Added value:
Following relentless rainfall over a prolonged period, England and Wales suffered the wettest autumn since 1727. The month of October saw rainfall of almost 180(mm) more than double the average for October. The Environment Agency estimated the event was on the same scale as the great floods of 1947. In York, the river Ouse rose almost 5.5(m) above its river defences and over 3000 residents were evacuated from their homes. Halcrow were commissioned to map the extent of flooding throughout these regions as quickly as possible and at very short notice in order to capture accurate data in highly sensitive areas.

Innovation:
The procedures adopted were to mobilise aircrews and equipment to capture Aerial Photography of the River Aire, Ouse, Medway and Uck, some 350km² of photo mapping. With previously established flight planning in place, a specialist air survey consultant was able to mobilise at first light to undertake the photo coverage of the swollen rivers and surrounding flood plains.

To provide maximum mapping benefit it was captured with a 60% forward overlap and 25% lateral overlap. The scale of the exposed photography was 1:3,000 and 1:5,000, at very low altitude 1,500’ and 2,000’ respectively. The specified scale of photography provides a high quality product solution suitable for subsequent map production with high order positional tolerance. The resulting photographic imagery has subsequently been used in the generation of flood envelopes depicting the limits of the flood event. Mapping has also included the flood debris lines where they exceed the photographed floodwaters. The flood envelope was mapped as two dimensional (2D) and three dimensional (3D) polylines superimposed on Ordnance Survey 1:10,000, or 1:2500 digital mapping. By incorporating various densities of ground control (xyz) using Global Positioning (GPS) equipment we generated a 3D polyline defining the absolute altitude of the envelope perimeter at the time of flying. The resultant flood map production was enhanced further by the inclusion of specific flood track marks surveyed by the ground teams, enhanced by digital photography.

Team work:
The aerial photography data capture was sub contracted to Blom/Simmons Mapping who have a number of planes to conduct this type of survey. Halcrow staff throughout the UK had significant involvement in a range of activities to assist the Environment Agency during this period of crisis. The survey teams were the most prolific, with staff deployed throughout the worst affected areas of Yorkshire, Sussex, Kent and the Home Counties. Teams worked relentlessly during the period between 31October 2000 and mid December. Both field teams and mapping personnel embraced cutting edge technology, introduced innovation and developed state of the art methodologies to achieve the client specification in a timely and cost effective manner. The benefits of the data set compilation are huge to the Environment Agency and to insurance institutions, local authorities and homeowners.