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Return Address: City Clerk's Office City of Renton 2001102200 1055 South Grady Way CITY OF RENTON Bs 0519 Renton,WA 98055 s0 2001 OF 002 9.00 KING COl#yTY WA BILL OF SALE Property Tax Parcel Number: 162305-9006-05 Project File#: NEel .Intersection: Edmonds Ave. 150 Edmonds Ave. NE. Reference Number(s)of Documents assigned or released:,Additional reference numbers are on page Grantor(s): Grantee(s): 1. The quadrant Corporation 1. City of Renton,a Municipal Corporation 2. a Washington Corporation The Grantor,as named above,for,and in consideration of mutual benefits,hereby grants,bargains,sells and delivers to LCIP the Grantee,as named above,the following described personal property: o WATER SYSTEM: Lend Size Type 0 1825 L.F.of 12 D1 Water Main cN` 3129 L.F.of 8 DI Water Main cv L.F.of Water Main a 9 each of 12 Gate Valves 15 each of 8 Gate Valves 13 each of Fire Hydrant Assemblies N SANITARY SEWER SYSTEM: Length Size Type q 265 L.F.of 24 PVC Sewer Main 3050 L.F.of R >' PVC Sewer Main L.F.of Sewer Main 1 each of 60 Diameter Manholes _9 each of 48 Diameter Manholes each of Diameter Manholes STORM DRAINAGE SYSTEM: Length Size Type 900 L.F.of 30, 36 HDPE Storm Line 1560 L.F.of 15,18,24 HDPE Storm Line 4307 L.F.of 8, 12 NPPF. Storm Line 55 each of T-1 CONC Storm Inlet/Outlet 23 each of 48 CONE Storm Catch Basin 7 each of 62, 72 C11htC Manhole STREET IMPROVEMENTS: (Including Curb,Gutter,Sidewalk,Asphalt Pavement) Curb,Gutter,Sidewalk 8;800 L.F. Asphalt Pavement: 17,388 SY or L.F.of Width STREET LIGHTING: t #of Poles 33 E f�a ,z-pit . By this conveyance,Grantor will warrant and defend the sale hereby made unto the Grantee against all and every person or persons,whomsoever,lawfully claiming or to claim the same. This conveyance shall bind the heirs,executors, administrators and assigns forever. H:\FILE.SYS\FRM\84HNDOUTABILLSALE.DOC\MAB Page 1 Form 84 0001/bh IN WITNESS WHEREOF,I have hereunto set my hand and seal the day a d year as written below. By* The Qun grant Jrnrnnratinn 0, Peter Qrser 4%J • UI INDMD UAL FORM OF A CKNO WLEDGMENT Notary Seal must be within box STATE OF WASHINGTON )SS COUNTY OF KING ) I certify that I know or have satisfactory evidence that signed this instrument and acknowledged it to be his/her/their free and voluntary act for the uses and purposes mentioned in the instrument Notary Public in and for the State of Washington Notary(Print) My appointment expires: Dated: REPRESENTATIVE FORM OF A CKNO WLEDGMENT Notary Seal must be within box STATE OF WASHINGTON )SS COUNTY OF KING ) I certify that I know or have satisfactory evidence that cis �- signed this irshvment,on oath r r stated that helsheithey was/were authorized to execute the instrument and Q acknowledged it as the and C=> C=5 of to be the free and voluntary act of such cv party/parties for the uses and purposes mentioned in the instrument. cv _-- Notary Public in and for the State of Washington r-- Notary(Print) o My appointment expires: CZ Dated: CORPORATE FORM OF ACKNOWLEDGMENT Notary Seal must be within box STATE OF WASHINGTON )SS COUNTY OF KING ) On this 7th day of September . 2001,before me personally appeared M. MgTr1�� Peter Orser to me known to J 1 be_F.xar,,tiVP VirP PrPcident of the corporation that S1ON ,*. Q ++ executed the within instrument,and acknowledge the said instrument to be the free i +j and voluntary act and deed of said corporation,for the uses and purposes therein � .o NOTARN p mentioned,and each on oath stated tha hashe was authorized to execute said fPUBLICinstrument and that the seal affixed is the corporate seal of said corporation. tt� yy Notary Public in and for the State of Washin ton ����•••���= Notary(Print) GL. • /Vl�i-t501� My appointment expires: I Z—/5 0 Dated: q T/0 Page 2 SFNT BY: ,LAW OFFICES; 253 272 9876 4;SEP-6-01 9:53AM; PAGE 4/4 PROJECT CLOSING L4 FmW Coat Data FINAL COST DATA AND INVWMRY and Uvmfo" CITY PROJECT NUMBERS: WTR 'S SUBJECT: LIBERTY RI - - WWp- PHASE 1 SWP' Name of ptnjoct TRO- . TED- Tp CxlyofRetton FROM: GARY MERLINO CONSTRUCTION CO, INC. Plant Review S&dOR 9125 loth AVE. S. ptutning/Bbildin&?WAic Wol s SEATTLE WA 98108 200 MM Avenue South Redm!4 WA 98055 DATA _ SEPTFMtBFR 6, ?nnt per the regwAt of the City of Renton,the following information is fvmisW conceming final costs for improvements installed for the above ref=ntred pr0j=t. �va�CYS'iT�M CONS7R1JGI1CIN�� Lmgth Sim Type 1825 L.F.of 12 DI WATERMAIN 3129 L.P.of _8 DI WATERMAIN L.F.of WATERIMAIN LIP.of aWATERMAIN EACH of t n t _ GATE VALVES _ 15 EACH of 8 DI OAIM VALVES EACH of �— ' ~~ OA'[E VALVES .r- 1-1 EACH of • FMB HYDRANF ASSElriBLIMS $ 37,050 (Coat d-Hm Hydrants mast be listed separately) $ 2 4 5,3 6 0 - Vida a E*maing and Sala Tax if appbcabta' f TOTAL COST MRWATM SYSIUM $ CANR'ARY ltngthSine Type sue 7y�e LF of 24 • _ PVC SEWERMAIN 9U0 LF.of 30, 36- PAC STORML� 3050 LP.of 8 PVC SBWBRMAIN 1560 LF.of 15,18,24 PVC STOR)NUM L.P.of ' SUWER MAIN - 4307 LF.of 8, 12• PVC STORI12m 1 . BA of 60 ' D[ SWM MANHOM 55 EA of T -1 CO N C SPOAM t291�/�DFTIt 19 EA of 48 ' DMMEM MANHOLES '23 BA of 48 ' CON G STORM CAT0MAS) oeerint.od Sala sxT _7 BA of �2' CONC _ sTORMGT�+� tFwtiab+e) S 310. 858 `gRvlw m T"` 443 88 - TOTAL CosT FoR SAmrARY SSWB,R SYs1TM s '�t , R 5 R T qp6cal O -�"---"�'� TOTALCASTO sTnRlrl txtA>r1Ac3s SYSTEM s 44 3, 889 - MRmr IMPROVEMMM: (iodadiag Club.aua".Sidewalk,Asphalt Pa"a"t and Sht a LighdnO 8,800 LF CURB & GUTTER, 4,500 SY SIDEWALK $ 987, 775 - . 17,' 388 STREETS Y� SlaNALIZATION: gnamrmg Eng.Design Costs•City>'attnit Pow.WA St Saks Tax) NONE STREET LIGHTING: (Including Bng..Dtsign Costs,aty Permit Foos.WA St Sales Tax) 33 LIGHT POLES $ 177, 881 - TED J. NOBLE P.E. (206) 76 ?9 2 Print signatory name i1 (SIGNATURE) form Si�ATLDOC/bl� / (Sigtatory rmtst be sud izcd&grit or owner of subject developmem) LIBERTY RIDG Renton, Washington swp-2�-z9oS Storm Drainage Report Prepared by: �, J. Scott R. Borgeson, P.E. ���' James A. Morin, P.E. Reviewed by: William J. Goggin, P.E. 28128 IoyN.� EXPIRES 1-27 -AMM TRIAD ASSOCIATES January 18 2001 TRIAD JOB # 00 - 022 Page 1 of 2 King County Building and Land Development Division TECHNICAL INFORMATION REPORT (TIR) WORKSHEET PROJECTPART 1 OWNER AND PART 2 PROJECT • • PROJECT • Project Owner Liberty Ridge LLC Project Name Liberty Ridge Address 9125 10th Ave 5, Seattle WA Location 25E Phone (206) 762-91 25 98108 Township William J. Goggin P.E. Range Project Engineer Section 1 6 Company Triad Associates Project Size 108 AC Address Phone 1814 115th A Upstream Drainage Basin Size 3 AC 425 821 -8448 PART 3 TYPE OF • OTHER Dx Subdivision 0 DOF/G HPA Shoreline Management 0 Short Subdivision 0 COE 404 0 Rockery 0 Grading 0 DOE Dam Safety 0 Structural Vaults 0 Commercial 0 FEMA Floodplain 0 Other 0 Other 0 COE Wetlands 0 HPA COMMUNITYPART 5 SITE AND DRAINAGE Community i Drainage Basin Cedar River PART 6 SITE CHARACTERISTICS 0 River 0 Floodplain 0 Stream 0 Wetlands 0 Critical Stream Reach 0 Seeps/Springs Depressions/Swales 0 High Groundwater Table Lake Dx Groundwater Recharge 0 Steep Slopes 0 Other 0 Lakeside/Erosion Hazard PART 7 SOILS Soil Type Slopes Erosion Potential Erosive Velocities Everett Variable Low Low Additional Sheets Attatched 1/90 TABLE OF CONTENTS Page INTRODUCTION.......................................................................................................................................I