This chapter discusses and
gives an overview of the structural requirements associated with the various
route options presented in the route selection report.
All the route options will
require new structures to carry the N22 over various obstacles ranging from
streams to rivers, existing roads and the reservoir. Likewise, all the routes will necessitate new overbridges to
carry existing roads and farm access tracks over the new N22.
This report summarises the
different structural requirements of the various route options and discusses
particular considerations associated with the major crossing over the
Carrigadrohid / Inishcarra Reservoir.
A summary of the
preliminary structure requirements, subject to revision at further design
stages, along the different route options is given in Table 20.1. The table provides details of the numbers of
each structure type. A more detailed breakdown of each structure, detailing
obstacle crossed, assumed span and width, is provided in Appendix 20.
Table 20.1 Summary of Structure Requirements
|
Route (Node Nos.) |
Numbers of Structure
Types |
|||||
|
Pipe Culvert |
Box Culvert |
River Bridge |
Underbridge |
Overbridge |
Reservoir Crossing |
|
|
Green (1-16) |
4 |
14 |
5 |
4 |
10 |
1 |
|
Yellow (1-15) |
6 |
10 |
10 |
4 |
12 |
1 |
|
Red (7-14) |
2 |
4 |
0 |
4 |
8 |
1 |
|
Blue (7b-15) |
2 |
10 |
2 |
2 |
12 |
1 |
|
L/01 (4-5) |
0 |
1 |
3 |
0 |
0 |
0 |
|
L/03 (9-13) |
0 |
0 |
0 |
1 |
1 |
1 |
|
L/04 (8-10) |
1 |
0 |
1 |
2 |
0 |
0 |
|
L/06 (12-12a) |
0 |
0 |
0 |
0 |
1 |
0 |
All route options require the
construction of a substantial new reservoir crossing. This structure is
discussed in section 20.3.
The green route (Nodes
6-17) carries the N22 to the north of the Lee Valley between Ovens and Macroom.
This area is less populated and comprises of rolling hills with some steep
valleys. Numerous regional and local roads are crossed along with a number of
rivers feeding into the reservoir system. Slightly fewer number of overbridges
are required as less accommodation works will be required for property adjacent
to the road.
The red route (nodes 7-14)
skirts the southern edge of the reservoirs before crossing the Carrigadrohid
reservoir near Classes. A higher percentage of over- and underbridges are
required over this half-length of the scheme to avoid stopping-off various
access roads and local roads. Also the road will rise steeply out of the Bride
Valley and then back down again to the Lee Valley and this route is likely to
require either significant earthworks or structural retaining measures to
enable the road alignment to meet the required design standards. This route is
considered to be a more expensive option in term of structures when compared to
the others.
Both the blue (nodes 7-15)
and yellow (nodes 7-15) routes run along the Bride Valley before passing north
to Macroom, with the yellow route following closely the existing N22 alignment
whilst the blue route keeps to the north of Mashanaglass. Both routes require a
higher number of structures per kilometre due to their alignments passing
through the more populated and densely farmed Bride Valley. The structures
along the yellow route will generally be more difficult to construct as the
scheme is often adjacent to the existing N22 and extensive traffic management
will be required. Both routes will require crossings of the River Bride and /or
its tributaries and the River Laney, which will need to be designed to minimise
impact on the environment. The yellow route also crosses the Sullane River just
before it discharges into the reservoir
Links 3 (Nodes 9-13), 4
(Nodes 8-10) and 6 (Nodes 12-12a) present no particular structural difficulties
other than the crossing of the existing N22 at Athsollis Bridge. This will
require traffic diversions in a confined area in order to allow construction.
To the west of Macroom,
the green route (Nodes 1-6) remains to the north of Sullane River Valley,
taking the route up onto the relatively high moorland. The watercourses at this
level are generally smaller and greater use of culverts, as opposed to bridges,
is anticipated. The use of culverts will have less impact visually on the
environment and will be more economic to construct.
The yellow route (Nodes
1-7) follows reasonably closely to the existing N22 alignment and the Sullane
River. This lower level alignment means that the watercourses are larger and
therefore more river bridges are required. Two skew crossings over the river,
close to one another, are needed near Lynchs Cross Roads. This route has a
higher density of structures per kilometre of road over this section when
compared to the green however there are no significant difficulties concerning
the structures over this length for either route.
At this stage in the design
process, it has been assumed that the river bridges will generally have a span
of 15 25m, making allowance for all appropriate environmental considerations
adjacent to the watercourse. Underbridges will carry the N22 over national,
regional or local roads and they will have open-aspect side spans that will
minimise their visual impact on the surrounding area. Overbridges will carry
regional and/or local roads over the new N22 and these will also have
open-aspect side spans. The culverts will be sized in accordance with the
environmental requirements. All structures will be designed to be aesthetically
pleasing and fit in with their surroundings whilst having a common approach to
detail and finish as agreed by the NRA and Cork County Council.
