Photograph: Lower culvert Photograph: Downstream face of lower culvert Photograph: Typical section of Ffrwd Wyllt watercourse

Top: Photograph through the lower culvert (nominal dimensions 1.82m high by 2.52m wide) looking downstream.

Centre: Downstream face of lower culvert.

Bottom: Typical section of the Ffrwd Wyllt watercourse.

1D river modelling study - Ffrwd Wyllt watercourse, Bryn, Port Talbot

HEC-RAS software

Bryn is a small village about 6km to the north-east of Port Talbot in south Wales. Morton-Roberts carried out a river modelling study on the Ffrwd Wyllt at Bryn to investigate if three proposed residential properties would be acceptable given the criteria set out in TAN15, the Welsh Government's guidance on developments and flood risk. The project as a whole incorporated a Flood Estimation Handbook (FEH) study of the catchment and 1D river modelling work using HEC-RAS software.

The Ffrwd Wyllt at Bryn is a steeply graded main river with a sandstone boulder and cobble bed which drains a small upland catchment and so probably justifies the description, 'wild stream', which is the English translation of its name.

FEH Study

TAN15 requires the consideration of 1 in 1,000 year return period floods so the FEH study followed the guidance for estimating long return period floods given in the Environment Agency's 'Flood Estimation Guidelines'. This approach is sometimes referred to as the 'ratio method' and involved utilising both the statistical and ReFH methods to estimate the peak flows in both the Ffrwd Wyllt and a tributary watercourse.

River modelling

The HEC-RAS 1D steady state model represented the Ffrwd Wyllt over a distance of some 400m and included the two culverts on the section of the watercourse. The culverts were found to represent significant restrictions to the flow and were predicted to be overtopped for 1 in 25 year return period floods. Apparently the highway authority installed 'armco' culverts within the arches of the original bridges some years ago which probably explains the problem.

Once the model of the present day watercourse was complete, a copy was modified to represent the future situation with the proposed development in place. This was then used to investigate the effect of the proposed houses on the flooding in the area. The affect of a blockage at the lower culvert was also investigated.


The model allowed an assessment to be made as to the acceptability of the proposed development with respect to the requirements set out in TAN15.

Further details of our services are given within the following pages:

Flood Estimation Handbook (FEH)
1D hydraulic river modelling
2D flood modelling

Alternatively please contact us for further details of our river modelling capabilities.


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