Memorandum                                                    

City of Lawrence

Utilities Department

 

TO:

David L. Corliss – City Manager

Diane Stoddard – Assistant City Manager

Cynthia Wagner – Assistant City Manager

FROM:

Philip Ciesielski – Assistant Director of Utilities

CC:

Dave Wagner – Director of Utilities

Mike Lawless – Assistant Director of Utilities

Beth Frailey Krishtalka – Assistant to Director

Date:

September 20, 2010

RE:

Department of Utilities Watermain Condition Assessment and Replacement Prioritization Model

 

Water Distribution System Background

The Department of Utilities operates and manages over 430 miles of water distribution and transmission main piping. These watermains range in age from 123 years to less than 1 year; in diameter from 2” to 36”; and are made from a variety of materials including ductile and cast iron, PVC, transite, steel and concrete. The service life of the various watermains and materials can range from 25 to 125 years. The life expectancy is influenced by the pipe material, surrounding soil condition, date of manufacture, construction methods and materials, adjacent utilities and construction activities and the operational conditions to list a few.

 

Watermain Replacement Program

The Department of Utilities Rate Plan and Capital Improvement Program historically budgets a yearly fund for the replacement of existing watermains within the distribution system. Recent propose budgets have included $1,216,700.00 in 2008; $1,265,300.00 in 2009; and $1,315,900.00 in 2010. The program is used to address the replacement of watermains due to pipe age, reliability, water quality and capacity issues. From 2005 – 2007 the program targeted the replacement of the over 100 year old watermains along Massachusetts Street, generally from 6th Street to North Park Street. The replacement of the 1950’s era cast iron watermain along Iowa Street from 9th to University has recently been completed along with adjacent watermains along Orchard Ln and University. As part of this project new watermains were installed along Harvard and Westdale to improve the redundancy of the distribution system in the area. Department staff have also self performed a number of watermain replacements during this period. The attached map highlights the watermain replacement projects, totaling 43,550 lf, completed since 2005 and those currently under construction.

 

Watermain Condition Assessment and Replacement Prioritization Model

Department of Utilities staff have continuously evaluated the condition of the water distribution system based on mainbreaks, pipe material and age, water quality, capacity and fire protection needs. However, there has not been a systematic, numeric prioritization process. After researching national and international documentation of similar efforts and reviewing the current systems used by the City of Olathe and WaterOne, staff have assembled and initiated a dynamic analysis and rating model using analysis tools and data from the GIS system.

The watermain data modeled includes pipe material, age, and diameter; soil condition; proximity to large or critical water users; repair costs; and GBA Master Series records of work orders and mainbreaks. The Department’s current historical data on mainbreaks is not comprehensive and only contains records back to 1998. The model is dynamic in the sense that the analysis can be run at any time to calculate the rating using the existing watermains in the system and the most current information in the databases. Map of Current Rankings

 

The rating model also includes a financial breakeven analysis based on the American Water Works Association Research Foundation AWWARF 90892 – Decision Support System for Distribution System Pipe Renewal. This analysis compares the Annual Break Rate to a Threshold Break Rate calculated using typical mainbreak repair costs, replacement cost of the pipe, number of breaks on the segment and the discount rate. The pipe segment meets the financial criteria for replacement when the Annual Break Rate is greater than the Threshold Break Rate. The current validity of this analysis is limited as the history of mainbreaks is not comprehensive and costs of repair have traditionally been tracked.

 

The model has been applied to 5,062 water distribution main segments totaling 365 miles and ranging in diameter from 4” to 12”. The individual pipe segments range in length from 1 ft to 5200 ft.

 

The following outlines the rating criteria and scoring factors. Higher scores indicate a more critical condition and result in a pipe segment of higher priority for replacement.

 

Pipe Age (years)

Score

0 - 20

0

21 - 40

1

41 - 60

2

61 - 80

3

81 - 100

4

100 +

5

Pipe Material

Score

PVC

1

Transite

3

Cast Iron

4

Ductile Iron

5

Pipe Diameter Criticality

Score

4" Diameter

1

6" Diameter

2

8" Diameter

3

10" Diameter

4

12" Diameter

5

 

Soil Corrosion Potential

Score

Low Corrosion Potential

0

Moderate Corrosion Potential

3

High Corrosion Potential

5

Proximity to Critical/Large User

Score

1000' From Large User

3

1000' From Hospital, Major Medical

5

Number of Breaks

Score

Zero Breaks

0

1 to 2 Breaks (Multiple # of breaks by)

1

3 to 4 Breaks (Multiple # of breaks by)

2

5 or More Breaks (Multiple # of breaks by)

3

Financial Break Even

Score

Annual Break Rate < or = Threshold Annual Break Rate

0

Annual Break Rate > Threshold Annual Break Rate

5

 

 

   

 

 

 

 

 

 

 

 

 

 

Annual Break Rate = (Number of Breaks for the segment)/(Number of years of break data for the segment)

Threshold Annual Break Rate = ln(1+i) / ln (1+ C/F)

 

Where

i = Annual discount rate (5% is used)

F = Replacement Cost (Based on cost per foot)

C = Repair Cost ($3,500 is used)

 

While the model does rate each pipe segment based on the criteria above due the number of pipe segments and the relatively few criteria numerous pipe segments are scored equally. The distribution of the individual pipe segments and their scores does not result in the ability to perform replacement projects in a strict sequential order. The geographic distribution of the ranked pipe segments provides another piece of information staff can use when reviewing the water distribution system and making recommendations on replacement and rehabilitation projects.

 

Ongoing Efforts

Staff are evaluating the model and its results to analyze and refine the current ranking criteria and scoring. The model results will also become more accurate as additional data is obtained and entered into the GIS and GBA Master Series, in particular, a longer and comprehensive history of mainbreak data and repair costs.

 

The model is not a stagnant effort and additional data sets to be added and scored have been identified. These include –

 

 

In an effort to enhance the existing pipe condition data set Staff are working on the implementation of Watermain Condition Survey to be completed anytime a watermain is excavated (attached sample). This information will provide a layer of data related to actual observations of internal and external pipe condition and help refine the condition assumptions currently being made primarily based on pipe age and material.

 

Rehabilitation Information

Other efforts are being directed toward the investigation of in-situ remediation technologies and processes in terms of their applicability to City of Lawrence watermains and installation conditions, success rates, installation and ongoing monitoring and maintenance costs. One of the most prevalent techniques is the application of retrofit cathodic protection on cast and ductile iron watermains. This is accomplished via the attachment of sacrificial anodes to the existing watermains as they are exposed for repair or tapping, or as a targeted program. In Canada and several northern U.S. states this technique has shown to be successful in reducing the break rates on cast and ductile iron mains since the 1980’s.