FLUMES
a hydraulic structure..
FLUME: Etymology
The term flume comes from the Old French word flum or from the Latin flumen,
meaning a river.
It was formerly used for a stream, and particularly for the tail of a mill race.
FLUME: Description & Use
It is a human-made channel for water in the form of an open declined gravity
chute whose walls are raised above the surrounding terrain, in contrast to a
trench or ditch.
They are not to be confused with aqueducts, which are built to transport water.
Flumes are used in transporting materials using flowing water.
FLUME: Types
Millrace
o A diversionary flume is used to transfer water
from one body to another, such as between two
reservoirs.
Log flume
o Log flumes use the flow of water to carry cut
logs and timber downhill, sometimes many
miles, to either a sawmill or location for further
transport.
FLUME: Types
Navigable canal flume
o In some nineteenth-century canals, a bypass
flume diverted water around a lift lock from the
level (or pound) above to the level below the lock,
so that the level below would have sufficient
water.
Recreational flume
o In competitive swimming, specialized flumes with
transparent sides are often employed by coaches
to analyze a swimmer's technique.
o The speed of the flow is variable to accommodate
the full spectrum of swimming styles and ability.
FLUME: Types
Flow Measurement flume
FLOW MEASUREMENT FLUME: Definition
are specially shaped, engineered structures that are used to measure the flow of
water in open channels.
are static in nature - having no moving parts - and develop a relationship between
the water level in the flume and the flow rate by restricting the flow of water in
a variety of ways.
FLOW MEASUREMENT FLUME:
Relationship with Flow Rate
Flumes can accelerate slowly, from a sub-critical (Fr<1)
flow to a supercritical state (Fr>1) by:
a change in elevation
a contraction of the sidewalls
or a combination of the two
FLOW MEASUREMENT FLUME:
Flow Rate Measurement
Accelerating slow flow to a supercritical state creates upstream conditions where,
under free flow conditions, the flow rate can be determined by measuring the
water level at a single, defined point in the flume (Ha).
The relationship between the water level at the point of measurement (Ha) and the
flow rate can be obtained by test data (short-throated flumes) or derived formula
(long-throated flumes).
FLOW MEASUREMENT FLUME:
Measurement Accuracy
When properly installed, flumes can be accurate to within +/-2-
5% (for the flume itself) - with overall system accuracy for a
typical installation being +/-10% (when all factors are
considered).
This accuracy is similar to that of weirs - although weirs are
slightly more accurate under ideal conditions.
FLOW MEASUREMENT FLUME:
Advantages (over weir)
The ability to measure higher flow rates than a comparably sized weir
Less head loss (generally 1/4th that of a weir)
The ability to pass debris more readily
Wide range of styles and sizes
Off-the-shelf availability
Smaller installation footprint
Less rigorous maintenance requirements
Also, most styles of flumes are resistant to changes or restrictions in the
downstream hydraulics (submergence) - something not found with weirs.
FLOW MEASUREMENT FLUME:
Sections
It typically consists of a converging section, a throat section, and a
diverging section.
However, not all sections need to be present.
FLOW MEASUREMENT FLUME:
Sections, Inlet
or the converging section (dark
blue) that restricts the flow then
is accelerated as it passes into
the throat.
it enters at a sub-critical state
[typically with a Froude number
(Fr) no more than 0.5] and it is
here that the point of
measurement (Ha) is located.
FLOW MEASUREMENT FLUME:
Sections, Throat
the part that accelerates the
flow to a critical / super-critical
state.
FLOW MEASUREMENT FLUME:
Sections, Outlet
The discharge or diverging section
(light blue) that slows the now
energetic super-critical flow and
allows it to transition into the
downstream channel.
While not all flumes have discharge
sections (H/Montana flumes), the
lack of this section means that the
resistance to submergence is
limited and that the downstream
channel may be susceptible to
scour and erosion as the flow spills
off the end of the flume.
FLOW MEASUREMENT FLUME:
Classes and Styles
In the approximately 100 years since Dr. Parshall began his
investigation on his improved Venturi flume, two distinct classes (and
numerous styles) of flumes have been developed.
Short-throated flumes
Long-throated flumes
FLOW MEASUREMENT FLUME:
Classes and Styles, Short-throated flumes
Short does not refer to the flume length but to the fact that flow is
controlled in a very specific region of the flume to produce the level-to-
flow relationship.
They also have a single, defined point of measurement and are empirical
devices. Intermediate or non-standard sizes of short-throated flumes
must be individually rated but their use in not recommended.
FLOW MEASUREMENT FLUME:
Classes and Styles, Short-throated flumes
FLOW MEASUREMENT FLUME:
Classes and Styles, Long-throated flumes
Flumes that control flow in a throat that is long enough to cause parallel
flow lines in the section of flow control.
Flumes that are somewhat more accurate than short-throated ones and
can be computer designed (eliminating the need for costly laboratory
rating).
Additionally, the point of measurement is not fixed, but can be taken
anywhere upstream of the drawdown zone of the throat.
FLOW MEASUREMENT FLUME:
Classes and Styles, Long-throated flumes
The following are long-throated flume styles:
FLOW MEASUREMENT FLUME:
Installation
earthen channels,
concrete canals,
below ground chambers,
or factory integrated into Packaged Metering Manholes.
end
