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SANITARY SEWER SYSTEM EVALUATION TEMPORARY FLOW MONITORING REPORT

TABLE OF CONTENTS SECTION

I. Introduction 1 II. Flow Data Analysis 5

a. Dry Weather Results 5 b. Wet Weather Results 7

III. Conclusions and Recommendations 10 LIST OF FIGURES

1 Location Map 1 2 Mainline Gravity System Inventory by Pipe Size 2 3 Flow Meter Schematic Diagram 4 4 Wet Weather Wastewater Flow (Meter Site 1) 7 5 Wet Weather Wastewater Flow (North Little Rock Site 3) 8 6 Project Area Ranking 11

LIST OF TABLES

1 Meter Locations 3 2 Dry Weather Flow Summary 6 3 Wet Weather Flow Summary 9

Note: This report presents an executive summary of the “Sanitary Sewer Infiltration/Inflow Analysis 2009 City-Wide Temporary Flow Monitoring Final Report” dated July 2009.

SANITARY SEWER SYSTEM EVALUATION

TEMPORARY FLOW MONITORING REPORT

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I. Introduction This report presents the temporary flow monitoring of the Sherwood wastewater collection system during the period March 17 through May 21, 2008. Sherwood is a city in Pulaski County, Arkansas (see Figure 1), and a suburb of Little Rock. According to the 2005 Census Bureau estimates, the population of the city was 23,149. Since then, the city has annexed the neighboring community of Gravel Ridge following voting in early 2008, and Sherwood's population as a result has increased to approximately 26,381 residents (including 3,232 Gravel Ridge residents), ranking it as the 16th largest city in Arkansas. The Sherwood collection system consists of approximately 346,913 linear feet (66 miles) of mainline gravity sewer. Figure 2 presents a summary of the system length by pipe size. The collection system is unique in that portions of the sewer within the City of Sherwood limits are maintained by North Little Rock. The service areas were established by a court order dated October 12, 1990. As a result of the court order, North Little Rock is a satellite contributor to the City of Sherwood while portions of sewer within the City of Sherwood are a satellite contributor to North Little Rock. The purpose of this project was to establish current dry and wet weather flows at key locations within the City, prioritize future sewer rehabilitation efforts and comply with the ADEQ consent order.

Figure 1 Location Map



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Figure 2 Mainline Gravity System Inventory by Pipe Size



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Table 1 presents the locations of each temporary flow meter. Figure 3 presents a wastewater schematic diagram that graphically represents the association between each flow meter placed in the collection system. Note that meter sites 3, 4 and 6 recorded flow originating from North Little Rock that pass through the Sherwood collection system. This flow monitoring study consists of eleven (11) meter sites. The flow meter sites determine the boundary and resulting linear feet of sewer within each monitored area.

Table 1 Flow Meter Locations

Meter Site Manhole ID Address Pipe Size (in.)

001 21300 6100 Hwy 167 12

002 21880 Wildwood Rd @ Heritage Ln 24

003* 24470 Henson Tennis Center 21

004* 24570 32 Greenview Cir 12

005 30990 1411 Silver Creek 12

006* 31830 721 Koehler Ave 8

007 10350 5620 Hwy 167 10

008 18982 1520 Stafford Rd 18

010 17750 22 Alicia Dr 12

011 None 10709 Brockington Rd 18

012 15390 19 Algonkin Place 12

*Note: Sites 3, 4 and 6 are on North Little Rock pipelines.



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Figure 3

Flow Meter Schematic Diagram Sherwood Flows

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4 3 2 Five Mile Creek

1

7 5

11 12 North Plant

South Plant

8

10

9

Sherwood Contribution

North Little Rock Contribution

NLR

*Note: Sites 3, 4 and 6 are on North Little Rock pipelines.



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II. Flow Data Analysis Wastewater flow data was obtained from the eleven (11) temporary flow meters in order to establish flow hydraulics at key locations within the collection system. The temporary flow meters used were Sigma Model 910 area/velocity flow meters and Sigma tipping bucket rainfall gauges manufactured by HACH. For analysis, rainfall totals from five (5) temporary rainfall gauges were evaluated. Storm events were investigated to establish the Rainfall Dependent Infiltration/Inflow (RDII). Normal annual rainfall is 50.86 inches. Sherwood recorded an average of 18.77 inches of rainfall during the monitoring period March 17 through May 21, 2009. The normal historical rainfall during this period would be expected to be approximately 11 inches. The average rainfall was approximately 170% of normal. Each meter was field calibrated prior to installation. Calibration of each meter is a simple procedure consisting of independent verification of the depth of flow and velocity. The flow sensors were secured to a steel mounting band that fits securely in the pipeline. The data logger for each site was installed in the top of each metering manhole and the meter was activated at user defined sampling intervals, typically 15 minutes. Routine maintenance and service was undertaken weekly to confirm normal operation.

a. Dry Weather Results

Total daily wastewater volume during dry weather was determined from analysis of the dry weather period. These days were representative of dry weather flow conditions with low groundwater potential. Dry weather data for each metering site is summarized in Table 2. Silt levels were negligible for all meter sites indicating that scouring velocity is adequate to prevent debris settlement and flow hydraulics are not reduced due to debris. The ratio of peak recorded liquid depth to the pipeline diameter (d/D) provides a method to gauge the capacity of the pipeline being utilized during dry weather. When the d/D ratio exceeds 50%, then the pipeline is over half full. Sites 2, 6, 7, 8 and 12 were observed to exceed a d/D of 50% during peak dry weather conditions. Overall, the collection system has adequate dry weather capacity to transport wastewater without any evidence of surcharge.



