Oakenholt Reservoir Auxiliary Spillway Hydraulic Design Summary June 2020

111854 – R04 Oakenholt Reservoir Auxiliary Spillway Hydraulic Design Summary June 2020

Oakenholt Reservoir Auxiliary Spillway Hydraulic Design Summary

Introduction

Oakenholt Reservoir is a water supply reservoir located on the North Wales coast near Flint. It is registered as a large raised reservoir under the Reservoirs Act 1975. The owner and Undertaker of the reservoir under the Act is Essity UK Limited. The reservoir supplies water to a paper mill site a short distance downstream.

A periodical inspection of Oakenholt Reservoir under Section 10 of the Act was undertaken by Mr Peter Kelham of Ove Arup & Partners Ltd. (Arup) in April 2015. In his subsequent inspection report of November 2015, Mr Kelham identified a potential deficiency in the capacity of the spillway channel to carry the design and safety check floods. In this regard, he made the following recommendation as to measures to be taken in the interests of safety:

“The flood study is reviewed before a model study (physical or computational fluid dynamic) is undertaken based upon a detailed topographical survey to assess the capacity of the existing spillway channel and an option study prepared to identify appropriate solutions for ensuring that the design and safety check floods can be safely accommodated without impacting on the safety of the dam. The model and option studies shall be presented to an All Reservoir Panel Engineer for a further recommendation when issuing a Section 10(6) Certificate”

In response to this recommendation Fairhurst were appointed to carry out an assessment of potential options to address the inadequacy of the existing spillway. As reported in “Oakenholt Reservoir – Preferred Option Report” (Fairhurst 2018), the preferred option was for the existing spillway to be augmented by an additional auxiliary spillway connected to the existing spillway tumble bay by means of an overflow weir designed to limit the flow down the existing spillway to an appropriate flow. It was identified that this spillway could be designed to accommodate the 1 in 10,000 year design flood and would bring benefits in the PMF however if the PMF were to be accommodated the existing spillway would have to be replaced.

Following on from this Fairhurst were appointed to carry out a risk based assessment of the reservoir as reported in “Oakenholt Reservoir – Overflow Capacity – Risk-based Assessment” (Fairhurst 2019). This report identified the consequences, costs and probability of failure for the existing situation along with the consequences, costs and probability of failure for various potential mitigation options. It was found that the reduction in risk as a result of implementing the preferred auxiliary spillway identified in the preferred option report was justifiable in relation to the cost designing and constructing the spillway. This assessment identified that the cost for replacing the whole spillway to allow it to accommodate the PMF was not economically justifiable. This finding was sufficient to satisfy Mr John Ackers (ARPE) and enable him to sign off the measure to be taken in the interests of safety and make the recommendation that the works to deliver the auxiliary spillway be completed no later than October 2021. The certificate which was sent to Natural Resources Wales reservoir section is given in Appendix A.

This summary provides details of the work that has been carried out to determine the geometry of the proposed auxiliary spillway and the tumble bay overflow weir.

Summary of Hydrology

In 2017 Fairhurst carried out a flood study for Oakenholt Reservoir. A 1D reservoir routing model was developed to assess the peak water levels in the reservoir and the peak outflows from the reservoir during the design and safety check conditions. Full details of the study that was carried out are provided in “Oakenholt Reservoir – Flood Study and Preliminary Options” (Fairhurst 2017). A summary of the results of the study are provided below.

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111854 – R04 Oakenholt Reservoir Auxiliary Spillway Hydraulic Design Summary June 2020

Table 1. Flow Estimates

Event Peak Reservoir Peak Outflow Level (mAOD) (m3/s)

PMF 33.11 47.8

1 in 10,000 Year 32.04 14.6

Existing Spillway Arrangement

The existing spillway for Oakenholt Reservoir comprises an overflow weir at 90 degrees to the dam crest at the eastern end of the dam. When this weir is overtopped water spills into a wide tumble bay before turning 90 degrees and discharging into a steep stepped spillway channel at its northern end. The existing spillway channel is relatively narrow and is open for around half of its length before becoming culverted down the mitre of the dam before discharging into the river below. It was previously identified as part of “Oakenholt Reservoir – Preferred Option Report” (Fairhurst 2018) that due to its width this channel does not have capacity for the design flood (1 in 10,000 year) or the safety check flood (PMF).

