Build Resilience

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The Bridge Inspection Procedure

The DMRB new document CS450 covers the inspection and reporting requirements for highway structures on motorways and all-purpose trunk roads. Highway Authorities are obliged to carry out General Inspections every two years and the interval for Principal Inspections is now risk based. The interval for Principal Inspections is normally every six years and structures which are scour susceptible (risk rating 10 to 100) to CS469 cannot have the inspection interval extended. Special Inspections are undertaken when the Engineer has cause for concern over the safety and integrity of the bridge. Special Inspections, such as an underwater examination may identify areas of deep scour or the undermining of a bridge support, however, it is only a condition report at any one time. To improve the resilience of our bridges we need to carry out risk assessments to forecast the location and depth of scour, the parts of the structure at risk and identify any necessary mitigation measures.

The new code CS469 “Management of Scour and other hydraulic actions on highway structures” assessment process does exactly this and proposes a theoretical scour depth that would occur from a 1:200 year return period storm. The Level 2 scour risk assessment uses the geometry of the bridge, the bed material and the hydrological characteristics of the river. The code also includes a section on the effect of debris build up and the stability and lateral movement of the stream or river channel

The outcome is a scour risk rating and scour risk score for the bridge;

Low scour risk rating at score of 10 or less

Medium scour risk rating at a score of 40 or 60

High scour risk rating at a score of 80 or 100.

Resilience

Resilience can be built in by any one or all the following hard engineering works

1. Carry out bridge and river maintenance by removing mature tree growth and by removing fallen trees upstream of the bridge.
2. Carry out de-silting of the waterway and possibly re-alignment of the river to ensure that all river spans and any useful flood relief openings or arches are clear and open.
3. Install river bed/invert scour protection in the form of either gabion baskets, or concrete bag work. However, in most cases and with the velocities produced by a 1:200year return period storm these two forms of protection do not generally last.
4. Installing a full concrete invert across the river bed with a downstream protective weir.
5. Providing additional waterway area in the form of more flood relief arches, thrust bored pipes or flood routing away from the bridge.

Resilience by Soft Engineering methods by Natural Catchment management

Several authorities are now looking into ways of holding back the flood waters by installing soft engineering features within the catchment such as;

1. On-line Storage.
2. Off-line Storage.
3. Re-Meandering of Watercourses.
4. Re-connection of Watercourses.
5. Leaky Dams (Woody dams, stone dams).
6. Riparian Tree Planting/Buffer Strips.
7. Contour Ploughing.

This names only a few recent proposals for improving catchments and the term “Greening the Grey” was used.

Please refer to Conwy County Borough Council of North Wales paper on “Natural Catchment Management for Wales” July 2018 for more information.

Protecting our Vital Infrastructure.

It is important that as many Highway Authorities are carrying out Principal, General and Special inspections on their bridge stock, that they undertake the next stage by completing scour risk assessments to CS 469. This would highlight the short comings of a bridge to pass the full flood waters from a 0.5% AEP, (1:200 year) return period storm. The only way to ensure the future integrity of the bridge and of the highway it carries is to remove the short comings, as above, build in resilience and hence protect our vital infrastructure.