Section 2

Paint Creek Interceptor Post-Rehabilitation Analysis

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YCUA 2007 Flow Metering

Technical Memorandum

Paint Creek Interceptor Post-Rehabilitation Analysis

February 18, 2008

Introduction The Ypsilanti Community Utilities Authority (YCUA) 1999 Sanitary Sewer Master Plan identified the Paint Creek Interceptor, which consists of parallel sewers, as being undersized during projected future peak flow conditions. The model prepared as part of the 2006 Update to the Sanitary Collection System Master Plan (2006 Master Plan Update) also indicated that projected future peak flows will exceed the capacity of the Paint Creek Interceptor. Based on Sanitary Sewer Evaluation Survey (SSES) findings, it appears that the flow meter data, upon which the hydraulic model was based, was recorded during a period of high inflows into the Paint Creek Interceptor due to a broken pipe along the interceptor that was experiencing a high amount of river inflows. The broken pipe has since been repaired. Following the SSES and 2006 Master Plan Update recommendations, the area was re-metered to project the peak flows through the Paint Creek Interceptor after the inflow and infiltration (I/I) removal. Pre- and Post-Rehabilitation Hydrologic Model Calibration Two hydrologic models were created using the i3DLab Antecedent Moisture Model. The first model was calibrated to the pre-rehabilitation flows monitored in 1999 and 2004. A second model was calibrated to the 2007 flow meter data. The second model utilized the same base flow set point, but the pre-rehabilitation wet weather inflow and infiltration were reduced by approximately 50% in order to obtain a very good match to the 2007 flow meter data. A comparison of the I/I flows predicted by the pre- and post-rehabilitation models to the observed I/I flows is presented in Figure 1. Figure 1 shows that the revised hydrologic model calibrated to the post-rehabilitation flows matches the 2007 data very well, and the pre-rehabilitation model significantly overestimates the inflow component of wet weather I/I, as expected due to the rehabilitation. This finding is consistent with the location of the pipe break at the Paint Creek sewer crossing (Figure 2) prior to rehabilitation. The pipe break was located above the low flow channel, so the base flows were not significantly affected. However, during times of high wet weather flows, the pipe break was subject to high amounts of river inflow—reflected by the reduction in the post-rehabilitation model and the overestimation of the peak inflow component by the model calibrated to the pre-rehabilitation conditions.

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Paint Creek Projected Future Design Peak Flow The Paint Creek Interceptor future design peak flow projected in this study was 8.96 cubic feet per second (cfs). The current projection is 47.8% lower than the peak flow projected in the 2006 Master Plan Update (17.17 cfs). The 2006 Master Plan Update projection was much higher because it was based on flow data collected before rehabilitation of the pipe break at the Paint Creek crossing, which was a significant source of wet weather river inflow. The lower peak flows measured after rehabilitation indicated that the pipe repair was very effective at removing the wet weather river inflows. The 10-year frequency peak wet weather flow, a major component of the future design peak flow, was greatly reduced by calibrating the hydrologic model to the flows metered after the pipe break rehabilitation. Each component of the projected future design peak flow is depicted in Table 1, and further discussed below. The Paint Creek Interceptor future design peak flow was projected by summing the existing base flow, the projected flow increase from future population growth, and the projected 10-year frequency peak wet weather flow (Table 1). This methodology was consistent with the methodology used to calculate future peak flows in the 2006 Master Plan Update. The average dry weather flow (2.29 cfs) and dry weather daily peak flow (2.75 cfs) was computed from 2007 flow metering data. The dry day peak factor (1.20) is the ratio of dry weather peak flow to average dry weather flow metered from June 11 through July 11, 2007. No significant rainfall events occurred during this period of time, so the dry weather period isolated the base sewage flow from flows influenced by rainfall dependent I/I. The projected flow increase from population growth (0.09 cfs) was based on projections used in the 1999 Sanitary Sewer Master Plan. Projections from the 1999 Master Plan indicated a population growth of 500 people in the Paint Creek Interceptor tributary area by the Year 2020. An additional average dry weather flow (0.08 cfs) was projected by assuming the additional 500 people will contribute sewage at a rate of 100 gallons per capita per day. The additional average dry weather flow was increased by the dry day peak factor (1.20) to produce the total projected additional dry weather flow due to growth of 0.09 cfs. The projected 10-year frequency wet weather flow (7.32 cfs) was determined from a statistical analysis. Using local temperature and rainfall data inputs, the post-rehabilitation hydrologic model was used to perform an 18-year (1988-2005) simulation. The Log Pearson Type III probability distribution was fitted to the annual peak flows predicted by the model (Figure 3).