The Inishcarra and
Carrigadrohid reservoirs were commissioned in 1957 and are owned and operated
by the ESB. The primary purpose of the reservoirs is electricity generation,
which is produced at the Inishcarra Generating Station situated beneath the
Inishcarra Dam. There is also a water
take-off point at Inishcarra that is operated by Cork County Council. The
critical reservoir facts and figures are detailed in Table 20.2.
Table 20.2 Reservoir Facts and Figures
|
|
Inishcarra |
Carrigadrohid |
|
Storage Volume (m3) |
29.9 Million m3 |
42 Million m3 |
|
ESB Ownership level
(m) |
48.49 |
63.74 |
|
Crest Level (m) |
49.32 |
64.51 |
|
Max. Operating Level
(m) |
47.29 |
63.74 |
|
Min. Operating Level
(m) |
42.99 |
58.29 |
|
Normal Operating Range
(m) |
44.79 46.79 |
59.29 61.79 |
(Note: All levels to Malin
Head datum (Pool Beg datum 2.71m))
The reservoirs store water
running off a large catchment area extending into the surrounding hills. The flow
of water down the Lee Valley is controlled and regulated through the use of
sluice gates at Carrigadrohid and a spillway at Inishcarra. The rainfall in the
catchment area is monitored on a 24-hour basis through the use of remote
rainfall measuring stations. The catchment area is prone to flash flooding
which in turn leads to rapid build-ups of water behind the dams. This volume of
water is managed between the two reservoirs and the discharge into the River
Lee downstream of the dams is co-ordinated to prevent flooding below the dams.
The two reservoirs are
crossed by four road bridges, three of which were built at the time of the dams
and are owned and maintained by the ESB. The fourth at Carrigadrohid is a
pre-reservoir structure. The ESB bridges (Rooves Bridge, Bealahaglashin Bridge
and Lee Bridge) are multi-span reinforced concrete structures. The reinforced
concrete beam and slab decks are supported by pairs of reinforced concrete
piers founded on the reservoir bed. At this stage it is unclear on the
foundation type for the structures. The Carrigadrohid bridge appears to be a
masonry multi-arch structure.
The following
site-specific constraints will need to be assessed and considered in the design
of the proposed bridge:
·
Wave levels of
approx. 300mm
·
Cork County Council
water take-off at Inishcarra and planning issues. Construction must not affect
the water quality. All development will need to go through the regulatory
planning process. The Lee Valley is identified as an area of Scenic Importance
and Recreational Importance and new structures will need to complement and
blend in with the existing environment.
·
Caoilte (Forestry Board)
Farran Wood. The section of reservoir adjacent to the wood is to be developed
into an international rowing venue.
·
South Western
Fisheries Board Coarse angling. The reservoirs are used extensively by
anglers and construction should not disrupt fish stocks and/or the anglers.
·
Cork Power Boat Club
water-skiing on the Dripsey Channel.
·
Carrig Waterski Club
water ski club at Caum upstream of the Carrigadrohid dam.
·
Kayak club
·
Sailing and pleasure
craft the reservoirs are used by dinghies and motorised craft. No boats
greater than 6 horsepower are allowed to use the reservoirs. Clearance should
be provided that is comparable to that under the existing structures.
·
Reservoir operation
the ESB need to be able to manage the water levels to cope with potential flash
floods from October to February. From March to September the ESB are able to
raise and lower the water levels to assist construction works.
The Reservoir crossings
identified by the different route options are summarised in Table 20.3 below.
Table 20.3 Reservoir Crossing Options
|
Route |
Start |
End |
Length over water (m) |
Max Water level (m) |
Max. depth of water at
crossing (m) |
||
|
Chainage |
Road Level |
Chainage |
Road Level |
||||
|
Green |
33+480 |
56.0 |
33+830 |
60.0 |
350 |
47.29 |
21.7 |
|
Link (Nodes 9-13) |
25+420 |
66.0 |
25+560 |
60.0 |
140 |
47.29 |
10.4 |
|
Red |
23+090 |
71.0 |
23+260 |
73.0 |
170 |
63.74 |
14.8 |
|
Blue |
22+500 |
72.0 |
22+770 |
72.0 |
270 |
63.74 |
10.9 |
|
Yellow |
20+740 |
72.0 |
20+940 |
73.0 |
200 |
63.74 |
6.6 |
(Note: All levels to Malin
Head datum (Pool Beg datum 2.71m))
This option has the
longest length of water crossing at 350m. The water depth is also greatest at
this location as the route is closest to the Inishcarra dam and the vertical
alignment of the route rises significantly from the north to the south. The
combination of these two factors means that the carriageway level is up to
12.8m above water level and 34.5m above the lowest reservoir bed level.
