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HomeMy WebLinkAboutSWP272051(3) SSOE, INC ENGINEERS ARCHITECTS SOUTH GARDEN AVENUE STORM DRAIN EXTENSION RENTON, WASHINGTON DRAINAGE REPORT February 28, 1992 eoosos�� e• to `L%" SOUTH GARDEN AVENUE STORM DRAIN EXTENSION RENTON, WASHINGTON • DRAINAGE REPORT INDEX I. DRAINAGE REPORT II. TIR WORKSHEET III. GARDEN AVENUE DRAINAGE STUDY IV. ADDENDUM NO. 3 GARDEN AVENUE NORTH DRAINAGE STUDY • SOUTH GARDEN AVENUE STORM DRAIN EXTENSION RENTON, WASHINGTON I. DRAINAGE REPORT .: ::.. . :: €GARDEN AVENUE SSOE SOB NOISU1T-03 ST( RIVI DRAIN EXTENSIC?I <:>:.: : FEBRUARY 28, T5�92 REN=N,WASHING?ON .:.::.:.:::.:;.. . DRAINAGE REPORT FOR SOUTH GARDEN AVENUE STORM DRAIN EXTENSION RENTON, WASHINGTON PREPARED BY SSOE INC. FEBRUARY 28, 1992 GENERAL: The present storm drain system that flows from North 8th Street and Garden Avenue North to the pond near the entrance to Gene Coulon Park (pond #3) is highly inadequate to convey the runoff generated by major storms. During extreme storms there is extensive flooding in the valley floor. In 1988, a pipeline parallel to Garden Avenue North and ending at approximately Lake Washington Boulevard was designed to help alleviate the flooding. In the same year 867 lineal feet, or about 40% of the pipeline was constructed. In 1991 the capacity of this storm drain system was analyzed as part of the design of a new truck plant at the PACCAR Renton site. The analysis clearly indicated that upgrading the existing storm drain system would be mutually beneficial to the truck plant construction and to the community as a whole. , PACCAR then retained SSOE to design the storm drain improvements. The storm drain extension project is to be constructed in two stages. The first stage, for which this report is written, is a pipeline parallel to Garden Avenue North that extends north about 1250 lineal feet from North 8th Street to the south end of the pipeline constructed in 1988. The second stage is a pipeline that extends from the north end of the 1988 pipeline to pond#3. This section of the pipeline is currently being designed. CORE REQUIREMENTS: Core Requirement #1: Discharge at the Natural Location The system will discharge water to the existing drainage system at pond #3. Core Requirement #2: Offsite analysis The design of this system utilized a study of the hydraulics and hydrology of the drainage basin tributary to and downstream of the proposed project. An initial study of the existing conditions used a steady state backwater analysis and predicted that flooding would occur under less than a 2 year storm. A copy of this report (Garden Avenue Drainage Study) is attached for reference. Subsequent to the initial report, the hydraulics and hydrology of the valley floor were analyzed using the EXTRAN module of the Stormwater Management Model (SWMM). The EXTRAN module offers a more detailed analysis of the valley floor drainage system, • allowing the dynamic routing of full hydrographs through the storm drain system. Once again the analysis indicated that the existing system would flood during major storms. 1 SQUTI�GARDEN AV IUE.<.::.. ..:.: :: . ::: ::.:::::: SS( E SOB NQ � 5fl1 -0 ST(3RM TJ►RAIN E�CTENSIN FEBRLTARY RENTQN, WASHfNGTpN • The EXTRAN model was then used to predict the drainage improvements provided by the entire storm drain extension project. The new pipeline will provide significant flooding reductions. A copy of this report (Addendum No. 3, Garden Avenue North Drainage Study) is attached for reference. As mentioned above, the exact alignment of the north portion of the storm drain extension is still being designed. Consequently, when the final alignment of the north end is set the analysis will be verified and adjustments made as necessary to meet (or improve upon) the hydraulic characteristics predicted by the current model. Core Requirement #3: Runoff Control The installation does not increase the amount of impervious area for the tributary basin and consequently does not need any flow control system. No biofiltration facilities are provided because the system merely conveys water that is collected off-site. Core Requirement #4: Conveyance System The conveyance system is designed to the standards of the King County Surface Water Design Manual (KCSWDM). The backwater analysis is performed as part of the EXTRAN analysis described above. Core Requirement #5: Erosion and Sedimentation Control Erosion and sedimentation will be controlled by several measures. First, silt fences will be provided at excavations where the earth surface is exposed. Second, backfilled trenches will be protected at the end of each day by a 6 inch top layer of clean crushed gravel surfacing in the parking lot and by temporary asphalt paving at driveway entrances. Third, existing catch basins will be protected to stop the entry of muddy water. Finally, the contractor is required to provide measures to prevent vehicles from tracking soil onto the public streets. 2 SOUTH GARDEN AVENUE STORM DRAIN EXTENSION RENTON, WASHINGTON II. TIR WORKSHEET Page 1 of 2 King County Building and Land Development Division TECHNICAL INFORMATION REPORT (TIR) WORKSHEET PART 9 PROJECT LOCATION � �A••' _ - / / • Project Owner City of Renton Project Name Card pxo Address Location Storm Drain Extension Phone Township 23N Range 5E Project Engineer Steve Wittmann-Todd Sectlon 8 Company SSOE Inc. Project Size NA AC Address Phone 206-822-2950 - Upstream Drainage Basin Size 620 AC • • PART 4 OTHER 0 Subdivision F-1 DOF/G HPA Shoreline Management [� Short Subdivision COE 404 a Rockery Grading DOE Dam Safety 0 Structural Vaults [� Commercial FEMA Fioodplain Other P�-] Other Utility 0 COE Wetlands HPA • r r - Community City of Renton yy Drainage Basin PART 6 SITE CHARACTERISTICS River ® Floodplain TnadP iiai-P annveyanrP ' Stream Wetlands Critical Stream Reach a Seeps/Springs C] Depressions/Swales ® High Groundwater Table [� Lake [] Groundwater Recharge Steep Slopes Other (� Lakeside/Erosion Hazard SOILSPART 7 Soil Type Slopes Erosion Potential Erosive Velocities ,,I N.A. 0 Additional Sheets Atlatched 1/9� Page 2 of 2 King County Building and Land Development Division TECHNICAL INFORMATION REPORT (TIR) WORKSHEET • PART 8 DEVELOPMENT LIMITATIONS . .:.�., �REFERENCE LIMITATION/SITE CONSTRAINT'' _.''I Q Ch.4-Downstream Analysis NA 0 0 a a Q Additional Sheets Attatched PART 9 ESC:REQUIREMENTS MINIMUM ESC REQUIREMENTS MINIMUM ESC REQUIREMENTS DURING CONSTRUCTION FOLLOWING CONSTRUCTION Q Sedimentation Facilities ® Stabilize Exposed Surface Q Stabilized Construction Entrance ® Remove and Restore Temporary ESC Facilities ® Perimeter Runoff Control ® Clean and Remove All Silt and Debris Q Clearing and Grading Restrictions ® Ensure Operation of Permanent Facilities Q Cover Practices 0 Flag Limits of NGPES ® Construction Sequence Other ® Other Stabilize exposed surface 1 • PART , . ' SURFACE Grass Lined Channel 0 Tank O Infiltration Method of Analysis � ® Pipe System Q Vault Q Depression Fxtran (� yl Q Open Channel Q Energy Dissapator Q Flow Dispersal Com0e6sad6n/MItlgatiori ;; r Dry Pond 0 Wetland Q Waiver of Eliminated Slid Storage Wet Pond 0 Stream Q Regional Detention NO Brief Description of System Operation Parallel el canyPynnrP system to upgrade ex�stjng conveyance system Facility Related Site Limitations Q Additional Sheets Attatched Reference Facility Umitatlon NA PART11 STRUCTURAL ANALYSIS PART 12 EAS EM ENTSrTR. [ 6rinage asemen(May require special structural review) Q Cast in Place Vault Q Other Q Access Easement Q Retaining Wall Q Native Growth Protection Easement Rockery>4'High Q Tract Structural on Steep Slope Q Other PART 14 SIGNATURE OF PROFESSIONAL I or a civil engineer under my supervision have visited the site. Actual site conditions as observed were Incorporated Into this worksheet and the r, / attatchments. To the best of my knowledge the Information provided here Is accurate. t 1/90 SOUTH GARDEN AVENUE STORM DRAIN EXTENSION RENTON, WASHINGTON III. GARDEN AVENUE DRAINAGE STUDY r_ ENTRANCO GARDEN AVENUE DRAINAGE STUDY Renton, Washington Prepared for PACCAR, Inc. Prepared by ENTRANCO 10900 NE 8th Street, Suite 300 Bellevue, Washington 98004 (206) 454-5600 October 1, 1991 t CONTENTS Page INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 METHODS Hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Backwater Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 ASSUMPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 RESULTS AND DISCUSSION Hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Backwater Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 CONCLUSIONS AND RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 APPENDICES A - Figures B - Hydrologic Summary and Calculations i FIGURES (Appendix A) Page 1. Vicinity Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.1 2. Drainage Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.2 3. Backwater Analysis Flow Routes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.3 4. Hydrologic Soil Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.4 5. Land Use Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.5 6. Operational Curve Flow Segment 1: Flow from Pond 1 to Lake Washington for Various Headwater and Tailwater Elevations . . . . . . . . . . . . . . . . . . . . . . A.6 7. Operational Curve Flow Segment 2: Flow from North 8th Street to Pond 