VICINITYMAP..........................................................................................................................................I DRAINAGECONCEPT.............................................................................................................................2 CITY OF RENTON AQUIFER PROTECTION AREAS EXHIBIT.......................................................6 DOWNSTREAMANALYSIS....................................................................................................................7 DOWNSTREAM ANALYSIS EXHIBIT................................................................................................8 DETENTIONCALCULATIONS..............................................................................................................9 OFF-SITE RELEASE.............................................................................................................................14 EXISTING CONDITIONS EXHIBIT...................................................................................................16 BASIN#2...............................................................................................................................................17 BASIN#1...............................................................................................................................................19 BASIN I DEVELOPED CONDITIONS EXHIBIT..............................................................................21 BASIN 2 DEVELOPED CONDITIONS EXHIBIT..............................................................................22 CONVEYANCE CALCULATIONS—PHASE I....................................................................................23 PIPE SIZING EXHIBIT—PHASE I......................................................................................................26 PIPE SIZING EXHIBIT—MODEL NAMES - PHASE I......................................................................27 EMERGENCY OVERFLOW PIPE CONVEYANCE CALCULATIONS..........................................28 PIPE SIZING EXHIBIT—OVERFLOW-PHASE I.............................................................................29 EMERGENCY OVERFLOW STRUCTURES......................................................................................30 EROSIONCONTROL.............................................................................................................................31 APPENDIX SCS Soils Map Table 3.5.213—SCS W. Washington Runoff Curve Numbers 2, 10,25,& 100-Year Isopluvials Liberty Ridge Infiltration Calculations Water Works Output King County Backwater Program Output Supplement No.1 Geotechnical Report Special Study, Proposed"Liberty Ridge"Residential Subdivision, September 8,2000 Geotechnical Report Special Study,Proposed"Liberty Ridge"Residential Subdivision Development, Renton, Washington, September 7,2000 Geotechnical Summary of Drainage System Construction Monitoring at the Site of the Proposed"Liberty Ridge"Subdivision(Former"Cedar Crest"Site)Renton, Washington,July 11,2000 Hydrogeologic&Geotechnical Services/Report Addendum prepared by GeoEngineers, Inc.,April 6, 1995 Hydrogeologic&Geotechnical Services/Report prepared by GeoEngineers,Inc.,October 24, 1994 Preliminary Geotechnical Report prepared by CHZM Hill,May, 1988 Preliminary Hydrogeologic&Geotechnical Report prepared by Golder Associates,January, 1988 Level 1 Drainage Study and Preliminary Storm Drainage Calculations for Cedar Crest Manufactured/Modular Home Community prepared by Barghausen Consulting Engineers, Inc., Revision Dated November 4, 1994 Hearing Examiner's Report and Recommendation/File No. LUA00-123, PP, ECF,December 21,2000 Hearing Examiner's Report and Decision/File No.LUA-95-023,CUN/MHP/ECF,May 23, 1995 STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page i INTRODUCTION The proposed project is a 436 lot single-family residential subdivision on an approximately 108-acre site located north of Maple Valley Highway (State Route 169), east of and abutting Edmonds Avenue and the La Colina subdivision,west and south of the King County Maintenance Facility site and other King County- owned property, and south of N.E. 3rdAth Street, in the City of Renton, Washington. More generally, the site is located in Section 16, Township 23 North, Range 5 East, W.M. (Please refer to the Vicinity Map located below). The project site is naturally separated into two drainage basins and both basins will be addressed within this report. Approximately 86 acres of the site will be developed, while approximately 22 acres will remain undeveloped. The site has been used for a gravel mining operation in the past and it has been mined of sand and gravel material to a depth of approximately 60' below the original ground surface. The current surface and underlying foundation material is primarily medium to dense fine to medium-grained glacial outwash sand with traces of silt and gravel, which allows fair to good infiltration of surface water, depending on the varying silt content of the sand and the in-place soil density. A detailed description of the on-site soils can be found in a report entitled Report Hydrogeologic and Geotechnical Services Proposed Residential Development, Renton, Washington October 24, 1994 prepared by GeoEngineers, Inc. concerning the subject property. A copy of that report is included in the Appendix. NF PARK DR O 0 4 SITE 51 NE 4TH Sr ,yE 3RD GREENWt�OD C1 TY OF RENTON CEpq�, 9! � t �P PO 9� VICINITY MAP Not to Scale STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 1 DRAINAGE CONCEPT The storm drainage system for the proposed project has been designed with reference to standards found within the 1990 King County Surface Water Design Manual (KCSWDM)and as shown on the Preliminary Plat Plans, which were submitted to the City in November, 2000, and the Phase I Construction Plans that accompany this report. Plans for the remaining six phases will be submitted in the future,though this storm drainage report is intended to be applicable to all of the project's phases. Currently,the majority of the on- site and off-site (upstream) stormwater infiltrates into the ground. The remainder is conveyed, through channels that were constructed as shown on the May, 1998 Grading/Temporary Erosion and Sedimentation Control Plans for the Cedar Crest project, a 402-unit planned manufactured home community, which was previously proposed for the subject property. The Cedar Crest plans were approved for construction by the City of Renton in 1998. The existing drainage channels convey stormwater to an existing Type II catch basin located approximately 500 feet south of the northwest corner of the site. From this structure, the water flows via a 30-inch diameter underground pipe to an open basin that is located south of N.E. 3`d/41' Street, just outside the northwest corner of the site. A description of the existing conveyance system downstream from this point can be found in the Downstream Analysis section that follows this section. The site lies within two separate City of Renton Aquifer Protection Zones (see City of Renton Aquifer Protection Areas at the end of this Drainage Concept section). The boundary line between the two zones runs east to west across the property, segmenting the southern portion of the property into APA Zone 1, while the northern portion is classified as APA Zone 2. This division line is also shown on the Basin 1 