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HomeMy WebLinkAboutSWP272884(11) © E N T R A N C O RECEIVED ' FEB - 1 = 70"Safte%van ft. 1 ' Houser Way North Renton, Washington REGIONAL DRAINAGE ANALYSIS ' Prepared for City of Renton Prepared by ENTRANCO ' 10900 NE 8th Street, Suite 300 Bellevue, Washington 98004 (206) 454-5600 January 1993 1 ' CONTENTS Page INTRODUCTION 1 MODELING 1 ' CURRENT CONDITIONS 6 EFFECT OF HOUSER WAY PROJECT ' ON STORMWATER SYSTEMS Drainage Requirements 8 ' IMPROVEMENT ALTERNATIVES ' Description of Alternatives 9 Cost of Alternatives 12 ' Cost Reduction Options 13 ' RECOMMENDATIONS FOR FURTHER STUDY 14 REFERENCES 14 ' APPENDIX - Planning Level Cost Estimate ' 91024;REPORTSi REGDRAIN(1!27193) ahw ' FIGURES Page 9 ' 1. Vicinity Map 2 2. Flow Comparison Points 4 3. Schematic of Pipe System Modeled with EXTRAN 5 ' 4. Conceptual Drawing of North Houser Swale 10 ' TABLES Page ' 1. Current Conditions of Houser Way North Drainage System 7 ' 2. Comparison of Houser Way North Alternatives: 100-Year Storm 11 ' 3. Comparison of Houser Way North Alternatives: 25-Year Storm 13 ' 91024 REPORTS REGOPAIN(1/27/93)l ahw )I ' HOUSER WAY REGIONAL DRAINAGE ANALYSIS INTRODUCTION ' This report describes the results of a hydrologic and hydraulic modeling effort to verify the existing conditions in the vicinity of the proposed Houser Way Relocation ' Improvements project and to analyze options for regional runoff conveyance in that area. The project area runs parallel to and just west of 1-405 in Renton, Washington (figure 1). Planned improvements will extend Houser Way through currently vacated ' right-of-way from North 8th Street south to Sunset Boulevard, resulting in loss of both an existing wetland and an open channel. Runoff from developed upland areas is con- veyed through the Houser Way area on the valley floor, by way of the open channel in ' the Houser Way right-of-way or through one of a number of pipes along the hillside, and is eventually discharged to Lake Washington. Entranco refined an existing EXTRAN storm sewer model of the region to provide increased detail within the project area, and used the refined model to analyze how the stormwater is conveyed through the existing system. Using the results of the model runs, three alternatives to mitigate ' the expected changes in the system due to the Houser Way project were evaluated and costs determined for the three alternatives. It is important to note that this analysis focuses on the Houser Way project as it impacts regional drainage patterns. Project impacts on site drainage, such as increased runoff and need for detention and treatment, are beyond the scope of this report and would ' require further analyses. ' MODELING The hydrology of this basin had been characterized in earlier studies (Entranco 1991). ' For the Houser Way analysis, two changes were made in the hydrological calculations to further refine this analysis. First, the drainage basins around Houser Way were fur- ther subdivided into several subbasins and the flows for the design storms were calcu- lated for these subbasins. Hydrographs for all subbasins were determined using the King County method, as delineated in the King County Surface Water Design Manual (1990), using 2-, 10-, 25-, and 100-year 24-hour rainfalls of 2.0, 2.9, 3.4, and 3.9 ' inches, respectively. The second refinement was to extend the Houser Way area stormwater conveyance systems in the model to capture flows from more individual ' subbasins on the hillside to the north and east of the site. The net result of these im- provements is to more accurately model the function of the systems in the immediate vicinity of Houser Way. ' 910241 REPORTS/REGDRAIN(1/27/93)/ahw 1 Shattuck I'Ave 8 C 0 z MI 0(tt Ave $ 10 a4lslan113 C: w .I . s z tOrFI3 Ave C o t Sm hats- Ave,eASn 0 4.7 Ave NZ > CD 1 r > ;K G) 'n X I FnS Z Ins Ave N nil Ave10 'A -4 m z Main Ave 0 m 0 1111111m. 0 Poll Ave —IT z 0 III ve n Park Z Ave N Park a ?w az •) QI Renton n Ave S Garden 0 n6re.1 ve S t"It, M allow Ave 114 Garden Ave > S CWI) ct( (n m Ave fk I hgh Ave A10,se, > (A f%I,S •tA in z In Ke Avil NE n X z z 10, Blvd In f, z i.j()njp..ey Ave > =i Vu. Ave;: e in Z PL sur)30r Monterey Ao,,I,!I(!y 'IF Z Capil 0 _j� in ve NE rude I ve NF Aberdeen Ave ;0 Gm. Fyn." Ave n Blaine Ave NE 1�1;,Ee- NE Carries Fma ne Ave ID Carries Ave N-) C,, 'a Day Ave z m 0 Da ton En Ave Ir-7-1 T 0 m NFen ale ,-- C:I A - 12Z 1/0 A NE It Ave HE 0 ev"'. 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The model of that particular event predicted water levels at North 8th Street and Garden Avenue North which matched observed levels, and predicted flooding in the places where ' flooding was observed during the storm. The current model formulation was improved by the inclusion of information from recent surveys conducted by Tudor Engineering and SSOE. ' Two assumptions were made concerning the condition of the lower section of the drainage system. First, the open channels which start south of Lake Washington ' Boulevard and travel through Gene Coulon Memorial Beach Park, hereafter referred to as the "pond system" (see figure 2), were assumed to be well maintained and free of ' overgrown bushes. This results in a lower resistance to flow than would occur if the ponds were not maintained. The effects of the assumption of maintained channels were tested using a King County synthetic 25-year storm event. Flows through the ' channels with unmaintained sides were predicted to be about 5 percent less than the maintained flows. The maximum flow through the Garden Avenue North lines was 15 