1 for Various Headwater and Tailwater Elevations . . . . . . . . . . . . . . . . . . . . . . A.6 8. Operational Curve Flow Segment 3: Flow from Garden Avenue North Junction to the Cedar River for Various Headwater and Tailwater Elevations . . . . . . . . . . . . . . . . . . . . A.7 9. Operational Curve Flow Segment 4: Flow from PACCAR Outlet to Garden Avenue North Junction for Various Headwater and Tailwater Elevations . . . . . . . . . . . . . . . . . . . . A.7 10. Operational Curve Flow Segment 5: Flow from Lower PACCAR Junction to North 8th Street for Various Headwater and Tailwater Elevations . . . . . . . . . . . . . . . . . . . . A.8 11. Operational Curve Flow Segment 6: Flow from Middle PACCAR Junction to Lower PACCAR Junction for Various Headwater and Tailwater Elevations . . . . . . . . . . . . . A.8 12. Operational Curve Flow Segment 7: Flow from the Inflow to PACCAR to Middle PACCAR Junction for Various Headwater and Tailwater Elevations . . . . . . . . . . . . . A.9 TABLES Page 1. Predicted Peak Flow Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 ii GARDEN AVENUE NORTH DRAINAGE STUDY INTRODUCTION This report is a hydrologic and hydraulic analysis of the Garden Avenue North drainage basin in Renton. Water in the basin flows through the Gene Coulon Memorial Beach Park into the southeast corner of Lake Washington adjacent to the Cedar River (figure 1). The basin includes a developed upland area of approximately 980 acres which slopes down to the flat valley floor along the Cedar River. Large storm events have caused flooding in some intersections, most notably the intersection of North 8th Street and Garden Avenue North and at the Houser Way un- derpass below Park Avenue. Cross connections exist in the stormwater system in the vicinity of Garden Avenue North and North 8th Street, so that stormwater that would nor- mally flow down Garden Avenue North could discharge to the Cedar River instead. This report analyzes flow through the pipe system, considering the effects of tailwater to the system. METHODS Hydrology At the direction of the City of Renton, the Santa Barbara Urban Hydrograph (SBUH) method was used to compute runoff rates using the Water Works software program. The basin was divided into 18 main subbasins, as delineated by the City of Renton, with some of these divided further along the pipe runs on the valley floor (figure 2). The east basin area (subbasins 6, 7, 8, 9, 10, 16, and 17) drains to the storm system at the east end of North Eighth Street. The north basin area (subbasins 2, 3, and 4) flows into Pond 1 on the south side of Lake Washington Boulevard. Off-site flows from subbasin 15 flow into the storm system running through the PACCAR site. The PACCAR site was divided into six additional subbasins based on in- formation provided by SSOE, Inc. Flows from these subbasins were added along the pipe run through the site. Backwater Analysis At the direction of the City of Renton and PACCAR, the King County Surface Water Management Backwater program was used to analyze the hydraulic pipe flow in the pipes. A backwater analysis was done along the two interconnected drainage corri- dors to determine water surface elevations corresponding to various stormflows (figure 3). One backwater pathway begins at Lake Washington and runs through the Gene Coulon Memorial Beach Park. It then crosses under Lake Washington Boulevard and runs up Garden Avenue North to North 8th Street, where the cross connection with the Cedar River outfall occurs. From here it runs east up North 8th Street to the base of the hill near Houser Way North. It also branches from North 8th Street and runs through the PACCAR site. 91031-60 Garden Ave.Drainage Study(100/91) 1 The other backwater pathway begins at the Cedar River and runs east up North Sixth Street to Garden Avenue North, then north to North 8th Street where Garden Ave- nue North jogs east. The backwater pathway then runs east to the continuation of Gar- den Avenue North, where it joins the first backwater pathway. At several locations an analysis was done to determine what flows would be ex- pected given various headwater and tailwater elevations (locations are shown on figure 3). ASSUMPTIONS The existing pipe network was modeled using information provided from various sources. The City of Renton provided a Pipe Attribute compendium of the Renton stormwater system. This was supplemented with field survey data collected by Entranco. The survey verified the system beneath Garden Avenue North, and collected cross-sections and invert elevations from the channel and culverts between Lake Washington Boulevard and Lake Washington. SSOE, Inc. provided information on the pipe network within the PACCAR property, as well as the pipe network on North 8th Street. Northwest Hydraulics provided the 2-, 10-, 25-, and 100-year water surface ele- vations for the Cedar River, which were 20.1, 21.7, 22.3, and 22.74 feet, respectively, at the approximate location of the outfall. Lake Washington water levels were provided by the U.S. Army Corp of Engineers. Water levels are regulated during the year and the lake is kept lower during the winter than the summer. A typical winter-time elevation is 13.3 feet. The channel from Lake Washington Boulevard to Lake Washington was modeled assuming that the channel will be maintained by cutting back the blackberries and keeping the culverts clear. Times of concentration were estimated using the criteria described in the King County Surface Water Design Manual. Flow components were divided into sheet flow, shallow concentrated flow, and then open channel or pipe flow. Estimates were made using aerial photography, topographic maps, and a pipe network map supplied by the City of Renton. Flow paths are shown in figure 2. Twenty-four hour precipitation totals were taken from isopluvial maps in the King County Surface Water Design Manual. The precipitation totals are 2.0, 2.9, 3.4, and 3.9 inches for the 2-, 10-, 25-, and 100-years, 24-hour storms respectively. Soil types were estimated using the USDA Soil Conservation maps for King Coun- ty. The hydrologic groups are shown in figure 4. Land use was estimated using aerial photographs supplemented by field verifica- tion (figure 5). Hydrographs from the upper basin were mostly added directly, neglecting the trav- el time between subbasins. This was done to simplify the calculations, and decrease the chance of computational errors. The lag times were small, from one to 12 minutes, which reflected the expected velocities in the pipe and open channel systems. Sub- basins 8, 10, and 16 were lagged by 30 minutes to reflect the attenuation between where the flows enter the valley floor, and North 8th Street. Pipe capacities were checked from the east basin to the valley floor. 91031-60 Garden Ave.Drainage St udy(10/1/91) 2 To check the output of the backwater model, headwater and tailwater elevations were compared with actual observations during large storm events. Flooding was ob- served at the corner of Garden Avenue North and North 8th Street during the January 9, 1990 storm. It was estimated that the flooding reached an elevation of approximately 27 feet during that storm event (Steve Whitman-Todd, pers. comm.). Flooding observations were also made during the January 9, 1990 storm by John Hobson of the City of Renton at the following locations: • Flooding occurred on Garden Avenue North between North 8th Street and North Sixth Street to near the top of the curbs. is Flooding occurred on Houser Way below the Park Avenue overpass, also to near the top of the curbs • Pond 1 on the east side of Lake Washington Boulevard rose nearly to the edge of the pavement, but did not overtop the road. • At North 8th Street at Houser Way, where the east basin reaches the valley floor, the manhole covers were nearly lifted off the ground from the water pres- sure. RESULTS AND DISCUSSION Hydrology Detailed summary information and documentation of the hydrologic results are contained in Appendix B, and are summarized below. Peak flow rates for various design storms are shown in table 1. The east basin flows represent those delivered from 581 acres of upland areas to the North 8th Street storm drain. Flows for the north basin represent the combined flows from 360 acres of upland areas delivered to Pond 1. The valley flows are the combined peak flow rate from 54 acres of tributary area located on the valley floor. PACCAR flows are the com- bined peak flow rate for the 76-acre PACCAR site. Flows for subbasin 15 represent the peak rate of off-site drainage crossing the PACCAR site. Flows from subbasin 15 are based on a tributary area of 38 acres, or 10 acres less than previously assumed for the SSOE, Inc. analysis. The reduction in tributary area is based on the discovery that runoff collected by 1-405 is diverted from the basin and does not contribute to the flows crossing the PACCAR site. It is worth noting that the existing drainage pattern for the subbasin is not clearly defined. It is possible that an additional eight acres above 1-405 are also diverted away from the PACCAR site; however, this area was not eliminated as a part of the current analysis since the infor- mation available to date is not conclusive. 