The U.S. Environmental Protection Agency (EPA) has
emphasized Stormwater and Combined Sewer Overflow (CSO) Control Policies in the
National Pollutant Discharge Elimination System (NPDES) and Municipal Discharge
permitting processes. These policies have been implemented to promote
attainment of minimum water quality standards in the receiving waters affected
by wet-weather municipal discharges. In many cases, the Stormwater and CSO
Policies and NPDES permitting processes require municipalities to develop and
implement Best Management Practice (BMP) control strategies and discharge
monitoring programs for planning, compliance, and reporting purposes.
The regulatory focus on long-term Stormwater and CSO control
programs increases the need for accurate flowrate measurement and monitoring
systems that may be deployed at multiple locations within municipal networks.
Flow monitoring data collected from key locations over a range of wet-weather
collection system flows frequently reveal significant differences in comparison
to flow predictions resulting from hydraulic system computer models such as the
Stormwater Management Model (SWMM). This real-world flow data is valuable for
verification of the overall modeling approach and can highlight specific
collection system features that may require more detailed analysis for
appropriate BMP implementation.
Real-world flow data is also useful for calibration of
computer model flow network response to wet-weather events. However,
high-accuracy flow measurements are critical to meaningful system modeling and
analysis, since the propagation of uncertainties (errors) through model flow
networks can rapidly grow to unmanageable proportions.
Ultrasonic transit-time flowmeters can be used to good
effect for meeting specific site monitoring and documentation requirements by
providing highly accurate and continuous flowrate measurement during dry- and
wet-weather conditions. Transit-time flowmeters include bi-directional (reverse
flow) measurement capability and can be configured for multiple acoustic paths,
making them highly accurate over a wide range of changing water level and flow
conditions, as well as in locations where other measurement methods cannot
reliably function.
In addition to
providing the data needed for system modeling and evaluation, accurate flow
information is valuable for regulatory reporting and compliance documentation;
planning and evaluation of Stormwater & CSO control alternatives; alerting
operators to CSO system malfunctions; optimizing operation of treatment
facilities; allocating contributory system user costs and billings; and pacing
chemical treatments for discharges.
Requirements for CSO and Stormwater Flowmeter Systems
Flowmeters for Stormwater and CSO monitoring typically are
required to operate under both free flowing (i.e., in partially filled conduits
or open channels) and surcharged (pressurized) conditions. Additional
measurement requirements can arise at locations subject to backflow, reverse
flow, or tidally governed hydraulics. The need to accurately determine
flowrates over such a wide range of conditions places stringent requirements on
the methods and technology that can be successfully utilized in these
applications. Methods that derive flowrate from measurements of water level
only (using stage vs. flowrate relationships) are simply not capable of meeting
these requirements.
A more suitable approach is developed from consideration of
the hydrodynamic continuity equation, with a derived principle that applies to
flow through any conduit section; i.e., flowrate is equivalent to
multiplication of a true average current profile velocity times the
cross-sectional area of the flow. Since flows in collection system pipes and
conduits range from partially full through surcharged conditions, measurement
of water level is used to determine the cross-sectional flow area (based upon
the geometry of the conduit), and water velocities are measured to estimate the
corresponding true average flow velocity.
Since flow velocity profiles in surcharged conduits are
significantly different than under free-flow conditions, a
“compound” approach that automatically selects an appropriate
integration method for computing representative velocity profiles and resultant
averages will provide better flowrate accuracy. Conduit sections that are well
removed from upstream bends, obstructions, or other flow disturbances will
exhibit “fully developed” flow velocity profiles. The multiple-path
transit-time method can provide accurate flowrate measurement even at sites
where flow profiles are not well developed.
The Accusonic multiple-path ultrasonic transit-time
flowmeters discussed here have been installed worldwide for high-accuracy flow
measurement in more than 2,000 large pipes, open channels, and collection
system project applications. The flowmeters, which operate in clean water or
raw sewerage environments, have been in use since the 1970s. Many Accusonic
systems have been installed in large sewers and collection system pipes and
channels, as well as at wastewater treatment plants for high accuracy and
reliable flow measurement. A number of US and Canadian municipalities have
established real-time monitoring networks comprising multiple flowmeter sites
using Accusonic flowmeter systems.
Because Accusonic flowmeters use relatively high-power
ultrasonic pulses for flow measurement, they are capable of operating in large
pipes and channels flowing with relatively high concentrations of suspended
sediments, as is common in sewer flows. Accusonic has developed proprietary
ultrasonic signalling techniques that maintain good signal strength under
sewerage conditions and are certified for intrinsically safe installation in
NEC Class I, Division 1 explosive atmosphere locations.
