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Planning Application Supporting Statement
November 2022
Cringle Reservoir is an impounding reservoir owned and operated by the Manx Utilities Authority (MUA). The reservoir spillway is an original, nearly 80 -year old structure. The spillway is designed to act as an overflow for the reservoir, diverting excess water from the reservoir to the downstream river.
Recent inspections by the appointed All Reservoirs Panel (ARP) Inspecting Engineer have highlighted the poor condition of the spillway and its need for refurbishment to mitigate the risk of the structure being undermined during a heavy rainfall/flood event. It has also been noted that stone masonry walls are not any longer recommended for spillways as they cannot reliably withstand the high flow rates or the high velocity flows. Disintegrated spillway structure may subsequently lead to the erosion of the dam and its failure. The reservoir, given its volume over $25,000 \mathrm{~m}^{3}$, imposes a threat to life and property in the event of dam breach. The proposed works are therefore required in the interest of reservoir safety and this is a mandatory requirement as specified by the Water Act 1991.
The refurbishment of the spillway will require the installation of reinforced concrete walls and a floor.. In the design of the new spillway, MUA have endeavoured to minimise the visual change as much as is reasonably practicable. The existing quartz capping will remain intact wherever the new walls are of the same height as the existing ones.
The spillway refurbishment will require:
The height of the new walls will remain the same, i.e. as the height of the existing walls with exception for the walls around the spillway bend. The new walls either side of the bend will be raised by 650 mm above the ground. This is required to contain the flow of water that sweeps round the bend during an extremely heavy rainfall event.
The proposed works have been consulted with the following third parties:
No major objections have been raised. The works will have to be undertaken taking into account the ecological value of the grass surrounding the spillway- this can be managed by the contractor during the works.
The reservoir water level during the construction works will have to be lowered and this might impact angling activities. The activities will be managed and agreed with DEFA and anglers prior to and during the works.
The public footpath around the reservoir will need to be partially closed during the works. The closure will include a 15 m -long section over the upper bridge that connects the reservoir western bank with the dam. The rest of the path will be accessible.
An area within the reservoir site which will be closed off for the public is the grassy area between the dam and Corlea Road immediately adjacent to the spillway and the stilling basin. The site access for the construction vehicles will be provided from Corlea Road via two private gates.
The construction works will take up to 5 months and are planned for the spring/summer 2023.
This project is to upgrade the invert and the walls of Cringle Impounding Reservoir (IR) spillway so that it can safely convey the design flood flows as recommended by the All Reservoirs Panel (ARP) Inspecting Engineer.
Figure 1 Cringle IR Spillway
Cringle Reservoir is a well-recognised water feature in the south of the island located within Cringle Plantation, south-west of South Barrule Mountain.
The reservoir design was commissioned by the Southern Water Board in the 1920s and the construction started in 1938 however it had to be postponed until after WWII. The reservoir was commissioned in 1946 and remained operational until 2011. At that point the capacity of the reservoir was insufficient for the growing water demand in the south of the island. The reservoir is no longer used as a potable water resource however it is still a Manx Utilities' asset that falls under the legal obligations of Schedule 3 of the Water Act 1991.
The reservoir site is shared by two local authorities: Arbory and Malew.
Cringle Reservoir is an impounding reservoir with a capacity of approximately 144,000m³ and a surface area of 1.8 hectares at the top water level of 172.21 m AOD. It was formed by the construction of a 160 m long embankment dam across a tributary of the Silverburn River that drains the southern side of South Barrule Mountain. The reservoir is about 250m long at its longest point.
A spillway is a structure that can safely pass excess water from a reservoir to the river downstream of the reservoir dam.
Cringle IR's spillway channel is located on the west and south-west side of the dam. The water spills over a weir and discharges into a tumble bay then sweeps past the dam and under the bridge to follow the south-west mitre of the dam. Then, the water flows into a polygonal stilling basin from which it passes over another weir into a natural water course which is the Silverburn River.
Figure 2 Spillway - Upstream View
The Silverburn River, downstream of the reservoir, passes through undeveloped, open countryside for about 4.0km before reaching Grenaby hamlet and then continues for another 4km before it flows into the sea in Castletown.