Construction in such depth of water will be difficult and therefore it is
likely that the span will be increased to a maximum in order to position the
piers either outside of the reservoir or in shallow depths of water adjacent to
the bank.
Both banks of the
reservoir are bounded by open farmland that will enable easy access for
construction.
This option is in an
exposed location that will have a significant visual impact on the Lee Valley.
The larger span will result
in an increased cost of structure.
The proposed reservoir
bridge for this route option is 200m across the water and it lies close to the
Bealahaglashin Bridge carrying the existing N22. At this point the reservoir
lies within a well-defined natural channel that is wide but fairly shallow at
only 6.6m maximum depth.
The proposed structure
would follow a similar vertical alignment to the current N22 giving similar
vertical clearances between soffit of bridge and water level.
This location is outside
any area of scientific interest and reservoir use is already constrained by the
two existing bridges at either end of this channel and the shallow depths of
water.
This route option presents
the NRA and Cork County Council with the best opportunity to deliver a landmark
gateway structure that may assist in adding to the identity of the local area
and the town of Macroom. The new crossing will have considerably longer spans
than those of the existing structures.
The shallow depth of water
and easy access to the site will ease construction.
This option has a
marginally longer length (170m) of crossing than the link option and the
maximum depth of water at this location reaches 14.8m. At this stage it is envisaged that the
crossing will have a significant major span in order to keep the pier
construction out of the deeper water and minimise any restriction on the use of
he reservoir in the vicinity.
The crossing at this point
is exposed to view by the communities either side of the water and thus the
impact of the bridge will be more significant.
Construction of the
proposed bridge in the location is relatively straightforward however
construction traffic will disrupt the minor roads nearby.
The length of this option
over the water is 270m and is the second longest crossing option. The maximum
water depth is 10.9m at this location and the reservoir bed is likely to rise
fairly steadily either side of the original river channel. This topography will
enable the relatively straightforward construction of piers within the
reservoir either within cofferdams or from temporarily backfilled bunds
protruding from either side.
This option places no
particular constraints on construction other than the need for temporary access
roads to enable site works.
The location of the
structure means that it is reasonably well hidden from many of the existing
viewpoints however it will be in an area of scientific importance.
The longer length of
structure will increase the cost of construction of this option.
This option produces the
shortest length of structure at 140m. At this location, the reservoir passes
through a narrow gorge that has quite steep sides on either bank.
The proposed structure
will also be constrained by the proximity of the R618 running along the north
side of the reservoir. This route will need to be kept operational and it is
therefore envisaged that the bridge will span this road in addition to the
reservoir.
The steep-sided banks will
make construction more difficult however the screened position of the crossing
means that the bridge will be well hidden from much of the surrounding area and
is therefore less intrusive on the environment.
The shorter length and
shorter span will result in a lower cost of structure.
The design of the
structures, whatever the selected route option, will consider all the
constraints appropriate to that particular location.
It is likely that all of
the reservoir crossing options, with the exception of possibly the link route,
will require piers to be located within the reservoir. A full survey of the
submerged topography will be required after selection of the preferred route
however various construction techniques such as cofferdams, temporary earth
bunds or reduced water levels within the reservoir during foundation
construction will enable pier construction to be undertaken.
The preferred construction
material for the bridges will be concrete due to the local availability of
materials. If however a landmark structure is proposed for the reservoir
crossing, a more lightweight construction material, such as steel, may be
utilised.
The use of pre-cast or
pre-fabricated elements will need to be kept to within current transport and
road haulage restrictions. All routes, except for the yellow route require
access along narrow local roads and tracks and therefore long beams and
structural elements will be difficult to transport to site. The yellow route
will allow relatively greater access along the existing N22 for large elements.
All routes involve
crossings that are off-line from the existing N22 and therefore provide a clear
site. The red route is located close to the Caum Water-ski club and therefore
this may partially restrict construction. The Link Route bridge passes close to
the shore-hugging R618 scenic drive and so this will involve traffic management
during bridge construction.
The blue and yellow routes
between Macroom and Ovens require considerably more construction along the line
of existing roads. This will produce more disruption to traffic and potentially
to the local population adjacent to the works. Consideration of these aspects
will result in longer design and construction programmes along with increased
associated costs.
All of the proposed routes
result in realistic and viable structural demands. It is considered that all of
the structural requirements can be satisfactorily engineered to meet the
Clients requirements.
A key requirement for the
structures is that they complement this area of scenic and recreational
importance and therefore emphasis must be given to their environmental and
aesthetic design. This objective can result, through good design, in a positive
enhancement to the locality on some of the routes (yellow and link) and no
adverse effect on the remaining routes (green, red and blue).