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monitors for the city’s eight regional The Net RDII is defined as each top 29 sub-basins will result in a priority
customers. The data collected from sub-basin’s specific contribution to the list for performing SSES investigation
Nov. 23, 2009 to April 23, 2010, was cumulative RDII volume. The Net RDII and rehabilitation for the Phase II
used to calibrate the hydraulic model is determined by calculating the volume projects.
and to determine the initial SSES of RDII measured at each flow monitor
basin priorities based on observed Net and removing the volume of RDII Completed Work
Normalized Rainfall Dependent Inflow measured at upstream flow monitors.
and Infiltration. The Net RDII for each sub-basin is then The city will generally utilize the
normalized by dividing the net RDII following rehabilitation techniques
The Phase II flow-monitoring by the total linear footage within each to perform the gravity sewer line
program will continue to assess respective sub-basin. Normalization of rehabilitation: Cured-in-Place Pipe,
changing flow conditions and provide the RDII allows for comparison of each CIPP Lateral Rehabilitation, pipe
continued insight to the system basin against one another by providing bursting, open cut replacement, external
response to a larger number of storms. a unit volume rather than an overall point repair and internal point repair.
The ongoing efforts associated with volume of RDII. The following projects have recently
Phase II represent a long-term flow- Phase I and II SSES Project Priorities been completed under the Phase I SSES
monitoring program and provide the using these methods.
data necessary for subsequent model Utilizing the Phase I flow-monitoring
validation and evaluation of the pre- program, the sub-basins were ranked Chattanooga Creek 4: The project
and post-conditions of the rehabilitation by Normalized Net RDII. The top involved a Sanitary Sewer Evaluation
projects. five ranking Normalized Net RDII Survey on approximately 62,000 linear
contributing basins were included in the
The city maintains permanent Phase I SSES projects. Continued on page 10
monitors at the CSOTFs, pump stations
and the Moccasin Bend plant. While The decision methodology for
these monitors were not selected for use sub-basin schedule prioritization of
in SSES prioritization, they can be used the Phase II projects will be based on
to supplement the monitors deployed several prioritization parameters, which
specifically for SSES. include: RDII, sanitary sewer overflows,
risk to impaired streams, relative age,
Developing SSES Project Priorities environmental justice area concerns
and other factors. The prioritization
The SSES project prioritization is criteria will be incorporated into a
split up into two phases: Phase I SSES scoring matrix. These scores will be
projects are to be completed within normalized and weighted to ensure
five years of EPA’s approval of the SSES comparability and relative importance
Work Plan, and Phase II includes SSES of each parameter in relation to the
projects to be completed within 15 years other parameters. The application of this
of EPA’s approval of the SSES Work methodology to the city’s next 24 of the
Plan.
Initial RDII Analysis
The first phase of SSES work involves
flow monitoring the wastewater
collection and transmission systems to
determine which areas of the WCTS
have excessive RDII, in order to
determine which areas of the WCTS
need further assessment and SSES work.
Rainfall Dependent Inflow & Infiltration
is defined as the portion of inflow and
infiltration that is directly influenced
by the intensity and duration of a
storm event. The response generated
by this component of inflow and
infiltration is an increase in the system
flow during and after a rainfall event.
This extraneous water enters the sewer
system in direct response to rainfall
through storm drains and other sources
such as leaky manhole covers and
defective sewers.
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