NEWSLETTER

Scour is the natural action of flowing water eroding the material over which it flows. In rivers it is a continuous process, but in those with bed material that is other than natural rock a local equilibrium tends to exist under normal flow conditions between material being eroded and that being deposited. In rising flood conditions local erosion is greater than local deposition, leading to a temporary deepening of the river bed that is usually reversed as the flood abates and deposition becomes greater than erosion. After the flood has passed, an equilibrium state almost identical that which existed before the flood is re-established. In rock bedded rivers the process seldom reaches equilibrium, but, unless the rock is very soft, geological time scales are necessary for the removal of appreciable quantities of material. This kind of scour is often referred to as "general scour", since it affects the whole bed width of a river.

When a bridge is built across a river, the supports (abutments and piers) are quite often placed within the watercourse. The local obstruction to flow that they create increases both the stream velocity and the turbulence within the flowing water. These two effects combine to increase scour locally, particularly immediately alongside the obstruction. This is usually referred to as "constriction scour" and is the main source of problems with bridges and other hydraulic structures.

Clearly, the local removal of support in front of, and, in exceptional cases, underneath, a bridge support is undesirable and can cause the sudden collapse of the structure. It has been suggested that, on average, within the UK bridge scour is responsible for the loss of one bridge each year. Many of you will be aware of the tragic loss of life caused by a bridge collapse in Portugal in early March 2001 that appears to have been triggered by the failure of a pier. Whilst it too early to draw any firm conclusions about the actual cause of this collapse, the most usual reason for a pier failure is scour.

It is possible to undertake theoretical calculations, normally based on laboratory tests, which give an indication of possible scour depth. These are used when new hydraulic structures are being designed, as foundations can then be located deeply enough to resist scour. For existing structures, methods have been developed which can predict possible scour depths and prioritise action to vulnerable structures. However, until remedial action can be undertaken, vulnerable structures remain at risk and this is where SIM should be of assistance.

The creation and subsequent backfilling of scour holes, which usually occurs during serious flood events, makes the actual maximum depth of scour difficult to determine and hence a structure owner has no way of actually knowing how close it came to being fatally damaged. Whilst a number of systems which could assist in measuring scour depths in real time (radar, sonic fathometers, sliding collar devices and tell tales) have been tested, principally by the Federal Highways Administration in the USA, most are not really successful in highly turbulent and sediment laden water. Unfortunately, those that are seem to be unable to record the backfilling of a scour hole and hence a return to safe conditions.

The challenge to SIM experts is to try to devise something that will fulfil this need. It should be robust, reliable, self diagnostic, vandal proof, be capable of being remotely interrogated and be able to transmit both alarms and stand downs to an owner. Can you come up with the right answer?