HomeMy WebLinkAboutmem_thunderhillsflowanalysis_Memo 2_20150224
CITY OF RENTON
Technical Memorandum No. 2
Flow Analysis
Thunder Hills Sewer Interceptor
February 24, 2015
Prepared By:
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
TECHNICAL MEMORANDUM NO. 2
February 24, 2015
Contents
1.0 BACKGROUND ................................................................................................................. 1
2.0 MODEL DEVELOPMENT AND FLOW ANALYSIS ............................................................... 1
3.0 ALTERNATIVES HYDRAULIC ANALYSIS ............................................................................ 3
FIGURES
FIGURE 1: THUNDER HILLS INTERCEPTOR SEWER TRIBUTARY AREA
FIGURE 2: EXISTING THUNDER HILLS INTERCEPTOR SEWER
FIGURE 3: INFILTRATION AND INFLOW ANALYSIS – PEAK FLOWS IN THUNDER HILLS
INTERCEPTOR
FIGURE 4: 2012 ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PLAN
FIGURE 5: 2012 ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PROFILE (PART A)
FIGURE 6: 2012 ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PROFILE (PART B)
FIGURE 7: ULTIMATE ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PLAN
FIGURE 8: ULTIMATE ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PROFILE (PART A)
FIGURE 9: ULTIMATE ANALYSIS – EXISTING THUNDER HILLS INTERCEPTOR PROFILE (PART B)
FIGURE 10: 2012 ANALYSIS – EXISTING TALBOT HILLS SEWER
FIGURE 11: 2012 ANALYSIS – EXISTING TALBOT HILLS SEWER PROFILE
FIGURE 12: 2012 ANALYSIS – EXISTING TALBOT HILLS SEWER WITH THUNDER HILLS
DIVERSION PLAN
FIGURE 13: 2012 ANALYSIS – EXISTING TALBOT HILLS SEWER WITH THUNDER HILLS
DIVERSION PROFILE
FIGURE 14: ULTIMATE ANALYSIS – EXISTING TALBOT HILLS SEWER PLAN
FIGURE 15: ULTIMATE ANALYSIS – EXISTING TALBOT HILLS SEWER PROFILE
FIGURE 16: ULTIMATE ANALYSIS – EXISTING TALBOT HILLS SEWER WITH THUNDER HILLS
DIVERSION PLAN
FIGURE 17: ULTIMATE ANALYSIS – EXISTING TALBOT HILLS SEWER WITH THUNDER HILLS
DIVERSION PROFILE
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FIGURE 18: ULTIMATE ANALYSIS – TALBOT HILLS SEWER IMPROVEMENTS WITH THUNDER HILLS
DIVERSION PLAN
FIGURE 19: ULTIMATE ANALYSIS – TALBOT HILLS SEWER IMPROVEMENTS WITH THUNDER HILLS
DIVERSION PROFILE
FIGURE 20: TALBOT HILL SEWER RECOMMENDED IMPROVEMENTS
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TECHNICAL MEMORANDUM NO. 2
February 24, 2015
1.0 BACKGROUND
The Thunder Hills Sanitary Sewer Interceptor was originally installed in 1965 and serves a
portion of the City’s Rolling Hills neighborhood, southeast of Renton City Hall. The
portion of the Thunder Hills Interceptor from Grant Avenue South to Benson Road South
runs within or adjacent to Thunder Hills Creek. Over the years, erosion created by
Thunder Hills Creek has compromised the maintenance access road in various locations
and has required stream bank stabilization. One example of this occurred in November
of 2009; a storm drain inlet became plugged in the lower reach of the stream, leading
to extensive damage to the access road, where a portion of the existing sewer is
located. To alleviate future erosion issues at this location, an overflow channel,
reinforced with large boulders and rip rap, was constructed later that year to protect
the access road, sewer, and storm drain.
Due to the risk created by the ongoing erosion issues in Thunder Hills Creek, an
alternatives analysis is being conducted to determine the best method for serving this
area in the future. A portion of this analysis is to determine the projected peak flow
rates from the tributary area, and assess the impact of those flows on the existing sewer
as well as any proposed alternatives.
