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Because of the drought conditions in 2000 and 2001, it
was not an ideal time to measure I/I flows. The precipitation that
did fall and the resultant flow monitoring data did provide some
useful information as direct inflow, or water that flows to the
sanitary sewer system as a result of precipitation through direct
connections such as roof drains, direct catch basin connections,
and leaky manholes. However, since rainfall events were light in
intensity and duration this information was not characteristic of
normal winter conditions.
The drought conditions also kept the groundwater from
rising to its normal levels. This precluded usual conditions where
water rises over sewer pipes and causes infiltration through leaks
or breaks in sewer pipelines. High groundwater conditions can also
cause inflow due to saturated soil conditions because this situation
prevents the ground from absorbing precipitation, which flows horizontally
and often enters sanitary sewer lines.
Below normal rainfall levels during the fall and winter
of 2000/2001 resulted in the need to repeat flow monitoring during
the 2001/2002 wet season in order to accurately define infiltration
and inflow levels in local collection systems and their impact on
the King County regional sewer system.
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Traffic Control During Installation
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Flow Meter Installation
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Out in the Field - Finding and Fixing
I/I > Flow Monitoring
Flow Monitoring
 Open
channel flow monitors were used to gather sewer hydraulic information
and were installed in manhole locations throughout the County during the
winters of 2000/2001 and 2001/2002.
Flow metering of the Local Agency systems provides the technical framework
upon which the successful Regional I/I Control Program will be built.
Working in coordination with each of the Local Agencies, the Earth Tech
team completed the installation of 807 flow meters associated with implementation
of the Regional I/I Control Program. Coordination with Local Agencies
has included verifying the size and location of local collection systems,
modeling and mini-basin configuration, flow meter location approval, traffic
control and safety plans, permitting, and fieldwork scheduling.
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Tier 1
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Tier 2 & 3
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Tier 4
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Tier 5
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75 long-term flow meters were installed in the King County interceptor
system. These meters operated for a total of 14 months capturing
new baseline flow data for the County's interceptor system.
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104 modeling basin and Local Agency boundary flow meters were installed
on or near the County's interceptors to measure and document actual
flows from each of the Local Agencies. Modeling basin meters also
verify wastewater flow data for utilization in the I/I Program's
modeling effort.
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628 Local Agency "mini-basin" flow meters were installed throughout
the Local Agencies to measure and document actual flows from Local
Agency basins containing an average of less than 22,000 lineal feet
of collection mains.
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Pre- and post-rehabilitation flow meters will be installed in specific
mini-basins before and after pilot project rehabilitation work is
completed. They will serve to provide the technical foundation for
determining the actual effectiveness of the pilot project I/I rehabilitation
efforts.
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The correlation of all five tiers of flow monitoring serves to establish
a direct correlation between each Local Agency's wastewater flows and
the effect of rainfall on infiltration and inflow entering the system.
The effects can then be measured and compared downstream to evaluate King
County wastewater system alternatives and associated cost saving possibilities.
The monitors are capable of acquiring depth of flow from two
sources:
- the primary source is a top-of-the-pipe ultrasonic
depth sensor
- the secondary source is a submerged pressure depth sensor.
The monitors measure velocity using Doppler peak velocity technology.
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Redundant Ultrasonic Depth Sensor
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Pressure Depth Sensor
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Doppler Velocity Sensor
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This sensor contains four separate ultrasonic sensors, which can
send and receive signals. Monitors operate all transducer combinations,
and then derive a single depth reading based on the composite results
of all firings. The sensor is mounted at the crown of the pipe where
ultrasonic waves are sounded at the surface of the flow. Echoes
bounce back and are received by the appropriate crystal pairs. The
time required for the signal to return to the sensor is directly
proportional to the distance traveled.
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The pressure depth measurement records the difference between atmospheric
pressure and the weight of water flowing above the sensor to measure
the depth of flow. Pressure depth sensors are not used as the primary
means of depth measurement because of their propensity for drift,
for hysteresis, and for damage caused by over-pressurization due
to high storm flows. This depth measurement methodology is typically
employed as a back-up to the ultrasonic depth sensor in lines where
surcharge conditions are expected.
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The velocity sensor sends an ultrasonic carrier signal through
the cross section of the flow directly above and upstream of its
beveled face. This signal is reflected off particles moving through
the flow and intercepted back at the sensor. Flow information will
be stored in data loggers installed in the manholes. Information
will either be gathered manually once per week in the field by downloading
information into laptop computers, or information will be transmitted
via telephone back to the central Program office for review and
analysis.
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