Proposed Design Concept

The proposed auxiliary spillway will connect via an auxiliary overflow weir to the side of the tumble bay opposite the existing reservoir overflow weir. The auxiliary overflow weir will discharge into concrete spillway channel which will convey flows in parallel with the existing spillway. This will limit the flow rate in the existing spillway channel to reduce the risk of flows leaving the existing spillway channel and eroding the mitre of the dam. The auxiliary spillway will extend approximately 25m beyond the toe of the dam and discharge down the steep valley side to re-join the watercourse below. The auxiliary spillway in conjunction with the existing spillway will then have capacity to accommodate the 1 in 10,000 year flood flow and will bring benefits in the PMF however will not be designed to accommodate the full PMF flow. Figure 1 provides a sketch of the proposed arrangement with arrows depicting the proposed hydraulic operation.

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111854 – R04 Oakenholt Reservoir Auxiliary Spillway Hydraulic Design Summary June 2020

Figure 1. Proposed Spillway Arrangement

Overflow Weir Design

A 1D hydraulic model of the existing reservoir overflow weir, the existing tumble bay and spillway channel, and the proposed auxiliary overflow weir has been constructed to determine the required auxiliary overflow weir crest level. To maximise the potential high flow conveyance into the auxiliary spillway channel while also minimising the frequency of operation, the weir will extend over the full 16.5m length of the tumble bay.

Various weir levels have been tested and it has been determined that a weir level of 31.38mAOD in conjunction with the existing spillway capacity will result in the reservoir overflow weir maintaining free discharge beyond the 1 in 10,000 year flow.

The model results indicate that with this weir level 6.1m3/s of the 1 in 10,000 year flow will pass down the existing spillway and 8.6m3/s of the 1 in 10,000 year flow will discharge into the auxiliary spillway.

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111854 – R04 Oakenholt Reservoir Auxiliary Spillway Hydraulic Design Summary June 2020

Additionally the model results showed that for the auxiliary overflow weir to operate as intended the water level downstream of it in the auxiliary spillway channel could not be higher than 31.8mAOD.

The modelling indicates that the auxiliary spillway will only become operational in the 1 in 30 to 1 in 50 rainfall return period events.

Auxiliary Spillway Channel Design

Further hydraulic modelling was carried out using a 1D model of the proposed auxiliary spillway channel to determine the channel width and bed levels required to convey the 8.6m3/s required in the 1 in 10,000 year event.

The hydraulic modelling of the auxiliary spillway identified that a 1.2m deep, 5m wide channel was sufficient to comfortably convey the required flow. The highest water levels within the channel occur at the upstream end where the overflow weir discharges into it prior to the flows accelerating down the channel.

Based on this assessment, a bed level of 30mAOD was selected at the upstream end of the channel. Based on the hydraulic modelling carried out this bed level results in water levels below the level at which the auxiliary overflow weir transitions from free discharge to drowned flow.

To reduce velocities in the curved portion of the spillway and to minimise the earthworks excavation required, the upper 67m of the auxiliary spillway has been set to a 1:200 gradient. The normal depth velocity for a 5m wide rectangular channel with a 1:200 bed slope is 2.9m/s. At this velocity given the large radius of the bend there is not expected to be significant super elevation of water levels on the outside of the bend.

The proposals are presented on drawings 111845-2000 PL, 2001 PL and 111845-2002 PL in Appendix B.

Spillway Discharge Arrangement

At its downstream end the spillway will discharge down the steep valley side to the watercourse. The angle at which the spillway approaches the slope allows the flow to be distributed along a large area of the slope thereby reducing the erosion potential. Exit velocities are expected to be approximately 6m/s based on the normal depth condition in the channel, therefore the slope which the spillway discharges to will require gabion basket protection to avoid erosion of the slope. As the return period which the auxiliary spillway will operate on is fairly low it is considered that this arrangement is acceptable.

