Hydraulic and Erosion Control
- Weirs, Culverts & Groynes
Weirs
In sloped and mountainous areas, channel erosion can be a
significant problem due to the high kinetic energy of the
flows. A designer could consider a transverse weir structure,
rather than a full channel lining which may be too expensive
or not appropriate to install.
Weirs, placed transversely across a water course dissipate
the stream energy to a level at which it ceases to move soil
particles forming the bed and banks of the water course.
This is achieved by reducing the gradient of the slope by
installing a weir, or series of weirs, so that the change
in level occurs through a series of shallow steps.
Gabions and Reno mattresses form ideal weir structures, due
to their flexibility, simplicity of construction and units
can be easily amended on site to overcome unforeseen conditions.
If the flow conditions change, gabions can be added to the
existing weirs, controlling the flow as necessary.
Gabion weirs soon collect soils and vegetation can grow from
within the stability of the stone fill. This is a positive
environmental bonus that distinguishes gabions from more traditional
concrete weir structures.
Weirs can be in three forms;
Weir type |
Use |
Comment |
Vertical Face |
Used in series in mountainous areas, or alone in
flatter terrain |
Can accommodate large bedloads if crest
is capped to resist abrasion. Reduction of kinetic energy
into a dissipation basin is important. |
Stepped Face |
Used for small drops and rivers with low bedload |
Offer greater dissipation than vertical faced weirs.
Gabion steps could be eroded by heavy bed-load. |
Sloped Face |
Used for high discharge and rivers with low bedload |
For high discharge large structures, typically on
major rivers, with fine grained soils of poor bearing
capacity. For taller weirs (30-45ft) sloped weirs with
greater stability and improved hydraulic behavior are
recommended. |
Our Macra 2™ 2002 design software is used to design
gabion weir structures. Please contact us if you are interested
in our assistance on your weir designs.
When designing weirs, be sure to check….
- Structural stability
- Hydrostatic Stability
- Scouring where the weir meets the channel banks
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Culverts
Where a water course encounters a culvert, constriction or
narrowing of the channel, the flow conditions change and become
more erosive.
Where this occurs, erosion will be focused where the flow
enters and leaves the constriction. This erosion can be limited
by Reno mattresses, gabions or RECPs installed in the channel
and on the banks.
The flow will be guided into and out of the constriction
by wing walls or abutments.
The advantages of using gabions and Reno mattresses for
these applications are:
- Ease of installation even on sites with difficult access
- Possibility of finding suitable fill materials locally
- Flexibility
- Units can be easily amended on site to overcome unforeseen
conditions
- Ease of maintenance
- Wing walls / abutments can be designed as retaining structures
- Adaptability of gabions to suit pipe and culvert opening
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Groynes
Groynes are structures which protrude into a water course
to divert the flow away from the bank and redirect the flow
energy.
Groynes slow the water flow, causing it to lose energy between
groynes and deposition occurs. The amount of deposition is
mainly dependent upon the spacing of the groynes relative
to their length, with a ratio of 1:4 to 1:6 (length / spacing)
being the average.
Groyne Type |
Uses |
T Head |
Best deposition/siltation |
Straight |
Good deposition/siltation |
Angled upstream |
Preferable with small sized bedload, small gradient,
wide channels |
Angled downstream |
Preferable where large sized bedload is carried in
high water conditions |
Gabions form ideal groyne structures as they monolithic structures
offering strength, are easy to construct, and flexibility.
This allows the units to conform to the stream bed
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