91031-60 Garden Ave.Drainage Swdy(101,91) 3 0 Table 1 Predicted Peak Flow Rates Peak Flow Rate (cfs) Location 2-year 10-year 25-year 100-year East Basin "' 94.0 155.0 191.0 228.0 North Basin 'z) 47.0 78.0 98.0 118.0 Valley Floor "1 20.0 30.1 35.7 41.3 Paccar Site '41 9.5 14.8 17.9 21.0 Subbasin 15 11) 11.4 17.6 21.8 25.5 ") Peak flow rates at east end of North 8th Street. Includes runoff from subbas- ins 6, 7, 8, 9, 10, 16, and 17. Hydrographs from subbasins 6, 7, 9 and 17 were added directly. Hydrographs from subbasins 8, 10, and 16 were added after lagging them by 30 minutes. j2) Peak flow rates at Pond 1. Includes runoff from subbasins 2, 3, and 4. Hy- drographs were added directly. '3) Combined peak flow rate added to the pipe system on the valley floor. In- cludes runoff from subbasins 11 and 18. Hydrographs were added directly. '4) Combined peak flow rates from subbasins on PACCAR site. 'S) Based on a 38-acre basin size, which excludes some of the 1-405 corridor. Peak flows from subbasins 6, 7, and 17 may be limited by a 24-inch concrete pipe that connects to North 8th Street. Under full head conditions, this pipe would allow ap- proximately 116 cfs. The peak flows from these areas range from 63 to 150 cfs. If this pipe were to overflow, stormwater would flow overland along the same pathway as sub- basins 8, 10, and 16. Diverting the flow over 116 cfs to subbasins 8, 10, and 16 and lagging it 30 minutes to reflect the travel time yields nearly the same flows at North 8th Street. The May 1988 report by the City of Renton indicates a 21-inch pipe that may limit flows from these same basins to 30 cfs. However, the only 21-inch pipe found between the basins and the valley floor had a slope of 39.9 percent and enough capacity to car- ry the 100-year peak flows. Peak flow rates appear to be very high compared to the capacity of the drainage system in the valley floor. The results should be viewed with several points in mind: • The peak rates are for a very narrow band of time, on the order of 10 minutes. The majority of flow from these storms is considerably less. • The flows are based on an SCS Type 1A storm distribution hydrograph. Actual storm distributions will vary. • Lag times and attenuation between the subbasins could also be more signifi- cant than were calculated. 91031 b0 Garden Ave.Drainage Study 00/1,91) 4 Backwater Analysis The stormwater system was divided into seven segments, including three seg- ments within the PACCAR site (figure 3). Operational curves were produced for each of these seven segments (figures 6 through 12). The operational curves show what the expected flow would be through the segment, given various headwater and tailwater elevations. According to the backwater model, the pond system between Lake Washington Boulevard and Lake Washington regulates the amount of flow down Garden Avenue North. As the level in Pond 1 rises, the flows to the lake increase, and the flows down Garden Avenue North decrease. By balancing the flows, and assuming the contribution to Pond 1 from the north basin was the 2-year peak flow rate of 47 cfs, approximately 75 cfs would flow through the North 8th Street pipe if the headwater at PACCAR was 31.5 feet. With 31.5 feet of head in the pipe under North 8th Street, water would be overtopping catch basins in the PACCAR property and flowing to the low areas in the northwest corner where the elevation is below 26 feet. The northwest corner of the PACCAR site and the adjacent area to the west pro- vide a storage area for stormwater from the drainage system. A rough estimate shows approximately 14,400 ft3 of storage at an elevation of 26 feet, and approximately 126,000 ft3 at an elevation of 27 feet. This does not include Garden Avenue North and the area beyond, which also provide significant storage. It is difficult to determine storage capacity above 27 feet without additional survey data. Stormwater may spread out laterally or find an overland pathway, and may not rise significantly above this level. Flood storage in this area coincides with the previously mentioned observations made during the January 9, 1990 storm. However, the model predicts that storage is needed in this area even for relatively frequent storm events. According to the model, the water surface elevation at the PACCAR stormwater discharge to North 8th Street would remain nearly constant for the 2-, 10-, 25-, and 100- year storms, as floodwater spilled out of grates and ponded in low areas. The cross connection to the Cedar River has limited capacity—less than 6 cfs even when Garden Avenue North is flooding. Tailwater effects lower the pipe capacity to about 4 cfs. The existing storm system through the PACCAR site has one section with an 18- inch pipe. According to the model, that pipe is too small to carry the 2-year storm from subbasin 15, even if the subbasin was decreased by an additional eight acres as pre- viously discussed. The flow would be limited to 7 cfs if there was no backwater. The rest of the PACCAR storm system is limited by the tailwater at North 8th Street. A tailwater elevation of 28 feet at North 8th Street would limit flows to 7 to 8 cfs, given a headwater elevation of 28.5 feet. Headwater above 28.5 feet would produce flooding in the PACCAR site. 91031-60 Garden Ave.Drainage Study(tOn.911 5 Stormwater can still be discharged from Subbasin 15 to the PACCAR site until the water surface elevation reaches approximately 31.5 feet in North 8th Street. However, this will apparently occur at less than the 2-year peak flow, and flooding would be oc- curring on the PACCAR site at that water surface elevation. The results may reflect limitations in the conservative approach used for the analy- sis. The backwater modeling approach specified for the analysis is based on the as- sumption of constant flow rates. As such, it does not incorporate simulated runoff vol- umes and variation of flow rates over the duration of the hydrograph. The maximum flow rates predicted by the SBUH model occur over a relatively short time step, and one could expect that some of the volume of the pipe network and/or surface storage would be available for attenuation of the maximum flow. A more sophisticated pipe routing analysis could give different results. CONCLUSIONS AND RECOMMENDATIONS The hydrologic and hydraulic analyses conducted indicate that runoff through and from the PACCAR site is severely restricted by limited capacity in the downstream pipe and channel network. As such, the site essentially would serve as flood storage for flows passing through the site from the south, flows generated on-site, and possibly for flows entering the North 8th Street storm drain. The capacity of the North 8th Street drain can be exceeded by design flows generated by upland areas. Under high flows, once the North 8th Street line surcharges, flows could backup onto the PACCAR site even if no flows were being generated by the site or subbasin 15. Caution should be exercised in the strict interpretation of this conclusion. These results would imply that flooding of the site is a relatively frequent occurrence. SSOE, Inc. has indicated that they have not observed flooding, suggesting there may be some uncertainty associated with the simulated peak flows and/or the hydraulics of the sys- tem. There are no flow recordings available that would allow the predicted peak flow rates to be checked against actual data. In addition, the analysis may not be accurate- ly representing the potential attenuation of the stormflows by pipes or surface storage in areas outside of the detailed study area. Another possible bias introduced by the study methods is the assumption of steady-state flow conditions. That is, peak flows delivered to the valley floor are as- sumed to be constant, with no regard to storm volume. Consideration should be given to using a more sophisticated hydraulic analysis of the valley pipe system. A model such as EXTRAN (of the SWMM model) could provide a more realistic definition of the expected water surface elevations and help to determine if, in fact, flow discharges from the PACCAR site for specific design storms. Additional survey would be required to accurately define flood storage areas throughout the valley floor. If the results produced by the current analysis are used to define site development conditions, then compensatory storage may have to be provided to prevent aggravating downstream flooding that now occurs. The volume of storage could be related to the extent of flood storage now available on the site under extreme conditions. 40 91031-60 Garden Ave.Drainage Siudy(10/1/91) 6 Appendix A FIGURES ! ! i LA S N > pan Avo N � 1 a rby O (lams Z Avo N r+ < 4 at O % ells z Ave o Pell Ave to :A>: a.A¢1 Park ZAve N Park �ty�r��ip;tpl. y i• s¢t �ur �_" Y�y Garden `'' -t Ave N Z �¢t• � •��� h%adnw Ave N Garden � - IA Ave N N Z ° # M r • a . Fact Y o O H r i I n c m, LM ,Y.—s Aw NF 11.0 fasno\A »�• >•�Yv�o�d rn ¢M m Ke newicl' Ave NE r �•;-pro; tj'� s m x M .inceln Z .o�nsot F 61vd Ave m r N m 1, 11 D PL NE Sonsof ° Monterey �'Aonlerey ~ z Ca ri A Aberdeen ¢y� m ve NE r n e bordn- ve NE n,. Blain Ave z a�b lame n tiF JLtTa �< S m z n _ Big Q Z Blaine m Ave NIE ' NE < �e� NE rr-r n �L. Blaine Ave m nxi•n v m R' m Camas Ave NE Camas` - y CAve C �n*1jry�� Q ,¢��Q� N Da Ion, L ry Darn n NE ^ Z Yrx' m O rn a � r •�:..;. o torrx.na° I,non,ls A�.