Developed Conditions Exhibit and the Basin 2 Developed Conditions Exhibit, both of which are located in the Detention Calculations section of this report. Since infiltration of stormwater is restricted within APA Zone 1, all of the infiltration facilities for this project have been sited within APA Zone 2. In 1995, a mobile home community was proposed for this same property,and as a part of the City's review of that proposal, the Hearing Examiner concluded that all of the stormwater-generated on-site would be required to be infiltrated. This stipulation can be referenced in the Conditions of Approval for Cedar Crest (see the Hearing Examiner's Report and Decision, May 1995 included in the Appendix). For the Liberty Ridge project that is now proposed for this site, the storm drainage facilities have also been designed to satisfy this condition, though this was not a condition aired during the latest hearing for the Liberty Ridge project that is now proposed for this site. Please also see the Liberty Ridge Hearing Examiner's Report and Recommendation, December 2000,also located in the Appendix. The site has been divided into two basins, with Basin #1 covering the northern portion of the site, while Basin#2 covers the remainder of the site. The storm drainage facilities of each basin are interconnected to achieve storage and on-site infiltration of stormwater for all design storm events. STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 2 The stormwater generated within Basin#2 will be collected in a tightline conveyance system and directed to the water quality/detention/ infiltration facility("Pond C"), which is located in the central area of the site, within the overhead power easement that bisects the site in a north-south direction. There is also an additional detention/infiltration facility located in Basin 41 ("Pond B"), which receives stormwater exclusively from Basin #2. Basin #2's stormwater will first enter a three-celled wetpond that will provide water quality enhancement prior to discharging into infiltration Pond C. Once the level pool in Pond C reaches a predetermined elevation, a percentage of the flow will be conveyed to infiltration Pond B. Additionally, if the water level in Pond B exceeds its design overflow water surface, an overflow riser will convey stormwater from Pond B to infiltration Pond A. Pond A is an existing water quality-detention- infiltration facility that is located along the western property boundary of the site and south of the proposed Road A. All of the stormwater generated within Basin #1 will be collected in a tightline conveyance system and directed toward Pond A, which also receives the overflow of Pond B, as noted above. Stormwater from Basin#1 will first enter a proposed three-celled wetpond(cell 2 and cell 3 already exist). This water quality facility will provide water quality enhancement prior to discharging the water into infiltration Pond A. All of the infiltration ponds (A, B, and C) also provide live storage detention to allow the 2, 10, and 100- year/24-hour storm events to be released entirely through infiltration. The rate of infiltration for each pond has been determined by GeoEngineers Inc. The hydrogeology of the site is described in three reports entitled: 1) Geotechnical Summary of Drainage System Construction Monitoring at the Site of the Proposed "Liberty Ridge"Subdivision(Former "Cedar Crest"Site, Renton Washington,July 11, 2000, 2) Geotechnical Report Special Study, Proposed "Liberty Ridge"Residential Subdivision Development, Renton, Washington, September 7, 2000, and 3) Supplement No.] Geotechnical Report Special Study, Proposed "Liberty Ridge" Residential Subdivision, September 8, 2000 The recommendations presented within these reports, as well as a previous report for this site by GeoEngineers, Inc. entitled Report Hydrogeologic & Geotechnical Services, Proposed Residential Development, Renton, Washington, October 24, 1994, form the basis for the infiltrative drainage design presented within this report. All of these reports have been included in the Appendix of this report. At the recommendation of GeoEngineers, Inc. (refer to the report entitled Geotechnical Summary of Drainage System Construction Monitoring at the Site of the Proposed "Liberty Ridge" Subdivision (Former "Cedar Crest"Site, Renton Washington, July 11, 2000 in the Appendix),the majority of Pond A and an adjacent gravel infiltration blanket of approximately 78,000 s.£ in area and 4' in depth have already been constructed. The infiltration blanket extends to the north from the northwest corner of Pond A, STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 3 providing an additional area for the distribution of infiltrating waters and thereby increasing the effective infiltration area of Pond A. For this reason, Pond A serves as the primary infiltration pond for the entire project site. It receives all stormwater that is not infiltrated by either Ponds C or B. This existing gravel blanket will be extended over the bottom area of Pond A, as described in GeoEngineers' July 11, 2000 report, and as shown on the Construction Plans. Finally, there are two bypass areas of Basin #1 that will be conveyed via the Emergency Overflow Pipe to the natural discharge point in the northwest corner of the site. One area is 3.87 acres lying along the northeast site corner. It will remain an undeveloped slope. The other area is a 1.47-acre naturally graded low area in the northwest corner of the site that is significantly lower than the remainder of the site. The goal of the proposed Liberty Ridge drainage system is to provide stormwater infiltration in order to recharge the groundwater aquifer, to prevent adverse impacts to downstream areas, and to maintain the overall hydrologic balance of the site and surrounding area. These goals are met as follows: 1) Currently, off-site (upstream) stormwater contributes to the downstream flow that exits the site to the northwest. The flow will be intercepted and piped into the existing drain that discharges into the natural ravine to the northwest of the site. 2) The on-site generated stormwater that does not infiltrate during storm events exceeding the design parameters will also be intercepted and piped into the existing drain that discharges into the natural ravine to the northwest of the site. 3) Existing groundwater base flows that contribute to the downstream flow in the natural ravine through the existing drain will be maintained. The Conditions of Approval for Cedar Crest questioned how the project site and neighboring areas would be affected by infiltrating stormwater in the infiltration ponds rather than distributing it over the entire site. This question has been addressed in the enclosed GeoEngineers reports. The reports state that the stormwater that infiltrates in the existing condition, both on-site and off-site (upstream), travels naturally toward the northwest corner of the site. The report dated July 11, 2000 discusses the location of the constructed gravel filter blanket as follows: "During gravel mining operations, a permeable zone of gravel was encountered north of Pond A. This material is a sandy gravel. This gravel zone is where much of the previous runoff from the site naturally infiltrated. To take advantage of this gravel zone, a connecting gravel infiltration blanket has been constructed from the north edge of Pond A approximately 750 feet north to connect it to the gravel zone." Based on GeoEngineers report, it appears that the stormwater that will be infiltrated on-site in the developed condition will follow the same general groundwater flow pattern toward the northwest corner of the site as the current undeveloped groundwater flow condition. Infiltration Pond A and the adjoining STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 4 infiltration blanket are situated back from the west property line of the site and are oriented to spread out in the north-south direction to better disperse the infiltrated flows to closely match the existing condition. STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 5 ' �Y1•i.L.W>•�NVY��;1}••'Q>`�+.y,,,L;','y',J S,•�ti}•�r�,•( r W! ';A:•ei:e��,:> LhL�'l y;,�r,.r myy;,,;,r•o :}�it•, �' .