2.0 MODEL DEVELOPMENT AND FLOW ANALYSIS
The Thunder Hills Sewer Interceptor collects sewage discharge from predominately
single and multi-family residential areas southeast of I-405. It conveys these flows
through approximately 3,100 feet of 10-inch and 12-inch concrete, AC, and DI pipe
from the intersection of South 18th Street and Grant Avenue South to I-405. See Figure
1and Figure 2. On the southeast side of I-405, the sewer increases to an 18-inch
diameter concrete pipe, and crosses underneath I-405 to Benson Road South. From
there, the interceptor flows to the west, joining with drainage from the Talbot Hills
service area and discharging to the King County interceptor sewer located north of the
South Grady Way/East Valley Road intersection. Beginning at Grant Avenue South, the
pipe alignment roughly parallels Thunder Hills Creek on the east side, and crosses under
the stream approximately 1,700 feet northwest of the street. The sewer then parallels
the stream on the west. A portion of the sewer is located under an existing gravel
access road, while other portions are located in the stream bank.
The City of Renton’s Mike Urban sewer hydraulic model was updated to include sewer
rim and invert elevations obtained during the topographic survey for this project.
Although much of the horizontal pipe alignment adjacent to Thunder Hills Creek
between Grant Avenue South and I-405 was determined with the use of locating
equipment, attempts to pothole or otherwise determine pipe inverts and locations were
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unsuccessful. Consequently, the pipe depths between manholes were estimated and
interpolated. Where the terrain was uniform (under access roads and similar features),
pipe depths were generally assumed to be constant, and consistent with the grade.
Sewer inspection videos collected as part of the project did not show any abrupt grade
changes in the sewer between manholes. This method of estimating sewer depth was
duplicated as much as possible in the model to minimize backwater effects.
In order to determine projected flow rates, Ultimate population projections were
determined. For Mini-Basin 3, projections were developed from the PSRC Land Use
Baseline projections for the year 2040. As directed by the City, an additional 25% was
added to the 2040 population estimates as a factor of safety. The PSRC projections for
Mini-Basin 45 (which is tributary to the Thunder Hills interceptor) for the year 2040 showed
large decreases in single and multi-family residential population, which is inconsistent
with the City’s Comprehensive Plan land use designations and current development
activity for the basin. Because of this, Stantec developed Ultimate populations for the
portions of Mini-Basin 45 tributary to the Thunder Hills Interceptor by applying the growth
percentages calculated for Mini-Basin 3 based on PSRC estimates. The population
projections for the Talbot Hill Sewer tributary mini-basins are listed in Table 1. The mini-
basins that are tributary to the Thunder Hills interceptor are shown on Figure 1.
Table 1: Population Projections for Thunder Hills Sewer Interceptor Tributary Mini-Basins
2012 Population (PSRC)
Ultimate Population (2040+25%)
2012 Population (PSRC)
Ultimate Population (2040+25%)
Mini-Basin 3 (Tributary to the Talbot Hills Area) Mini-Basin 45 (Tributary to Thunder Hills Interceptor)
Single Family Residential 1,012 1,660 20 33
Multi-Family Residential 1,307 1,836 1157 1,626
Employment 95 248 N/A N/A
There were several factors affecting the sewer model hydraulic analysis for this project.
Calibration of the entire City of Renton Sewer Model based on King County’s 2010 flow
monitoring data has not yet been completed. Therefore, the I/I parameters for this
analysis were based on the previous model calibration (which used 2001/2002 flow
monitoring data). The previous calibration effort included flow monitors at each of the
mini-basins, which provided more detail, whereas King County only monitored at the
Model Basin level in 2010. For the Ultimate model analysis, I/I parameters have been
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increased by 28% to simulate increases in I/I due to pipe deterioration over the planning
period.
3.0 ALTERNATIVES HYDRAULIC ANALYSIS
To be consistent with previous modelling efforts, the hydraulic analysis was conducted
using the November 5, 1998 storm, which approximately represents the 20-year flow
event for this region of the Renton Sewer Service Area. For current (2012) population
projections and I/I parameters, results of the hydraulic analysis show a peak flow rate of
approximately 680 gpm entering the Thunder Hills Interceptor in MH#5320051(MH2325)
on Grant Avenue South, northeast of South 18th Street. There are approximately seven
tributary sewer connections to the interceptor between MH#5320051(MH2325) and
MH#5320035(MH2309) at I-405. The peak design flow rate at MH#5320035(MH2309) is
approximately 865 gpm.
Results of the Ultimate hydraulic analysis show a peak flow rate of approximately 870
gpm entering the Thunder Hills Interceptor in MH#5320051(MH2325). The peak
projected Ultimate flow rate at MH#5320035(MH2309) is approximately 1,125 gpm.