percent less for the unmaintained model, and water levels at North 8th Street and Garden Avenue North were predicted to be 0.8 foot higher. Second, the capacity of the culverts connecting the ponds was assumed to be ex- panded as part of mitigation for the PACCAR site development. The added culverts include two 54-inch pipes from the farthest upstream channel (Pond 1) to the next (Pond 2), and one 84-inch pipe from Pond 2 north to Pond 3. These culverts have ' been installed in the system. These two assumptions are critical to the modeling re- sults, as they increase the conveyance through the pond system which, in turn, in- creases the conveyance through the entire stormwater system. ' The model also included a critical 9P 9 assumption regarding predicted flooding to the east P 9 ' of 1-405. At the uppermost point of the pipe which connects to the North 8th Street storm sewer line from the east, flooding is predicted to occur during high flows. In this version of the model, this point is called junction 1207 (see figures 2 and 3). Due to ' the formulation of the EXTRAN model, the water which leaves the system as "flooding" disappears from the model, and is artificially lost. Since the flooding was predicted to be significant (over 12 acre-feet for the 25-year storm), an alternate pathway for this ' water was modeled so that the water would not be lost. Based on topographic information and previous studies (City of Renton 1988), it was assumed that this water would appear as street flooding until it reached a swale along 1-405, eventually ' discharging into the channel along the Houser Way alignment approximately one-third mile south of North 8th Street. Field investigations since the modeling was completed indicate that this precise route probably would not be taken, but that overflow would still ' discharge to the Houser Way North right-of-way via several pipes which cross under 1-405. ' 91024/REPORTS/REGORAIN(1/27/93)1 ahw 3 ' C:4CADDODGN491024-220ENVOFIG8.DGN LAKE Gene Coulon WASHINGTON Memorial Beach 0 1/8 1�4 Park 1 / 0 MILE I m \ G � Pond 5 1 Pipe 401 NE 12th ST ♦♦ L E G E N D •' moons Existing Pipe System Pipe 154 ♦ •o 900 Pond System Qom' •� . pipe 430 Q�' � • '.� 900 ® Areas of Modeled Flooding <Z> + ��•�� Z • • 405 > Pipe 152 Pipes '. Q 102 & 603 y w W O ! � f . O w ' G m z Z ' > a N 8th ST ,......... ......... ■■ sell Y Q a- tPipe 201 n O L ' � Z ' N 5t ST a z \ w N 4tr ST \ tjj � JZ ❑ 405 HOUSER WAY REGIONAL DRAINAGE ANALYSIS ' Figure 2 E N T R A N C O FLOW COMPARISON POINTS 4 LAKE \'A5HIN67ON ILEGEND t o PIPE JUNCTION 5002 ' ® 5TORA6E JUNCTION 5001 2000 JUNGT ION NLNffR 4M 1 400 1 ' 1 PIPE/OPEN CHANNEL 3002 I NFLOW OR OUTFLOW 300 1 2002 1404 ' 1 00 1 200 1 1 40 1 1 4OZ 1403 1406 461 1 405 ' 1 1 50 liOb 1460 1� 1 430 1 450 1 45 1 601 F. 1 1 01 1952 1 G02 1 43 I 11 5 1 1 432 1 433 c 1 453 ' IIOZ 1403 1434 rn 1454 �— II 52 1103 1604 1 435 A 1 1 04 105 D ' un 1404 I936 ^ 1153 1105 IUn�vJl A 1607 1437 z 1439 1106 m 1606 1436 ' 11 54 1 1 07 Z 1609 1711 1741 1742 NOT TO SCALE 11 06 m 1 a 0 F 110,11 109 Z 1 64 1 1 7 1 0 1 740 II99 1699 0 0 0 0 N N N N 1300 161 2 1 201 1 ZOZ 1 203 1000 1 301 N.8,b\. St. 750 1253 1 35 1 1 302 m o v 1 Z5Z 1 35Z 1 303 1 6o I 1 353 1 700 1 Z5 1 1309 1602 1629 165G 1250 1305 1603 1857 0 o b Po 1M4 1 306 1 604 — — — — 1 656 1 730 GEDAK - - - - - -- - - 1645 ' RIVER 1 307 1 605 1 659 1 326 1 31 9 1 6" 1 647 1 160 1 327 1 320 1 607 1 64 1 ON ' 1 326 1 32 1 1 bob - - - - - 1 642 INFLOIitS Tz VALLEY FLOOR 015TKIE�ITFD T $ 1 643 EQUALLY TO &WT I a45 O O O 4 Z V IN VICINITY. 1U4 1 645 ' Ociolrer 6, 1 992 ' ROUSER WAY NORTH REGIONAL DRAINAGE ANALYSIS lNTRANCO Figure Schematic of Pipe System Modeled with Extran ' 5 ' Although this assumption is based on precedent and results in a more reasonable simulation than had the loss of stormwater volume been ignored, it must be refined ' before the final design of any system along Houser Way. Also, it is unlikely that all of the predicted flooding would occur at this point. The pipe systems further upstream were added to the EXTRAN model in an effort to check the functioning of the pipe sys- tem. The model predicted these pipes to be overloaded, resulting in flooding which de- creases the flow which actually reaches junction 1207. This may indicate that there is some attenuation of high flows within the pipe system in the east basins, and that the ' actual flows to junction 1207 are somewhat lower than the modeled flows. The hydrog- raph method used is unable to take into account such attenuation, but the modeled re- sults can be assumed to represent a worst case scenario. CURRENT CONDITIONS ' Predicted water levels and pipe flows at various storm recurrence intervals for the exist- ing conditions model runs are shown in table 1. It is important to note that these re- sults are not necessarily comparable to observed storm events, as planned upgrades to the system were included in the model. In addition to the assumed upgrades to the ' pond system, the conveyance through the system is upgraded by the completion of the 72-inch line down Garden Avenue North. This line greatly increases the flow capacity to the pond system, and the net result of all of the improvements is a lessening of the ' backwater conditions which have led to street flooding in the past. The predicted depth of the stormwater in the system (which limits the capacity of the ' upstream pipes) is shown for two important locations: the PACCAR outfall at North 8th Street and Garden Avenue North (junction 1000) and Pond 1 (junction 1001). ' Under the modeled current conditions, the North 8th Street line reaches capacity during rainfall events approximately the size of the 10-year, 24-hour storm. Instead of passing larger flows, this pipe serves to create a restriction on the pipes entering the system at the intersection of North 8th Street and Houser Way, and introduces a backwater situ- ation upstream. The pond system, on the other hand, does not appear to reach capac- ity even during the 100-year storm. However, the increasing levels in the ponds do ' begin to show backwater effects on the pipes coming in from Garden Avenue North and the North Basin for the larger storms, as shown by the "plateau" of peak flows in these tsystems. 