The spillway channel is approximately 135m long, 4.5m wide at the bottom and approximately 1.6m deep. The spillway channel has Manx stone walls throughout but its bed varies along its length. The upstream section, approximately 21m long, has a stepped concrete slab floor with a slightly dished surface. The floor below the stepped section has a masonry finish with a randomly placed boulders set into the surface.
Spillway photographs can be seen in Appendix B.
PROJECT NEED Recent mandatory inspections, as required by Schedule 3 of the Water Act 1991, have confirmed that the spillway is in poor condition and failure to repair/improve it will lead to the increased risk of failure during a severe flood event. Failure of the spillway could lead to the dam being undermined which may subsequently release water stored in the reservoir. Given the stored volume of $144,000 \mathrm{~m} 3$, human lives and property downstream of the reservoir would be endangered.
The existing spillway has been operating since the reservoir was built in 1946. The spillway has been regularly maintained and inspected as required by the Water Act 1991. As stated in Section 3 of the Water Act the impounding reservoirs are required to be inspected by an Inspecting Engineer and Supervising Engineer every ten years and annually, respectively.
The inspections have confirmed that the spillway walls and floor are in poor condition and will further deteriorate. The issues found are as follow:
An abstract from the letter issued by the Inspecting Engineer containing the advice is quoted below: ". . . in my opinion, it is unlikely that the spillway would be able to survive the passing of the design flood. In these circumstances (and perhaps under lesser floods too). I envisage that there would be erosion of the invert and that the walls would fail in a progressive manner. This could lead to erosion of the natural ground on either side of the structure and, more worryingly, failure of the dam embankment itself."
Additionally, stone masonry walls are not recommended nowadays. Masonry spillways cannot reliably withstand high flows and high velocities. The fast flow may inject the water behind the stone through open joints, loose or missing mortar causing the stone to dislodge. This leads to the structure being disintegrated and subsequently to an erosion of the ground behind the structure.
It has been concluded that the improvement works must be undertaken in the interest of reservoir safety. The project has therefore been commissioned to reinstate the spillway and to ensure its safe operation.
Various options for the spillway restoration have been reviewed including repointing the walls, spraying them with concrete or installing precast concrete walls. The options considered have been reviewed by the Inspecting and Supervising Engineer. It has been concluded that the most suitable solution will be:
Figure 3 shows the extent of the works proposed within the spillway. Spillway floor repairs will include only the invert downstream of the upper concrete stepped section (only the blue area on Figure 3 as the upstream stepped section is considered to be in good condition). The walls however will be lined with the new structure throughout the entire length of the spillway.
Figure 3 Extent of the proposed works An engineering consultancy with a specialisation in the field of impounding reservoirs has been appointed to design the new spillway. It should also be noted that any alternation to the existing reservoir structure must to be reviewed and approved by an ARP Engineer in order to comply with the Water Act 1991. The design therefore is being delivered by the expert consulting engineer and simultaneously reviewed by ARP Engineer. A set of the planning drawings showing the proposed design has been prepared and issued with the planning application. Site photographs are included in Appendix B.
The existing spillway walls on both sides of the spillway chute are masonry stone walls. The proposed design involves lining the existing walls with cast in situ reinforced concrete all the way from the downstream end of the tumble bay up to the stilling basin. The existing wall will remain intact forming a base for the new walls.
The height of the new walls will match the height of the existing spillway walls expect for a section of 23 m where the wall on both sides of the spillway will be raised. The section coincides with the spillway bend which changes the spillway direction to more south east to follow the south mitre of the dam.
Computational Fluid Dynamics (CFD) modelling simulations were performed to assess and compare the resulting flow phenomena between the existing spillway configuration and the proposed lined channel spillway configuration for the design flow condition.
The figures below illustrate the flow phenomena in the relined channel for the 1 in 10,000 flood event.
Figure 4 Water flow in the spillway - lined new spillway, 1 in 10,000 year flood event In summary, the velocities along the channel remain largely the same when compared with the existing channel. Slight velocity increase is visible towards the end of the chute upstream of the stilling basin but this is not a concern.