Approximately 700 gpm of this flow rate is the result of I/I. The estimated tributary area
to the Thunder Hills Interceptor Sewer, as shown in Figure, is 264 acres, and the
projected I/I contribution is approximately 4,200 gpad. The attached Figure 3 shows
the sanitary and total peak flows discharging from MH#5320035(MH2309).
Based on the Population projections listed in Table1, the peak projected per capita flow
rate is 215 gal/PE/day. For both the 2012 and Ultimate analysis, the results show no
surcharging for most of the pipe alignment between MH#5320051(MH2325) and
MH#5320035(MH2309), with Q/Qfull <0.50. Only a short section of 10-inch pipe directly
upstream of MH#5320049(MH2323), approximately between Station 20+40 and 20+65,
projects any surcharging. For the 2012 analysis, this section shows moderate
surcharging (Q/Qfull between 1.0 & 1.2) of approximately 0.1 feet above the pipe
crown. The Ultimate analysis shows more significant surcharging (Q/Qfull>1.2) of
approximately 0.5 feet above the pipe crown. However, if the pipe is steeper than
estimated, this result may be inaccurate. Additionally, a highly conservative roughness
value for this pipe was used in the model, resulting in an overestimation of the level of
surcharging. Even still, the level of projected surcharging here does not appear to pose
a significant risk of impacts to the upstream system or side sewers.
The results show no surcharging for the existing 18-inch sewer which runs under I-405
between MH#5320035(MH2309) and MH#5305033(MH2307), with Q/Qfull <0.50.
Since the actual slopes of the pipes are not known, it is difficult to determine the
accurate capacity of the pipes. However, assuming that the modeled profile is
relatively accurate, the capacity of most of the pipes in the interceptor exceeds 1,300
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gpm, with the exception of the pipe upstream of MH#5320049(MH2323), which has a
capacity of 1160 gpm. Based on the plan and profile capacity analysis results shown in
the attached Figures 4 through 9, it is unlikely that the interceptor will need to be
upsized to convey projected Ultimate peak flows.
Since the stream bank has a history of erosion, and past events have damaged the
pipe, it may be necessary to line the interceptor to prolong its lifespan. Since the liners
are typically made of plastic resin, lining the interceptor should improve the smoothness
of the interior of the pipe over the existing materials. However, the improved flow
characteristics from the smoothness could be offset by the decreased internal pipe
diameter, depending on the required liner thickness used to reinforce the pipe. An
analysis was conducted to determine the effects of the liner, assuming that a 6mm liner
was added to the 10-inch and 12-inch pipe. This would decrease the diameter in each
pipe by approximately 0.5 inches. Results show a net improvement in flow
characteristics in the pipe, with a minimum flow in the 10-inch pipe upstream of
MH#5320049(MH2323) increasing to 1,290 gpm, and capacity in the rest of the Thunder
Hills Interceptor exceeding 1,500 gpm.
One alternative under consideration is to divert a portion of the flows which are
currently tributary to the Thunder Hills Sewer Interceptor into the Talbot Hill Sewer. This
would be accomplished by installing approximately 980 lineal feet of new 8-inch sewer
main on South 18th Street between MH#5320052(MH2326), which is located at the
intersection of Grant Avenue South, and MH#5320168(MH2680), which is located
southeast of Eagle Ridge Drive South. This system discharges northwesterly through
approximately 2,400 feet of existing 8-inch diameter sewer (from South 18th Street, near
Eagle Ridge Drive South), ending on Smithers Avenue South between South 15th Street
and South 14th Street.
The existing (2012) peak flow rate in this portion of the Talbot Hills sewer system is 460
gpm, and the estimated 2012 peak flows for the diversion are 660 gpm, which would
increase the overall peak flow rates into the Talbot Hill sewer service area to 1,120 gpm,
creating capacity problems in the downstream sewer. With the diversion, modeling
results show major surcharging in the existing 8-inch sewer main in South 18th Street from
MH#5320168(MH2680) to MH#5320167(MH2679), moderate surcharging on Benson
Road South between MH#5320160(MH2672) and MH#5320159(MH2671) and between
MH#5320158 (MH2670) and MH#5320157(MH0083), and major surcharging between
Benson Road and Smithers Avenue South from MH#5319085(MH2482) to
MH#5319084(MH2481). The existing system appears to be very near capacity for a 20-
year flow event, limited in particular by the 8-inch diameter pipe immediately upstream
of the 10-inch diameter pipe recently installed by WSDOT.