91024,1 REPORTS!REGDRAIN(1/27193)!ahw 6 r r Table 1 rCurrent Conditions of Houser Way North Drainage System Storm Recurrence Interval (years) r2 10 25 100 r Water level (ft) at PACCAR Outfall at North 8th St./Garden Ave. N 2.71 3.14 3.33 3.36 rPond 1 3.16 3.96 4.31 4.49 Flow (cfs) rNorth 8th St. 76.9 82.7 82.9 82.9 Garden Ave. N (54-inch) 24.3 37.8 47.0 49.0 rGarden Ave. N (72-inch) 87.9 149.9 146.8 155.4 North Basin (pipe 401) 49.8 83.1 89.8 90.4 r North Houser (pipe 430) 13.0 17.9 20.9 25.6 Pond system 204.4 276.7 316.7 343.9 Houser Open Channel 31.4 124.1 128.8 128.9 r rThe EXTRAN model also shows the amount of water which flows out of the system as flooding. Upsizing of the culverts in the pond system does not seem to change mod- eled street flooding relative to the preimproved model runs. The flooding at the Houser Way underpass is estimated at 0.1, 0.7, 1.4, and 2.5 acre-feet for the four design storms. No flooding is predicted along Garden Avenue North, where most of the re- gional stormwater flows to the pond system. However, the backwater conditions caused by the North 8th Street pipe do lead to a large amount of flooding at North 8th Street and Houser Way (1.1, 8.1, 13.9, and 22.7 acre-feet). Areas of predicted flooding r are shown in figure 2. As noted before, one of the critical assumptions for this model- ing was that the overflow water from the area on the hillside is routed to the channel along Houser Way. It is primarily this flow which seems to result in the flooding in the rvicinity of the intersection. Again, it is not certain how much of this flow is actually at- tenuated in the systems on the hillside. Any attenuation would probably lead to lower predicted flooding on the valley floor. rIn comparison, the 25-year storm for the existing conditions without the P larger and cul- verts (but with the assumption of maintained ponds) predicted 1.2 acre-feet of flooding rat the underpass, 11.7 acre-feet at North 8th Street/Houser Way and 2.8 acre-feet on the hillside, although this run produced 10 percent less flow in the system than the im- proved condition run. The lower amount of flow is because this run used HSPF r91024/REPORTS I REGDRAIN(1127m)/ahw 7 ' modeling to determine the hydrographs, and this accounts for attenuation more than does the SBUH hydrographs. It is important to remember that flooding in this system is dictated both by high water levels downstream from the flooding areas and by high flow levels. Under the Houser ' Way underpass, the highest flow conveyed without flooding was predicted to be 82 cfs, which is approximately the peak flow of the 10-year storm. Likewise, the maximum predicted conveyance through the North 8th Street pipe is about 96 cfs before flooding, which is the flow predicted for the 10-year storm. However, with higher water levels at the outlet, these same flows would cause flooding, and less flow could be safely con- veyed. Because of the water level/flow interaction, it is difficult to establish flow levels ' above which flooding will occur for any one specific pipe section. ' EFFECT OF HOUSER WAY PROJECT ON STORMWATER SYSTEMS ' On a regional level, the effect of the Houser Way road improvement will be to alter an area along the railroad tracks south of North 8th Street, which currently serves as an open channel for storm flows. It is probable that, in order to construct the roadway, this channel will need to be replaced by an underground pipe. The drainage alternatives considered involve converting the channel to a pipe and providing some other im- provements to the system. Drainage Requirements ' The City of Renton drainage requirements are the same as those define d in the King ' County Surface Water Design Manual. For this study, Core Requirements 1-4 apply, as paraphrased and discussed below: ' 1. Runoff "must be discharged at the natural location". The intent of this requirement is to ensure that baseflows of streams are not changed by drainage changes, so the flow between pipes within a stormwater system is of lesser concern. Depending ' on interpretation, the natural location for discharge from this area could be consid- ered to be at a specific point within the stormwater system, at Pond 1 or at Lake Washington. Since the entire system from Houser Way to Pond 1 consists of piped ' storm drains, the natural discharge of this system reasonably can be assumed to be considered to be at Pond 1, which is the assumption used for this analysis. ' 2. Upstream and downstream areas which would be impacted by the changes to the drainage system must be analyzed. This study would presumably fulfill much of the analysis requirement. ' 91024/REPORTS i REGDRAIN(1/27.193)/ahw 8 ' 3. Restrict postproject peak flows to preproject levels. The regional analysis does not consider an increase in the amount of runoff generated in the project area, but this ' requirement applies in that the 100-year, 24-hour peak flow after improvement will be constrained to be no more than 0.5 cfs more than the pre-improved 100-year flow. Also, some regional biofiltration may be required as part of the Houser Way project to replace that occurring in the existing channel. 