•• �',{, JJ:I� �J�.•.:4•J•4❖:t•.� � 'ta'ti�:•> '': �I:�'T'�sr�J��•����}'�ti;:v'y;�.F:•tir.'.: '4'�4:•'�::'J'�>.,. >•�:;�:� ' . •• =.='•sa'••�t ��.4•ti:•,-:"•k ,�✓.,�3.•.}L'{��.}.Pr.�,•'•1;:•:�•r�5�"' _':4:'✓.:;.J�r• `+LY W:i�4•M) ". `. ..r ya t; :tV:}f. 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However, an Emergency Overflow Pipe system has been designed and installed to convey the stormwater that would be discharged by Ponds A, B, or C in the event of system failure or a storm in excess of the 100-year design storm event. This existing pipe, which was constructed as part of the Cedar Crest project, is also designed to convey the runoff from the on-site bypass areas and the off-site (upstream) tributary area. A 30-inch diameter pipe follows the alignment of Road A until it makes a jog to connect to a catch basin that is located west of Edmonds Avenue NE, approximately 500 feet south of the northwest corner of the site. This structure is labeled as CB OV-]A on the Downstream Analysis Exhibit(next page). From this structure, the stormwater continues through approximately 157 feet of 30-inch underground H.D.P.E. pipe to an open pond that is located on the south side of N.E. P/4`h Street,just outside the site's northwest corner. Stormwater from this basin overflows into a 48-inch diameter corrugated metal pipe riser and then flows through approximately 168 feet of 48-inch corrugated metal pipe at roughly a 5 percent slope. This pipe discharges into a drainage channel that is well defined, heavily vegetated, and extends westerly for approximately 300 feet. There, water in the channel discharges into an open pond that is approximately 100 feet long by 50 feet wide and approximately 3 to 5 feet deep. The pond is located along the south margin of N.E. 3`d/4" Street and east of Blaine Avenue N.E., adjacent to the entrance into the Mount Olivet Cemetery. Stormwater exits this pond through an overflow structure with a birdcage,where it then enters into a system of approximately 1,250 feet of pipe(18" diameter pipe followed by 24" diameter pipe) that runs along the southern margin of N.E. 3`d/4t' Street, eventually discharging into a tightline conveyance system near I-405. By that point, the Liberty Ridge site makes up less than 10 percent of the contributing basin and, accordingly, further downstream drainage systems were not studied. However, the system was traced on City maps to the Cedar River. Note that the above-described downstream flow path was observed while it was conveying heavy rains during an October 13, 1998 site visit by Triad Associates. The drainage course was working properly and no signs of erosion or flooding problems were evident. A similar summary of downstream conditions is included in the November 4, 1994 Level 1 Drainage Study and Preliminary Storm Drainage Report by Barghausen Consulting Engineers, Inc., included in the Appendix. STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 7 PORTIONS SECTONS 16, TOWNSHIP 2:3 N., RANGE 5 E., W.M. TRIAD ASSOCIAT all UV-.-t L-d&-,w L-a U.rm•k sm ueu uw,a r u.d,a"m 7d 425t2IdN! 7¢Im-5 kp 1 l / m W / W - - o� , SCALE: 1 = 200 cc ® 0 100 200 400 - E _ iIc Liu 1 530 L.F. APPROXIMATE \ CONVEYANCE SYSTEM - , ':, - ' -TfREAM 48" CUP Q BASED ON CITY OF RENTON ,- S RISER AS-BUILT DRAWINGS DATED - ' ' ' 0 LF' ` \ MAY 17�1965. STORM PIPE , " - ' - ANNEL�MiTE�_ .5.0 `F f ,t; AND 24 TO 7HE WES OF TY OR. POND �APPROX A P l 0- FORMER SITE ENTRANCE EX. CB W PAVED DEBRIS BARRIER {; ""!: :• fir' 1 " ' D MOUNT OLIVET CEME TER Y BL A INE _ MONTEREY AVENUE f DR. N.E. J lik I ' P 2WWAGP�R �\ PRO7ECI SURVEYOR L `� auw t cvava•t � �i PRO]&T ENGRIRRB 2 I�, 2]F�EY L QAC ASA 7 • i \ ➢ROIECT UNDSCAPR ARCRIMT DAYS /' �l SCAfE ROWZ:f'=200' VERY.:N/ i LIBERTY RIDGE SITE { SrW NOT VWD UN=SIGNED AND DATB JOB KWER 00-02 SNEEr NURRER 10P 1 QC 2001 TRIAD ASSOCIATES DETENTION CALCULATIONS Basin #1, as shown on the Basin I Developed Conditions Exhibit provided at the end of this section, includes the north half of the developed site, as well as the full width of Edmonds Avenue from N.E. 3`1/41h Street to the Plat of La Colina. Basin #2, as shown on the on the Basin 2 Developed Conditions Exhibit provided at the end of this section, includes the south half of the developed site as well as the approximately 4-acre future park area in the southwest corner of the site. The specific areas, which were included in the facility-sizing calculations,have been summarized in the table below. The table shows the areas calculated for each of the different land covers in the developed condition for the areas of each basin and the areas that are tributary to each pond. SUMMARY OF AREAS USED IN STORM DRAINAGE CALCULATIONS TABLE Basin #1 7 Basin#2 Pond Sizin Location Impervious Pervious Impervious Pervious A C&B Area Area Area Area (acres) (acres) (acres) (acres) CN=98 CN=68 CN=98 CN=68 - On-Site Areas - Interior Roads&Sidewalk 8.15 e 5.75 X X Alleys 1.31 0.66 X X Impervious Area on Lots Basin#1/Basin#2 11.54 9.64 X X 40' Wide Lots 1,925 s.f./lot 191/80 lots 45' Wide Lots 2,200 s.f./lot 30/12 lots 50' Wide Lots 2,385 s.f./lot 29/62 lots 60' Wide Lots 2,855 s.f./lot 0/32 lots Detention/Infiltration Pond A Incl.surrounding area 3.42 X Water Quality Pond A 0.45 X Detention/Infiltration Pond B Incl.surrounding area 0.90 X Detention/Infiltration Pond C Incl.surrounding area 1.88 X Water Quality Pond C 0.52 X Pond Access Roads Gravel 0.15 0.15 X X Future Park Area North of S.E.3`"Street 0.50 2.89 X Bypass Area(Slope in N.E.corner of site 3.87 Pervious Future Park Area S.W.corner of site 3.76 1 X Pervious Grass/Landscaped Areas 20.24 13.82 X I X On-Site Acreage Totals 22.10 30.42 16.72 20.36 Sub-Totals 52.52 37.08 Total On-Site Impervious 38.82 Total Pervious _ 50.78 Total On-Site Area 89.60 - Off-Site Areas - Edmonds Avenue Adjacent to Plat of Liberty Ride 1.54 X _ Edmonds Avenue Portion of Adjacent La Colina Plat 0.20 X Total Impervious 23.84 16.72 Total Pervious 30.42 20.36 IFT- Totals 54.26 37.08 Total of On-Site&Off-Site Areas 91.34 ,xisting Conditions Tributary Area: 50.39 Ac.(area tributary to Pond A)-0.20 Ac.(Edmonds Ave.off-site) +3.87 Ac.(N.E.slope bypass area)+37.08 Ac.(area tributary to Ponds B&C) +1.47 Ac.