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Photograph - River channel

River Pelenna modelling study

Proposed residential development

This study was carried out to assess the flood risk to a proposed development of three detached houses on the site of a former HGV garage situated close to the River Pelenna to the east of Neath in south Wales. The study involved the use of Flood Estimation Handbook and Flood Modeller Pro software and was carried out in the context of TAN15, the Welsh Government's document governing new development and flood risk.


The length of modelled river was fairly steeply graded with a bed comprising sandstone cobbles and boulders. Approximately 75m downstream of the site itself the river was crossed by a three span reinforced concrete road bridge and just downstream of this, a small tributary flowed into the river from the west.

FEH Study

A Flood Estimation Handbook (FEH) study covering both the River Pelenna and the tributary catchments was carried out to produce input hydrographs for the river modelling work. Both the statistical and ReFH methods were used to produce hydrographs of various return periods up to once in 1,000 years. The work followed both Natural Resources Wales' Good Practice Guidance and the Environment Agency's 'Flood Estimation Guidelines' and utilised the approach sometimes referred to as the 'ratio method'.

1D − 2D linked model

A 1D − 2D linked hydraulic model of the River Pelenna in the region of the proposed development was constructed using Flood Modeller Pro software. The model was based on a topographical survey of the river channel together with a detail survey of the bridge, both of which were carried out as part of the study.

The models showed that during extreme floods the river downstream of the site would overflow through an urban area before rejoining the main river channel some 150m further downstream. The restriction on the flood water caused by the houses and boundary walls was represented within the 2D model by topographical feature shapefiles.

Further details of our river modelling capabilities

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

Flood Estimation Handbook (FEH)

Hydraulic river modelling

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

Photograph: River channel looking upstream, River Pelenna, near Neath.