� n mt NF P H F m r. Forr felo ;n pt+ Zrc. m fZn in 2 D D K CT U U ° For Clt •o Glenn woo r M � Z H n .r' ^dd/e v Av NE w S NE to N D ,�dar� `o Cr v �'i Ir rv+00d Hanin Ic Ave 1 Harri ton vAve NE'S' r C71. ndef H v W t E d.Z T ,D to { (�� Index Av rn nnx C Index A > z= O•O rr u n o o _ NE .lerk•son Avq NE .�� ,���, m S -< �� .�n Jellersom r n ? n r-,tom �+ Kir.land Lane tlE •o y ? ` ve NE yy o ' Ave NE m-MT n'.m ++ 0 2J y N F Kirklan L nmvood A` a �, N ip QQ Gs z r� e NE H z ,1 h ® Monroe Ave NE Hp C y z x M N H toy rnr`o f m m JT NEr m C> m irveNE 3Sany o n n N M,te ►T�o n o J m )I, is Ave.NF � Ave Ave NE z 'o 3S SZt + n 8 0 •L > >7 m Z � NE i«ce Pl. � r Z � OH A r H x�� n ''a`d Pierce, NI m r- NE :� 3S and 9 �' mt-)o AVER o rl M M r 7 '� 02'.en Ave ve NE H v, n lie.dmond Z r^ n R132 H iedtnond Ave NF ;) Z rn r^ rn0 � 0 m oNm �ot^ �' 3 a n n -� ?`' m Shelton w ci (/)zShcllon A 3N and x,un•'U� 0 n w rri. n Tn� yrm b m •, mt � Union ~11rtonAm wV o E Z v,shunve SE r C rn P n, 2 Z m z m CT Vasho L C G•n id m to rn !n r _ *lo' 'C+ r1n A° r. n D ACAD\91024-20\91024-20.DWG 09-20-91 PTK C-1 Ll [�[ o 0 0.25 0.5 s oa NE 20th ST [ LEGEND MILE 2 ['No BASIN NUMBER F1 BASIN BOUNDRAY a ........ SHEET FLOW -- SHALLOW CONCENTRATED FLOW NE 12th ST ® [ 0� [ —� PIPE FLOW 9 z [ 9 < O 0 18 s ❑ w w 6 • Z [J :> o Q cr a N 8th ST 0 I � z NE U 0 Q © 0 Z z w l� [ NE-y UE I I ago [ [a GARDEN AVENUE DRAINAGE STUDY Figure 2 DRAINAGE BASINS ENTRANCO _ A.2 ACAD\91024-20\PACCAR.DWO 09-17-91 PW i 0 /8 / MILE ■ ®■ J Sirs NE 12th ST LEGEND , N0. Basin Number 9 Basin Boundary v SEES Backwater Analysis Flow Routes ® 25 V 900 �■ Upper Basin Flow Input >i 18-5 ®i Operational Curve Flow Segment / Z i < 11-3 • 18-4 iw • o o: • m 18-3 G y • s 9r � 9 1-2 11-2 r ti ; 18-2 y IL 73 Z 11-1 18-1 SEEN l o ,0 Q N 8th ■ ST tiS 4 � � � i 11-4 i 14-�4 14-6 p 14-5: I 13 •. 10 14-1 ■ 14-3 - �3 r w r y o • 3� �D � GARDEN AVENUE DRAINAGE STUDY ,V IZ,q Figure 3 BACKW TER ANALYSIS FLOW ROUTES 'i LENTRANGO 1G51 t7 A.3 r ACAD\910-4-20\91024-20.DWG 09-20-91 PTK + + / / + + + + + + + + + + + + + Al G 0.25 0.5 + + + �� / / /� /� + + + + + + + / �/ j/ / + + t + + + + + + + t + f + + + + + + + + + + MILE + + / / • / / j�j. + + + + + + + + + + + + + + + i + �j + + + + + + + � + + + + + + + + + + + + + + + + + + + + + + + LEGEND + / �/ /.�j //. //� + + + + + + + j + + + + + + + + + + SOIL GROUP + �� / + + + + + + + + + + + + + + + + + + + + + + + + + + A + + / . / + + + + + + + + + + + + + + + + + + + + + + + + / �/ + ++ + /� / /�� + + + + + + + + + �� � + - + + + + + + + + + + + + + + + + + + + + + + - - - + + + + + + + . + + + + C + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 'r + + + + + + + + + + + + + + + + + + + + + + / + + + + + + + + t + + + + + + + + + + + + + + + + j� + + + + �� + �J�' + + + t + + + + + + + + + + + + p/ + 0 + + + + + + + + + + + + + + + + + + -+ + + + + + + + , + + + + + + + + + + + + 4 + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + — + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + t + + + + t + + + + + + + + + + + + + + +a + o + + + + + + + + ++ + ��— + + + + + + + + + + + + + + + + + -t + + + + + I+ +_. + + + + + + + + + + + + + + + + + + ' + + + + + + + + + + T + + + + + + + + + + + 4 + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + t + + + + + + + + + + + i- + + + + + + + i� + + +y + + + + + + + + + + + + + + + + + + t + + + + + + + + + + + + +Z + + + + o + + is + + + + + + + + + + + + + + + + + + + + + + I + + + a , ST+ + + + + + + + + + Z + + + + + + + + + + + + + I + + + + + + + + + + +¢ + + + + +2 + + + + + + + + + + + + + + + + +I + + + + + + + Y + + + + + + + -+ + + + + Z + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 4 + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + _ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + •}� + + + + + + + + + + + + + + + + + + + + + + - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + i + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 40 + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + i + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +¢ + + + + + i- + + + + + + + + + + + + I> + + + + + + + + + + + + + + + + + + + + + / + + + + + + + + + + i + + + + + + + + + + + + + + + +Z + + + / + + + + + -+- + + + + + + > + + + + + + + + + + + + + + + + + + // ' + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + t + + + + + + + + + + + + + + + + + + + + + + + + + + + 4- El + + + + + + + + + + + + + + + / / + ++ + + + + + + + + + + + + + + + +' + + + + + + + + + + + + +❑ + + + + + + + + + + + 4 + + + + + + + + + + F + ++ + + + + / + + + + + + + + + + + i• + + + + + + + + + + + + + + + + // // + + + + + + + + 4 + + + + + + + + + + '+ + + + + + / / + + + + 'i. + + + + + + +- + + + + + + + + + / + + + + + + + + / / / // + + + + 4 + 'F + + + + + + + + + + + + + + + + + + + + + + + + + + / / / + + + + + + + + + + GARDEN AVENUE DRAINAGE STUDY Figure 4 C.' HYDROLIC SOIL GROUPS >a� E_NTRANCO ACAD\91024-20\91024-20.DVG 09-20-91 PTK + + + + + + + + + + + + + + + 1\1 �_ - - - 4 + + + + + + + + + + + + At MILE _ + + + + + + - - + + + + + + + + - - - + + + + LEGEND + + + + + + + + + + + - - + + a COMMERCIAL / INDUSTRIAL J + / + + + + + + + + + + + RESIDENTIAL + + + + + + + MULTI-FAMILY /// // 12 S + + + _ — — + + + + + + FOREST //�j//////j/ + + + + + + + + + m - + + + + + b + / + + + + + + + + _ // + + + + + + + w, + o + + + + + + + + + + + + + + + + + + + + 42 + + + + + + 10E + + + + + + + + + + // + + + + l _ + + + + cl - GARDEN AVENUE DRAINAGE STUDY _.._ Figure 5 1 LAND USE CATEGORIES E? EIITNANGO A C Figure 6 Operational Curve Flow Segment 1: Flow from Pond 1 to Lake Washington for Various Headwater and Tailwater Elevations Overflow 260 - - - 250 1 240 w 230 U -l--W =24.5 Ft 220 � I I HW - 231.5 Ft 200 — — — ----e—! I 0 L 190 3 LL tao HW = 22.5 Ft 170 110 HW = 21.5 Ft Gran 150 12 - 13 14C gW G� G�tN �' 16 �F ✓k i L, Level of Lake Washington w ° _Eiguce._.Z_._. P�aperatforral�rve-flow-Segment 2: Flow fromNorth-8ttr Street to Pond-1- - for Various Headwater and Tailwater Elevations 100 �, — se H6�✓_ 28 0 F go t 80 HW'27.0F .� 70 t __ `26.0 Ft 00 v� -(i ! W -=-5.0 F-- t 0��Z/off 0 60 —{ _. ---------- -- — -- --- .. _ -- 30 20 10 r 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5 Level of Pond 1 A6 Figure 8 Operational Curve Flow Segment 3: Flow from Garden Avenue Junction to the Cedar River for Various Headwater and Tailwater Elevations 5.6 - C 5.4 5.2 7 \ I 5 ' -- LP 4.8 4.6 n I u 4.4 I \ .91 a 4.2 2j0 3.8 HW= 26.0 Ft 0 3.6 30 3.4 2S0Ft LL 3.2 -� i 3 2.8 2.6 I 2.4 22 1 B.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5 22.0 22.5 23.0 Level of Cedar River Figure 9 Operational Curve Flow Segment 4: Flow from PACCAR Outlet to Garden Avenue Junction for Various Headwater and Tailwater Elevations 130 I 120 eve"ljpw 110 I HW=33.0 Ft �_ 100 -� 90 HW= 31.0 pt 80 a a 70 HW= 29.0 Ft r � I 60 r 50 3 0 ti 40 - 30 HW = 27.0 Ft 20 10 0 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28 Water level at Garden do N 8th A.7 Figure 10 Operational Curve Flow Segment 5: Flow from Lower PACCAR Junction to North 8th Street for Various Headwater and Tailwater Elevations 16 15 Overflow 14 13 N W-28.5 Ft 12 11 10 0 0 9 0 7 y 4 6 o U- 5 .0 4 r 3 2 1 0 26.0 26.5 27.0 27.5 28.0 Water level at PACCAR and N Sth Street Figure 11 Operational Curve Flow Segment 6: Flow from Middle PACCAR Junction to Lower PACCAR Junction for Various Headwater and Tailwater Elevations 16 15 14 13 HW = 32.0 Ft 12 O n 11 k `e/7/04, 10 W_ 31•0 Ft 0 a 9 a 8 yw, 3 0 0• Fj 0 7 r 6 0 U- 5 4 y� 3 1P 9 2 .0- f 1 0 26.0 26.5 27.0 27.5 28.0 28.5 29.0 Water level at first junction A.8 Figure 12 Operational Curve Flow Segment 7: Flow from the Inflow to PACCAR to Middle PACCAR Junction for Various Headwater and Tailwater Elevations Overflow 6 HW�3r'SFt _ 5 Nw,31.0 Ft 0 a 4 rn J0,6 = r 0 3 r yi a LL- o� f 0 29.0 29.5 30.0 30.5 31.0 Water level at second junction A.9 Appendix B HYDROLOGIC SUMMARY AND CALCULATIONS 9/23/91 ENTRANCO ENGINEERS , INC . PAGE 1 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i t i i ------------ BASIN RESULT SUMMARY YDtIME---- -RATE- - TIME----- HYDROGRAPH AREA ID ---CF-- AC-FT --CFS- -WIN- HOURS METHODOLOGY ACRES zzszszszsszsazasssaa■aaazazszaz■asaaazzassaaazaazzzszasazaazzzzau■■aa az■ i1-11 418595 9.51 14.81 491 8.17 SION METHOD 55.1➢ 11-2 26➢271 5.97 8.99 491 8.17 SION METHOD 55.11 11-25 519821 11.71 18.18 491 8.17 SINK METHOD 55.11 1fill 612581 13.83 21.63 491 8.17 SION METHOD 55.19 ll.1A 43148 ➢.99 2.53 481 9.11 SION METHOD 7.11 11.18 65562 1.51 3.82 481 8.11 SOON METHOD 7.11 11.1C 78➢98 1.79 4.53 481 8.11 SOON METHOD 7.11 ll.1D 91671 2.18 5.24 481 8.11 SOON METHOD 7.11 11.2A 77691 1.78 4.51 411 8.11 SION METHOD 12.61 11.21 118138 2.71 5.79 481 8.19 SION METHOD 12.61 11.2C 141616 3.23 8.15 481 8.81 SION METHOD 12.61 11.2D 163238 5.75 9.32 48➢ 8.11 SIIN IETHOD 12.61 11.3A 12958 ➢.31 1.78 481 8.11 SION METHOD 2.11 11.38 19662 1.45 1.18 481 8.➢1 SI➢N METHOD 2.11 11.3C 23422 1.54 1.41 481 9.11 SION METHOD 2.11 11.3D 27194 1.62 1.62 481 8.11 SION METHOD 2.11 11.4A 32639 1.75 2.13 481 8.11 SOON METHOD 5.31 11.48 49614 1.14 3.16 481 8.11 SOON METHOD 5.31 11.41 59195 1.36 3.63 111 8.11 SIGH METHOD 5.31 11.40 68612 1.58 4.21 481 8.