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Table 2 Dry Weather Flow Summary



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b. Wet Weather Results

For the 2009 monitoring period, rainfall was recorded at five gauging sites with five major storm events that ranged from 0.67 inches to 2.15 inches. Figure 4 presents data from Site 1 for the storm event of April 12-13, 2009. The high peak flows are a characteristic of wet weather inflow. Note that the meter site surcharged to a depth of approximately 75 inches and velocity data indicates that flows were restricted due to a backwater condition originating from the North Little Rock sewer.

Figure 4 Wet Weather Flow

Meter Site No. 1

The wet weather analysis summarizes the system responses observed during the wet weather periods. The analysis computes the volume of wet weather I/I, also known as rainfall dependent I/I (RDII), which entered the collection system during the storm. Table 3 presents a summary of the wet weather system response and compares this response to dry weather conditions. From this data, the discrete RDII was calculated for each storm event. The total discrete RDII for each



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meter basin was normalized using the average rainfall which established the rainfall dependent infiltration/inflow volume per inch of rainfall. Total recorded RDII per inch of rainfall was determined to be 3.02 mg/inch. Of this total approximately 1.56 mg/inch is associated with the Sherwood system (52%) and 1.46 mg/inch (48%) with the North Little Rock collection system. The amount of RDII originating from the North Little Rock collection system will prove problematic for Sherwood. High wet weather flows originating from North Little Rock (Sites 3, 4 and 6) will require significant increases in capacity for those sewers in Sherwood that must transport these flows through the Sherwood system. Figure 5 presents the flow from North Little Rock as recorded at meter site 3. These peak flows must be transported through the Sherwood system before discharge into the North Little Rock interceptor serving the Five Mile Creek treatment plant. In addition, downstream restrictions where Sherwood flows discharge into North Little Rock ( sites 1, 2, 5 and 7 for example) may create sufficient backwater resulting in the potential for sanitary sewer overflow within the City of Sherwood. The City of Sherwood has no responsibility or control over these downstream sewers. Likewise, the City of Sherwood discharges into North Little Rock sewers for ultimate treatment at the Five Mile Creek treatment facility. Under the existing ADEQ order, Sherwood has initiated a comprehensive program to identify and rehabilitate the sanitary sewer to reduce wet weather infiltration/inflow.

Figure 5

Wet Weather Flow North Little Rock Site 3



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Table 3

Wet Weather Flow Summary



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III. Conclusions and Recommendations Extraneous water from infiltration/inflow (I/I) sources reduces the capacity of the collection system to transport wastewater and may result in Sanitary Sewer Overflows (SSO’s). The presence of infiltration/inflow within a collection system is an indicator of system deterioration that if left unchecked will continue to increase and reduce system capacity. Infiltration occurs when existing sewer lines undergo deterioration. Groundwater may enter the collection system through defects such as open pipe joints, cracks, broken pipe, dropped joints, etc. Inflow (Rainfall Dependent Infiltration/Inflow – RDII) normally occurs when rainfall enters the sewer system through yard drains, missing cleanout caps, roof leaders, manhole covers and frame seals, storm sewer cross connections, etc.

The reduction in infiltration/inflow by rehabilitation and on-going operation and maintenance will extend the life of the collection system assets and restore capacity. This report quantifies the extraneous infiltration/inflow (I/I) and characterizes its location and impact on the collection system. Figure 6 presents the ranking of Sherwood areas for subsequent testing and rehabilitation.

Following is a summary of conclusions and recommendations:

1. Dry weather flows are adequately transported and treated without overflow and no surcharging was observed during dry weather.

2. Wet weather rainfall induced infiltration/inflow (RDII) averages 1.56 million gallons per inch of rainfall for the City of Sherwood and 1.46 million gallons per inch of rainfall for that portion of North Little Rock that is tributary to Sherwood.

3. All sites surcharged during the recorded rain events except Sherwood meter sites 5 and 11.

4. North Little Rock contributes significantly (48%) to the total wet weather inflow passing through the Sherwood collection system.

5. The City of Sherwood has entered into an agreement with ADEQ to assess and rehabilitate its wastewater collection system. Data collected during the flow monitoring will be used to prioritize subsequent testing and rehabilitation efforts to reduce wet weather infiltration/inflow originating from the City of Sherwood collection system.



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Figure 6

Project Area Ranking

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