High Frequency Risk Reduction: Rock View (2022)

Detention tank shaft construction: (left) jacking ring and (right) roof beams installation - Courtesy of United Utilities

In the PR19 agreement, water companies were encouraged to provide resolution to long-standing flooding problems being experienced by customers – but allowed a less prescriptive approach on the actual projects defined in the Water Company’s Price Review submission. This new process allowed the water companies a degree of discretion on the number and frequency of flooding areas to be addressed in the AMP7 period and provided an incentive in resolving flooded areas based on when in the AMP they were delivered. This article provides a technical and engineering focussed precis of one of United Utilities’ early start ‘High Frequency Risk Reduction’ (HFRR) projects to reduce the flooding risk of properties in Rock View, Melling, in Liverpool, and the factors that were considered in delivering this fast-tracked project.

Background

The overall programme of flood alleviation projects to be delivered by United Utilities during AMP7 is entitled ‘High Frequency Risk Reduction’ (HFRR). The purpose of the programme is to resolve long standing (decades in many cases) issues experienced by residents that previously had been overlooked or not deemed sufficiently high enough priority to be included in previous flooding investment programmes.

The HFRR selection process was based on several factors including frequency of flooding experienced, number of properties affected and whether the flooding was internal or external in nature. It is worthy of note that previous flood impact assessments had often resulted in reduced priority being given to the resolution of external flooding events, whereas the HFRR assessment output generally concluded these events to be almost as traumatic as internal flooding especially when the frequency was high.

Rock View flooding

Rock View is in the village of Waddicar in the parish of Melling within the Metropolitan Borough of Sefton on Merseyside. The village has a population of around 3,500 and is located between the M57 and M58 motorways and close to the larger conurbation of Kirkby. Although an urban area, the houses of Rock View back onto rural farmland, with the houses constructed around 20-30 years ago.

Detention tank shaft construction: (left) excavation and under-reaming for base slab and (right) dowelling of rings into concrete collar – Courtesy of United Utilities

Project need

Flooding from the sewer network has been recorded for many years but due to other higher priority cases and needs elsewhere, implementation of a solution to address the deficiency in the network had been deferred. Records showed that 15 properties experienced internal and/or external flooding approximately twice each year on average. The HFRR assessment identified the project need and the urgency of flood alleviation measures required based primarily on the frequency of reported events.

Solution development

Extensive hydraulic modelling was undertaken initially by United Utilities Ltd and its design partners and then by its AMP7 Capital Delivery Partner, Advance-plus (an MWH Treatment/Murphy Group joint venture), to understand the root cause of the flooding and subsequently identify and develop potential solutions to resolve the issues. The options were hydraulically modelled and considered at the preliminary optioneering phase, namely:

1. Option A: Upsize of sewers between flooded area and proposed detention/pumped storage tank, involving approximately 135m of new 450mm diameter sewer and 1,900m³ storage tank, to provide 1-in-30 year protection from flooding.
2. Option B: As ‘A’ but 1,750m³ of storage to provide 1-in-20 year protection.
3. Option C: As ‘A’ but 1,400m³ of storage to provide 1-in-10 year protection.
4. Option D: As ‘A’ but 1,100m³ of storage to provide 1-in-5 year protection.
5. Option E: Surface water separation of the largely combined existing network.

However, from topographical survey information it was found that the levels of the proposed new manholes would be lower than the lowest flooding location in Rock View, which directed the design to only consider protection of 30 years, or new flooding would be introduced at the new works site, namely Option A above.

Analysis of the existing sewers showed that extensive separation of the network would be needed to a significant distance downstream of the flooding area and there were no substantial (sub) catchments identified for SuDS-type beneficial outputs. Hence, Option E was discounted from further assessment.

Detention tank shaft construction – roof beams installed – Courtesy of United Utilities

Once the level of protection had been decided the detailed design development proceeded and by looking at options that disconnected the Rock View sewer catchment from the main sewer network and link this directly to the new storage tank it was possible to reduce the scope of works required significantly, thereby achieving efficiency savings.

The final recommended solution required the following upgrades to the network:

• Pumping station/detention tank of 600m³.
• New sewers to transfer the Rock View catchment to the detention tank of 135m of 300mm diameter.
• High-level overflow between the Rock View sewer and the main Waddicar Lane catchment.
• 100 metres of 100mm diameter rising main from the pumping station back to the existing Waddicar Lane catchment.
• Duty/standby pump capacity of 8.5 to 13.5 l/s (depending on the level of storage in the detention tank) – which is below the maximum figure of 15 l/s that could be accommodated in the downstream sewer network.

It was calculated that this arrangement would provide approximately 135 hours of storage (at 3 x dry weather flow), which far exceeded the Asset Design Standard of 4 hours of storage where an emergency overflow to watercourse (or elsewhere) could not be provided, as was the situation in this instance.

The location of the storage tank/pumping station proved to be a complex and at time controversial decision. Due to the topography and existing layout of the area, only two locations were practical, namely, in the agricultural land to the northwest of Rock View and within a green landscaped area in a relatively new housing estate that is used for recreation by residents and children of the area.