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Hydraulic Model Results The Environmental Protection Agency Storm Water Management Model (EPA SWMM) hydraulic model results indicate that the Paint Creek Interceptor system has fewer over-capacity problems than was determined for the pre-rehabilitation condition in the 2006 Master Plan Update. Previously, an approximately 10,000 foot section was predicted to be up to 127% over-capacity when the pre-rehabilitation projected future peak flow was routed through the Paint Creek Interceptor. When the post-rehabilitation projected peak flow (8.96 cfs) was routed in the hydraulic model, the results of the simulation indicated that approximately 6,000 feet of the Paint Creek Interceptor was up to 46% above capacity (Figure 4) as a result of significant inflow reductions from the rehabilitation. The section of the Paint Creek Interceptor system (West Side Sewer and East Side Relief Sewer) that does not have adequate capacity to handle the future design peak flow is depicted in a plan view from the EPA SWMM hydraulic model (Figure 5). Pipes shown in red are at or above capacity under the future design peak flow. Profile plots of the East and West Side Sewers (Figure 6), bounded by the nodes labeled in Figure 5, depict the water surface elevations predicted by the EPA SWMM hydraulic model under the future design peak flow. The hydraulic model predicts that the original, 12”/15”diameter West Side Sewer will be surcharged by about one foot or less under the future design peak flow. Between roughly 700 to 7000 feet from the origin of the profile plot scale, the 18” diameter East Side Relief Sewer lies at an invert elevation approximately 4 feet below the invert of the West Side Sewer. The parallel East and West Side Sewers are interconnected, so the deeper relief sewer will be surcharged by a maximum of about 4 feet (approximately 10 feet below the ground surface) even if the original Interceptor is flowing at capacity within the approximately 6,300 foot section. It is not clear whether this level of surcharging would have an adverse effect on any part of the local collection system. The slight surcharging of the original, West Side Sewer under the future design peak flow indicates that the effects on local sewers may be minor. If further investigation indicates that the shallower West Side Sewer will not significantly affect the local collection system, then the deeper East Side Sewer will not be expected to affect the local collection system either. August 19-20, 2007 Storm Analysis The August 19-20, 2007 storm was analyzed to determine whether or not the future design peak flow was observed during the largest rain event of the 2007 flow metering period. The strategy is to evaluate the performance of the Paint Creek Interceptor during an event that produced at least the future design peak flow during the temporary flow metering study. If no local backups or sanitary sewer overflows (SSOs) occurred during such an event, the interceptor would be considered satisfactory, and no additional improvements to bring the system into capacity would be recommended. However, the Paint Creek Interceptor peak flow (4.54 cfs) during the August 20, 2007 storm was less than the future design peak flow (8.96 cfs). A review of YCUA rain gauge data indicated that there was less rainfall accumulation in the YCUA service area than would be expected to produce a 10-year frequency flow.

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The cumulative daily rainfall amounts measured by the three YCUA rain gauges were relatively uniform (Table 2). The gauges recorded an average of 1.15 and 1.17 inches of total rain on August 19 and 20, respectively. The rain gauge located at the Martz Road Pump Station recorded 0.6 inches of rainfall during the August 20 event, but the data indicated that this rain gauge malfunctioned on this day. Upon inspection, the gauge was restricted by debris, so the August 20 data from the rain gauge located at the Martz Road Pump Station was disregarded.

