . 00 min CN. . . . : 68 . 00 98 . 00 ABSTRACTION COEFF: 0 .20 TC. . . . : 10 . 00 min 10 . 00 min PEAK RATE: 0 . 17 cfs VOL: 0 . 08 Ac-ft TIME : 480 min BASIN ID: EXW100 NAME : 100-YR STORM EXIST WEST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 0 . 65 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 3 . 90 inches AREA. . : 0 . 57 Acres 0 . 08 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 65 . 00 98 . 00 ABSTRACTION COEFF: 0 . 20 TC. . . . : 17 . 61 min 17 . 61 min PEAK RATE : 0 .11 cfs VOL: 0 . 07 Ac-ft TIME : 480 min SECTION IV: CONVEYANCE SYSTEM ANALYSIS & DESIGN: The tightlined stormdrainage system was checked for capacity using the rational method for both the 25-year, 24-hour and the 100-year, 24-hour storms. Flows for each subbasin collected by each catch basin were calculated using a conveyance spreadsheet. See attached subbasin map and "Catchbasin Sub-basin Areas" spreadsheet on the following pages. A conveyance spreadsheet for both storms are attached on the following pages. All pipes within the stormdrainage system are under capacity without taking into consideration backwater effects. The pipe running from CB 4 to the detention vault is the only pipe that will experience backwater effects. The calculated flow from the conveyance sheets were used for backwater analysis of the stormdrainage system. Attached is the backwater analysis for the 100-year storm for the pipe running between CB 4 and the vault. See the backwater spreadsheet following the conveyance spreadsheets. The headwater elevation falls below the rim of CB 4 thereby, ensuring adequate capacity. • f 6 In > f j r r. C. •, y CI1H0 � ciirttri � `-- — - ?' snN ST VA It NES AVE.' I�. rrxF rr air W�,tr �'� � wn It I.,/ -- u F �Lrt r;'r• p Ia t y , url.um0 t ltl /t L' i I rxr �/ ///� `` •1•. 4.40 1. , CONC. PAD i =I PAO �I ' i Ot % i/ I 41. Cil 00 ►,� , 00 ( / i '� �` L N' oa•e, , 1 211.9 ti ti • WHISPERING PINES Catch Basin Sub-basin Areas Sub- Total Asphalt/Concrete Grass Composite Basin Area Area Area C Value (Acres) (Acres), C=0.90 (Acres), C=0..25 4 1 43 :: 0.28 1.15 0:38 5 0 26. 0.07 0.19 0.43 6 0.26 0.47 • AREASHTALS 8/5/98 STOR YI CC�I�T EYAN- CE �YST04 DESIGN: LOCATION: KING COUNTY 24-HR RAINFALL: 3.4 INCHES OB NAME: WHISPERING PINES JOB NUMBER: 98098 PREPARED BY: GRB DESIGN STORM: 25 YEAR 1\G3t$31EYT�1::;; RLYOFF - IltPERi E4U$ .;:TIttEOF.: $.SLYFa23.:.. .. TR!BLT�RY � FPL� .� PIPE � .P(PE�:� AE'I'EIAL� TR�tiEL �PIPECA CITYS[_':lYYf:tiRY ; 'Rifts s1E`iT - .:iiREa :. : .COfiFFiC1E`iT .iRE:i :: SOcjtDF EO'vC., ;'i,i7SYSFTY 'FLOIr:: DIAN*,NG5 DilistETER-�.:�SLO�&.:. 3.EYG'F3{ �uELOC[TY ; ............. .... 4E YY2E}'tlT.�>: G1KC)' (btl II`TES) ,'::fcN/FiR) '(CFS).':: ?`ii^ fl\CHF>Sl :(?ARE&i Fk fF7U.9 EF7'IS�G) pG�UTES) EeFS) (tYiSEC}_::::.EPERCEYT:j 6 EXCB 0.730 0.48 0.350 0.350 10.00 2.02 0.709 0.012 12 2.500 172 4.90 059 6.103 7.77 11.6% 5 6 0.260 0.43 0.112 0.462 10.59 1.95 0.902 0.012 12 10.000 49 8.78 0.09 12.205 15.54 7.4% 4 VAULT 1.430 0.38 0.543 0.543 10.00 2.02 1.100 0.012 12 25.000 26 12.41 0.03 19.299 24.57 5.7% 8/5/98 DODDS ENGINEERS, INC. PAGE 1 S...... V � 1� 17 1�7 1 i� \T11 LOCATION: KING COUNTY 29-HR RAINFALL: 3.9 IXCHES [JOB NAME: WHISPERING PINES JOB NUMBER: 98098 PREPARED BY: GRB DESIGN STORM: 100 YEAR !YC'R£;ilE\F.