Upstream face of the road bridge over local watercourse just upstream of the site. The pile of boulders on the right of the photograph is the toe of a
very large accumulation of debris brought down by the stream. Notice the damaged face of the bridge which included eroded concrete and exposed
reinforcement bars. The concrete section supporting the parapet rails had apparently recently been repaired signifying further damage in the past.
There was no corresponding damage to the downstream face of the bridge.
Flood Consequence Assessment,
Brecon Beacons National Park
Farm diversification scheme
Morton-Roberts produced a Flood Consequence Assessment (FCA) to TAN15 to support a planning application for the construction of a new building near
the confluence of a local watercourse and a main river within the Brecon Beacons National Park. The building, part of a farm diversification scheme,
was to be located close to the local watercourse. This stream was very steeply graded with a large bed load and flowed over an alluvial fan which meant
that if water over-topped the banks it was likely to flow away from the watercourse.
Hydrological Assessment and River Modelling Work
Morton-Roberts carried out a hydrological assessment of the flows in both watercourses using
Flood Estimation Handbook (FEH)
methods. We also constructed a model of the watercourses using
ISIS 1D river modelling software
to understand the flooding mechanisms in the area. A
2D Flood Modelling
exercise was also carried out to predict the extent of
the overland flow resulting from the flooding from the local watercourse. The upstream face of a road bridge over the local watercourse just upstream
of the site showed damage likely to have been a result of blockages caused by the large bed load of cobbles and boulders carried by the watercourse.
For the 2D modelling work it was assumed that this bridge was blocked during the course of the storm.
The modelling work allowed us to predict the depths and velocities of the flood water at the site for a 1 in 100 year return period (1% AEP) plus climate change event.
In addition we were also able to determine a safe evacuation route from the site. The modelling work also confirm that the proposed building produced
no detrimental effects for a 1 in 1,000 year return period (0.1% AEP) event.
As a result of the FCA the new building gained planning permission.
Further details of Flood Consequence Assessments
For further details of our services within the area of Flood Consequence Assessments and Flood Risk Assessments please see the following pages:
Flood Consequence Assessments (for use in Wales).
Flood Risk Assessments (for use in England).