➢1 SION METHOD 5.51 12 11 571476 13.11 31.12 481 8.11 SION METHOD 61.11 12-2 375275 8.62 19.89 481 8.11 SION METHOD 61.11 12-25 679681 15.61 35.65 481 8.01 SOON METHOD 51.1➢ 12111 789214 18.12 41.27 481 8.11 SBIH METHOD 61.11 13-11 813143 18.44 21.55 521 8.67 SIIN METHOD 91.11 13-2 518571 11.91 13.12 521 8.57 SIIN METHOD 91.11 13-25 963516 22.12 24.74 511 8.51 SIIN METHOD 91.11 13111 1124855 25.82 28.97 511 8.51 SION METHOD 91.81 14.1A 42511 1.98 2.67 481 8.11 SIIN METHOD 6.61 14.18 63937 1.47 3.96 481 8.11 SI➢N METHOD 6.61 14.1C 75871 1.74 4.67 481 8.11 SION METHOD 6.61 14.1D 87815 2.12 5.37 481 8.11 SBIH METHOD 5.61 14.2A 11511 1.85 4.79 481 8.11 SOON METHOD 12.51 14.28 121192 2.78 7.11 481 8.11 SIIN METHOD 12.51 14.2C 143693 3.31 8.39 481 8.11 SION METHOD 12.51 14.2D 166315 3.82 9.65 48➢ 8.11 SOON METHOD 12.51 14.3A 111766 2.34 6.1E 481 8.11 SIIN METHOD 15.81 14.31 153161 3.51 9.12 481 8.11 SOON METHOD 15.91 14.3C 18162E 4.17 11.64 481 8.11 SION METHOD 15.81 14.5D 211223 4.85 12.26 481 8.11 SION METHOD 15.81 14.4A 69351 1.59 2.38 491 8.17 SINK METHOD 15.11 14.4E 114161 2.62 4.18 491 8.17 SION METHOD 15.11 14.4C 139851 3.21 5.17 491 8.17 SOON METHOD 15.11 14.0 165912 3.81 6.17 491 8.17 SION METHOD 15.11 14.51 11111 2.17 4.21 411 8.11 SIIN METHOD 28.11 14.58 154146 3.54 7.14 481 8.11 SION METHOD 21.11 14.5C 188332 4.32 8.81 481 8.11 SOON METHOD 21.11 14.50 222153 5.12 1➢.48 481 8.11 SOON METHOD 21.11 14.6A 22762 ➢.52 1.63 521 8.67 SION METHOD 6.11 14.68 41172 1.92 1.22 491 8.17 SOON METHOD 6.11 B.1 9/23/91 ENTRANCO ENGINEERS , INC . PAGE 2 BASIN RESULT SUMMARY BASIL -----VOLUME---- -RATE- ----TINE----- HYDROGRAPH AREA 1D ---CF-- AC-FT --CFS- -Ill: HODIS RETHODOL06Y ACIES i t■titttittiit■ttiitiitiitttitt.ittittititti■iti■tifit■■ti■tittt 14.6t 51152 1.15 1.57 490 8.17 STUN METHOD 6.11 14.60 61435 1.39 1.93 491 8.17 SBUN METHOD 6.11 15-11 423421 9.72 22.61 481 8.11 SI➢N METHOD 49.11 15-2 275192 6.32 14.73 481 8.➢1 SIIH METHOD 48.19 15-25 517➢94 11.64 27.14 48➢ 8.➢1 SION METHOD 48.11 15111 591384 15.58 31.51 481 8.19 SBUN METHOD 48.➢➢ 15AII 331178 7.58 17.57 48➢ 8.11 SBUN METHOD 38.11 15A2 213961 4.91 11.4/ 481 8.➢➢ SION METHOD 31.11 1SA25 595933 9.19 21.94 481 8.11 SION METHOD 38.11 15A99 462248 11.61 25.48 481 8.1➢ SBUN METHOD 38.11 15911 261827 5.99 14.43 481 8.11 SBUN METHOD 51.11 1592 168711 3.87 9.34 481 8.1➢ SBUH METHOD 31.11 15125 312925 7.18 17.31 481 8.11 SION METHOD 3➢.11 15B99 365451 8.39 21.19 481 8.11 SINH METHOD 31.1/ 16-11 629738 14.46 25.17 491 8.17 SION METHOD 87.➢1 16-2 386256 8.87 15.11 491 8.17 SBNN METHOD 17.11 16-25 771179 17.71 51.13 491 8.17 SION METHOD 87.11 161➢1 915311 21.➢1 37.25 491 8.17 SBUN METHOD 87.11 11-11 171115 22,31 37,21 191 1,17 SION METHOD III,11 17-2 612122 14.15 22.98 491 8.17 SION METHOD 125.11 17-25 1181591 27.12 45.59 491 8.17 SION METHOD 123.01 17111 1391792 31.95 53.71 491 8.17 SBUN METHOD 123.11 18.1A 29887 1.69 1.87 481 8.11 SINN METHOD 4.91 18.18 45527 1.15 2.12 481 8.11 SION METHOD 4.91 18,1C 54283 1.25 3.35 481 8.11 SION METHOD 4.91 18.1D 63168 1.45 3.87 481 8.8➢ SION METHOD 4.9➢ 18,2A 3➢512 1.11 1.91 481 8.01 SBNN METHOD 5.11 18.2E 46461 1.17 2.88 481 8.11 SBUN METHOD 5.11 18.2C 55347 1.27 3.42 481 8.10 SOON METHOD 5.11 19.2D 64362 1.48 3.95 481 8.01 SOON METHOD 5.➢➢ 18.3A 32961 1.76 2.16 481 8.➢1 SBNN METHOD 5.41 18.31 51211 1.15 3.11 481 8.11 SBUN METHOD 5.41 18.3C 59851 1.37 3.69 481 9.00 SBIN METHOD 5.49 18.3D 69535 1.61 4.27 481 8.➢➢ SBUN METHOD 5.4➢ 18.4A 42717 1.98 2.67 481 8.11 SBIH METHOD 7.11 18.48 65151 1.49 4.13 481 8.11 SION METHOD 7.If 18.4C 77561 1.78 4.78 491 1.11 SIIN METHOD 7.I1 18.4D 91111 2.17 5.54 481 8.11 SION METHOD 7.11 18,5A 25642 1.59 1.61 481 8.11 SION METHOD 4.21 11.58 39/51 1.91 2.42 481 8.1➢ SBUN METHOD 4.21 18.5C 46557 1.17 2.87 481 8.➢1 SINN METHOD 4.21 18.5D 54189 1.24 3.37 481 8.11 SION METHOD 4.29 19-11 385155 9.84 17.14 481 8.11 SI➢N METHOD 51.11 19-2 241324 5.52 11.49 491 8.17 SION METHOD 51.11 19-21 468312 II,75 21.99 411 1,I I SIIN METHOD II.11 191➢/ 552917 12.59 24.92 481 8.11 SION METHOD 51.11 2 11 313114 7.19 12.31 491 8.17 SION METHOD 61.11 2-111 476414 11.94 19.57 491 8.17 SBUN METHOD 61.1➢ 2-2 182922 4.21 9.75 481 8.➢1 SBNN METHOD 61.11 2-25 3928➢2 9.➢2 15.81 491 8.17 SION METHOD 61.11 B.2 9/23/91 ENTRANCO ENGINEERS . INC . PAGE 3 --i--iiO---st------hats♦----------i t•t•ti---------------------------- BASIN RESULT SUMMARY BASIN -----VOLBME---- -RATE- ----TIME----- HYDROGRAPH AREA ID ---CF-- AC-FT --CFS- -Rix- HOURS METHODOLOGY ACRES ■■asssaassssssscssssassssasaaassesaaa.sssaaaassasssasassaassaaaassaaasaaa 3-11 129569 16.75 29.17 491 8.17 SOIN METHOD 116.➢1 5-111 1978273 2k.75 44.17 491 8.17 SBUH METHOD 116.11 3-2 441017 11.12 17.22 411 8.17 SBBN METHOD 116.10 3-25 9➢115k 29.69 56.41 491 8.17 SBUH METHOD 116.11 4-1➢ 137➢327 31.46 36.8➢ 511 8.50 SOON METHOD 18k.➢l. 4-1➢9 1981895 45.51 54.36 491 8.17 SBUN METHOD 184.11 4-2 845984 19.42 22.12 511 8.59 SBNH METHOD 184.➢1 4-25 1673351 38.41 45.45 491 8.17 SBUH METHOD 184.➢1 6-11 1099296 25.24 41.15 491 8.17 SION METHOD 139.1➢ 6-1➢1 1576969 36.21 58.42 490 8.17 SBUH METHOD 131.11 6-2 6841➢8 15.71 2k.33 491 8.17 SBUH METHOD 139.➢➢ 5-25 1336655 51.69 49.23 491 8.17 SBON METHOD 139.11 7-11 786493 18.16 26.51 491 8.17 SION METHOD 112.1➢ 7-1➢➢ 1131361 25.97 38.77 491 8.17 SBNH METHOD 112.➢1 7-2 488646 11.22 16.04 491 8.17 SBOH METHOD 192.➢1 7-25 95762k 21.98 32.59 491 8.17 SBUH METHOD 1➢2.11 8-11 292255 6.71 13.78 481 8.11 SBUH METHOD 37.➢1 8-1➢1 417814 9.59 19.91 481 8.11 SBNN METHOD 37.1➢ 8-2 183117 4.21 8.51 181 8.11 SBUH METHOD 11.11 8-25 354589 8.14 16.81 481 8.19 SBNN METHOD 37.19 9-11 198751 4.56 7.92 491 8.17 SBUH METHOD 38.➢1 9-111 3➢9312 7.1➢ 13.➢7 49➢ 8.17 SBNH METHOD 38.91 9-2 1➢9961 2.52 3.93 49➢ 8.17 SBUN METHOD 38.11 9-25 252846 5.81 11.42 49➢ 8.17 SBUH METHOD 58.➢1 B.3 0 GARDEN AVENUE D R A I N G E PEAK FLOWS 2 7-S E P VA��EY FLLoP,, Sl)bbaSIh IS STORM EVENT 2 10 25 100 BASIN 11 . 1 2 . 53 3 . 82 4 . 53 5 . 24 11 . 2 4 . 51 6 . 79 8 . 05 9 . 32 11 . 5 0 . 78 1 . 18 1 . 4 1 . 62 11 . 4 2 . 03 3 . 06 3 . 63 4 . 2 SUBTOTAL 9 . 85 14 . 85 17 . 61 20 . 38 STORM EVENT 2 10 25 100 BASIN 14 . 1 2 . 67 3 . 96 4 . 67 5 . 37 14 . 2 4 . 79 7 . 11 8 . 39 9 . 66 14 . 3 6 . 08 9 . 02 10 . 64 12 . 25 14 . 4 2 . 38 4 . 08 5 . 07 6 . 07 14 . 5 4 . 23 7 . 14 8 . 8 10 . 48 14 . 6 0 . 63 1 . 22 1 . 57 1 . 93 SUBTOTAL 20 . 78 32 . 53 39 . 14 45 . 77 STORM EVENT 2 10 25 100 BASIN 18 . 1 1 . 87 2 . 82 5 . 35 3 . 87 18 . 2 1 . 91 2 . 88 3 . 42 3 . 95 18 . 3 2 . 06 3 . 11 3 . 59 4 . 27 18 . 4 2 . 67 4 . 03 4 . 78 5 . 54 18 . 5 1 . 6 2 . 42 2 . 87 3 . 32 SUBTOTAL 10 . 11 15 . 26 18 . 11 20 . 95 STORM EVENT 2 10 25 100 BASIN 15 14 . 73 22 . 61 27 . 04 31 . 5 15A 11 . 4 17 . 57 21 . 84 25 . 48 15B 9 . 34 14 . 4 17 . 3 20 . 19 GRAND TOTAL 50 . 08 77 . 04 92 . 16 107 . 29 BA SHEET NO. 1 OF ? e ENTRANCO ENGINEERS, INC. JOB NO. I - 1 o 2 4_-2 o PROJECT CALCULATIONS FOR OF GO�l�c.lT �-r;v�1 MADE BY (Tk' DATE CHECKED BY DATE �- To�7at, 13�1� 5►.1-ect - rao�v - ��' GO�ilca.11�2o-"f6v r-(.o�v OnE�.I C,�}c,rd�lGt,1PIpE-FW��----- -I-IME SwreI k (c'f1M£ �-ro Slt7P6 K" -t1ME %G.�j 1700 b.0 45 0.1S 17 0 o.0-1 , 1 1 1,0 1700 0.0 S4- 4°- - 3 _-- 30o O. 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JOB NO. k - f� PROJECT CALCULATIONS FOR 11MG-r, Or- MADE BY VlY— DATE 09-1 1-9' CHECKED BY DATE 9 'fp7.41. 1 �kect �t o�v CA4aU,o\' 60►,J49.l-9,WT5;o FLOW onra.l=c�c�liJGt;%P��-�l oiv' CMInI) �►� t 1� E '. Lo PYIK3: 10 Swra aLZ qlsLofs i }2.2 14-1 �)oO 0.2qo 0.15 I 150 0 2'll 0.9 020 I �.Z iI j,2 3o0 0.009 0.011 h.4 ?00 0.009 277; ,6.I :,00 0.005 0,011 6.4 o.o0s z- Z.2 Igo 0 oos I4-z --'- 1 . I_ I� O,OZo 0.011 2.1 _ _ 75� P. �00 o.ozo 0.011 3.1 o.oZo sl t.1 �1 �-T;�_. �.6 i�3-2. 300 O.9*2w 0.0(1 3-7 2-gb 0.020 4.b 'I�_3 300 0.020 0 01► 3.-1 2SD O.U2o 27 (, { 1 _� :; ;. .� Ji-.�_.._... 17.0 10-4- 300 0,o2o 0.01) 3.1 300 D.OZO 2-7 1,3 — -- -' -� I-r 44- 3o o 0.02D 0,011 3� 3� 0,020 27 I`5 t f =4 Zo.1 �1- 3o0 0.OvS 0.011 �,4 _ _ . 245v f ODOS! f4 G- �-- 1 L_p1.. (_ r 14 i� B.6 e ENTRANCO Garden Avenue Renton, Washington DRAINAGE STUDY ADDENDUM Prepared for PACCAR, Inc. Prepared by ENTRANCO 10900 NE 8th Street, Suite 300 Bellevue, Washington 98004 (206) 454-5600 October 16, 1991 DRAINAGE IMPROVEMENT ALTERNATIVES To determine whether improving any segments of the drainage system would allevi- ate existing capacity problems, five alternatives were selected for further analysis. PACCAR has developed storm drainage improvement plans that may be construct- ed in the near future. These plans include the construction of a bypass from subbasin 15 around the PACCAR site. The plans also include the construction of an east/west interceptor across the PACCAR site. The following alternatives analysis assumes the construction of this bypass and in- terceptor. Alternative 1 - Enlarge culverts between Pond 2 and Pond 3 (Burlington Northern Railroad culverts) to three 72-inch concrete culverts Enlarging these culverts would improve the flow somewhat between Pond 1 and Lake Washington- The flow through this segment would increase approximately 15 cfs (from 125 cfs to 140 cfs) with a Pond 1 headwater elevation of 20.5 feet (figure 13). The flow through the segment would increase from 220 cfs to 260 cfs with a Pond 1 headwater elevation of 24.5 feet. Flow improvements are summarized in table 2. Table 2 Flow Improvements under Alternatives 1 and 2 Pond 1 Flow (cfs) Headwater Improved Improved Elevation BNRR Lake Wash. (feet) Existing Culverts Blvd. Culverts 19.5 90 100 100 20-5 125 140 150 22.5 15 - 210 220 24.5 220 260 270 Alternative 2 - Enlarge culverts between Pond 1 and Pond 2 (Lake Washington Boulevard Culverts) to three 