Unfortunately access for a new sewer to the agricultural land was heavily restricted, i.e., it would need to pass alongside/between two houses, this coupled with similar restrictions for site traffic during construction largely ruled out this location. Hence arrangements were developed to site the tank in the landscaped area.

Detention tank shaft construction – M&E installation – Courtesy of United Utilities

Rock View HFRR Project: Supply chain – key participants

• Delivery contractor: Advance-plus JV (MWH Treatment/J Murphy & Sons)
• Shaft construction: Active Tunnelling Ltd
• Electrical installation & MCC: Lloyd Morris Electrical Ltd
• Mechanical installation: STAL (UK) Ltd
• Pumps: KSB Pumps
• Precast concrete shaft rings: FP McCann
• Precast concrete beams & cover slab: Macrete (Ireland) Ltd

Detention tank & sewer construction

The sewer construction was undertaken using traditional open-cut trenching techniques. However, poor ground and wet-weather conditions influenced progress and resulted in delays on the TTRO (Temporary Traffic Regulation Order) within Waddicar Lane. The original cl32 programme indicated 44 days to install the sewer in this main highway but due to the above issues along with working around existing services, the actual work was extended to 118 days.

Waddicar Lane is a main highway through the borough, and although the delay caused issues for the Highway Authority (Sefton MBC), they understood the issues that were presented to the construction team and granted the required extensions to the TTRO. However, it was made clear that any further extensions would not be forthcoming and lane penalties would have been levied. Thankfully the work was completed without this being incurred.

The pumping station/detention tank structure to provide the 600m³ of storage was provided via a 12.5m diameter caisson shaft construction, 9m deep (to top of the base slab). The shaft was constructed by Active Tunnelling Ltd. Work commenced in late-August 2020 and took 8 weeks to complete the caisson element. To provide the necessary stabilisation against flotation the shaft ring was supplemented by a 1.35m x 1.35m concrete collar around the circumference of the top ring, which was subsequently dowelled into the shaft with 90 (No.) 32mm diameter stainless steel bars, 700mm into the shaft equally spaced around the shaft.

Detention tank shaft construction – butterfly benching installed – Courtesy of United Utilities

Although the works were generally built to United Utilities asset standards, there were two instances where Asset Standard Deviation (ASD) requests had to be submitted – these were all subsequently accepted by the United Utilities knowledge team. The ASDs were:

• Use of smaller diameter chamber for one of the new manholes, namely 1050mm diameter in place of the standard 12mm diameter. This was required to install the new sewer alongside existing large diameter surface water sewer within the housing estate road, thereby avoid unnecessary and expensive additional diversion costs.
• Reduction in the diameter of the rising main riser from the pumping station. The normal Asset Standard is for a 100mm riser and flow velocity at normal flow of 1.8m/s to reduce the risk of blockages in the rising main. The use of a 100mm diameter riser at the design flow of 10l/s was calculated at 1.27m/s, hence consideration was given to reducing the riser to 80mm diameter, giving a velocity of 1.98m/s. This was accepted largely based on the consideration that large items that could enter the pumping station wet-well were unlikely or the residential housing estate, and hence blockage risk would be low.

Detention tank shaft construction – roof slab installation – Courtesy of United Utilities

As mentioned previously, the siting of the detention tank on green recreational land within the housing estate was not received with universal acceptance. This was heightened since the works were resolving an issue in another part of the village some 200-300m away. As a result, customer liaison had to be particularly vigilant. Both United Utilities and the contractor appointed a dedicated representative to liaise and communicate with residents and other stakeholders. Whilst the loss of the green area for a full summer was one of the main concerns, the overriding issue was how the area would be left once the works were completed.

The initial proposal included a new lay-by on the edge of the site to facilitate maintenance vehicles to service the asset. This would have taken up approximately 165m² of the site, or over 10% of the main field. Through negotiation with United Utilities Operations, it was agreed that this could be deleted from the scope and alternative processes could be used for the relatively few times each year that maintenance would be needed.

However, whilst none of the detention tank would be above ground level (other than the kiosk and pressure relief column), the United Utilities asset standard stipulates 1m of concrete hardstanding around each access cover/opening. Once this was installed there was strong objections from residents and users of the green area focussed on: health & safety for children etc user the area, aesthetic eye-sore of so much concrete and reduced amenity use. These concerns were considered and following discussion with stakeholders and United Utilities operations, it was agreed that most of the hardstanding could be removed apart from around the main accesses for pump removal and person-access into the tank, due to health and safety concerns and the capability of being able to wash down the area far easier should sewage contamination be deposited on the ground. Whilst this was not accepted by all, it was a compromise that enabled satisfactory conclusion of the works.

(left) Rock View site before construction and (right) Rock View site after construction – Courtesy of United Utilities

Outcome

The Agreement to Operate (ATO) was required on or before 31 March 2021 to achieve the required output delivery date to claim the ODI (Output Delivery Initiative). Although the storage tank and new sewer was put into service under an Interim ATO (i.e. operated by the contractor until full ATO could be achieved) the benefit of the new infrastructure was realised by this date. Full ATO was achieved on 7 June 2021 once all the telemetry and control configuration had been accepted by United Utilities Operations team as end-users.

Since being commissioned there has been no further reported flooding at Rock View.