Download appendix b - Brisbane City Council

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APPENDIX B
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Figure B2
Multi-cell culvert
Figure B3
Culvert inlet, (Plan View)
Figure B4
Section A - A
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Hydraulic Structures
To assist in the development of the above conditions in multi-cell culverts, and also to
assist in the control of sediment flow, an inlet weir can be formed. Sediment training walls
can be designed to direct all trickle or base flows to one ‘wet’ cell as shown in Figures B3
and B4.
The typical height of the training wall that is placed in front of the ‘dry’ cells is 0.3 - 0.5
metres. For hydraulic reasons, the training walls need to be located at least four times the
height of the wall away from the culvert entrance as shown in Figure B4.
Drop structures
Traditional vertical and near-vertical drop structures usually cause serious impediments to
fish passage.
To avoid disruptions to aquatic corridors, several small drop structures should be built
rather than one large structure. In areas where fish movement is required, rock chutes or
grouted boulder chutes should be used.
Weirs
Weirs create upstream backwater regions where velocities are reduced and coarse
sediment transported by flood events is allowed to settle. An energy dissipation pool is
normally created downstream of the weir and this pool can provide significant aquatic
habitat benefits. Unfortunately, the accumulation of aquatic species in these areas is
usually the direct result of the fish not being able to migrate past the weir.
To improve fish migration past weirs, fish ladders need to be installed. Fish ladders are
different from the open channel fishways employed in riffle, chute and drop structure
design because they do not need to operate in high sediment load conditions.
The design of fish ladders is not discussed in this document. Reference should be made
to the local Fisheries Authority (ie QDPI, Fisheries Group) for advice on the design and
installation of fish ladders.
Migration of terrestrial
wildlife
Waterway corridors provide important corridor links with bushland corridors for various
bird and terrestrial species. It is important for the needs of the terrestrial movement
corridor to be clearly identified in relation to bushland links and open space corridors.
Terrestrial wildlife movement corridors are required along waterways and across catchment
boundaries for migration, breeding, territory expulsion, predation and genetic transfer.
The need for terrestrial migration paths along the waterway should have been established
during the planning stage of the proposed works.
Within Brisbane City Council, terrestrial wildlife movement corridors should be sized in
accordance with Council’s Ecological Assessment Guidelines.
Discontinuities within the corridor should be avoided. These discontinuities can be
formed by roads, culverts, certain types of drop structures and by the removal of essential
riparian vegetation.
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Where transport infrastructure (road/rail,etc) crosses environmentally sensitive areas with
riparian vegetation, then the Bridge/Culvert should provide adequate spans to maintain a
continuous riparian zone.
The vegetative needs of terrestrial wildlife movement corridors vary significantly from
species to species, but generally canopy trees at regular spacing are required for birds
and animals such as sugar gliders, and understorey plants are required to protect grounddwelling animals from predators.
Arch structures
It is important to make sure that a dry path is formed under the arch and to make sure
that any future sedimentation that may occur under the arch does not result in the loss of
this dry animal migration path.
Bridges
When properly designed, bridge structures generally provide the least interruption to
terrestrial movement.
To allow for terrestrial wildlife movement, the bridge should span the main channel plus a
portion of the floodplain on each side of the channel. In some cases, terrestrial movement
considerations may apply only to one side of the main channel. However in most cases, it
is usually extremely important for each side of a permanently wet stream bed to be
considered as a separate terrestrial movement corridor with its own specifications and
requirements.
In some locations it may be essential for appropriate vegetation to be established under
the bridge to provide cover for terrestrial wildlife migration. This may require special
stormwater drainage consideration to allow the regular supply of rainwater to the plants.
Chutes
In most cases, chutes can be surrounded by riparian vegetation corridors that can act as
terrestrial movement corridors.
Design consideration would be the same as for the rest of the watercourse
Culverts
Terrestrial wildlife movement through culverts can be affected by a number of factors, but
the most important requirement is for the formation of a dry migration path. In single-cell
culverts, the production of a dry migration path normally requires the construction of a low
flow channel or elevated ‘fauna’ path.
Fauna path
To allow for fauna movement in single-cell culverts, a dry migration path may need to be
established. A fauna path may consist of a 0.5 – 1.0m wide, stone pitched path elevated
above the base flow water level (Photo B2).
Fencing
Fencing, such as Koala-proof fencing, can be used to direct animals to the culvert to avoid
animals moving over the road.
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Photo B2
Lighting conditions
‘Dry’ fauna path in a single-cell box culvert
When aquatic passage is provided, light should be encouraged to enter the culvert or, in
the case of a multi-cell culvert, to enter the wet cell(s). In multi-lane roads, stormwater
drop inlets can be installed into the median strip to allow the entry of light as well as
stormwater.
As a general rule, if the road is wide enough to need a raised or painted median, then it is
wide enough to need the introduction of a skylight into the dedicated migration cells.