72-inch concrete culverts The results for this alternative are very similar to those described under Alternative 1 (table 2). A headwater elevation in Pond 1 of 20.5 feet would increase flows to ap- proximately 150 cfs, slightly more than in Alternative 1 (figure 14). A headwater eleva- tion of 24.5 feet in Pond 1 would increase flows to 270 cfs, also slightly more than un- der Alternative 1. Alternative 3 - Enlarge the Garden Avenue line to 72 inches Enlarging the Garden Avenue pipe would dramatically increase its capacity under high headwater conditions at the upper end. For example, if the water level in Pond 1 is 20.5 feet, and the headwater at North 8th Street is 28 feet, the conveyance capacity would increase from 80 to 180 cfs (figure 15). A headwater elevation of 25 feet with the 91031-60 Garden Ave.Draw%&9 a Addendum(10-16.91) 1 same water level in Pond 1 would increase flows from 56 to 80 cfs. As the tailwater in Pond 1 rises above 21 feet, the capacity of the Garden Avenue line decreases. A Pond 1 water level of 24.5 feet would reduce the capacity to 45 cfs given a headwater of 25 feet. Headwater above 25 feet would cause flooding. Table 3 shows a comparison of the existing Garden Avenue flow capacity, with the future capacity if upgraded as in Alternative 3. The table assumes that 100 cfs is enter- ing Pond 1 from the North Basin, and contributing to a rise in water level there. Under this scenario, the system would overflow above a headwater of approximately 26 feet and a flow of 107 cfs. Table 3 Garden Avenue Flow Capacity and Pond 1 Elevation under Alternative 3 N. 8th Street Pond 1 Pond 1 Headwater Existing Flow Existing Future Flow Future Elevation Garden Ave. Level 72" Culvert Level (feet) (cfs) (feet) (cfs) (feet) 25 55 21.5 75 22.5 26 60 21.8 107 23.7 27 70 22.3 overflow overflow 28 72 22.5 overflow overflow Alternative 4 - Enlarge the North 8th Street line to 72 inches Enlarging the North 8th Street line to 72 inches also would increase the convey- ance capacity of this segment under high headwater conditions. Figure 16 shows that if the tailwater at Garden Avenue was 25 feet, and the headwater at the PACCAR exit to North 8th Street was 29 feet, the pipe capacity would increase from approximately 66 cfs to 110 cfs. The capacity of the pipe would remain at approximately 110 cfs for tail- water elevations at Garden Avenue up to 28 feet. Headwater elevations above 29 feet would increase the pipe flow more dramatically. However, because of the low grate elevations on the Paccar site, increased head would also cause flooding there. Alternative 5 - Construct a separate storm drain beginning at Houser Ave- nue and North 8th Street, and extending northward along Houser Avenue to Pond 1 A 72-inch storm drain constructed along this route could serve to convey most of the water from the east basin directly to Pond 1. A headwater of 29 feet would produce a flow of approximately 115 cfs to Pond 1, even if the tailwater in Pond 1 was as high as 24.5 feet (figure 17). A headwater of 31 feet would increase the flow to 220 cfs for the same tailwater condition. A few inches of additional headwater would increase capac- ity to handle the entire east basin for the 100-year event. Additional survey information would be required to determine if the system could be constructed to achieve those headwater conditions. 91031-60 Garden Ave.Orawuap Addendum(10.16-91) 2 Alternatives Discussion It is apparent that some combination of these alternatives will be needed to allevi- ate the capacity problems. Alternative 5 appears to be one of the most promising alter- natives for solving the entire basin flooding problem. However, the drainage system would then be limited by flow through the pond system. There is not sufficient capacity through the lower ponds to accommodate a new storm line, as well as the existing lines from Garden Avenue and the north basin. Under current conditions, the flow through the pond system could reach 220 cfs, with a corresponding water elevation of 24.5 feet in Pond 1. However, when the water level in Pond 1 is 24.5 feet, the capacity in the Garden Street line is reduced to less than 25 cfs, and the capacity of the line from the north basin would be similarly reduced. It appears that the 2-year storm could pass through the system without improvements between Pond 1 and Lake Washington. The 2-year storm would include 99 cfs from the east basin; 49 cfs from the north basin; and 59.5 cfs from subbasin 15, PACCAR, and the rest of the valley floor. The 10-year storm would raise the Pond 1 water level above 24.5 feet and therefore cause flooding in the Garden Avenue line. The other alternatives are also best not considered in isolation. For instance, the effect of increasing the size of the Garden Avenue line would be somewhat negated be- cause of the capacity of the pond system. As the water level in Pond 1 rises, the ca- pacity of the Garden Avenue line decreases, no matter how large the pipes are. For example, with a headwater elevation of 26 feet, the capacity decreases from 125 cfs to 85 cfs and then overflows as the tailwater elevation rises from 21 to 24 feet. The opti- mal solution will be some combination of these individual alternatives. Construction by PACCAR of the subbasin 15 bypass and the east/west interceptor will help to manage the runoff from the PACCAR site. These improvements do not alter the basic conclusions of the backwater analysis of the alternative downstream drainage improvements. High headwater elevations in the North 8th Street pipe would still cause flooding in the PACCAR site. Backwater analysis with the Extran model should help to more clearly understand to what extent peak flows from the east basin may impact the PACCAR site. It may also be possible to trade increased conveyance of the drainage system with compensatory storage requirements. For example, increased conveyance would reduce flood levels in the PACCAR site. This would translate to lower compensatory storage requirements. 91031-60 Garden Ave.Draw%aW Addendum(10-16-91) 3 Figure 13 Alternative 1: Flow from Pond 1 to Lake Washington with Three 72-inch Culverts Installed Between Pond 2 and Pond 3 (Burlington Northern Railroad Culverts) for Various Headwater Elevations 300 T E3 E3 Overflow 13 280 24.5 Ft 260 240 220 22.5 Ft T A v 200 O 1 0. 180 L U 7 0 160 -1 r 20.5 Ft 3 140 0 LL 120 -j 19.5 Ft 100 so -� 18.5 Ft 60 12 13 14 15 16 17 Lake Washington level Figure 14 Altemative 2: Flow from Pond 1 to Lake Washington with Three 72-inch Culverts Installed Between Pond 1 and Pond 2 (Lake Washington Boulevard Culverts) for Various Headwater Elevations 320 B Overflow 300 I 280 J 24.5 Ft 260 0 240 r 220 M Ft • 0 200 Q. rn 180 0 r 160 20.5 Ft 0 140 L i 120 19.5 Ft too so- so 18.5 Ft 1 12 13 14 15 16 17 Lake Washington level Figure 15 Alternative 3: Flow From North 8th Street to Pond 1 if Garden Avenue Pipe is Enlarged to 72 Inches for Various Headwater Elevations 200 Overflow 190 180 28 Ft 170 160 27 Ft 150 J o i a 140 -{ P 130 -1 26 Ft 0 120 -j r 110 3 0 100LL- - 90 I 25 Ft 80 I 70 -; 60 -1 50 40 18.5 19 19.5 20 20.5 21 21.5 22 22.5 23 23.5 24 24.5 Pond 1 Level Figure 16 Altemative 4: Flow from PACCAR Outlet to Garden Avenue Junction if North 8th Street Pipe is Enlarged to 72 Inches for Various Headwater Elevations 450 Overflow 400 33 Ft 350 300 a 31 Ft a t 250 P 7 0 t 200 1 0LL- I 150 29 Ft 100 28 Ft so 27 Ft 0 24.5 25 25.5 26 26.5 27 27.5 26 Water level at Garden Avenue Figure 17 Alternative 7: Flow through New 72-Inch Storm Line Along Houser Avenue from North 8th Street to Pond 1 for Various Headwater Elevations 320 Overflow 300 33 Ft 280 I 260 240 31 Ft 220 I a 200 { a I L 180 w 0 160 L ' 140 _ 120 29 Ft 100 28 Ft so 60 40 20 27 Ft 0 1&5 19 19.5 20 20.5 21 21.5 22 22.5 23 23.5 24 24.5 Pond 1 Water Laval e iu31-o Cwdm Awt oraw"e Addendum(Io-1&e1) 6 PACCA.R — Garden Avenue Drainage Analyis — Oct 29 , 1991 Houser Avenue Bypass Alternatives The Houser Avenue bypass alternative was further examined , with an analysis of four different pipe sizes . The bypass extends from Houser Avenue and North 8th Street along Houser Avenue to Pond 1 . The pipe would be approximately 2700 feet long , with 8 catch basins . The upper invert elevation would oe approximately 30 . 4 feet , and the lower invert in Pond 1 would be 16 . 