Dolwen Reservoir (2021)

Hybrid piling/excavation solution through dam core viewed from the left side of the spillway - Courtesy of MMB

Dolwen Reservoir is a Category A Large Raised Reservoir as defined under the Reservoirs Act 1975. It is located on a tributary of the River Elwy, 0.7km upstream and north of Plas Uchaf Reservoir, near Abergele, Conwy, Wales. It was constructed in 1905 and has a capacity of approximately 288,185m³. Dŵr Cymru Welsh Water commissioned principal designer Arup and principal contractor MMB under the Welsh Water Capital Alliance framework to carry out the re-construction of the existing spillway to ensure commitments were met in-line with the Reservoir Safety Act 1975. Section 10(6) MITIOS Measures.

Background and project driver

Dolwen Reservoir was inspected in January 2013 under Section 10(3) of the Reservoirs Act 1975 by the Inspecting Engineer. The Section 10(3) report included the following recommendation for ‘measures to be taken in the interest of safety’ which are enforceable and mandatory:

“A a topographical survey of the embankment crest together with hydrological and hydraulic studies and investigations are carried out using the latest methodology to determine the adequacy of the spillway to pass the design flood event and safety check flood (PMF) and current freeboard provision on the earth embankment dam and appropriate measures implemented as required.  The structural adequacy of the free-standing right-hand wall at the toe of the dam also needs to be assessed when passing the design flood event.”

Spillway Options Report

A flood study, conducted by Arup, concluded that the existing spillway was incapable of passing a PMF storm without the dam being overtopped. A Spillway Options Report, also produced by Arup, confirmed with a more detailed analysis the lack of capacity in the spillway it also included a high level structural assessment of the existing spillway chute which concluded the spillway was in fair condition, with evidence of spalling and cracking to the spillway invert. This presented a number of options to bring the reservoir into compliance with the standard industry guidance, Floods and Reservoirs Safety (4th Edition), and concluded that the preferred option is to ‘rebuild spillway on existing alignment’, selected using a multiple criteria analysis.

Concrete pour to stilling basin walls – Courtesy of MMB

Furthermore, in 1998 leakage through the dam was observed and subsequent investigations showed leakage through the raised concrete extension to the puddle clay core. In 2000 a horizontal bellmouth pipe was fitted onto the inlet to the top draw-off pipe to lower the water level to below the concrete core wall to reduce leakage. This reduced the storage capacity of Dolwen Reservoir to around 221,000m³.

The options report concluded that the spillway weir crest level should be lowered to 167.84mAOD to formalise the lowered top water level. The options report also proposed a 6 or 7m wide spillway, tapering to 5.5m wide. The outline design was developed based on this lowered weir crest, with a consistent 6m wide spillway following the horizontal alignment of the original spillway.

The scheme

The scope of work was as follows:

• Remove the existing masonry and concrete spillway.
• Lay cross drainage beneath proposed spillway structure.
• Install mass anti-flotation concrete base beneath main spillway structure.
• Installation of main RC spillway.
• Installation of back of spillway wall drainage system.
• Removal, preservation, and reinstallation of existing puddle clay core.
• Reinstatement and landscaping.

(left) Temporary works installed, excavation of the existing upper chute spillway and (right) completion of stilling basin walls and commencement of lower chute – Courtesy of MMB

Civil works to spillway

Works on the spillway included:

• Installation of excavation temporary works to maintain embankment stability.
• Removal of all masonry and concrete of existing spillway as per the agreed construction sequence agreed with the ARPE (All Reservoirs Panel Engineer).
• Installation of cross drainage. This consisted of a 225mm diameter perforated HDPE pipe installed in class 6Z filter material and surrounded by a geotextile membrane.
• Casting of mass concrete slabs with U-bar dowels set into the mass concrete for joint between mass concrete and spillway base.
• Construction of RC concrete main spillway. This includes the movement joints, spillway slab, spillway walls, down-stands, sloped key walls through the core, reinforced heels, chute blocks, baffle blocks and weir baffle.
• Installation of new back of wall drainage. 225mm diameter perforated HDPE pipe installed in class 6Z filter material and surrounded with 900mm inspection chambers on the right-hand side of the spillway, and 1200mm manholes on the left hand side at each intersection of cross drainage and back of wall drainage.
• A 750mm wide type 6Z filter material drain was installed behind the right and left walls respectively from the clay core reinstatement joint down to the corner of the stilling basin walls with Type 1 geotextile separation/ filtration layer surround.
• Reinstatement of the existing clay core to a minimum elevation of 170.61mAOD with a minimum overlap length of 1.0m either side of the concrete core wall, and with minimum thickness of 0.75m. 1.0m of class 6Z filter material each side. Installation and compaction was successfully completed by hand under the supervision of the ARPE and their representative.
• Backfill temporary excavations.
• Works were then complete with the installation of hand railing to the top of the spillway walls and new boundary fencing/hedge planting.

Looking upstream at the stilling basin/outlet of the newly constructed spillway – Courtesy of MMB

Environmental consideration

As part of the enabling works a badger set had to be re-located prior to any works taking place on site. Hedgerows were removed on a temporary basis with new hedges to be planted upon completion.

Dolwen Reservoir Spillway: Supply chain – key participants

• Client: Dŵr Cymru Welsh Water
• Principal contractor & temporary works design: Mott MacDonald Bentley
• Principal designer: ARUP
• Reinforced concrete build: Doohans Civil Engineering
• Temporary works: Sheet Piling (UK) Ltd

Site challenges

Due to concerns raised on the condition of the existing diaphragm wall through the dam crest, the designers, along with the assigned ARPE decided on lowering the top water level by 1m. This, coupled with significantly higher forebay walls and 1.5m depths of anti-flotation mass concrete, resulted in the required excavation depth of approximately 7.5m through the existing dam crest and puddle clay core. Due to the severe gradient of the adjacent hillside this depth of excavation would have a high risk of ground stability issues and as such would likely require temporary and potentially permanent reinforcement.

As temporary works designers on the project, MMB worked collaboratively with the site operational team, ARUP’s designers and the ARPE, to devise a hybrid solution to use a conventional sheet piling installation through the dam core and a significantly reduced battered excavation design was developed. The team worked tirelessly to ensure the safety of both the dam itself and the workforce were at the forefront of their minds throughout the temporary works design development whilst managing water levels.

Management risks due to Storm Dennis (1:100 year flood event) and overcoming the global Covid-19 pandemic, which presented the team multiple programme and logistical related obstacles in the form of material supplier issues and staff/labour attendance inconsistencies.

The completed spillway with landscaping on-going – Courtesy of MMB

Project status

As of December 2020, the spillway construction was complete and a preliminary completion certificate was issued by the All Reservoir Panel Engineer under the Dams and Reservoirs Act 1975 with further monitoring of the dam to be carried out by Welsh Water’s Dam Safety Team. The spillway is now in full operation and is able to spill.