The projected Ultimate peak flow rate in this portion of the Talbot Hills sewer system is
600 gpm, and the projected Ultimate peak flows for the diversion are 850 gpm, which
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would which would increase the overall peak flow rates into the Talbot Hill sewer service
area to 1,450 gpm, and create or exacerbate capacity problems in the existing
downstream sewer, which already would have moderate to major surcharging due to
capacity limitations in a few locations during Ultimate peak flows. In addition an
increase to the capacity problems shown in the 2012 analysis, modeling results show
moderate surcharging on Benson Road South from MH#5320164(MH2676) though
MH#5320160(MH2672), and moderate to major surcharging between Benson Road and
Smithers Avenue South from MH#5319085(MH2482) to the recently installed manhole on
the east side of the Talbot Pond, directly west of MH#5319084(MH2481) and between
MH#5319083(MH2480) and MH#5319082(MH2479). The remainder of the existing 8-inch
sewer along this flow path should be able to accommodate the projected flow rates,
since the pipe grades are much steeper than those with projected capacity problems.
See the attached Figures 10 through 17 for plan and profile views of the capacity
analysis results for the existing downstream sewer through Talbot Hill for existing and
Ultimate conditions. Table 2 contains are summary of the projected flows into the
Thunder Hills Interceptor and the Talbot Hills sewer under each of the scenarios
described above.
Table 2: Peak Flow Rate Projections for Thunder Hills Sewer Interceptor and Talbot Hills Sewer
2012 Population (PSRC)
Ultimate Population (2040+25%)
2012 Population (PSRC)
Ultimate Population (2040+25%)
Existing Sewer With Upstream Flows Diverted to Talbot Hills Sewer
Thunder Hills Interceptor 865 gpm 1,125 gpm 210 gpm 285 gpm
Talbot Hills Sewer 460 gpm 600 gpm 1,120 gpm 1,450 gpm
Possible upgrades to the existing pipes along the Talbot Hill sewer diversion route were
identified and modeled. Approximately 225 LF of existing sewer from
MH#5320168(MH2680) to MH#5320167(MH2679) was upsized to 12-inch diameter pipe,
approximately 730 LF of existing sewer from MH#5320164(MH2676) though
MH#5320159(MH2671) was upsized to 10-inch diameter pipe, and approximately 285 LF
of existing sewer from MH#5319085(MH2482) to the new manhole west of
MH#5319084(MH2481), and from MH#5319083(MH2480) to MH#5319082(MH2479) was
upsized to 12-inch diameter pipe. Results of the analysis show that these upgrades
would have the capacity to convey the peak flows diverted from the Thunder Hills
Sewer Interceptor. See the attached Figures 18 and 19 for plan and profile views of the
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capacity analysis results for the upsized piping through Talbot Hill. Figure 20 shows the
recommended improvements.
To determine the priority and phasing of the proposed upgrades along the Talbot Hill
sewer diversion route, the results of the analysis were compared to the Ultimate model
analysis results to determine which improvements need to occur before the diversion
sewer is installed, and which can be installed at a later time. Based on this comparison,
we recommend the improvements between MH#5320168(MH2680) and
MH#5320167(MH2679), between MH#5320160(MH 2672) and MH#5320159(MH2671),
and between MH#5319085(MH2482) and the new manhole west of
MH#5319084(MH2481) be made prior to the installation of the diversion sewer.
Although the section between MH#5319083(MH2480) and MH #5319082(MH2479) does
not show capacity problems under existing conditions, we recommend that it be
upsized in conjunction with the proposed Talbot Hill Sewer Relocation. The
improvements between MH#5320164(MH2676) and MH#5320160(MH2672), upsizing
approximately 500 LF of existing sewer to 10-inch pipe, may be delayed. However, that
section of sewer should be monitored, and upsized before it reaches capacity.
Temporary diversions during dry weather could be feasible for construction; however,
large wet weather events could cause issues for diversion, even in the immediate
future.
The modeling analysis results described above for the diversion were also considered in
the in the Talbot Hill Sewer Relocation – SR167 IC/DC Project Alternatives Analysis
Report, dated October 14, 2014. In general, for the alternatives in that study, piping
sizes across I-405 would need to be upsized to accommodate the increased flow rates
from the diversion.
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