Lizard run
Lizard runs are approximately 100mm wide strips of timber bolted to the waterway
embankment side of a culvert cell. They are introduced to a culvert to enable smaller
reptiles to move through the culvert at an elevated height.
Lizard runs should be located approximately 300 mm below the cell’s obvert. The lizard
run must extend from ground level at the upstream wing wall, through the culvert to
ground level at the downstream wing wall (Figures B1 and B2).
Median traffic barriers
Where practical, any traffic barriers installed along the median should contain breaks to
allow animals to pass over the road. The design of these breaks needs to be discussed
with both fauna and traffic safety experts.
Side wall roughness
Ideally the culvert cell wall adjacent to the watercourse bank should be roughened with
texture paint, grout or other suitable material to allow for the movement of fauna such as
lizards.
Any increase in culvert wall roughness must be taken into account in the hydraulic analysis
when designing the culvert. However, it is noted that in a typical road culvert, side wall
friction represents only around 12% of the total head loss.
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Street lighting
Many animals migrate only at night. To assist in the migration of these animals, street
lighting adjacent to culverts should be fitted with metal shields to prevent the lighting of
the culvert entry/exit.
In some circumstances it may be desirable to paint the concrete wing walls and apron in a
dark colour (dark green) to minimise the reflection of light.
Vegetation
Bank vegetation should be extended up to the edge of the culvert. This is especially
important if a ‘lizard run’ or ‘fauna path’ has been installed.
In a critical flood control region, this bank vegetation may need to consist only of flexible
(non-woody) species that provide minimal hydraulic resistance (ie. no shrubs).
Wet and dry cells
Most culvert designs will have requirements for some degree of aquatic or terrestrial
movement. Thus, most multi-cell culverts will require both wet and dry cells (Photo B3
and Figure B2).
The dry cells may be elevated above normal bed level, or may be aligned with the
elevation of the adjacent floodplain. Dry cells can also have a dual use as pedestrian
tunnels and bikeways.
Photo B3
Wildlife habitat
Multi-cell, multi-use, wet and dry cell culvert
As a general rule, the cells of a culvert should not be modified to provide or promote the
habitat of birds and terrestrial animals. The reason for this is that these habitats and the
residing animals will likely be destroyed during flood events.
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However, culverts and the bed erosion often associated with culverts can provide
desirable habitat areas for reptiles.
Drop structures
Drop structures can cause breaks in terrestrial movement corridors if the structure extends
across the full width of the channel and floodplain.
Breaks in terrestrial movement corridors can usually be prevented by providing rough,
sloping faces to drop structures, especially near the banks of the main channel and within
any floodplains.
Impacts on riparian
and floodplain
vegetations
Vegetation restrictions often exist immediately upstream and downstream of waterway
bridging structures. In critical flood control areas, vegetation species that may interfere
with floodwaters should not be located within the following areas:
(i)
upstream of a bridge or culvert within a radius equal to the total bridge or culvert
opening width;
(ii) downstream of a bridge or culvert within the zone defined by a 1 in 4 expansion of
the outlet jet and for a distance equal to three times the floodwater depth;
(iii) between the bridge or culvert opening and the floodplain;
(iv) any areas judged necessary by hydraulic modelling.
Bridges
Traditionally, vegetation has not been placed under bridges because it is felt that there
would be insufficient light. However, with the appropriate selection of plant species, this
should not be the case in most circumstances.
If plants are introduced under bridges, whether on the abutments of or the banks of the
watercourse, then it is important for stormwater runoff from or around the bridge to be
directed to these plants.
Drop structures
In the Brisbane region, a number of gabion drop structures have either suffered damage
to the wire baskets or have been invaded by undesirable weed species such as vines.
Vine invasion can be a problem in natural channels because the vines can damage native
trees. Gabion structures are generally recommended only if suitable vegetation can be
integrated into the design.
Public access, movement
safety requirements
Public safety during storm and flood events is normally assessed based on the depth and
velocity product, ie the product of the depth in metres times the flow velocity in metres
per second.
In most cases, the depth velocity product requirements will not apply to Natural Channel
Design works because it is normally unreasonable to expect the public to pass across a
waterway channel during a flood event. However, at recognised public access points and
crossings, such as pathways, bikeways, causeways, culverts and bridges, consideration
must be given to the public safety and thus the depth velocity product at these locations.
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In all cases, public safety should be treated on a case-by-case situation giving due
consideration to Council policies, community expectations, engineering and economic
practicality, the risk of harm and an appropriate storm event or frequency.
Drop structures
Drop structures can form a public safety hazard. In most cases, the maximum drop height
should be 1m for public safety reasons.
Because fish can swim at burst speed for only short distances, the greater the flow velocity
down the chute, the shorter the distance must be between rest areas (i.e. pools).
However, tailwater conditions must be checked to see when the chute becomes drowned.
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