Conclusion

Fairhurst were appointed to carry out the hydraulic design for an auxiliary spillway to augment the existing spillway capacity for Oakenholt Reservoir.

Hydraulic analysis has been carried out to confirm the geometry of the auxiliary overflow weir and spillway channel.

The final arrangement comprises a 16.5m long auxiliary overflow weir with a crest level of 31.38mAOD which discharged into a 5m wide spillway channel. The spillway channel has a gradient of 1:200 for the first 67m to minimise velocities around the bend. After this it transitions to a gradient of 1:20 following the existing topography before discharging down the valley side to the watercourse. The channel depth varies between 2.5m at the upstream end to 1.2m at the downstream end.

A gabion mattress will be used to protect the valley side from erosion from the moderately high velocities and discharge volumes at the exit of the spillway.

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111854 – R04 Oakenholt Reservoir Auxiliary Spillway Hydraulic Design Summary June 2020

Appendix A

Section 10(6) Certificate

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RESERVOIRS ACT 1975 INSPECTING ENGINEER’S CERTIFICATE UNDER SECTION 10(6), AS TO THE CARRYING OUT OF SAFETY RECOMMENDATIONS

I, John Christopher Ackers, Consultant to Fairhurst, of Cornwall Buildings, 45–51 Newhall Street, Birming- ham B3 3QR, being a member of the All Reservoirs Panel, appointed by Essity UK Ltd to supervise the carrying into effect at the reservoir known as OAKENHOLT RESERVOIR, situated about 2km southeast of Flint in the County Borough of Flintshire at NGR SJ 262 711, of measures taken in the interests of safety recommended in a report made on 19th November 2015 by Peter Kelham, am satisfied that those measures have been carried into effect.

Signature of Engineer Date of Certificate 19th February 2019

Explanatory note The table below lists the specific measures and describes the means by which each has been implemented.

(i) The flood study is reviewed A detailed topographic survey was undertaken by Williamson before a model study (physical Technical Services Ltd in February 2017 (see Drawing No or computational fluid dynamic) WTS/2864/01) and was revised with additional survey information is undertaken based upon a in June 2017 and May 2018. detailed topographical survey Fairhurst report dated August 2017 and entitled to assess the capacity of the Oakenholt Mill Flint – Hydraulic modelling assessment existing spillway channel… contained the requisite review of the flood study and assessment of the spillway capacity. I determined that a model study was not required in order to assess the capacity of the existing spillway with sufficient accuracy to allow the preferred option to be reliably identified and designed. …and an option study prepared The above report also contained a preliminary options study. to identify appropriate solutions Fairhurst report dated November 2018 entitled for ensuring that the design and Oakenholt Reservoir – Preferred option report safety check floods can be identified appropriate solutions and a further Fairhurst report safely accommodated without dated February 2019 and entitled: impacting on the safety of the dam. Oakenholt Mill Flint – Overflow capacity – Risk-based assessment helped to refine the appropriate option and design standard with respect to the accommodation of the design flood and the safety check flood. The model and option studies I have reviewed the above reports, as a result of which I shall be presented to an All recommend implementation of Option 4A, as defined in the Reservoir Panel Engineer for a November 2018 report and shown on the following drawings further recommendation when dated November 2018: issuing a Section 10(6) 11845/2001 Plan and cross sections Certificate. 11845/2002 Long section I further recommend that these works be completed no later than the end of October 2021, but preferably within two years. (ii) Joints in the spillway be Completed in September 2017 and photographic evidence thereof resealed. provided to the Supervising Engineer in February 2018 confirming the same.

JCA:Oakenholt-s10-6-cert-2019-02-19 19/02/2019 111854 – R04 Oakenholt Reservoir Auxiliary Spillway Hydraulic Design Summary June 2020

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111854 – R04 Oakenholt Reservoir Auxiliary Spillway Hydraulic Design Summary June 2020

Appendix B

Scheme Drawings

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