(Lot C depression bypass area)=92.61 Ac. STORM DRAINAGE REPORT FOR LIBERTY RIDGE-Page 9 The detention calculations were performed using Engenious Systems Inc.'s hydrology program Water Works. Hydrographs were developed using S.B.U.H. methodology with a King County Type I-A (User 1) 24-hour rainfall distribution. Separate S.C.S. curve numbers were used for the impervious and pervious portions of the site. Using the onsite existing condition hydrographs, the allowable release rates were determined for the site in accordance with City of Renton standards. Developed condition hydrographs were then generated for the bypass areas in order to verify that the developed bypass flows did not exceed the pre-developed peak runoff rates for each of the 2, 10, and 100-year,24-hour storm events. Infiltration Rates According to Core Requirement #3, Section 1.2.3 of the KCSWDM, "[t]hree basic methods for peak rate runoff control are possible: detention, retention, and infiltration." This project proposes to use infiltration to meet this core requirement. Further, in this section of the KCSWDM (under the heading "Infiltration Facilities"), the manual states "the factors of safety for infiltration systems are incorporated within the methods of analysis and design standards described in Section 4.5." GeoEngineers has studied the soils present on the proposed project site over several years, and published several reports that address the infiltration rates anticipated for the soils in the vicinity of each of the proposed infiltration ponds. In their report entitled Geotechnical Summary of Drainage System Construction Monitoring at the Site of the Proposed "Liberty Ridge"Subdivision (Former "Cedar Crest" Site, Renton Washington, July 11, 2000, which is included in the Appendix, GeoEngineers recommends a design infiltration rate of 8 inches per hour for Pond A (over the base area of the pond) and a design infiltration rate of 4 inches per hour for Ponds B and C. As mentioned previously, an infiltration blanket of approximately 78,000 s.f. in area and 4' in depth has already been constructed as a part of the construction of Pond A. This construction work was performed during the site grading operations in accordance with the approved Cedar Crest Grading/Temporary Erosion and Sedimentation Control plans prepared by Triad Associates (sheets I through 21, dated August, 1995 and revised May 7, 1998). In the report entitled Report Hydrogeologic and Geotechnical Services Proposed Residential Development, Renton, Washington, October 24, 1994, GeoEngineers recommended the construction of this gravel infiltration blanket in order to take advantage of the site's natural underlying gravel soils for improved infiltration. GeoEngineers, in conjunction with Construction Testing Laboratories, Inc., monitored the construction of the infiltration blanket's construction. In their later report entitled Geotechnical Summary of Drainage System Construction Monitoring at the Site of the Proposed "Liberty Ridge" Subdivision (Former "Cedar Crest" Site, Renton Washington, July 11, 2000, they summarize the intended purpose of the gravel blanket as follows: "During gravel mining operations a permeable zone of gravel was encountered north of Pond A. This material is a sandy gravel. This gravel zone is where much of the pervious runoff from the site naturally infiltrated. To take advantage of this STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 10 gravel zone, a connecting gavel infiltration blanket has been constructed from the north edge of Pond A approximately 750 feet north to connect it to the gravel zone." The report goes on to address the current condition of the blanket and their recommendations as follows: "It will be necessary to extend the infiltration blanket under infiltration Pond A in order to permit large flow of water into the blanket. As Pond A and the blanket are presently constructed, the area of flow into the blanket is limited to the end area of the blanket that intersects with Pond A. After the blanket is extended under Pond A (at the same thickness as the existing portion of the blanket), the new portion of the blanket should be covered with nonwoven geotextile and a minimum of 18 inches of washed concrete sand or similar material. Note that the infiltration rate from Pond A is limited by the percolation rate through the material over the geotextile. For washed concrete sand, a design rate of 8 inches per hour is recommended over the base area of the pond." GeoEngineers' report entitled Geotechnical Summary of Drainage System Construction Monitoring at the Site of the Proposed "Liberty Ridge" Subdivision (Former "Cedar Crest" Site, Renton Washington, July 11, 2000, addresses the infiltration rate that they recommend for the design of Ponds B and C in the following paragraph: "We have also reviewed the design percolation rate for Ponds B and C. Providing that they are cleaned out down to native soil, the previously recommended rates of 4 inches per hour are recommended for design." The following quotation refers to GeoEngineers' recommendations contained in their report entitled Report Hydrogeologic and Geotechnical Services Proposed Residential Development, Renton, Washington October 24, 1994, which provides an analysis of the on-site soils: "Stormwater infiltration rates for the site soils were calculated based on the grain-size distribution of select soil samples and their corresponding soil textures. The U.S. Department of Agriculture Textural Triangle provided in the Stormwater Management Manual for the Puget Sound Basin, published in February 1992, was used to determine the soil textures and the infiltration rate. Representative soil samples were collected at the elevations of the proposed infiltration pond bottoms below the filter. The theoretical stormwater infiltration f rate for the soil samples analyzed is about 8 inches per hour. This rate of infiltration would occur until the underlying soil is saturated and the water table is mounded above the bottom of the pond. When the top of l the mound is above the bottom of the pond,the effective infiltration rate is equivalent to the dissipation rate of the mound. Important factors affecting the dissipation are the size and shape of the infiltration area, depth to the water table, and other factors." The report goes on to specifically address the infiltration rate for the areas of Pond B (identified in their report as `area B')and Pond C(identified in their report as `area C'): "For percolation area B, we calculate a dissipation rate of 4 to 6 inches per hour. Based on this, a design percolation rate of 4 inches per hour is recommended. The dissipation rate for area C is calculated to be 4 inches per hour, and a percolation rate of 4 inches per hour is recommended." The infiltration rates recommended for all three ponds do not exceed the maximum rate of 8 inches per hour for this Vashon glacial outwash soil as specified in Table 4.5.2 of the KCSWDM. Due to KCSWDM Section 4.5's required factor of safety of 2.0 (although the EPA test was not used, 2.0 is the more conservative of the two factors specified in the KCSWDM), an infiltration rate of 4 inches per hour was STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page I 1 used for the subsequent detention calculations for the design of Pond A and an infiltration rate of 2 inches per hour was used for the design of both Ponds B and C. Pond Operations and Interconnection The stormwater infiltration needs of Basin #2 are served by Ponds B and C, which are interconnected by a pipe conveyance system. Pond C serves as the primary water quality-detention-infiltration facility for Basin #2. It also provides detention and infiltration in its most northern cell. Pond B provides additional detention and infiltration