4. New pipe systems are to be designed to convey the 25-year peak flow. The pipes ' for this study were designed for such a rate, although, as mentioned earlier, those flows will have to be confirmed before final design. ' IMPROVEMENT ALTERNATIVES ' Description of Alternatives Three alternatives were considered to improve the functioning of the Houser Way ' drainage system. For this analysis, we assumed that: • Road placement will require filling the channel. ' • A 72-inch pipe will be necessary to convey the modeled flows. • Four 400-foot-long sections of 72-inch pipe will be set at the slope of the exist- ing ground surface. ' Alternative 1 is to replace the 42-inch pipe under North 8th Street with a 72-inch pipe. Alternative 2 is similar to this, but also involves removing the cross connection between ' the 54-inch and the 72-inch lines at North 8th Street and Garden Avenue North. Alternative 3 is to build a 10-foot wide swale and 48-inch overflow pipe along Houser ' Way North, north of North 8th Street, to divert some of the flows from the North 8th Street line and pick up drainage from the hillside. A conceptual drawing of this alter- native is shown in figure 4. The swale would have the same surface overflow rate as ' the existing channel for the 2-year flow, giving it comparable biofiltration capacity, as biofiltration is dominated by physical (settling) processes. ' The resulting predicted flows and water levels under each of these alternatives for the 100-year storm are shown in table 2. ' Replacement of the open channel with a 72-inch pipe is not expected to affect the drainage system compared to existing conditions. However, the existing flooding also continues, as well as the existing conveyance, so the replacement cannot be consid- ered to be a drainage improvement. 1 ' 91024/REPORTS i REGORAIN(1/27/93)/ahw 9 V / i Q D 17 — � O1 /Q �r a►e 1 Y O NGU5EK WAY N 5WALE ' 6YPA55/G��RFLG`J7 P I PE ' HOUSER WAY NORTH REGIONAL DRAINAGE ANALYSIS ® ENTRAHCO Figure Conceptual Drawing of North Houser Swale ' 10 ' Table 2 ' Comparison of Houser Way North Alternatives 100-Year Storm Alternatives' 72-Inch Existing Pipe 1 2 3 Only ' Height at junction (ft) PACCAR Outfall at North ' 8th St./Garden Ave. N 3.36 3.36 7.07 5.51 3.39 Pond 1 4.49 4.50 5.03 5.15 4.68 ' Flow (cfs) North 8th St. 82.9 82.8 204.6 211.0 82.8 ' Garden Ave. N (54-inch) 49.0 47.5 73.3 63.4 66.5 Garden Ave. N (72-inch) 155.4 155.2 161.6 190.5 148.8 ' North Basin (pipe 401) 90.4 90.5 86.4 86.3 89.7 North Houser (pipe 430) 25.6 27.1 28.2 28.6 47.4 Pond System 343.9 342.9 411.4 424.8 366.4 ' Houser Channel 128.9 128.6 129.4 129.4 129.0 1. Alternative 1 is the North 8th Street upgrade. Alternative 2 is the North 8th Street upgrade with ' removal of the cross-connection at North 8th Street and Garden Avenue North. Alternative 3 is the bypass Swale. Alternative 1, increasing the size of the North 8th Street pipe, leads to increased con- veyance and reduced flooding throughout the system, eliminating the flooding at North 8th Street and Houser Way North with only a minimal (0.08 acre-foot) amount of flood- ing on Garden Avenue North. One major drawback for this option is that it leads to greatly increased water levels at the intersection of North 8th Street and Garden Avenue North (junction 1000), where drainage from the PACCAR site enters the sys- tem. The higher water levels could lead to restricting the flow of water off of that site and result in further flooding. In fact, the predicted flooding on the PACCAR site in- creases from 0.55 acre-foot to 2.0 acre-feet due to the enlarged pipe system. ' Alternative 2 improves on Alternative 1 by removing the cross-connection between the Garden Avenue North lines at North 8th Street. The result is increased conveyance ' through the two Garden Avenue North systems (a total of 254 cfs vs. 235 cfs, 204 cfs for existing) and down the North 8th Street line while the water level at the PACCAR outfall is 1.5 feet lower than in Alternative 1. This alternative does lead to some ' 91024 r REPORTS 1 REGDRAW(1/27193)!ahw 11 ' flooding (0.5 acre-foot) from the North 8th Street line, and the water level at junction 1000 is still 2.15 feet higher than under the existing conditions. However, the predicted ' increase in flooding on the PACCAR site is only 0.15 acre-foot. Alternative 3, the bypass swale, like the first two alternatives, also reduces the pre- dicted flooding at North 8th Street and Houser Way North, although it does not elimi- nate that flooding. Additionally, it results in a slightly improved conveyance down Garden Avenue North, but does not affect the maximum flow down the North 8th Street ' line. Overall conveyance through the system is improved by about 23 cfs, and there is no effect on flooding on the PACCAR site. As mentioned above, the swale would re- produce the biofiltration from the existing channel, but there are some uncertainties re- garding this option, including the size of swale which can be built in the space available. This alternative could provide additional wetland functions which may be required as ' mitigation when the wetland is removed. These alternatives were run for the 25-year storm as well, and the results are shown in table 3. One of the more important results for the 25-year storm analysis is that Alternative 2 does not lead to increased flooding on the PACCAR site for this storm. ' It is