8 feet . Bypass Bypass Capacity Pipe Size 2yr 10yr 25yr ' 100yr inches cfs cfs cfs cfs 36 60 60 60 60 48 99 lie 110 110 60 99 150 150 150 72 99 163 190 190 -------------------------------------------- Exisiting Garden Avenue Capacity 2yr 10yr 25yr 100yr cfs cfs cfs cfs 74 72 72 71 -------------------------------------------- Enlarged Lake Washington Blvd Culverts Bypass Garden Avenue Pipe Size 2yr 10yr 25yr 100yr inches cfs cfs cfs cfs 36 74 74 74 '4 48 70 65 60 60 60 70 35 35 35 72 70 35 25 25 -------------------------------------------- Enlarged Lake Washington Blvd & BNRR Culverts Bypass Garden Avenue Pipe Size 2yr 10yr 25yr 100yr inches cfs cfs cfs cfs 36 80 78 75 76 48 73 74 71 71 6 78 '0 6 : 68 72 78 63 6 6' SOUTH GARDEN AVENUE STORM DRAIN EXTENSION RENTON, WASHINGTON IV. ADDENDUM NO. 3 GARDEN AVENUE NORTH DRAINAGE STUDY e TECHNICAL MEMORANDUM ENTRANCO TO: Dick Bangert PACCAR, Inc. FROM: Ralph Nelson, P.E. Entranco DATE: December 18, 1991 SUBJECT: Addendum No. 3 Garden Avenue North Drainage Study Entranco Project No. 91029-60 INTRODUCTION Entranco has been retained by PACCAR to study the Garden Avenue North drainage system in Renton, Washington. This report is a continuation of drainage studies com- pleted to date concerning the proposed PACCAR site development, runoff from the North Renton Drainage basin, and the Garden Avenue North drainage system. The Garden Avenue North drainage study began with a steady-state backwater analysis of the Garden Avenue North storm drain as it relates to the proposed PACCAR site de- velopment. Flows produced by the King County hydrology model as a part of the Park Avenue Drainage study (in progress) were used as input. The steady-state analyses in- dicated that even design flows as low as the 2-year return rate were capable of causing wide-scale flooding on the valley floor. This condition severely limits the discharge of stormwater from the PACCAR site. Results of this initial analyses were presented in the Garden Avenue North Drainage Study (October 1, 1991). A second study was conducted to determine what drainage alternatives were available for the PACCAR site development. The study examined several potential alternatives, including enlarging the culverts for the lower ponds, bypassing storm flows down North Houser Avenue, and enlarging the existing Garden Avenue North storm drain. Each al- ternative was evaluated using the steady-state backwater model. The results demon- strated that a combination of improving the capacity of the lower ponds system along with an increase in the flow capacity of the valley storm drain system could help to re- duce flooding on the valley floor. The results of the second study were summarized in a Drainage Study Addendum (October 16, 1991). After the second study was completed, Entranco began to analyze the valley floor drainage using EXTRAN of the Stormwater Management Model (SWMM). The EX- TRAN model offered a more detailed analysis of the valley floor drainage system, ak lowing the dynamic routing of full hydrographs through the storm drain system. The dif- 91029-60 Garden Ave.Drainage Study Add.#3(12.18-91) (001 Terence between the EXTRAN modeling and the previously used steady-state modeling, is that the EXTRAN model allows conditions to be tracked throughout the duration of a storm, whereas the steady-state analysis permits only a snapshot of conditions at one flow rate, and assumes that flow rate is maintained for an indefinite length of time. The EXTRAN model developed for the study represents over 115 pipes and 120 struc- tures (manholes or catch basins) for the principal drainage system on the valley floor. Information used to define the system was obtained from a combination of sources in- cluding field survey, existing plan sets, and the City of Renton data base. The EXTRAN model was used to evaluate and further define potential drainage alterna- tives available for the PACCAR development. A November 21, 1991 Addendum to the Drainage Study evaluated the need for pumping stormwater off the site, as well as for Barron's bypass alternatives. This potential solution was abandoned in light of new in- formation that allowed a more viable solution. This new solution is the subject of this third Addendum. This analysis addresses flooding and drainage patterns for "existing", "interim" and "fi- nal" conditions for the PACCAR site and related drainage on the valley floor. The "in- terim" conditions represent the PACCAR site development conditions during the con- struction period until the "final" drainage alternative can be implemented. The information presented in this Drainage Study Addendum summarizes the results of the EXTRAN modeling for the various conditions examined. EXISTING CONDITIONS Description The Existing Conditions case assumes that the PACCAR site and drainage system ex- ists as it did during the winter of 1990 (per SSOE plans showing "Existing Conditions"). The south half of the site is largely covered by impervious surface, while the north half of the site is predominately bare earth. Included are open ditches, cross connections with off-site drainage, and an 18-inch restrictor in a storm drain passing runoff across the site. The Garden Avenue North drainage system, including the lower ponds, is assumed to be maintained and fully operational. Please note that the assumption of these main- tained conditions is critical to the results of this analysis. Drainage/Flooding Conditions As noted in our earlier studies, the existing valley floor drainage is undersized with re- spect to the peak flows expected for extreme storm events. In the vicinity of the PACCAR site, flooding occurs from two sources: 1) storm drains conveying runoff from other parts of the drainage basin back up onto the PACCAR site, and 2) when these drains back up, runoff generated on the PACCAR site is contained on-site, and there- fore contributes to site flooding. 9102MO Garden Ave.Drainage Siudy Add.K3(12.18-91) tJ02 In the vicinity of the PACCAR site, flooding occurs principally at the northwest corner of the site, which is the lowest point on the property. For the 25-year design storm, it is estimated that up to 16 acre-feet of water would be found on the PACCAR property. This water comes from both on-site and off-site drainage. Some of this water comes out of the North 8th Street storm drain. Currently there is a 24-inch diameter pipe con- necting the North 8th Street storm drain and a drainage ditch in the northwest corner of the PACCAR site. When the North 8th Street drain is full, water flows onto the PACCAR site through the existing ditch "outlet'. During the 25-year storm event, as much as 45 acre-feet would be discharged from North 8th Street into the PACCAR drainage ditch. Another source of flooding on the PACCAR site under existing conditions is the storm drain crossing the site from subbasin 15 (located to the south of PACCAR) to the North 8th Street storm drain. Under low flows, runoff crosses the PACCAR site and enters the North 8th Street storm drain approximately midway between North Houser Way and Garden Avenue North. During high flows, stormwater in the North 8th Street drain backs up into the storm drain on the PACCAR site, causing flooding. Flooding from in- lets and manholes probably travels as shallow sheet flow across the site to the north- west corner. Flooding created on the PACCAR site under existing conditions promotes flooding on adjacent properties. As flood waters build up on the northwest corner of the property, water begins to spill over onto the low-lying adjacent areas along Garden Avenue North (west of the PACCAR site). Street flooding is predicted along North 8th Street (the northern boundary of the PACCAR site) during the 25-year design storm. Approximately 4.7 acre-feet of flooding is predicted, with most of the flooding occurring at the intersection of North 8th Street and North Houser Way. Under existing conditions, no flooding is predicted for Garden Avenue North, down- stream from the PACCAR site. These predictions may not be representative if the low- er pond systems are not maintained. INTERIM CONDITIONS Description The Interim Conditions case assumes that the PACCAR site exists as defined by the July 18, 1991 Grading Plans submitted to the City of Renton. Specific conditions in- clude: 1) the South Houser bypass is in place, carrying drainage from off-site areas (subbasin 15) around the PACCAR site before joining the Garden Avenue North drain- age system at the intersection of North 8th Street and Garden Avenue North; 2) the east-west interceptor is in place, carrying runoff from the southern half of the PACCAR site, north to the intersection of North 8th Street and Garden Avenue North; 3) runoff from the northern half of the PACCAR site is collected in two sedimentation ponds be- fore being discharged at the intersection of North 8th Street and Garden Avenue North; and 4) the existing ditch "outlet' at the northwest corner of the site is removed and the site is regraded according to the plans. 91029-60 Garden Ave Drainage Study Add.