capacities through its interconnection with Pond C. Pond B is entirely a detention and infiltration pond that is sized to contain storm events up to and including the 100-year storm. All of the flows entering Pond B will have already been treated by the wet pond portion of Pond C. Overflow during storm events exceeding the 100-year storm will be conveyed from Pond B to Pond A. Pond A serves the water quality-detention-infiltration needs of all of Basin #1, while also providing additional detention and infiltration capacity for Basin#2, for storm events exceeding the 100-year storm. The details of the operation of this proposed system are explained as follows. A flow control structure containing a 24" diameter riser will be located at the north end of Pond C. This structure receives flows from Pond C via a single 24" pipe. Once the level pool in Pond C reaches an elevation of 246.00, water will begin discharging through the riser via a single 4.00"-diameter orifice. This flow will be conveyed to Pond B through a 24" diameter pipe. Under normal circumstances, the riser will serve as a flow regulator, allowing a portion of the stormwater flows to be conveyed to Pond B while the remainder is infiltrated in Pond C. Computer simulation modeling of Pond C showed that the 100-year design storm water surface elevation of Pond C would be 249.91. In order to provide additional storage capacity in this pond as an additional factor of safety, the riser will have a top elevation of 251.00, the design overflow water surface elevation of Pond C. As the water surface of Pond C exceeds this elevation, additional flows will be conveyed via this riser and 24" pipe to Pond B until the water surface reaches the emergency overflow water surface elevation of 251.50. A 48"diameter catch basin with an open top will also be located in the northwest corner of Pond C with a rim elevation of 251.50, which will serve as the emergency overflow route for Pond C, discharging via a 24" diameter pipe to a 30" diameter pipe located under Road A. This pipe is currently in-place on the site and it exits at the site's natural discharge point(as described previously in the Downstream Analysis section). Pond C has been designed to reach a water surface elevation of 252.12 while conveying the peak flow of 100-year design storm event for all of Basin#2. Pond B will receive stormwater exclusively from the riser control structure in Pond C, as described above. Pond B is designed to discharge most of the runoff that enters it via infiltration;however, it will have a flow control structure located near its western end. This structure will contain a primary riser with a rim set at elevation 245.50. Once the level pool in Pond B reaches this elevation, flow from Pond B will be directed to Pond A through the Basin #1 storm drainage conveyance system. When the water surface reaches the emergency overflow water surface elevation of 246.00, a 48"diameter catch basin with an open top and rim set at this same elevation will serve as the emergency overflow route for Pond B, discharging additional flows via a 24" diameter pipe to a 30" diameter pipe located under Road A. As noted above, this pipe is STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 12 currently in-place on the site and it exits at the site's natural discharge point(as described previously in the Downstream Analysis section). Pond A is designed to receive all of the stormwater from Basin#1 as well as the additional flow from Pond B, as noted above. Pond A has been designed with a three-celled water quality control facility to treat the inflow from Basin#1. The flow that leaves the water quality cells enters the storage and infiltration area of Pond A which will store and infiltrate all stormwater up to and including the 100-year storm event through the bottom of the pond and its interconnected gravel filter blanket. A primary overflow structure will be located in the northwest corner of the pond that will have a riser with a top elevation of 241.60. Additionally, an open-topped catch basin with a debris barrier grate will be located in this same vicinity and will serve as an emergency overflow structure, with a top elevation of 243.10. If the level pool in Pond A reaches this elevation during extreme storm events, additional flows will be conveyed to the 30" diameter pipe located under Road A. As noted above, this pipe is currently in-place on the site and it exits at the site's natural discharge point(as described previously in the Downstream Analysis section). Pond Detention Volume Analysis Ponds A and C have already been constructed and have been surveyed, providing us with accurate information on their storage capacity. Pond A will retain its current storage capacity and an improved link to the gravel blanket will be constructed to maximize its infiltrative capacity. The infiltrative portion of Pond C will be cleaned and restored to its original dimensions in order to maximize its infiltrative capacity. A pond near the location of the proposed Pond B has also already been constructed. However, this pond will be graded and relocated to provide the required design storage and infiltrative capacity for Pond B. Developed condition hydrographs have been generated for each of the 2, 10, and 100-year, 24-hour storm events for the 54.26 acres of Basin #1 and the 37.08 acres of Basin #2. The hydrographs were routed through the interconnected infiltration Ponds, A, B, and C, verifying that the design volume of these ponds is sufficient to control the storm flows. Refer to the Level Pool Table Summary near the end of this section. For the following calculations,the total precipitation for the 2, 10, and 100-year,24-hour storm events were found to be 2.00", 2.90", and 3.90"respectively, as shown on the respective King County isopluvials. The curve numbers used for the different landcovers are shown on Table 3.5.2B—S.C.S. Western Washington Runoff Curve Numbers. All of these exhibits are located in the Appendix at the end of this report. The infiltration rates identified above (in units of feet per second) were used to calculate the projected discharge rates at each stage of each pond (in units of cubic feet per second). The infiltration rates were applied to the surface area at each stage, which increase with increasing elevation due to the sloped sidewalls of all of three facilities. These calculations are summarized in a table entitled Liberty Ridge Infiltration Calculations, which is included in the Appendix. A summary of the off-site release and detention-infiltration pond calculations is provided below. STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 13 OFF-SITE RELEASE Onsite Existing Condition Hydrographs Total Area=92.61 Ac(See the preceding Summary of Areas table) Impervious Area = 0.00 Ac @ CN =98 Pervious Area =92.61 Ac @ CN=68 (grassy open space) Time of Concentration= 14.89 minutes(See the Existing Conditions Exhibit on page 13) Reach 1: 130 ft Sheet Flow @ 32.3%, `n' =0.15 (prairie grass) Reach 2: 350 ft Shallow Concentrated Flow @ 4.1%,`ks' = 11 (pasture) Reach 3: 1,215 ft Channel Flow @ 2.7%,`ks' = 17(grassy swale) Existing Condition Hydrograph Summary Storm Precip. Peak Flow Volume Time of Peak Event [in] [cfs] [ft3] [min.] 2-Year 2.00 1.48 65,378 1440 10-Year 2.90 4.45 193,541 490 100-Year 3.90 15.88 383,978 480 Bypass Area Developed Condition Hydrographs Slope area along