Figure 5 Flow along the bend - velocities November 2022; Issue Rev. 1
Figure 6 Flow along the bend - water depth
The CFD model has predicted out of channel flow at the aforementioned spillway bend. The spill over is likely to occur on the non-dam side of the spillway. Figure 4, 5 and 6 above assume that the channel is already refurbished however the out of channel spill will also occur for the same flood event outside the existing, not refurbished channel. The extent of the spill for 1 in 10,000 flood event is up to 2.4 m away from the wall with a water depth up to 0.4 m . In order to prevent the water spill, it is required to raise the wall locally by 0.65 m above the existing wall height either side of the bend. Although, the model does not predict out of channel spill on the north side (dam side), water that comes down the spillway and hits the non-dam wall at the bend, rebounds and hits the opposite wall. The raised wall at this location will help to contain any water spray, it will future proof the structure in the event of more severe flows and allow the structure to maintain its symmetry. The ground exterior to the north and south wall along the higher wall sections will be raised and locally regraded to reduce visual impact. The raised wall at each end will be tapered to match the new walls of the same height as the current walls. The length of the wall raised above the current ground level will be approximately 23 m either side of the spillway. The ground profile alteration will subsequently require $110 \mathrm{~m}^{2}$ of soil. It is envisaged that the most of the required soil will come from the site. Spoil from drainage trenches and any other temporary works will be retained and used for the ground re-profiling works. Any additional required soil volume can be sourced from other construction sites managed by MUA. Everywhere else, the height of the new walls will match the height of the existing walls which is approximately 1.6 m . It is proposed that the existing Manx quartz stone capping will remain intact where the new walls are of the same height as the existing ones.
Figure 7 Refurbished Spillway - No Wall Height Change - Cross Section The two parallel sections of the raised wall will lose the capping feature as the old walls will have to be buried behind the new ones to raise the exterior ground to the new level. The existing timber fence will be replaced with new stainless steel hand railing - 1.1 m high above the top of the new walls.
The existing spillway floor is partially concrete and partially masonry. The first 50 m measuring from the weir upstream of the tumble bay is made of concrete. The section from the upper bridge is a concrete stepped section. The concrete lining is considered to be in good condition and is not required to be refurbished. The remaining 70 m is predominantly masonry with large boulders protruding above the finished level of the spillway floor. Condition of the masonry invert has been reported as poor and the floor needs to be repaired. Therefore, the invert between the end of the stepped section and the end of the spillway chute (just upstream of the stilling basin) will be relined with cast in situ reinforced concrete. Similarly as with the wall, the existing channel invert will largely remain intact creating a base for the new floor. The exception will be narrow channels within the existing floor that will be excavated to install new underdrainage. The boulders currently visible within the spillway floor will be removed if required. There is no intention to use the boulders within the new invert.
Consideration has been given to the change in the visual impact the new spillway may have when compared with the current, stone structure. Re-building the spillway using stone is not an option as it would not ensure the long term robustness required. Stone masonry spillways are not a recommended option nowadays as they cannot reliably withstand the high flow rates and the high velocities. This means that the structure integrity may be compromised during severe rainfall events which may lead to structure disintegrating and subsequent ground erosion and potentially dam failure. The possibility of cladding the concrete spillway walls with stone or stone panels has been looked into. It has been concluded that the use of stone cladding would require significant ongoing maintenance. No warranties could be obtained from stone panel manufacturers that their products and especially adhesives used can withstand the high velocity flows for years to come. These products have been found to be unsuitable for the application within the spillway. The spillway must be a robust, functional and reliable structure complying with modern standards. The spillway must be able to safely pass the flows ensuring the safe operation of the reservoir while protecting properties and infrastructure downstream. The proposed concrete spillway will meet these requirements. The proposed design makes provision for retaining the white quartz stone as a wall capping where possible. The new walls will be cast against the existing ones leaving the quartz stone in their current location. This should provide some resemblance and connection between the old and the new spillway. The Isle of Man Strategic Plan 2016 and Environment Policy 1 states the following: The countryside and its ecology will be protected for its own sake. For the purposes of this policy, the countryside comprises all land which is outside the settlements defined in Appendix 3 at A.3.6 or which is not designated for future development on an Area Plan. Development which would adversely affect the countryside will not be permitted unless there is an over-riding national need in land use planning terms which outweighs the requirement to protect these areas and for which there is no reasonable and acceptable alternative.
Bringing the spillway up to modern standards should be considered as "an over-riding national need" as it will allow for the safe and robust operation of the spillway and the reservoir for years to come. It will continue to protect lives, properties and livelihoods and maintain the reservoir as a valuable public amenity.
