The EDIBON Sediment Transport Demonstration Channel "CAS" allows demonstration of the full range of bedforms that arise in a mobile bed as the slope and / or flow are increased.
This unit can play a useful role in any course concerting the mechanics of open channel flow and sediment transport.
The unit consist of an inclinable channel mounted on a base plate, supported by two supports, with a discharge tank and recirculating pump. This tank is in the basic hydraulic feed system (FME00/B) that contains the recirculating pump.
The channel sides are transparent allowing the observation of bed profile changes, and a section of one side is provided with graphical grid markings to permit quantitative assessments to be made of bedform dynamics.
A water level gauge is included to measure the head over the channel discharge weir and therefore to deduce flow rates from a calibration chart. For demonstrating scour effects of structures on rivers beds, solids models, as an adjustable undershot weir and bridge pier are supplied.
To start a demonstration, sand is placed along the channel bed, between the inlet tank and the overfall discharge weir. Water is circulated around the system at one of the flow rates. The slope of the channel is adjustable.
Flow over a mobile sand-bed
(associated bedforms with increasing flow intensity and sediment transport rate)
- Lower Regime (exhibed bedforms):
- Plane- bed (no motion).
- Ripples and dunes.
- Washed out dunes.
- Upper Regime(exhibed bedforms):
- Plane- bed (with motion).
- Chutes and pools.
- Anti- dunes.
- Breaking anti-dunes.
- Standing waves.
Flow over fixed, gravel-bed
- Although the channel can not transport gravel, this can be used to investigate flow resistance in gravel and polder-bed rivers.
- We can calculate the flow resistance coefficients, using equations such as those of Bray, Limerinos, Hey, Lacey, Thompson and Campbell and Bathurst and the results compared to the actual values obtained by observation.
- We can examine the structure of turbulence in the flow, using dye injection, interesting for the dune bedform configuration and clearly demonstrates separation on the lee face.
Fixed, smooth bed flow:
(the channel may be used without sediment on the bed to demonstrate several flow phenomena and equations)
- Rapid, super- critical flow- dominance of inertial over gravity forces, shock waves from flow obstructions.
- Governing equations of open channel flow-Reynold’s number, Froude number, continuity, Bernoulli’s equation, weir equations.
- Tranquil, sub-critical flow- movement of surface waves upstream against flow.
- Hydraulic jump- transition from super to sub critical flow, air entrainment, mixing.
- Flow measurement- using sharp crested weirs.
- If the discharge in the channel changes quickly, there is no sufficient 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 will occur for the same discharge on the rising and falling limbs.
Data collection and numerical evaluation (computational work).
- In addition to illustrating flow and sediment phenomena, we can use the channel for basic data collection and numerical evaluation of the following:
- Flow resistance: Manning, Chezy and Darcy-Weisbach friction factors for several bedform configuration.
- Bedform prediction: Velocity-Hjulstrom diagram, Suspended load-movement by suspension, Shields parameter - Bogardi diagram, Stream power-Simons and Richarson charts, Boundary shear stress-Leeder chart.
- Initiation of motion: Shields diagram, Hjulstrom’s curve.
Mechanics of sediment transport.
- We can observe the movement of grains, starting from a plane- bed with no motion, 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.
Aspects and features of tanks
- We can observe the deposition of sediment load and the resulting patterns of grains within the sand body may be identified.
- Scour under boils and vortices in the flow is observed under both the lower and upper regime bedforms. Obstructions may be introduced to represent bridge piers, sills, revetments, etc, and the resulting pattern of scour examined.
- Behaviour study of the connection to the drain of a channel with sendiment.
- Turbulence study by means coloration.
- Calculation of water flow.