northeast property boundary: Total Area=3.87 Ac(See the Summary of Areas table) Impervious Area =0.00 Ac @ CN=98 Pervious Area =3.87 Ac @ CN—68 (grassy open space) Time of Concentration=30.44 minutes (See the Basin 1 Developed Conditions Exhibit at the end of this section) Reach 1: 275 ft Sheet Flow @ 6.2%, `n'=0.15(prairie grass) Reach 2: 630 ft Shallow Concentrated Flow @ 1.5%,`ks' = 11 (pasture) Reach 3: 460 ft Channel Flow @ 2.5%,`ks' =42(pipe) Reach 4: 370 ft Channel Flow @ 1.2%,`ks'=42(pipe) Reach 5: 290 ft Channel Flow @ 0.5%,`ks' =42(pipe) Reach 6: 340 ft Channel Flow @ 2.7%,`ks' =42(pipe) N.E.By ass Area Developed Condition Hydro ra h Summary Storm Precip. Peak Flow Volume Time of Peak Event [in] [cfs] [ft3] [min.] 2-Year 2.00 0.06 2,732 1440 10-Year 2.90 0.16 8,088 760 100-Year 3.90 0.49 16,046 490 STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 14 Depression area in northwest corner of property: Although there are not any current plans being proposed for the development of this area, it has been modeled here as 100 percent impervious, in order to account for its potential development in the future. This documents that even with a land use with an intensive impervious coverage, the allowable release rate for the site will still not be exceeded. Total Area= 1.47 Ac(See the Summary of Areas table) Impervious Area = 1.47 Ac @ CN=98 (pavement) Pervious Area =0.00 Ac @ CN=68 Time of Concentration=6.30 minutes (The calculated time of concentration was less than the minimum allowed of 6.30 minutes) N.W. Depression Area Developed Condition Hydro ra h Summary Storm Precip. Peak Flow Volume Time of Peak Event [in] [cfs] [ft3] [min.] 2-Year 2.00 0.71 9,468 470 10-Year 2.90 1.05 14,240 470 100-Year 3.90 1.43 19,559 470 Offsite Release Summary Using the existing condition hydrographs,the following allowable release rates for the site were determined. 2-Year Allowable Release Rate =2-yr Pre-developed flow = 1.48 cfs 10-Year Allowable Release Rate = 10-yr Pre-developed flow =4.45 cfs 100-Year Allowable Release Rate= 100-yr Pre-developed flow= 15.88 cfs The hydrograph sums(not arithmetic sums)for the bypass areas(as shown below)were then compared with the allowable release rates in order to check that the flows discharged from the site in the developed condition did not exceed the allowable rates. Please refer to the Water Works output included in the Appendix for the hydrograph data,hydrograph summary table,and actual program output. 2-Year Bypass Area Release Rate =Sum of developed flow from both areas=0.71 cfs 10-Year Bypass Area Release Rate =Sum of developed flow from both areas= 1.13 cfs 100-Year Bypass Area Release Rate=Sum of developed flow from both areas= 1.86 cfs STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 15 PORTIONS SECTION 16, TOWNSHIP 23 N., RANGE 5 E., W.M. BYPASS AREA 1.47 AC. -j ' " c 1 TRIAD ASSOCIATES '__ t 'k L ': F . 'mar �w�" �E� .3� } 'ram✓ t y- }. l Rrym,v,nf �y „y t a„ ,:,,k",,... -„�•n�,�o,,:,,,�,,;,.,..A•. - r �V }<�• i .,r,-. ! •^. i! r x x•kx .:u:.:-.;,.nyY'O�� S .,/ / :i a! UWL 1aL-d U. s; "` - w�'• .. f {-'""" g .+N, :.:p§p;_.. .: .a,r.�: V�f s�wipe Td I use W is LrkhdI d YAB9.-"t �rr � � - a � ) +,:�•^.. �"�� � �' t t. Td 4r1Z1${/J fa 173R1.74E1 }} f E ,.....-; 5 : -._..::-,.._ - •� #-"' � - �' ��_ "`x�.x�,' r�u tM eoo.un.mse ...-wa—.•m " SL D �.xx..' � O HEET F S - 32.3% ^" �W g y :_ �_x>•.._.�,^�,. �_�fir.-^.:",r-:�=` _ t �3. .�V 3 r. 1 m :IQ V r R q: Tc — 14.96; MIIV s .� Bey was-xv�-., � � -y .!..xt c;;:Y:x rs-'fir 2;s t' Y•t' co A uJ LQ "POND B i OFF-SITE TRIBUTARY AREA - 2.15 AC BASIN 1 OFF-SITE TRIBUTARY AREA = 0.86 AC. BASIN 2 OFF-Sl TE TRIBUTARY AREAag 0.95 A C. L s muoA 4 PROJECT SURVEYOR a ILI PROJWT a ENGINEER� C .E7FiEl'L COK ASA PROJECT LANDSCAPE ARCRI7ECT i • i =c.z SCAIE: BORIL:i"=J00 VERT:NIA t tf > i t1 ` t ot� - SCALE: 1 " = 300' A+ ��• 0 150 300 600 STAMP NOT VAUD UNLESS SIGNED AND DATED 6 JOB NUMBER 00-022 SBE67 NUMBER 10F 1 ©2001 TRIAD ASSOCIATES BASIN #2 Developed Condition Hydro2raphs for Area Tributary to Ponds C& B Total Area=37.08 Ac(See the Summary of Areas table) Impervious Area = 16.72 Ac @ CN =98 Pervious Area =20.36 Ac @ CN=68 (lawn) The following time of concentration figure is based on an approximate flow path from the east end of Basin 92 to Pond C via the proposed conveyance system. Please reference both the Basin 2 Developed Conditions Exhibit(located at the end of this section) and the Preliminary Plat Plans (that accompany this report)for the basis of the following values used in the computation of the time of concentration. Time of Concentration= 10.74 minutes Reach 1: 90 ft Sheet Flow @ 29.0%, `n' =0.15 (lawn) Reach 2: 235 ft Shallow Concentrated Flow @ 3.4%,`ks' = 11 (lawn) Reach 3: 1,830 ft Channel Flow @ 2.2%,`ks' =42(pipe) Basin#2 Developed Condition Hydro ra h Summary Storm Precip. Peak Flow Volume Time of Peak Event [in] [cfs] [ft'] [min.] 2-Year 2.00 7.51 122,065 480 10-Year 2.90 12.20 204,522 480 100-Year 3.90 19.01 306,879 480 Infiltration Ponds C and B Level Pool Routing The following level pool table summary represents the results of routing the Basin#2 tributary area 2, 10, and 100-year hydrographs (for the developed condition) through the network of infiltration Pond C and Pond B in order to check the adequacy of the constructed ponds. The flow values shown below for the flow into Pond B were generated using Water Works to model the flow through a 4.00"-diameter orifice in a riser in the proposed storm drain manhole that will be located at the north end of Pond C. Hydrographs were generated for these flows, which are conveyed to Pond B as its only inflow. Please refer to the Water Works output located in the Appendix for the hydrograph data, hydrograph summary table, and actual program output. INFILTRATION POND C-LEVEL POOL TABLE SUMMARY Description Inflow Storage Discharge P.Stage Volume Outflow P.Time [CFSI ID ID [FT CF] CFS Min Pond C-2 Year Developed 7.51 PONDC COMBC 246.69 47,434 1.167 1020 Pond C- 10 Year Developed 12.20 PONDC COMBC 248.13 103,170 1.574 1460 Pond C- 100 Year Developed 1 19.01 1 PONDC I COMBC 1 249.91 183,972 1.950 1460 INFILTRATION POND B-LEVEL POOL TABLE SUMMARY Description Inflow Storage Discharge P. Stage Volume Outflow P.Time [CFS ID ID [FT CF] [CFS Min Pond B -2 Year Developed 036 PONDB COMBB 241.08 6,076 0.265 1560 Pond B - 10 Year Developed 0.63 PONDB COMBB 243.19 21,402 0.409 2050 Pond B - 100 Year Developed 0.86 1 PONDB I COMBB 1 244.90 1 37,776 0.551 2470 STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 17 Live Storage As shown in the Level Pool Summary above, Pond C has a 100-year design storm water surface elevation of 249.91 and provides 183,972 cubic-feet of storage at this elevation. Infiltration Pond C has actually been constructed to provide 240,000 cubic feet of storage at the design overflow water surface elevation of 251.00, or 130%of the required storage volume. Included in this volume is the additional storage capacity provided by the wet pond portion of Pond C, which provides an additional 56,000 cubic-feet of storage between the live-dead interface elevation of 246.60 and the maximum water surface elevation of 251.00. As shown above, Pond B has a 100-year design storm water surface elevation of 244.90 and provides 37,776 cubic-feet of storage at this elevation. Infiltration Pond B has actually been constructed to provide 43,300 cubic feet of storage at