The project involved the survey of four bridges, all located on the River Wear close to the environs of Durham City:
- Sunderland Bridge
- Croxdale Bridge
- Old Elvet Bridge
- Framwelgate Bridge
The purpose was to gather information on the existing scour located around the bridge piers, foresee developments in order to assess the present situation and act to ensure the stability of the bridges.
As a result of the increasing number of flooding incidents experienced in the country, it is crucial to understand the hydrodynamics around bridges and other structures so that the controlling bodies can plan and undertake repair and maintenance works. In this context, Durham’s County Council’s requirement was to gather topographic, bathymetric and geophysical data of the four bridges so that informed decisions could be made as to the stability of the bridges with regard to scour. Data captured would be stored and compared to previous and future information to have a clear understanding of the bridges’ behaviour throughout the years, monitor the progress of potential scours and determine the impact that external forces are having on the scour.
A multiple-survey program was carried out in order to produce a model of the bridge structures and surrounding riverbeds, including:
- Hydrographic and Topographic Survey techniques to determine riverbed levels and the main physical features, including plan and elevation drawings of each bridge structure above the waterline.
- High Resolution Sonar Scanning to capture geo-referenced imagery of the below waterline bridge structures. Once the water depth exceeded approximately 1 meter, the aim was then to define the extent of any scour hole, provide profile sections and monitor the integrity of the pillar footings, rip rap or rock armour.
- Geophysical survey using Ground Penetrating Radar (GPR) and 2D Electrical Imaging to provide a detailed plan of each bridge footing and historical scouring around the structures.
The surveying was carried out during the summer of 2010 under favourable weather conditions. As a number of survey techniques were used to capture data, logistics were arranged so that instrumentation required for a particular data capture was undertaken on various site visits as not all equipment was carried during the one mobilisation. In general all field surveying went favourably with no problems or delays encountered other than a rise in river levels due to a heavy overnight rain, which made data capture hazardous and therefore we demobilised until water levels receded.
- For each bridge the survey extended over a larger area, including top of slope on both banks, river inverts and full elevations of both bridge faces.
- For each bridge the integrity of the bridge structures – above and below the waterline – and the surrounding areas was investigated.
The use of cost-effective and non-intrusive Geophysical techniques allowed investigate anomalies under the waterline, map the sub-surface and diagnose soil types and underground structures.
- The use of High Resolution Sonar Scanning techniques allowed us investigate the pier integrity and scouring for each bridge to a deeper level. This then allowed us determine whether the riverbeds were sloping or flat, the difference in the material used for construction and construction phases, the presence and location of possible debris, the extent of scour holes, the integrity of the exposed footing and of the whole underwater structure. HR Sonar Scanning is a more innovative and cost-effective non-intrusive method compared to divers or surveyors entering the water.