possible that other drainage alternatives may achieve the same results, such as replacing one large pipe with two smaller ones, but these three alternatives were con- sidered the most viable. ' A concern about increasing the conveyance through the system is that increases in peak flows will lead to a requirement to provide detention. It is unknown whether the ' area exists to provide the detention for these alternatives, which would increase the peak flows through the pond system by as much as 82 cfs. The cost for such a facility in this area could be expected to be significant. ' Of the three alternatives evaluated to improve drainage in the vicinity of Houser Way, Alternative 2 appears to be superior to Alternative 1, as the removal of a cross connection seems to improve drainage in the Garden Avenue North lines. Alternative 3 does not reduce flooding as much as the other two, but it provides biofiltration and does not affect the drainage off of the PACCAR site. Cost of Alternatives ' A planning level cost analysis was conducted for the three alternatives. The costs for Alternatives 1 and 2 were the same ($1,996,000), as the cost of removing the cross connection was assumed to be negligible. Alternative 3 would cost $2,824,000. These are planning level costs, and are subject to change before final design. The calcula- tions for these costs are shown in Appendix A. ' 910241 REPORTS/REGDRAIN(1127193)/ahw 12 Table 3 Comparison of Houser Way North Alternatives ' 25-Year Storm Alternatives' 72-Inch Existing Pipe 1 2 3 Only Height at junction (ft) PACCAR Outfall at North ' 8th St./Garden Ave. N 3.33 3.34 6.95 5.08 3.20 Pond 1 4.31 4.30 4.89 4.98 4.44 ' Flow (cfs) North 8th St. 82.9 82.9 209.0 203.2 82.7 ' Garden Ave. N (54-inch) 47.0 48.9 73.5 62.0 42.0 Garden Ave. N (72-inch) 146.8 149.1 169.1 188.9 153.8 ' North Basin (pipe 401) 89.8 90.0 86.0 85.9 89.4 North Houser (pipe 430) 20.9 20.9 21.1 21.3 39.6 Pond System 316.7 321.3 391.9 402.2 335.9 ' Houser Channel 128.8 117.1 117.1 117.1 117.1 Flooding (acre-ft)2 ' Houser & North 8th St. 13.9 15.6 0.0 0.0 5.4 North Houser 1.4 1.5 1.5 1.5 2.3 PACCAR Site 0.3 0.3 0.5 0.3 0.3 ' 1. Alternative 1 is the North 8th Street upgrade. Alternative 2 is the North 8th Street upgrade with removal of the cross-connection at North 8th Street and Garden Avenue. Alternative 3 is the bypass swale. 2. See figure 2 for locations. Cost Reduction Options ' Entranco also investigated conceptually various options to reduce the high costs of these alternatives while providing equivalent drainage functions. ' It is important to note that most of the predicted peak flow in the planned 72-inch Houser Way North pipe is due to the overflow from the eastern basin. Without this overflow, the peak flow through the channel would be reduced by more than 60 per- cent-a 42-inch pipe may be sufficient to convey the flows. The eastern overflow could ' 91024/REPORTS l REGDRAiN(1127/93)/ahw 13 be reduced by installing a regional detention facility on the hillside. It is estimated that using a 42-inch pipe along Houser Way would save about $440,000. ' A second cost-saving option would be to use two smaller pipes instead of one large pipe to improve the system. Using a 42-inch and a 48-inch pipe along Houser Way North would save $115,000 in material and excavation costs, and installing a 48-inch pipe parallel to the existing system along North 8th Street would save $310,000 over replacement with a 72-inch pipe. ' It appears that the most economical measure would be to develop a regional detention facility, as it may be eligible for cost-sharing with other agencies. This would allow ' downsizing of conveyance facilities through the project while possibly reducing the flows and flooding in the valley area. RECOMMENDATIONS FOR FURTHER STUDY There are at least two drainage issues regarding the Houser Way Improvement project which will be important to clarify when proceeding with that project. The first is the pre- viously mentioned drainage/overflow off of the hillside, and the resulting flooding on the ' valley floor, the amount of which must be verified before final design. The second issue is that a portion of the project area currently drains to a bypass pipe which runs through the PACCAR site. Since the flow through the bypass is limited, peak runoff flow rates ' may be restricted and additional detention may be required in that part of the site. ' REFERENCES Entranco ' 1991 Garden Avenue Drainage Study. Prepared for PACCAR, Inc. October 1, 1991. ' City of Renton 1988 North Renton Basin. Interim Drainage Study to Address Development West of 1-405. May 1988. ' 91024/REPORTS/REGDRAW(1/27/93)/ahw 14 ' Appendix PLANNING LEVEL COST ESTIMATE SHEET NO. OF - ENTRANCO ENGINEERS, INC. JOB NO. PROJECT - - ' CALCULATIONS FOR ,1 ,�1„ �, / ,-,�c - 7- MADE BY DATE 6 Z-3 Z CHECKED BY DATE ' A L1c,= rt .vA7 1vc:S 04 Liz O Z `/ CEO L l= 0 != D 12 A i N,4 y c-' S I,✓A C c= S o L !_ v 1= O 7 - 7 /eft V,4VLT 1 V S t✓A Cc: /NC. � i S T i2c/G (/r2L--- ' n SHEET NO. / OF Z ENTRANCO ENGINEERS, INC. 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MADE BY / 1-;4 DATE 6 Z q L CHECKED BY DATE 7 ;t/ T /Z i - c ' l - 1oIc ic ( c-ss r/-/ 's l=vlZ. 2 /=/0 '� 7-, 000 �c 01ucrs10,,/ S T 12 L/C Tv!2(:— /=c'rZ P /:1 = "' � ?0, 000/L=f4, ' 1,v UT $ /a, 000 /Clil C C3 T YI-)C: 12 - q6 FvrZ �l8 /�/i-c.