When assessing flood risk, even a few cm height difference can make the difference between water entering a home and causing damage or staying safely outside! In order to attain that accuracy special surveys are needed.
The height of the ground in the St Denys area has been determined by a spot survey along the length of Priory Road and by laser ranging (“Lidar”) over the whole area from an aircraft. Heights have been determined for positions near homes which are not on Priory Road using the Lidar survey, after being calibrated using the spot survey.
Important: The resulting spot height values refer to the land around properties or, for roads the lowest point, that is the rain water gutter next to the kerb. The height of the ground floor level in a property will be significantly higher than the land outside the property – this must be taken into account when assessing flood risk.
Index map for residential roads at risk of flooding
The image shows selected land spot heights superimposed on aerial photography of the St Denys part of Southampton (image courtesy Southampton City Council). A number of residential areas have been highlighted: the Belsize Flood Resilience Project area (near Priory Road Hard), Janaway Gardens, Pettinger Gardens, Priory Avenue, and Saltmead. In each of these areas there are low lying sections of road, pathways, or open ground, which may flood during a very high tide. Clicking on each area will open a large scale map of that area in a new window. The maps show house numbers and nearby spot heights.
Inspection of the larger scale maps shows that the road at most risk of direct flooding from the river is the part of Priory Road near the Public Hard where there is a history of flood events. For Janaway Gardens only a small area of the western most part of the road (near the Boat House) is at significant risk. The area is used for turning and/or car parking. For Pettinger Gardens the northern most part of the road is lowest. Here, as at Janaway Gardens, the grass area bordering the river does flood during high tides but the houses are raised above that level. The middle section of Priory Avenue is lower than that nearer the river suggesting that flooding is more likely to be caused by water emerging from the rain water gullies in the road – which might happen if a flap valve opening onto the river does not close properly. Both the Priory Road and the Kent Road railway bridges have dips in the road that might similarly flood. The area near Saltmead around the Kent Road railway bridge is particularly low lying but isolated from the river by higher ground.
Low land road heights don’t mean a property will flood!
Modern buildings have been built at a level higher than the surrounding ground. For example, the top left photograph shows the Quay2000 apartment block which has been built on a grass plinth which is substantially higher than the surrounding walkway. Even in older houses the ground floor will be perhaps 0.3m to 0.5m higher than the surrounding ground level. That extra amount must be added to the land height values before deciding whether a property is at risk of flooding.