�.l: i� RLlO£P �IlIP£R4EQU& 71!lEOF''. �R.YLtFitt 'FRIF1U�Rti � :RFPL�� PIPE :'�AC1'FI,�[ii.-�IRatiEI. ...PIPE.G�PACITY:SU>�I�IARY:i':: YII'ESItC'I[ENi� : :.AREA COEFEiC1E\F AR$A''.............SUDYUF' CO C:::::. IN V i.:� - .Py .SE..SFTY� ..FL01 3....... S;.OPE: 3.ENG'fTf' GBLOCITY: `Flt[E: Q(FCLL} Sp'IlLL) :.:QEAC7YQfFtFL3.l: TO e. ��.. .. ...�.....�. ..:: ..... .....(....... YA..C):. .f......)...Ei,MR).'' 6 EXCB 0.730 0.48 0.350 0.350 10.00 2.39 0.836 0.012 12 2.500 172 5.17 0.55 6.103 7.77 13.7% 5 6 0.260 0.43 0.112 0.462 10.55 2.31 1.066 0.012 12 10.000 49 9.17 0.09 12.205 15.54 8.7% 4 VAULT 1.430 0.38 0.543 0.543 10.00 2.39 1.297 0.012 12 25.000 26 13.27 0.03 19.299 2457 6.7% i 8/5/98 DODDS ENGINEERS,INC. PAGE 1 KW T A ER C' 'AL LA. OB NAME: WHISPERING PINES PREPARED BY: G.Brooks OB NUMBER: 98098 DESIGN STORM: 100 YEAR ENTRANCE ENTRANCE EXIT OUTLET INLET APPROACH BEND JUNCTION PIPE PIPE MfiNNING'S OUTLET INLET PIPE FLOW VELOCITY TAMWATER FRICTION HGL HEAD HEAD CONTROL CONTROL VELOCITY HEAD HEAD HEADWATER FROM TO FLOW LENGTH DIA. n ELEVATION ELEVATION AREA VELOCITY HEAD ELEVATION LOSS ELEVATION LOSS LOSS ELEVATION ELEVATION HEAD LOSS LOSS ELEVATION CB CB (CFS) (FEET) (IN) I VALUE (FEET) (FEET) (SQ FT) (FT/SEC) (FEET) (FEET) (FEET) (FEET) (FEET) (FEET) (FEET) (FEET) (FEET) (FEET) (FEET) (FEET) RIM EL JVAULTJ 4 1 1.30 26 12 0.012 253.98 260.48 0.79 L66 0.04 260.91 0.03 261.48 0.02 1 0.04 261.54 1 261.48 1 0.00 I 0.00 0.00 I26L54 263.38 8/5/98 DODDS ENGINEERS,INC. PAGE 1 SECTION V: BASIN AND COMMUNITY PLAN AREAS: • The site is in the Newcastle Community Planning Area and Cedar River Basin. The site is split by a subbasin ridge; the west portion of property is located in the East Lake Washington Subbasin and the east portion of the property is located in the May Creek Subbasin. • SECTION VI: EROSION/SEDIMENTATION CONTROL DESIGN: Erosion control of this site during construction will be accomplished by utilizing temporary silt fence, interceptor swales along with a sediment pond. Silt fence will be installed in the west basin. Silt fence, interceptor swales along with a sediment pond will be installed in the east basin. Calculation of the required surface area of the pond along with required depth is calculated below. Design of Sediment Pond Area Collected= 1.89 AC Onsite Area= 1.54 AC, CN= 72 (bare ground) Offsite Area= 0.35 AC, CN= 68 (grass) Q10= 0.16 cfs (see following pages for basin summary) Required Surface Area= 1250 SF/cfs * QIo = 1250 * 0.16 =200 SF The minimum required settling depth is 2 feet and the minimum sediment storage depth is 3 feet. The required side slopes are 3:1. To accomodate the 5 feet of required depth and to allow the bottom to be 10-feet X 10-feet, the surface area of the pond was increased to 1600 SF or 40 feet X 40 feet. 10/7/98 11 : 34 :54 am Dodds Engineers, Incorporated page 1 WHISPERING PINES DEI JOB NO. 98098 ----------------------- BASIN SUMMARY BASIN ID: EROS010 NAME : EROSION CONTROL EAST BASIN SBUH METHODOLOGY TOTAL AREA. . . . . . . : 1 . 89 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPElA PERV IMP PRECIPITATION. . . . : 2 . 90 inches AREA. . : 0 . 35 Acres 1 . 54 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 68 . 00 72 . 00 ABSTRACTION COEFF: 0 . 20 TC. . . . : 10 . 00 min 10 . 00 min PEAK RATE : 0 . 16 cfs VOL : 0 . 11 Ac-ft TIME : 480 min