This section summarises all information collected and consultations undertaken that have been addressed and considered while delivering the initial design.
MUA own the land within which the reservoirs and associated infrastructure such as the spillway are located. The land registry number for the land is: 43-00013_1. The land adjacent to MUA's land belongs predominantly to DEFA. Appendix A contains the land ownership plan as per the land registry.
It is a statutory requirement that all large reservoirs (of a volume above $25,000 \mathrm{~m}^{3}$ ) have inundation plans. The Flood Plan for Cringle IR shows the area downstream of the reservoir that would be affected if an uncontrolled water discharge from the reservoir was to take place. The areas affected include:
The above information emphasises the need for the works.
Southern Area Plans (Map 1 constraints; Map 2 Landscape) have been reviewed in order to determine any planning implications on the above design. The following have been noted:
The Area Plan for the South (that came into operation in March 2013) drafted under The Town and Country Planning Act 1999 describes the reservoir as a recreational site due to it being a fishing lake. Given that the reservoir site is of ecological importance as stated on the area plan, Department of Food, Environment and Agriculture (DEFA) Ecosystem Policy Unit (EPU) have been consulted to understand any implications and constraints that the status may have on the proposed works. The following points have been raised by DEFA Ecosystem Policy Unit:
It is therefore important to protect the grassland in the area. Some turf either side of the spillway will have to be removed to install the proposed drainage. The turf will be stored in a suitable environment and looked after until it can be relaid. Given that vehicle access to the site will be required, it may be prudent to also remove the turf from the access track. The access track up to the spillway will be clearly marked to stop any site vehicle going off the marked access route.
Any alternative option preventing the grassland from being damaged will be considered prior to commencing the works on site.
Common lizards are listed on Schedule 5 of the Wildlife Act 1990 and are protected from being intentionally or recklessly killed, injured or removed from the wild. Additionally, they are protected from disturbance and it is an offence to damage or destroy their place of shelter. There is a very low risk that lizards may be utilising cracks and crevices in the spillway walls or the varied vegetation between the wall and short turf grassland. As short grassland is not suitable for lizards, Ecosystem Policy Unit have suggested during the initial consultation that the grass is kept short in the immediate vicinity of the spillway to discourage lizards from living there. This recommendation has been implemented by MUA's Ground Maintenance Team. The lizards hibernate over the winter but are active during the spring and the summer. EPU has further suggested that the area is checked for any potential refuges by suitably qualified ecologists prior to commencement of the works. Any sightings of lizards in the work area should be reported to DEFA and the works stopped immediately.
The requirement will be written into the construction contract.
The above points raised by EPU will be written into the Project's Scope document which will be issued with the project tender and subsequently with the contract. The contractor appointed to undertake the works will be required to prepare a Construction Environmental Management Plan. The Plan will be required to contain Method Statements for the following:
Furthermore, DEFA Fisheries Division - Inland Fisheries have been consulted with the view to understand DEFA's concerns and recommendations with regards to proposed works in the vicinity of the reservoir and the downstream river as both being a fish habitat. The contractor undertaking the works will be required to ensure that Silverburn River, downstream of the spillway, is not polluted with concrete residues while the walls are being cast. Any concrete spill, wash waters or water used to cure concrete will have to be contained and removed so that it is not released into the river. As stated earlier in the section, the requirement of protecting the ecology of the river will be written into the Scope document and will form part of the project documentation upon the award of the contract. The contractor will be required to prepare a Construction Environmental Management Plan which will discuss proposed method of containing any concrete spill or residue that may pollute Silverburn River. The works will be advertised prior to their commencement on site and all the details provided that may affect the anglers or members of public.
It is envisaged that the Cringle Reservoir site will be accessible to members of public and anglers throughout the duration of works except for the grass area adjacent to the spillway and with some restrictions when it comes to crossing the dam. Please refer to drawing: CS-ACM-ZZ-XX-DR-CE-2001 for more details.
The members of public will be able to access the site via the usual entry points:
Members of public, anglers and walkers will therefore be able to enter the site and use it however with some minor changes or restrictions. The fishing ramp on the west bank of the reservoir will remain open. Mountain bike trails will also remain unaffected. Cringle Reservoir is also a popular venue for various public events. Permissions to organise any event are managed and given by MUA. The works will be planned with the intention to avoid any annual events and any ad hoc ones will be managed by MUA accordingly.