= � / o, vvv lc= q , 1 t 1 POND 4 V j/ 12.E `t.F2-2 12.E2-2 7.Da- 1, 7,D8-3' POND 3 12.02-3 1 12.D2- Q 12.E2-4 2.E2- 12.F2-1 .E2-3 /PON 12,02-I 12.E3-3 2-2 I 12.E3-2 12.f 3-1 9 ol(P T \ 12,D3-11 12,F3,2 12,03- D3-10 12.E3- 12.D3-5 12,D3- 12.03-6 1'l. 3-7 12.D3-4 r 12.03-3 / �9 ✓ 12.D4-7 2 1 F4-1 / 12,E4-S 2.C4-2 t i.:+4-9 I2.D4-12 v ►`1 �� 2.F4-2 12,04-14 tt 12.04-I1 .E4-6 ,4 12.04-15 12.E4-4 12.D4-10 O 4- j 12.E5-4 V 12,F5-8 12.05-4 12.E5-6 12.E5-3 12.C5-4 f05-5 17 C N R - 2 12.D5-8 ;i �� 12.C5-5 012,E5-2 n 12.E5-1 � � .FS 5-3 �I 7 ,2I11\VI- 12.E5-7 12 5-5. 1 1 12.C5-9 12.C5=7- �'--12 - 1 t2C5-� E5- _ ` 12 G5- 12.C5-3 12. 12.D5- 12.OS-1 S-3 t 2 C5-2 1 12.Da-1 1 t;J6-2 � 12.06-I 7 12.D6-10 12.D6- 12.F5-I' 12.DS-8 1:. 12.06-3 { n J_I 12.D6-7 �j 12.06-13 12.0a-6 1 = - 12.06-15 12.06-4 12.F6-5 12,F6-' 1 12.D6-16 1 6-1 12.07-1 12.F6-1 12.07-I' l 12,F6-4 12.07 12.F7-2 - 18 R - 22 12.07-. _7-5 12.C7-4 12,C7-6 12.07-1 N R -2 6 12.D7-2 1 •L 12.D7-1 l 12,F7-4 Rr 23 ' 1 t 12.F7-3 12,D8-1 R - 20 N R - 29 12,C8- 12 C8- 12.D8 12,D&-2 !, I 1 ,fa-I \ •, '2.C8-)' 1 .C8-2 12.C8-I 11 p8_7 _ 0 e1, ENTRANCO E t 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 cis (from 125 cis to 140 cis) with a Pond 1 headwater elevation of 20.5 feet (figure 13). The flow through the segment would increase from 220 cis to 260 cis 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 (cis) Headwater Improved Improved Elevation BNRR Lake Wash. (feet) Existing Culverts Blvd. Culverts 19.5 90 100 100 20.5 125 140 150 22.5 /Sb-1-& 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 cis, 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 A. 1 . 91031-60 Garden Ave.orawu&W Addendum(10.1691) 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.Or&w%aW Addendum(10-1"1) 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. g1031-60 Garden Ave.damage Addendwn(10-16A1) 3 Figure 13 Alternative 1: Flow from Pond 1 to Lake Washington with Three 724nch Culverts Installed Between Pond 2 and Pond 3(Burlington Northern Railroad Culverts) for Various Headwater Elevations 300 13 Overflow 13 E3 280 24.5 Ft 260 1= 240 1 e 220 , 2-5 Ft v 200 , o i 0. L 180 rn � 0 160 1 20.5 Ft 3 140 0 tL ; 120 too 19.5 Ft 80 18.5 Ft 60 12 13 14 15 16 17 Lake Washington leval Figure 14 Alternative 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 El Overflow 300 28 24.5 Ft 260 0 Ft 240 r 22.5 � 220 c 0 200 a oa 180 7 0 160 20.5 Ft � 140 120 100 19.5 Ft so 6'0 18.5 Ft • ' 12 13 14 15 16 17 . ,_.. Lake Washington level Figure 15 w- Alternative 3: Flow From North 8th Street to Pond 1 if Garden Avenue Pipe is Enlarged to 72 Inches for Various Headwater Elevations 200 19 0 Overflow 180 28 Ft 170 160 0 e 0 0 27 Ft 150 a a 140 a 0. 130 26 Ft 0 120 -j y 3 110 j ti 100 i 90 25 Ft 60 1 70 -i i 60 -i 50 1 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 Alternative 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 ON 7 O L 200 M 0 ISO 29 Ft too 28 Ft 50 27 Ft . . 0 24.5 25 25.5 26 26.5 27 27.5 28 Water,level at Gorden Avenue _. j' Figure 17 Alternative, 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 n. 200 -� a j L 180 a 0 160 r 140 J 120 -' 29 Ft 100 28 Ft 80 60 j 40 J 20 27 Ft 0 18_5 19 19.5 20 20.5 21 21.5 22 22.5 23 23.5 24 24.5 Pond 1 Water Leval 2I03160GrdewA.-Gra" OAddMdwncl0-%4Q 6 ENTRANCO GARDEN AVENUE DRAINAGE STUDY ' Renton, Washington Prepared for PACCAR, Inc. 1 ' 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 1 ASSUMPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 RESULTS AND DISCUSSION ' Hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Backwater Analysis 5 ' CONCLUSIONS AND RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 ' APPENDICES A - Figures B - Hydrologic Summary and Calculations ' 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 rTABLES 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-16-compute runoff rates using the Water Works software program. The basin was divided into 18 main subbasins, as delineated by the City of Rento 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 Oirection of the City of Renton�nd PACCAR, the King County Surface Water Management Sac water 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(10/1/91) 1 ' 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 Study(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. • 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. AND RESULTS A 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 Study(101/91) 3 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 (2) 47.0 78.0 98.0 118.0 Valley Floor (3) 20.0 30.1 35.7 41.3 Paccar Site 01 9.5 14.8 17.9 21.0 1 Subbasin 15 (5) 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. ' (2) 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. ' (5) 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 1 A storm distribution hydrograph. Actual storm distributions will vary. ' • Lag times and attenuation between the subbasins could also be more signifi- cant than were calculated. r ' 91031-60 Garden Ave.Drainage Study(10/1191) 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 W of storage at an elevation of 26 feet, and approximately 1 126,000 W 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(1011(91) 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 analysi8�js 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 rThe 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. 91031.60 Garden Ave.Drainage Study(10/1/91) 6 Q � x W c � � � Q u' � � r � � � � � � � � � � � � ' � � � T LL x (ll QC ^•inV 6JV - ` Q U .. 0�1 W '• w W '•Wl• Z ld.. 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SHEET FLOW --- SHALLOW CONCENTRATED FLOW NE 12th ST —�► PIPE FLOW oo� 18 sg [ 9 © Z 11 y *,z / Q ' ` a 1 I N 8th ST Ell z I Y 1 © NE Z - o Z � W 5 � D � cl a po GARDEN AVENUE DRAINAGE STUDY ' Figure 2 DRAINAGE BASINS ' ENTAANCO A.2 ' ACAD\91024-20\PACCAR.DWG 09-17-91 PTK 0 1/8 1/4 ♦♦♦ . MILE ■ i ■ NE 12th ST LEGEND N0 Basin Number 179 Basin Boundary Oman Backwater Analysis Flow Routes . 