the design overflow water surface elevation of 245.50, or 115% of the required storage volume. Both of these ponds have been designed with additional storage capacity in order to provide an additional factor of safety over and above the required factor of safety that is applied to the infiltration rate used for these calculations. Water Quality Water quality will be provided through the use of dead storage. According to King County standards, the required water quality volume is equal to the total runoff from the developed condition 24-hour design storm event using 33%of the 2-year, 24-hour precipitation. (0.33)(Pzn)_(0.33)(2.00 in)=0.67 inches Water Quality Hydrograph Summary Storm Precip. Peak Flow Volume Time of Peak Event [in] [cfs] [ft3[ [min.] Water Quality 0.67 2.03 28,835 480 The City of Renton requires that when a bioswale is not used in conjunction with dead storage, as the King County standards require, the required dead storage volume shall be 1.5 times the volume of the water quality storm event. Therefore, the required volume to be used for water quality storage is 43,253 cubic feet. The three-celled wet pond portion of Pond C will actually provide 45,800 cubic feet of water quality storage(from elevation 242.90 to 246.60). The wet pond is also required to have a surface area of at least 1% of the developed impervious area. The impervious area in the developed condition is 16.72 acres,therefore 0.17 acres(or 7,405 square feet)of wet pond surface area is required. The wet pond will have a surface area at elevation 246.60 of 17,600 square feet(0.40 acres). STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 18 BASIN #1 Developed Condition Hydrographs for Area Tributary to Pond A Total Area=54.26 Ac(See the preceding Summary @Areas table) Impervious Area =23.84 Ac @ CN=98 Pervious Area =30.42 Ac @ CN=68 (lawn) The following time of concentration figure is based on an approximate flow path from the slope area immediately south of the proposed emergency access road to Pond A via the proposed conveyance system. Please reference both the Basin 1 Developed Conditions Exhibit(located at the end of this section)and the Preliminary Plat Plans (that accompany this report) for the basis of the following values used in the computation of the time of concentration. Time of Concentration= 17.86 minutes Reach 1: 235 ft Sheet Flow @ 8.7%, `n'=0.15 (lawn) Reach 2: 1,650 ft Channel Flow @ 2.4%,`ks' =42 (pipe) Basin#1 Develo ed Condition Hydrograph Summary Storm Precip. Peak Flow Volume Time of Peak Event [in] [cfs] [ft3] [min.] 2-Year 2.00 9.57 175,026 480 10-Year 2.90 15.46 294,520 480 100-Year 3.90 24.19 443,323 480 In addition to the stormwater runoff generated in Basin 41, Pond A receives flows from Pond B. Pond B's primary discharge structure has been designed so that this only occurs for storm events which exceed the 100-year design storm, after the design overflow water surface for Pond B is reached. Since storm events with return frequencies greater than 100 years are beyond the scope of this analysis,the flows which may be contributed to Pond A from Pond B were not considered in the sizing of Pond A. This interconnection is intended to reduce the potential for downstream erosion by further increasing the duration over which larger storm events will be discharged from the site, and by providing another means for flows from Basin #2 to be infiltrated if a failure were to occur at Ponds B and/or C. Infiltration Pond A Level Pool Routing The following level pool table summary represents the results of routing the Pond A tributary inflow 2, 10, and 100-year hydrographs (for the developed condition) through infiltration Pond A, in order to check the adequacy of the constructed pond. Please refer to the Water Works output located in the Appendix for the hydrograph data,hydrograph summary table,and actual program output. STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 19 INFILTRATION POND A- LEVEL POOL TABLE SUMMARY Description Inflow Storage Discharge P.Stage Volume Outflow P.Time [CFS] ID ID [FT[ [CFJ [CFS[ [Min] 2 Year Developed 9.57 PONDA COMBA 236.31 22,442 2.905 570 10 Year Developed 15.46 PONDA COMBA 237.84 72,729 3.218 920 100 Year Developed 1 24.194 1 PONDA I COMBA 239.88 175,219 3.622 1450 Live Storage As shown in the Level Pool Summary above,Pond A has a 100-year design storm water surface elevation of 239.88 and provides 175,219 cubic-feet of storage at this elevation. Infiltration Pond A has actually been constructed to provide 278,000 cubic-feet of storage at the design overflow water surface elevation of 251.00, or 159% of the required storage volume. Included in this volume is the additional storage capacity provided by the wet pond portion of Pond A, which provides an additional 70,000 cubic-feet of storage between the live-dead interface elevation of 239.10 and the maximum water surface elevation of 241.60. Pond A has been designed with additional storage capacity in order to provide an additional factor of safety over and above the required factor of safety that is applied to the infiltration rate used for these calculations. Water Quality Water quality will be provided through the use of dead storage. According to King County standards, the required water quality volume is equal to the total runoff from the developed condition 24-hour design storm event using 33%of the 2-year,24-hour precipitation. (033)(P,n)=(0.33)(2.00 in)=0.67 inches Water Quality Hydrograph Summary Storm Precip. Peak Flow Volume Time of Peak Event [in] [cfs] [ft3] [min.] Water Quality 0.67 2.56 41,113 480 The City of Renton requires that when a bioswale is not used in conjunction with dead storage, as the King County standards require, the required dead storage volume shall be 1.5 times the volume of the water quality storm event. Therefore, the required volume to be used for water quality storage is 61,670 cubic feet. The three-celled wet pond portion of Pond A will actually provide 66,000 cubic feet of water quality storage(from elevation 235.60 to 239.10). The wet pond is also required to have a surface area of at least 1%of the developed impervious area. The impervious area in the developed condition is 23.84 acres, therefore 0.24 acres (or 10,454 square feet) of wet pond surface area is required_ The wet pond will have a surface area at elevation 239.10 of 23,500 square feet(0.54 acres). STORM DRAINAGE REPORT FOR LIBERTY RIDGE—Page 20 PORTIONS SECTION 16, TOWNSHIP 23 N., RANGE 5 E., W.M. N POND A BASINS 1 & 22;X_ TRIAD ASSOCIATES lit *55 QQ Emm—mg -av M14 1150 1..16 rkj..&Al T.1 42MIAW 425M� TO?—W9.48M54 N BASIN 1 OFF—SITE TRIBUTARY AREA J ! , 0.86 A C. POND SIN ji 't BA SCALE: 1 200' 235 L.F. SHALLOW CONCENTRATED 0 100 200 400 ti FLOW p S 3.49 %. ct ' BASIN CELL 3 OFF—SIT TRIBUTARY AREA ',, ' 'a. All ,:' ; ' `/, ,` ..\ �.....- : = 0.95 AC. ,zi 'CELL 2 n 'o ZONE 2 'ip 4 uj sec ... AVAZONE Lu fza 71� _Az_ �k &L L I 74. V/ ......1�11 �f-, ANIVLL FL 0 c cc 90 L.F. UL -------- ROAD K SHEET FLOW rz . ......... .......... cc Z k, Dom4w.1 FRWICT MANAGIR f % V Z' M GGGW PE .............. 00 JUTICYL CIDL ASIA FRWBCT LANWAPS IRCHMECT DAM; ..... ..... S= RDM:1-200' NIA J-4 %A: A q % —SITE ADDITIONAL ON TRIBUTARY AREA DEVELOPED IN THE FU 110*11 AS A NATURALLY : srw Nor vam Lu Mf=SKM M DITO PED LANDSCA PARK) 00-022 3.76 A I.F I (C)2001 TRIAD ASSOCIATES I Sim Numm ` - PORTIONS SECTION 16, OVVNSx|p 23 N., RANGE 5 E W.M.