#3(12 18-91) (00 3 Drainage/Flooding Conditions The interim condition changes the existing drainage pattern primarily by eliminating some of the overflows from the North 8th Street storm drain. As a result, the extent of surface flooding on the PACCAR site is reduced. For the 25-year design storm, flood- ing on-site is approximately 8.8 acre-feet, approximately 36 percent less than that pre- dicted under existing conditions. Most of the flooding occurs at the northwest corner of the site. The cause of the flood- ing is a backwater or reverse flow condition created by high flows in the N. 8th Street storm drain. During the peak of the storm, the outlet from the site reverses flow direc- tion, diverting approximately 1 acre-foot of runoff onto the site over a 2.75 hour period. During this period, stormwater in the South Houser bypass, the east-west interceptor, and runoff from the northern half of the PACCAR site contributes to the flooding on-site (since it cannot be released into the North 8th Street storm drain). A reduction of flooding on the PACCAR site coincides with an increase in flooding off- site at another location. Under the interim conditions, flooding in the vicinity of North Houser Way and North 8th Street (upstream from the PACCAR site) was substantially increased over existing conditions. Street flooding at this location increased from 3.8 to 7.5 acre-feet. The increase in flooding may be caused primarily by eliminating cross connections between the PACCAR site and the North 8th Street storm drain. Under in- terim conditions as described above, the only hydraulic cross connection between the PACCAR site and North 8th Street occurs at the intersection of North 8th Street and Garden Avenue North. Under existing conditions, a cross connection exists for the storm drain carrying runoff from subbasin 15 (the storm drain replaced by the South Houser bypass). If the cross connection between the PACCAR site and North 8th Street was retained (approximately midway between North Houser Way and Garden Avenue North), the flooding at the intersection of North Houser Way and North 8th Street is predicted to be reduced by 85 percent (7.5 acre-feet reduced to 1.1 acre-feet). However, flooding on the PACCAR site would increase as additional runoff from North 8th Street finds another point of entry into the PACCAR drainage system. On-site flooding would increase by over 8 acre-feet, with most of the flooding occurring along the east-west interceptor. Flooding along the east-west interceptor probably would be due to the relatively low rim elevations of the manholes and catch basins. Reduced flooding on the PACCAR site appears to reduce the current amount of off-site flooding expected along the western boundary of the site (Garden Avenue North). Esti- mated flood storage in this area appears to be reduced from over 6 acre-feet under ex- isting conditions, to less than 1 acre-foot under the interim conditions. The reduced flooding in this area is attributed to lower flood levels on the PACCAR site resulting in less water spilling over to adjacent properties located to the west. The interim condition had no noticeable effect on the downstream drainage system (Garden Avenue North). The maximum predicted water levels in the manholes along Garden Avenue North were not substantially different from those predicted for the exist- ing condition (figure 1). 91029-60 Garden Ave.Orainage Study Add #3(12-18-91) 604 Conclusions/Recommendations The interim condition proposed for the PACCAR site apparently will have the greatest effect on the flooding occurring immediately upstream from the site, at the intersection of North 8th Street and North Houser Way. Removing or modifying some of the existing cross connections with the North 8th Street storm drain reduces the amount of flooding on-site, but increases the amount of street flooding immediately upstream. On-site detention could be required to mitigate any increases in off-site flooding. De- tention would have to be designed to divert some of the peak flows from North 8th Street onto the PACCAR site. The volume required would need to be established for the 10-year design storm. For comparison, the volume of on-site flooding for the 25- year design storm could be as high as 14 acre-feet. Another option would be to retain the existing cross-connections and allow the PACCAR site to flood during the interim period—much as it now does. Provisions would be re- quired to hold the water on-site and not allow it to flood adjacent properties any more than they currently are flooded. FINAL CONDITIONS Description The Final Condition case represents the recommended drainage alternative that will al- low the PACCAR site to be developed in accordance with the drainage requirements of the City of Renton. The recommended solution is the extension of a 72-inch storm drain along Garden Avenue North, parallel to the existing storm drain. The 72-inch storm drain would extend from North 8th Street, northward along Garden Avenue North before crossing beneath Lake Washington Boulevard and the Burlington Northern Railroad (BNR) tracks. On the north side of the BNR tracks, the storm drain would be diverted east to rejoin the Garden Avenue North drainage flowing through Gene Coulon Memorial Park. Existing storm drains crossing the proposed 72-inch pipe alignment were assumed to be joined with the new 72-inch diameter line. Cross con- nections are maintained between the 72-inch storm drain and ponds 1 and 2 in the vi- cinity of Gene Coulon Memorial Park. Storm drainage from the PACCAR site (including the South Houser bypass, the east- west interceptor and the on-site detention pond) would be collected at the northwest corner of the site, then discharged directly into the existing 48-inch storm drain on Gar- den Avenue North. Flows from North 8th Street would be diverted to the new 72-inch storm drain. A 48-inch cross connection would be provided between the existing Gar- den Avenue North storm drain and the proposed storm drain at the intersection of North 8th Street and Garden Avenue North. Approximately 700 feet of the proposed 72-inch pipe is already in place along the north- ern end of Garden Avenue North. 91029-60 Garden Ave.Drainage Study Add #3(12 18-91( G O 5 Drainage/Flooding Conditions The proposed Final Condition solution helps to reduce flooding along North 8th Street. At the intersection of North 8th Street and North Houser Way, the volume of street flooding would be reduced by 37 percent from existing conditions. As a result it ap- pears that the proposed solution compensates for any loss of flood storage on the PACCAR site as it relates to flows in the North 8th Street line. Flooding along the western boundary of the site (Garden Avenue North) is reduced un- der the final solution. Potential off-site flood volumes for the 25-year design storm are reduced from over 6 acre-feet (predicted under existing conditions) to less than 1 acre- foot (under the final solution). The reduction is attributed to the elimination of flooding on the PACCAR site. Under this condition, no flooding was predicted on the PACCAR site for the 25-year de- sign storm. The maximum water surface elevation at the point of discharge (at North 8th Street and Garden Avenue north) would be 23.7, which appears high enough to cause a tailwater effect, but not high enough to cause flooding on the site. Figure 2 shows the water surface elevation over time at the point of discharge. Water levels along the existing Garden Avenue North storm drain are predicted to be lowered by the addition of a parallel, 72-inch storm drain. Figure 3 compares existing water levels to those predicted for the final solution for the existing storm drain. In the 72-inch storm drain, water levels are predicted to range from elevations of approxi- mately 21 at pond 3, up to 23.64 in the first manhole at North 8th Street. The total flow through the lower ponds (in the vicinity of Gene Coulon Memorial Park) would be increased by approximately 25 percent by directing the 72-inch storm drain to pond 3 (north of the BNR tracks). Figure 4 compares the flow out of the lower ponds for the existing and final solution condition. Conclusions/Recommendations The proposed final solution would improve drainage on the valley floor and permit grav- ity drainage from the PACCAR site. A relatively minor increase in off-site flooding would occur along the western boundary of the site, but it appears that this could be mitigated in the final design (if necessary Direct discharge of the 72-inch storm drain into pond 3, immediately north of the BNR tracks, would improve conveyance through the lower ponds by as much as 25 percent. fi��ewterl�s rtiq rV!e�rel SvL� L) �tOf DGG0 Ct �61���:� • 91029-60 Garden Ave.Drainage Study Add.03(12-18-91) 606 Figure 1 MAXIMUM PREDICTED WATER LEVELS ALONG EXI mo GARDEN AVENUE STORK DRAM 37 36 35 i 34 u 33 1 32 31 o GROUND SURFACE 27 J i 26 j 25 24 23J 2 21 1001 1101 1102 1103 1104 1105 1106 1107 110e 1106 1000 1201 1202 1203 MODELED PIPE JJNC ION NN*0 M) + E)CISTMG CONDITION O MTER111 CONDITION Figure 2 WATER LEVELS AT N.8TH ST. AND GARDEN A. 28—YEAR DESIGN STOW 27 26 ! 25 , � 241 4 „ 23 21 f� 20 ^i rTr f*^'T*rTr- 0.00 2.00 4.00 6.00 6.00 10.00 12.00 14.00 16.00 16.00 20.00 22.00 24.00 TIME (HOURS) 0 EXISTING CONDITIONS + FMAL. SOLUTION /c'07 Figure 3 MAXIMUM PREDICTED WATER LEVELS ALONG DOSTm GARDEN AVENUE STORM DRAM 3a 37 -� 36 �(( 36 -i 34 I 33 32 31 -y .. 30 -1 z , 29 GROUND SURFACE 251 27 26 26 24 23 22 21 II , 20 1001 1101 1102 1103 1104 1105 1106 1107 1106 1106 1000 1201 1202 1203 MODEM PIPE JUNCTION N"ODES) + EASTMG CON=ON O MI/ 72—N GARDEN AVE Figure 4 FLOW AT GENE COULON MEMORIAL PARK (PREDICTED FOR 26—YEAR DESIGN STORM) 320 300 260 260 -j 240 -1 220 FINK SOLUTION 200 u 180 160 -� 140 10 EXWNG SON IGO 60 64 40 0 1. 0 2.00 4.00 6.00 &00 10.00 12.00 14.00 16.00 1&00 20.00 22 00 24.00 TIME (HOURS) �08 .a WATER SURFACE ELEVATIONS--- 100 YEAR STORM m w f 16 Inch dTcmeter pond outlet 28 m r 0 27 ro 0 26 �- v a ry rU a d 25 uu J 24 N 23 y� y 22 1 1 4 O 21 H 20 0.00 2.00 4.00 8.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.O0 11M E (HR) I� C7 M AND GARDEN + ONWE POND . o 0 i Corporate SSOE, Inc. 1001 Madison Avenue Toledo, Ohio 43624 419-255-3830 Fax 419-255-6101 Branch Offices 200 Mott Foundation Building Flint, Michigan 48502 313-238-5200 Fax 313-239-1180 1050 Wilshire Drive Suite 260 Troy, Michigan 48084 313-643-6222 Fax 313-643-6225 1001 Madison Avenue Toledo, Ohio 43624 419-255-3830 Fax 419-255-6101 624 Grassmere Park Drive Suite 28 Nashville, Tennessee 37211 615-833-8980 Fax 615-781-2844 3015 112th Avenue N.E. 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