900 -4- Upper Basin Flow input >♦ 18-5 ®i Operational Curve Flow Segment / z 1 1-3 ♦ ¢ 18-4 1? ♦ w � ♦ m s 18-3 G 12 „-2 �� , � g 2 18-2 y i Z ■ I 1 1-1 ; 18-1 a N 8th ST ' no: 1 i a 11-4 : j 14-4 : 14-6 p14-5♦♦ 14-1 14-3 � z � ¢ 14-2 Ma ' w � 1 GARDEN AVENUE DRAINAGE STUDY Figure 3 BACKWATER ANALYSIS FLOW ROUTES ENTRANCO A.3 'o' 9/1 ' , I� Wd_ '� fit` 11,MA AFi' `wr4r �► � -' ,'�,,.�"'�i..�_..__I,ii__�.I.,s ss!�►.���Iai�w�i�►� ,r_s�IA��IOI ' - ACAD\91024-20\91024-20.DVG 09-20-91 PTK + + + + + + + + + + + + + + t + + + t 4 t t + + + + t t t + t + �� 0 0.25 0.5 + + + + + + + + + + + + D := MILE + + + + + + LEGEND + + + + + + + + + + + - ___ + ❑ + C❑MMERCIAL / INDUSTRIAL / + + + + + + + + + + + RESIDENTIAL + + + + + + + + + + + + + I + + _ _ _ / + + + + + + + + + + + + + + + + - _ - MULTI-FAMILY �� E 12 S + + + - - - + + + + + + FOREST ���j��/�� + + + + + + + TEI + + + + ////• Yi /// + + + + + + + + + + + 21 l ' / — — // + + + + + - - - - - - - - - - - - Pir + + + + + + + Fl GARDEN AVENUE DRAINAGE STUDY Figure 5 LAND USE CATEGORIES ' ENTRANCO — — -- -- A.5 ' Figure 6 Operational Curve Flow Segment 1: Flow from Pond 1 to Lake Washington for Various Headwater and Tailwater Elevations Overflow 260 250 —� ' 240 .. 230 U I HW = 24.5 Ft 220 +-- -- - �-- — -- --� 210 r HW= 23.5 Ft I 0 200 t 190 ' 3 o � LL 180 I;W = 22.5 Ft 170 ' 160 HW = 21.5 Ft 150 I ---T —, 12 13 14 15 16 17 ' Level of Lake Washington ' Figure 7 Operational Curve Flow Segment 2: Flow from North 8th Street to Pond 1 for Various Headwater and Tailwater Elevations 100 90 Hw=28.0 Ft ' 80 v Hw=27.0 pt 70 Hw,26.0Ft a 60 J HW= 25.0 Ft 0 50 L r � 40 ' 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 ' A.5 ' Figure 8 Operational Curve Flow Segment 3: Flow from Garden Avenue Junction to the Cedar River for Various Headwater and Tailwater Elevations 5.6 5.4 � 5.2 ' S � OL 4.8 �Ix0 4.6 ' u 4.4 m 4.2 yty` a' 4 _�__- 2j 0 �r F 3.8 HW= 260Ftt 0 3.6 0 3.4 � 2S0 Ft LL' 3.2 3 ' 2.8 2.6 2.4 2.2 18.0 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 ' 110 H W= 33.0 Ft 100 90 HW=31.OFt �Ui SD Q� a ' 0 70 HW= 29.0Ft L U 0 60 L .L ' 50 3 0 ti 40 ' 30 HW = 27.OFt 20 10 o 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28 ' Water level at Garden do N Sth ' 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 HW,28. S Ft ' 12 t t 3 Ft 10 m a 9 r rn a 7 y 6 o 5 4 3 ' 2 1 0 ' 26.0 26.5 27.0 27.5 28.0 Water level at PACCAR and N eth 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 10 N�'31.0 Pt � 1 a a 9 a 8 33 0 Ft 0 r 7 3 6 0 LL 5 4 3 02 2 90 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 s HIv`3rsPt 5 HW,3r0F ' o t a. 4 / a Y rn $Fj 3 L t 3 ti� 0 2 \\1?0 .0X r ' o 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 d i i i i!d d d!!d i i i i i i!!i i i i!i------- BASIN RESULT SUMMARY ' IAS11 -----VOLIME---- -RATE- ----TIME----- HYDROGRAPH AREA ID ---CF-- AC-FT --CFS- -RIR- HONIS METHODOLOGY ACRES sass[sssasssassasssesss■ssassasssasasss■cssssssssassassessasssasassassass ' 11-11 418595 9.61 14.81 491 8.17 SOUR METHOD 55.11 11-2 261271 5.97 8.99 491 8.17 SOUR METHOD 55.10 11-25 519821 11.11 18.18 491 8.17 SOUR METHOD 55.16 ' 11111 602581 13.85 21.63 491 8.17 SIUH METHOD 55.11 11.1A 43148 1.99 2.53 481 8.11 SEIH METHOD 7.11 11.19 65562 1.51 3.82 481 8.11 SIUH METHOD 7.11 ' 11.1C 78198 1.79 4.53 481 8.81 SIIN METHOD 7.11 11.1D 91671 2.18 5.24 489 8.11 SOUR METHOD 7.11 11.2A 11690 1.78 4.51 481 8.11 SIUH METHOD 12.61 ' 11.21 111111 2.11 6.19 111 8.11 1111 METHOD 12.61 11.2C 141616 3.23 8.15 481 8.11 STUN METHOD 12.61 11.2D 163238 3.15 9.32 481 8.11 SIIN METHOD 12.61 11.3A 12938 0.30 9.78 481 8.11 STUN METHOD 2.11 11.38 19662 1.45 1.18 481 8.11 SIUH METHOD 2.11 11.3C 23422 9.54 1.41 481 8.11 SIUH METHOD 2.11 11.3D 27194 1.62 1.62 481 8.10 SIUH METHOD 2.11 ' 11.0 32639 1.75 2.13 489 8.11 SOON METHOD 5.31 11.48 49614 1.14 3.16 481 8.11 SOUR METHOD 5.31 11.4C 59195 1.36 3.65 481 8.11 SOUR METHOD 5.31 11.41 61112 1.58 4.21 481 8.11 SIUH METHOD 5.31 ' 12-11 511476 13.11 39.12 481 8.11 SIIN METHOD 61.11 12-2 375273 8.62 19.89 481 8.11 SOUR METHOD 51.11 12-25 679681 15.61 35.65 481 8.11 SIIN METHOD 61.11 ' 12111 789214 18.12 41.27 111 8.11 SOUR METHOD 61.10 13-11 813143 18.44 21.55 521 8.67 SOUR METHOD 91.11 13-2 518571 11.91 13.12 521 8.67 SOUR METHOD 91.11 13-25 963516 22.12 24.74 511 8.51 SIIN METHOD 91.11 13111 1124853 25.82 28.97 511 8.51 SOUR METHOD 91.11 14.1A 42511 1.98 2.67 481 8.11 SIIN METHOD 6.61 14.11 61911 1.47 3.91 411 8./1 SIIN METHOD 6.61 ' 14.1C 75871 1.74 4.67 481 8.11 SOUR METHOD 6.61 14.1D 87815 2.12 5.37 480 8.09 SOUR METHOD 6.61 14.2A 81511 1.85 4.79 481 8.1/ SOUR METHOD 12.51 14.21 121192 2.18 7.11 481 8.11 SOUR METHOD 12.51 14.2C 143693 3.30 8.59 480 8.11 SBIH METHOD 12.51 14.2D 166315 3.82 9.66 489 1.11 SOUR METHOD 12.51 14.3A 111166 2.34 6.II III 8.11 SBIH METHOD 15.81 14.31 153161 3.51 9.12 481 8.10 SOUR METHOD 15.81 14.3C 181628 4.17 11.64 481 8.11 SOUR METHOD 15.81 14.31 211223 4.81 12.21 110 8.11 SBUH METHOD 15.81 ' 14.4A 69351 1.59 2.38 491 8.17 SIIN METHOD 15.11 14.48 114161 2.62 4.18 491 8.17 SOUR METHOD 15.11 14.4C 139861 3.21 5.11 491 8.17 SOUR METHOD 15.10 ' 14.4D 165912 3.81 6.17 491 8.17 SIUH METHOD 15.19 14.5A 94419 2.17 4.23 491 8.17 SIUH METHOD 21.01 14.58 154146 3.54 7.14 481 8.11 SOUR METHOD 21.19 14.5C 118112 4.32 8.8/ 481 8.11 SIIN METHOD 21.11 14.5D 222953 5.12 11.48 481 8.11 STUN METHOD 11.11 14.6A 22762 1.52 9.63 520 8.67 SOON METHOD 6.11 14.68 49172 1.92 1.22 491 8.17 SOON METHOD 6.11 ' B.1 ' 9/23/91 