Flood plan Photograph: Bridge over river Photograph: River adjacent to deveopment site

Top: Flood plan produced as part of the study with the river flow being from the top to the bottom of the plan. The dark blue represents the 1 in 100 year return period plus climate change flood extent while the light blue represents the 1 in 1,000 year flood extent.

Centre: River channel adjacent to the development site.

Bottom: River bridge downstream of the development site.

Linked 1D − 2D river model,
River Pelenna, near Neath

Proposed housing development on brown field site

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. This was done in the context of TAN15, the Welsh Government's document governing new development and flood risk.

The river at this point 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 for floods with various return periods up to once in 1,000 years. This was done using both the statistical and ReFH methods. The modelling followed both Natural Resources Wales' Good Practice Guidance, GPG102, 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 topograhical 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.

Located on border of zones B and C2

The model showed that two of the three proposed properties would be located on the boundary of the 1 in 1,000 year flood extent with the third being wholly above the level of the same flood. Two of the proposed properties would therefore be on the border of flood zones B and C2 but with the third property being entirely within zone B.

Proposed Works

It was proposed to set the floor slab level of the proposed properties 600mm above the predicted 1 in 1,000 year flood level. It was also proposed to raise the ground level around the properties above this flood level to provide a safe means to evacuate the properties in the event of a flood. The ground levels between the properties and the river would be lowered slightly to ensure that there was no net loss of flood storage volume and so no detriment elsewhere.

The proposed works at the site were modelled to confirm that the new development would comply with the requirements of TAN15.

Further details of our river modelling capabilities

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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