Finavon: The Interim Edge in a Two-Season Scour Repair

The next flood reached the temporary works first

Transport Scotland's February 2026 assessment describes repeated damage at the A90 northbound bridge over the River South Esk. Storm Babet in October 2023 undermined abutments and eroded the bed. Emergency protection included rock mattresses and bags. January 2026 high flows then displaced bags, enlarged undercutting and cracked the underpass while permanent repairs were programmed for summer [1, p.3].

Emergency protection has to remain serviceable until the permanent works take over. During that interval, the remaining bags, exposed bed and undermined structure form a changing system. Reinstating a displaced unit requires a post-flood record of the support and adjacent protection that remain.

Define the repair from the post-flood surface

The proposed operation combined repositioning bags, replacing damaged or lost units and removing a lodged tree. The assessment specifies 4-tonne and 8-tonne bags or nets, an excavator and crane, existing access routes and approximately one week of daytime work [1, pp.3-4,9].

An effective work instruction would locate the remaining units, bed depressions, abutment exposure and obstructions against the same survey reference. That record lets the temporary-works designer distinguish a bag that can be repositioned from one whose support has been lost. It also gives the lifting planner the actual pickup and placement positions rather than an approximate reach from the road edge.

Bag mass is only one input to the lift. A long working radius can govern crane selection; lifting accessories add load, while the access and setup area govern the available machine position. Reused bags also need a condition and lifting assessment before handling. These checks belong ahead of the plant booking because they can change the crane, setup and delivery sequence together. HSE places the planning, supervision and execution responsibility with the crane user unless a contract-lift arrangement expressly transfers it to the hire company. The procurement instruction should identify which arrangement applies before the plant is ordered ^[3].

The permanent design leaves an interim edge

The permanent scheme, published on 24 June 2026, proposes 22 m of secant-pile wall on each bank and bed reinforcement over 1,722 m2. It divides the north and south works between summer 2026 and 2027 within a June-September in-water window. The concrete mattress is recessed into the bed at its upstream and downstream ends; the temporary edge between phases receives geotextile and rock-net protection [2, pp.4-7].

That intermediate edge has a different duty from the completed mattress. It must remain protected through the interval before the second half is installed, then permit a sound connection to the next phase. The first-phase release should therefore record its level, alignment and edge protection, together with access for inspection and the conditions that trigger repair. The following season's team needs that same record to identify what can remain, what must be removed and where the permanent connection starts.

The cofferdam creates another hydraulic interface while work proceeds. The assessment records modelling of higher velocities in the remaining open channel and provides for temporary scour protection. It assigns temporary-works design to the contractor, with Amey reviewing it as Principal Contractor and Temporary Works Coordinator [2, pp.27-28]. HSE guidance identifies working platforms and permanent works used temporarily as temporary works requiring design and management for foreseeable loads ^[4]. The design review consequently needs the construction-stage flow and support conditions alongside the final bed profile.

Price a usable water-level window

The assessment favours lower water levels and programming away from heavy rain. The river and the northwest access field are both flood-exposed [1, pp.11-12,21]. The nominal week should therefore be translated into shifts in which access, lifting and placement can all proceed safely.

The site team needs agreed water-level and weather triggers, a forecast-review responsibility and enough time to clear personnel and vulnerable plant before access deteriorates. Set those triggers through the site-specific temporary-works and lifting plans. The public assessment supplies no numerical stopping threshold; selecting one requires the access geometry, forecast response time and safe withdrawal route.

For pricing, separate productive shifts from mobilisation, weather waiting and demobilisation. State the assumed availability of the field access and the river conditions needed for each task. An extra dry day can add placement time; a dry day after the access track has flooded may add none. This distinction makes the programme and standby allowance respond to the same physical constraint.

Protect the river while restoring the bed

The February emergency assessment requires cleaned bags, controlled placement and NatureScot agreement before works in the River South Esk Special Area of Conservation. The permanent scheme records a completed HRA with mitigation, fish rescue before dewatering, continued fish passage in the adjacent channel and an ECoW appointed by the contractor independently of Amey [1, pp.16,19-22;2, pp.21-22]. NatureScot's HRA guidance places the assessment with the competent authority ^[5]. Each phase needs to carry the agreed mitigation into its actual isolation, pumping and rewatering sequence.

These requirements affect the work cycle. Provide a clean inspection and preparation area before a bag reaches the lifting position, then control its placement into the bed. If the method changes to reach a newly exposed area or remove an obstruction, review the resulting environmental and structural effects before treating it as a routine extension of the same shift.

After installation, record bag locations, levels, condition and the remaining exposed areas in a form that can be repeated after the next high flow. An inspection trigger should connect a river event to a prompt structural and bed check. That turns the emergency repair into an asset the owner can manage through its intended service interval, with a clear basis for further intervention.