Table 2. Cumulative Daily Rainfall ~ August 19-20 2007

Date Martz Road

Pump Station (in.)

Merritt Road Booster

Station (in.)

Pittsfield Township Hall

(in.)

August 19, 2007 1.08 1.12 1.24

August 20, 2007 No Data 1.07 1.27

Post-Rehabilitation Hydrologic Model Validation The August 19-20 rain event presented an opportunity to validate the Paint Creek Interceptor post-rehabilitation hydrologic model, which was calibrated to March-June 2007 data. The I/I flow predicted by the post-rehabilitation model produced a very good estimate of the observed flow (Figure 7). In fact, the peak I/I flow predicted by the post-rehabilitation model (3.344 cfs) matched the observed peak I/I flow (3.340 cfs) almost exactly. The level of accuracy of the post-rehabilitation model to predict the August 20 peak flow was excellent, especially since the August data was not used to calibrate the model. This validated the use of the model as a diagnostic tool to evaluate long term system performance. As expected, the August 20, 2007 peak I/I flow predicted by the pre-rehabilitation model (4.68 cfs) was significantly higher than the observed flow and post-rehabilitation model prediction (3.34 cfs) (Figure 7). This was expected because the pre-rehabilitation model was calibrated with flow data gathered before rehabilitation of the pipe break at the Paint Creek sewer crossing, which was the source of a large amount of river inflow. The over-prediction of flows by the pre-rehabilitation model was consistent with results from earlier 2007 storms, which were discussed in the “Pre- and Post-Rehabilitation Hydrologic Model Calibration” section of this memorandum, and demonstrates the effectiveness of the pipe break rehabilitation.

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Recommendations YCUA should perform manhole rehabilitation, continue flow monitoring, and update the hydrologic model to determine if the manhole repairs are sufficient to bring the Paint Creek Interceptor into capacity. The 2006 SSES found 310 defective manholes (256 with vented or perforated covers; 54 with structural defects), and estimated that these manholes may be the source of 2.09 cfs of total peak inflow into the Paint Creek Interceptor system. Removal of approximately 1.8 cfs is needed to bring the Paint Creek Interceptor within capacity, but this high rate of removal is probably optimistic. The 2006 SSES reported that the manhole rehabilitation would cost approximately $89,200. Post-manhole-rehabilitation flow metering will be needed so that the hydrologic model can be updated to project the new 10-year frequency flow. The subsequently revised future design peak flow projection would be routed through the hydraulic model to determine whether the projected 10-year frequency flow was reduced greatly enough to be within system capacity. Even though the August 20, 2007 storm did not produce the projected future design peak flow (8.96 cfs), evaluating the impact of an actual event with a peak flow equal to or greater than the 10-year frequency storm on the Paint Creek system remains an excellent strategy. If no local backups or SSOs occur during such an event, the interceptor could be considered satisfactory, and no additional improvements to bring the system into capacity would be needed. Based on this analysis, we recommend the following: 1) Perform the manhole rehabilitation recommended in the 2006 SSES. 2) Perform post-rehabilitation metering and update the model to determine the post-

rehabilitation 10-year flows to see if adequate flow was removed. 3) Continue to meter the Paint Creek Interceptor in 2008. YCUA owns flow meters to be able

to do this work. Monitoring a 10-year flow and assessing actual system performance would be a strong method for determining the adequacy of the interceptor capacity rather than relying on model projections alone.

4) Investigate the impacts the simulated Paint Creek Interceptor surcharging has on the local

system, and determine if the slight surcharging would adversely affect local service.