The footpath from Corlea Road and around Cringle Reservoir is one of the Public Rights of Way (PROW) managed by Department of Infrastructure (DoI) Highways Division. The PROW number is 634.
The footpath will remain unaffected by the works except for a 15 m -long section over the upper bridge connecting the west bank of the reservoir and the dam. No walking across the dam will be possible. The eastern side of the dam will however be accessible by walking all around the reservoir and back the same route. MUA have consulted Highways Division with regards to the planned works and the partial closure of the footpath. No objections have been raised. The contractor will endeavour to reopen the bridge as soon as the works under the bridge have been completed. The contractor will also be required to display advance notices on site to indicate the location of the partial footpath closure.
The construction site boundary will encompass the area immediately adjacent to the spillway and the spillway including the stilling basin and the upper bridge over the spillway. The construction site will be fenced off and closed for public. The upper bridge will be within the site boundary as the spillway under the bridge is required to be refurbished - as discussed in the previous section. It will however be stipulated in the contract that once the works under the bridge have been completed, the contractor shall make the dam fully accessible. There may be occasions when the contractors will need to close it again but those will be short term closures. Construction vehicles will be entering the site via two private gates at Corlea Road located either side of the river. The gates are used by MUA only.
Figure 8 Site Access and Use
The following consultations have taken place while preparing this statement:
Consultation with Inland Fisheries have focused on the following points: protecting fish upstream and downstream of the works, reservoir fish stocking and access to the reservoir. Protecting fish in the reservoir and the downstream watercourse is discussed in the earlier section titled: Environmental Appraisal. With regards to the access to the reservoir, the access for DEFA will remain unchanged i.e. via Cringle Reservoir Car Park at Corlea Road (south of the reservoir) following the public footpath. The construction access will be via two private gates either side of the river - the gates are used by Manx Utilities only. DEFA will be able to access the reservoir any time following a notice given to the contractor.
With regards to restocking and fishing during the works, it is not envisaged that either of the activities will be prohibited. It may only be constrained by the lower reservoir levels and the need to walk all around the reservoir to access the eastern reservoir bank - this is due to the partial closure of the public footpath over the upper bridge. The reservoir level will have to be sufficiently below the spillway weir - 172.21 mAOD - to prevent water discharging down the spillway during the construction phase. Furthermore, in the interest of safety, a lower water level will form a headroom to contain rainfall for a long enough period of time so that the contractor has enough time to safely evacuate the spillway and secure the works in the event of an unexpected extreme rainfall event. The exact level is to be yet determined. This will be further discuss with DEFA Inland Fisheries.
The reservoir site will remain largely accessible to the anglers and members of public throughout the duration of the works with some limitations:
The above constraints will be listed in a public notice given prior to the commencement of the works on site. The notice will be displayed at the Cringle Reservoir car park and Cringle Plantation Car Park both at Corlea Road and the plantation car park at Ronague Road - eastern edge of the plantation.
Flood Risk Management (FRM) Team shall be consulted with regards to any works taking place in the vicinity of rivers. The section of Silverburn River immediately downstream of the reservoir is a nondesignated watercourse and it only becomes a designated watercourse further downstream south of the Grenaby area. The consultations have been undertaken with the FRM Team with regards to the proposed work. No comments or objections have been received.
The DoI Highways Division has been consulted with regards to the required closure of public right of way around the reservoir. Details of the consultations are provided elsewhere within this statement. The closure is only partial hence the majority of the path will be accessible by members of the public. No objections have been raised.
Waste produced during construction works will be minor. The new spillway walls and floor will be cast onto the existing structure so no debris will be produced. There will be some minor excavations within the spillway floor to install the new underdrainage. The waste will be disposed of to a licenced site. Excavations either side of the spillway to install drainage pipework will reuse the as-dug material to reinstate the trenches. Turf removed will be reinstated as requested by DEFA's EPU and as discussed elsewhere in this statement.
Due to the nature of the works and their location downstream of the reservoir, the works are planned for late spring, summer or early autumn to minimise the risk of the reservoir over spilling.
The envisaged duration of the works is up to 5 months.
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