ENTRANCO ENGINEERS . INC . PAGE 2 saaaaas-------aces--aaaa--a-aaasass=-aaaa--aa:za:aaaa:w-aa:sa-a ' BASIN RESULT SUMMARY BASIN -----VOLUME---- -RATE- ----JIME----- HYDROGRAPH AREA IO ---CF-- AC-FT --CFS- -AIM: HORNS METHODOLOGY ACRES azazzzzzaza■.sass..zaaazzazzazasazzazazzaz zzzaaa az........agent......azaz 14.6C 58152 1.15 1.57 491 8.17 SBUH METHOD 6.11 14.61) 61435 1.39 1.93 498 8.17 SBUH METHOD 6.19 15-19 423421 9.72 22.61 481 8.18 SBUH METHOD 48.18 15-2 275192 6.32 14.75 111 8.11 SBUH METHOD 49.11 15-25 597994 11.64 27.04 488 8.81 SINN METHOD 48.88 15191 591384 13.58 31.51 488 8.81 SBUH METHOD 48.88 ' 15A11 331178 7.58 17.57 488 8.81 SBUH METHOD 38.81 15A2 213961 4.91 11.48 481 8.19 SBUH METHOD 38.11 15A25 395935 9.19 21.84 488 8.11 SBUH METHOD 38.18 ' 15A99 161111 11,11 15,11 111 1,11 SBUH METHOD 31,19 15911 261827 5.99 14.43 481 8.01 SBUN METHOD 31.18 15B2 168711 3.87 9.34 481 8.11 SBUH METHOD 31.11 15825 312925 7.18 17.39 481 8.81 SBNH METHOD 31.18 15899 355451 8.31 28.19 481 8.11 SBUH METHOD 31.11 16-11 629738 14.46 25.17 491 8.17 SBUH METHOD 87.11 16-2 386256 8.87 15.81 491 8.17 SBUN METHOD 87.81 ' 16-25 771879 17.18 31,13 491 8.17 SBUH METHOD 87.11 16181 915311 21.11 37.25 491 8.17 SHUN METHOD 87.81 17-11 974115 22.36 37.21 498 8.17 SBBH METHOD 123.11 ' 17-1 611122 14,15 22,98 191 1,17 SBUN METHOD 123,11 17-25 1181581 27.12 45.39 491 8.17 SBON METHOD 123.81 17111 1391792 31.95 53.71 491 8.17 SBUN METHOD 123.11 18.1A 29887 1.69 1.87 481 8.11 SBUH METHOD 4.98 ' 18.11 45527 1.15 2.82 111 8.11 SBUH METHOD 4.98 18.1t 54283 1.25 3.35 481 8.11 SOUR METHOD 4.91 18.11) 63168 1.45 3.87 481 8.18 SBNN METHOD 4.91 ' 18.2A 38592 1.71 1.91 111 8.10 SBUN METHOD 5.81 18.2E 46461 1.97 2.88 k81 8.91 SBUH METHOD 5.11 18.2C 55397 1.27 3.42 481 8.88 SOUR METHOD 5.81 11,211 14362 1,48 1,95 111 1,11 SBUH METHOD 1,11 18.3A 52961 1.76 2.16 111 8.18 SBUN METHOD 5.41 18.31 51211 1.15 5.11 481 8.11 SBUN METHOD 5,49 18.3C 59851 1.37 3.69 481 9.11 SBON METHOD 5.41 ' 18.38 69535 1.60 4.27 111 8,11 SBUN METHOD 5.49 18.4A 42717 1.98 2.67 481 8.11 SBNH METHOD 7.11 18.41 65151 1.49 4.13 481 8.11 SBUH METHOD 7./1 18.4C 77561 1.78 4.78 481 8.11 SBUH METHOD 7,11 18.41) 91111 2.81 5.54 481 8.11 SBUN METHOD 7.11 18.5A 25642 1.59 1.61 481 8.61 SBUH METHOD 4.21 11,51 19151 1.91 2,12 411 8,11 SBUH METHOD 1,21 ' 18.5C 46557 1.87 2.87 481 8.11 SBON METHOD 4.21 18.51) 54889 1.24 3.32 488 8.99 SBUN METHOD 4.21 19-11 385855 8.84 17.14 486 8.81 SBUH METHOD 51,11 ' 19-2 249324 5.52 11.49 491 8.17 SBUN METHOD 59.88 19-25 461312 11.75 21.99 481 8.11 SBNN METHOD 51.18 19111 552981 12.69 24.92 481 8.11 SBUN METHOD 51.11 ' 1-11 113111 7,19 11,11 191 1,11 SOON METHOD 68.11 2-111 476414 18.94 19.57 491 8.17 SBUH METHOD 68.88 2-2 182922 4.28 9.75 481 8.11 SBON METHOD 61.81 2-25 392882 9.92 15.81 491 8.17 SBUN METHOD 61.19 ' B.2 ' 9/23/91 ENTRANco ENGINEERS . INC . PAGE 3 s<::-----ease ss s--as sa te-axssa---ass----=--------te ax ss _=------ BASIN RESULT SUMMARY ' IASIM -----VOLUME---- -RATE- -- i1ME----- HYDROGRAPH AREA ID ---CF-- AC-FT --CFS- -KIN- HOURS METHODOLOGY ACRES csazxzxczszazzxccsszesaaczzasszxzxzcxxzsizeazc:xezzzcxaaacccexzzcsxzzxczz 3-19 129569 16.75 29.97 491 8.17 SRUN METHOD 116.11 3-111 1078273 24.75 44.17 491 8.17 SHUN METHOD 116.11 1-2 111117 11,12 17,22 191 8,17 SIUN METHOD 111,11 3-25 911154 20.69 36.41 491 8.17 SION METHOD 116.11 4-11 1370327 31.46 36.89 511 8.51 SHUN METHOD 184.11. 4-111 1981893 45.51 54.56 491 8.17 SION METHOD 184.11 ' 4-2 845984 19.42 22.12 519 8.58 SION METHOD 184.11 4-25 1673351 38.41 45.45 491 8.17 SBUH METHOD 184.11 6-11 1099296 25.24 41.15 491 8.17 SION METHOD 139.11 ' 6-111 1116911 11,21 58,42 191 8,17 SION METHOD 111,11 6-2 684118 15.11 24.33 491 8.17 SION METHOD 139.1/ 6-25 1336655 31.69 49.23 491 8.17 SION METHOD 139.11 7-11 716193 18,16 11,11 191 8,17 SIDH METHOD 112.11 ' 7-1/1 1151361 25.97 38.77 491 8.17 SION METHOD 192.11 7-2 488646 11.22 16.14 491 8.17 SION METHOD 192.11 7-25 957624 21.98 32.59 491 8.17 SION METHOD 112.11 8-11 292255 6.71 13,78 489 8.60 SION METHOD 37.11 8-111 417804 9.59 19.91 481 8.11 SKIN METHOD 37.19 8-2 183467 4.21 8151 481 8,11 SION METHOD 37.11 ' 8-25 111519 1,11 16,11 411 8,11 1111 METHOD 17,11 9-11 198751 4.56 7.92 491 8.17 SION METHOD 38.11 9-111 309512 7.10 13.97 491 8.17 SION METHOD 38.11 9-2 119961 2.52 3.93 491 8.17 SION METHOD 38.11 ' 9-25 252846 5.81 19.42 491 8.17 SION METHOD 58.01 ' B.3 1 , ' GARDEN AVENUE DRAINGE PEAK FLOWS 27-SEP VAL-L-E`( FCDof. � S,)bbasIh IS ' STORM EVENT 2 10 25 100 BASIN 1 11 . 1 2 . 53 3 . 82 4 . 53 5 . 24 11 . 2 4 . 51 6 . 79 8 . 05 9 . 32 11 . 3 0 . 78 1 . 18 1 . 4 1 . 62 ' 11 . 4 2 . 03 3 . 01 3 . 63 4 . 2 SUBTOTAL 9 . 85 14 . 85 17 . 61 20 . 58 ' 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 . 26 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 3 . 35 3 . 87 18 . 2 1 . 91 2 . 88 3 . 42 3 . 95 ' 18 . 3 2 . 06 3 . 11 3 . 69 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 ' B.4 SHEET NO. 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'4 -._. ioo 0.0Eb Z-J D.zi - i _._;. 6'7'3 -� - 3vo o.036 0.40 51,"► 4-00 0.079 1; 7 925- 31640 a,O . . 14z .fir __. _ 3 -f to 300 o.ob-t a.go qo,� ��v - o.o�� s �,� -- goo- -v v;v - -- 2E? -tea 8 Rio,S ..19 300 0,7-Ga o,a o z� zoo 0,29+- 20, 1 - boo 0,00s. 0,011 13 3aQ o 00 i o 15 -9�.9. 5U0 4 0(73 27 :- 3�b OPT— �4- 1- Z�S�- iT i i --77 Q SHEET NO. Z OF 2 1 �+ ENTRANCO ENGINEERS, INC. JOB NO. 1 PROJECT 19W�-' Ave- CALCULATIONS FOR lw;G or- GorJG l�r�'rlotil MADE BY ?IV- DATE 09-11-c�1 CHECKED BY DATE 'TO'f.Al. I C �.o�uv 60IQGG+.}SZA'f60 Flo. ! 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