(All Flows in CFS)

Existing Base Flow Computations

Item Description

Revised

Calculations Based

on New Analysis

Formula for Revised

Calculations

A Average Dry Weather Flow 2.3 metered

BDry Weather Daily Peak

Flow (Base Flow)2.8

metered

C Dry Day Peak Factor 1.20 B / A

Future Base Flow Computations

Item Description

Revised

Calculations Based

on New Analysis

Formula for Revised

Calculations

DEstimated Population

Growth500 1999 Master Plan

EAdditional Average Dry

Weather Flow at 100 gpcd0.08 1.547*100 * D / 1,000,000

FAdditional Dry Weather

Daily Peak Flow0.09 E * C

Design Peak Flow Computations

Item Description

Revised

Calculations Based

on New Analysis

Formula for Revised

Calculations

G Existing Base Flow 1.55 B - 1.2

H Future Base Flow 0.09 F

IProjected Design Wet

Weather Flow7.32 From Statistics

J Future Design Peak Flow 8.96 G + H + I

K Net Peaking Factor 3.79 J / (A + E)

Notes: Existing base flow (item G) for the revised calculations reduced by 1.2 CFS to

account for base infiltration flow included in the projected wet weather designflow (item I)

Table 1. Paint Creek Interceptor Design Flow Summary

Figure 1. Modeled and Observed I&I Flow Comparison

2007 Paint Creek Interceptor Flow Metering Period

0

1

2

3

4

5

6

7

8

3/26/2007 3/27/2007 3/28/2007 3/29/2007

I&I

Flo

w (

cfs

)

pre-rehab model

post-rehab model

observed

0

1

2

3

4

5

6

7

8

4/10/2007 4/11/2007 4/12/2007 4/13/2007

I&I

Flo

w (

cfs

)

pre-rehab model

post-rehab model

observed

0

1

2

3

4

5

6

7

8

4/30/2007 5/1/2007

I&I

Flo

w (

cfs

)

pre-rehab model

post-rehab model

observed

Figure 3. Paint Creek Interceptor Model Statistics

Projected 10-Year Frequency Peak Flow

0.01

0.10

1.00

0 1 2 3 4 5 6 7 8 9 10

Peak Flow (cfs)

An

nu

al

Pro

bab

ilit

y

Log Pearson Type III Distribution

Modeled

10-Year Peak Flow =

Annual Probability of 0.10

10

-Ye

ar P

ea

k F

low

= 7

.32

cfs

Ford

Pitt s

field

To

wn

sh

ip

City of Ypsilanti

Yp

sila

nti T

ow

nsh

ip

Pain

t Creek In

tercepto

r

88

%3

3%

23

%

34

%

60%

15

%1

4%

77%

156%

143%

25%

91

%

82%

68%

42%

11

%

38

%

113% 59%

37%

58%

97%

53

%

43

%

99%

94%

126%

110%

100%

162%

67%

107%

47

%

16%

64%

146%

119%

115%

149%

51

%

96%

81%

117%

95%

11

8%

65%

80%

13

%

111

%

22

%

70%

66%

30

%

21%

29

%

19

%1

7%

14

%

60

%

15%

66

%

14

%

14%

64%

59

%

115%

21%

38%

14

%

14

%

16%

34

%

14

%2

5%

14

%

64%8

2%

64%

118%

97%

Factory Street PS

Mart

z P

S F

orc

e M

ain

Figure 4Paint Creek InterceptorProjected Peak Flow Conditions

0 0.5 Miles

I

December 2007

Legend

Pipe Capacity Utilized

<30%

31% to 60%

61% to 90%

>90%

Ypsilanti Community Utilities Authority

Factory Street Pump Station Sewer District

Martz Road Pump Station Sewer District

Water Bodies

Figure 7. Modeled and Observed I&I Flow Comparison

August 20 2007 Storm

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

8/19/2007 8/19/2007 8/20/2007 8/20/2007 8/21/2007 8/21/2007 8/22/2007

I&I F

low

(cfs

)

0.0

0.5

1.0

1.5

2.0

2.5

Rain

fall In

ten

sit

y (

inch

es/h

ou

r)Observed

Post Rehab Model

Pre Rehab Model

Rain Gauge #2