Armfield Engineering Education
APPLIED HYDRAULICS AND HYDROLOGY
S8 MKII Sediment Transport Demonstration Channel - Issue 5
This improved version of the Armfield sediment transport teaching facility allows demonstration of the full range of bedforms that arise in a mobile bed as the flow and/or slope are increased.
The channel can be used to perform most of the experiments
and demonstrations usually undertaken in much larger laboratory flumes,
but at much lower cost and without the need for technician back-up. The
equipment is portable, and therefore may be used in the classroom as well
as in the laboratory. Although too small for research applications, this
demonstration flume can play a useful role in any course concerning the
mechanics of open channel flow and sediment transport, including those in
Departments of Civil Engineering, Geology and Physical Geography.
ORDERING SPECIFICATION
• A transparent, inclinable flow channel through which water can be re-circulated by a pump over a mobile bed to demonstrate the whole range of bedforms from incipient particle movement to bed wash-out.
• Three different discharge rates can be selected (and measured) within the range 0.2 to 0.6 litres/sec.
• The channel slope can be adjusted within the range 0-10%.
• The working section of the channel is 1.55m long, 78mm wide and
110mm deep.
• The equipment is self-contained and may be bench-mounted in either the classroom or laboratory by virtue of its portability.
• A model undershot weir and bridge pier are included for local erosion demonstrations.
• A water level gauge is supplied to calibrate the overshot weir.
TOPSummary of Demonstration Topics
> Fixed, smooth bed flow
> Flow over a mobile sand-bed
> Mechanics of sediment transport
> Depositionary features and facies
> Local scour
> Flow structures
> Bedform hysteresis
> Computational work
> Flow over a fixed, gravel bed
Description
The unit consists of an inclinable channel mounted on a frame, together with a discharge tank and recirculating pump. To commence a demonstration, sand is placed evenly along the channel bed, between the inlet tank and the overfall discharge weir. Water is circulated around the system at one of the three selectable flow rates. The channel slope is adjusted by means of a fine screw jack to which is attached an accurate slope indicator. The channel sides are transparent in order that bed profile changes can be readily observed, and a section of one side is provided with graphical grid markings to permit quantitative assessments to be made of bedform dynamics.
TOPDemonstration Capabilities
FIXED, SMOOTH BED FLOW
The flume may be used without sediment on the bed to demonstrate the following
flow phenomena and governing equations:
> Tranquil, sub-critical flow - movement of surface waves upstream against
flow
> Rapid, super-critical flow - dominance of intertial over gravity forces,
'shock waves' from flow obstructions
> Hydraulic jump - transition from super to sub critical flow, air entrainment,
mixing
> Turbulence - flow visualisation for example by dye injection from a
hypodermic syringe (not supplied)
> Flow measurement - using sharp crested weirs
> Governing equations of open channel flow -
Reynolds' number, Froude number, continuity, Bernoulli's equation, weir
equations
FLOW OVER A MOBILE SAND-BED
Sequence of bedforms associated with increasing flow intensity and sediment
transport rate. The following bedforms are exhibited (as discharge and/or
slope are increased):-
> Lower Regime
- plane-bed (no motion)
- ripples
- ripples and dunes
- dunes
- washed-out dunes
> Upper Regime
- plane-bed (with motion)
- standing waves
- anti-dunes
- breaking anti-dunes
- chutes and pools
MECHANICS OF SEDIMENT TRANSPORT
Starting from a plane-bed with no motion, the movement of grains can be
observed with emphasis on the following:-
- initiation of motion
- trajectory of initial motion
- movement by rolling and sliding (contact load)
- movement by hopping (saltation load)
- movement by suspension (suspended load)
A water level gauge is supplied to measure the head over the channel discharge weir to deduce flow rates from a calibration chart. Solid models of a bridge pier and an undershot weir are provided to demonstrate the scour effects on river beds of man-made structures:
DEPOSITIONARY FEATURES AND FACIES
The deposition of sediment load can be observed and the resulting patterns
of grains within the sand body (such as cross-bedding, foreset beds etc.)
may be identified. The significance of such features when found in geological
records can be discussed.
LOCAL SCOUR
Scour under boils and vortices in the flow is observed under both the lower
and upper regime bedforms. Artificial obstructions may be introduced to
represent bridge piers, revetments, sills or other man-made structures,
and the resulting pattern of scour examined. Two such models are included.
FLOW STRUCTURES
The structure of turbulence in the flow may be examined using dye injection
(dye injector not included). This is particularly interesting for the dune
bedform configuration and clearly demonstrates separation on the lee face.
BEDFORM HYSTERESIS
If the discharge in the flume changes quickly, there is insufficient time
for bedforms to adjust to the new flow regime. Hence, if a flood hydrograph
is simulated by increasing and then decreasing the discharge, different
depths (stages) will occur for the same discharge on the rising and falling
limbs. This effect is of major importance to gauging stations on sand-bed
rivers. It is easily and clearly demonstrated in the flume.
COMPUTATIONAL WORK
In addition to illustrating flow and sediment phenomena, the flume can be
used for basic data collection and numerical evaluation of the following:
ÇFlow resistance:
- Manning, Chezy and Darcy
- Weisbach friction factors for various bedform configurations
ÇBedform prediction:
- Hjulstrom Diagram (velocity)
- Bogardi Diagram (Shields parameter)
- Simons and Richardson Charts (Stream power)
- Leeder Chart (Boundary shear stress)
ÇInitiation of Motion:
- Hjulstrom's Curve
- Shields Diagram
FLOW OVER A FIXED, GRAVEL-BED
The flume cannot transport gravel, but can be used to investigate flow resistance
in gravel and polder-bed rivers. The flow resistance coefficients may be
calculated using equations (such as those of Bray, Limerinos, Hey, Lacey,
Thompson & Campbell and Bathurst) and the results compared to the actual
values obtained by observation. It is recommended that users obtain actual
gravel material from local sources, (Armfield cannot supply gravel).
BEDFORMS IN SAND
As water flows over sand in a river or on a beach it exerts a shear force
on the bed. If the flow is strong enough, sand grains are lifted to roll
and bounce along the bed. The shape of the bed responds to this motion by
transforming into ripples. As the energy of the flow and transport rate
of sand increase, the bedforms change. Ripples are replaced by larger dunes.
At higher energy still, the dunes are washed out to produced a flat bed and in extremely energetic flows anti-dunes appear.
Bedforms are important in affecting the flow of water and movement of sediment in rivers and on beaches. They also occur in deserts due to the movement of sand by the wind.
Bedforms are preserved when sand deposits are turned into sandstone by
geological processes. They are used to reconstruct environments under which
the sand was deposited.
TOP
Technical specification
Channel working section:
Length: 1.55m
Width: 78mm
Depth: 110mm
Discharge rate:
3 fixed flow rates between 0.2 and 0.6 litres/sec, selected by switch on
pump
Slope: 0 to 10%
Sediment diameter: 0.1 to 0.3mm
Weight of sand supplied: 15kg
Weight [including sand and water]:
S8MKII-A: 74kg
S8MKII-B: 78kg
requirements
Single phase mains electrical supply:
S8MKII-A: 220-240V/1ph/50Hz
S8MKII-B: 120V/1ph/60Hz
S8MKII-G: 220V/1ph/60Hz
First fill of water [approx. 22 litres]
TOPShipping specification
Volume: 0.6m³
Gross weight: 100kg
Overall dimensions
Height: 1.1m (to top of point gauge)
Width: 0.4m
Length: 2.5m
|
 |
|---|