The picture shows the topography along the west bank of the River Itchen determined from an aircraft using laser ranging (“Lidar”) and GPS positioning. While the land rises steeply inland towards Highfield, St Denys and the West Itchen area lies on the flood plain in the river valley. Walking around St Denys the land seems fairly flat. However, because any tidal flooding will be relatively shallow, even a 10cm height change can make the difference between a property flooding and staying dry.
This section presents land height maps based both on the combined surveys which will enable residents to determine whether their house is at risk. For roads the height values given are for the rain gutter next to the kerb, for other areas it is the general height of the land. The ground floor of houses will normally be significantly above these heights; this must be taken into account in estimating flood risk for a particular property.
What really matters is how high the land is relative to high water in the River Itchen. This is illustrated by discussing examples of high water events on the River Itchen, including:
- an Extra-high tide in St Denys: Photographs, taken on 14th December 2012, showing what happens in St Denys during one of the largest tides of the year – this one was 4.9m and caused water to start flooding through the rain water drains in Priory Road.
- Analysis of a Flood Event: Understanding why the river level on 10th March 2008 was amongst the highest yet recorded can help us predict future high water events.
- Flooding in Riverside Park : During the high water levels on 10th March 2008 there was flooding of the roads near Priory Road Hard and at Woodmill. This shows the scene in Riverside Park which is part of the river flood plain.
- Is Sea Level Rising?: changes in sea level discussed from a St Denys perspective by a local resident who is a retired oceanographer.
Technical details also presented in this section include the accuracy of the land height data and the allowance which must be made for the slope of the river surface between St Denys and the tide gauge at Dock Head.