Author: Brian Devlin, chartered engineer, managing director of CEI Collins Engineers This article is a preview of Brian Devlin's presentation, which is set to take place at 6:30pm on Tuesday, 9 December in 22 Clyde Road (organised by the Structures and Construction Division). 

Before discussing the effects of scour and the means by which they can be averted, it is important to have an understanding of the scour process. Scour occurs when the velocity of the water in the channel is great enough to pick up bottom materials and transport them downstream. The greater the water velocity, the greater the weight of material that can be moved. The total scour that occurs at a bridge can consist of three components: long-term channel degradation, contraction scour and local scour. Degradation is a long-term lowering or scouring of the bed of a waterway due to man-made or natural causes. These long-term changes usually occur over the full width of the river or stream and may extend over long reaches of a waterway. These changes may be due to natural trends of the waterway or may be the result of some man-made modification to the waterway or watershed. For example, urbanisation of the watershed or dredging of an area of the waterway may cause downstream degradation. In general, a bridge engineer will not be able to affect or control factors beyond the locale of a bridge, but the engineer needs to recognise that a bridge that has performed satisfactorily in the past could become endangered by degradation due to causes and changes in conditions outside the immediate vicinity of the bridge. In addition, if degradation has occurred at one bridge, other bridges over the same waterway may also be or become affected. Contraction scour involves the removal of material from the bed or banks of a waterway across all or most of the channel width in the immediate vicinity of the bridge. The most common type of contraction scour is caused by a reduction in the channel cross-section due to substructure elements in the waterway, or by the encroachment of abutments and approach embankments into the natural waterway opening. As a result of the contraction of waterway flow, there is a corresponding increase in flow velocity through the bridge, which then carries channel bottom and bank material away. Besides bridge piers and abutments, debris accumulations and vegetation growth in the channel or floodplain can also cause contraction scour by reducing the available waterway opening. [caption id="attachment_17676" align="aligncenter" width="640"] Figure 1: Scour at cylindrical pier[/caption] Local scour is usually restricted to a small part of the channel adjacent to an individual substructure unit. As water ‘piles up’ on the upstream face of a pier or abutment, downward and horseshoe-shaped vortices form around the bridge foundation element. The action of these vortices removes material from the stream or river bed adjacent to the foundation. If the transport rate of sediment away from the vicinity of the bridge is greater than the transport rate into the area, a scour hole will develop. Scour holes usually are deepest at the upstream nose of a foundation element and extend some distance along the sides. At the downstream end of the element, where the water velocities are generally less, transported material may sometimes be deposited. Factors affecting the depth of local scour are the width of the substructure unit, the velocity of the flow, the angle of the flow relative to the substructure unit, the shape of the unit, and the presence of any debris accumulation. With regard to local scour, the wider the pier, the deeper the scour hole that will generally develop. The effective width of the pier is also increased if the pier is not oriented in line with the waterway flow, or if there is an accumulation of debris at the pier. The degree to which a substructure unit is aerodynamic also greatly influences the vortex velocities formed at the upstream nose of the pier, and in turn, the potential scour depths. For instance, a square nosed pier will have scour depths which are about 10% greater than a round nosed pier and about 20% greater than a bevel nosed pier.

Inspection and monitoring of scour

Severe scour conditions can often be identified with underwater inspections or any of a number of topside monitoring methods. The greatest scouring occurs, however, during periods of maximum flow, when velocities are greatest. As flows diminish and velocities subside, scour holes may fill in partially or in full. Consequently, inspections during periods of lower flow rates may not indicate the true maximum scour depth that has occurred during higher flow velocities. Therefore, the best time to conduct scour field inspections is during the period of maximum flow, but conducting scour depth measurements during periods of high flow, although usually possible, may not always be practical. Water depth soundings are the most basic tool of the scour investigator. In water 5m or less in depth, the channel bottom configuration can typically be determined by sounding with a long graduated pole. Echo-sounders can also be used to take soundings from the bridge superstructure, but when practical, a boat should be used to permit the taking of soundings upstream and downstream of the bridge, as well as around the substructure units. To fully assess scour activity at a bridge, it is common to take soundings around each substructure unit, along the upstream and downstream faces of the bridge, and along lines up to 60 m upstream and downstream of the bridge. During periods of high flows, even in velocities 3m/s or more, it is possible with a trained and experienced crew, an appropriately equipped boat, and the proper sounding equipment to take soundings around a bridge and its substructure units. Care and skill, however, must be used to prevent the sounding vessel from colliding with the foundation elements, and the turbulence of the water may degrade the quality of the sounding data to some degree. Although the best time to assess the maximum depth of scour is during the flood event, this can not always be accomplished because of the difficulty and possible danger involved. As previously indicated, however, the scour hole may refill completely once flows subside and the scour investigation can more easily take place. One method to ascertain the maximum depth of the scour hole that occurred during the peak flows is the use of sub-bottom profiling equipment to evaluate the various layers of soil material present around the substructure element. The in-filled material within the scour depression will often be less densely compacted than the original bed material, and differences between newly placed and in-situ layers of channel bottom can be detected using ground penetrating radar. The applicability of these specialised methods, however, depends on conditions at the bridge site and the nature of the waterway bed material. There is always an element of risk associated with bridges remaining in service during flood events. However, innovative high resolution acoustic imaging technology can reduce both situations where bridges are closed too conservatively, as well as where bridges remain open for use, perhaps unwisely, simply because engineer-divers cannot safely enter the water to determine the foundation’s integrity. Underwater acoustic imaging can enhance condition assessment observations at any time, but its true value is most often demonstrated during flood events. Underwater acoustic imaging technologies have been utilised to supplement diving inspections at a variety of submerged structures over the past two decades, and implemented in emergency situations unsafe for diver investigations. The latest advancements in range and resolution now provide infrastructure owners with a diverse set of tools to ensure the safety of their bridges. Even in the most turbid waters with zero visibility, sonar can provide water depth data and high-quality images. Underwater acoustic images vary in quality, resolution, and dimensional perspective (2-D or 3-D) depending on the particular sonar device.

Multi-beam swath sonar

Multi-beam swath sonar beam arrangement allows detailed mapping of a very thin transverse section with each sonar pulse. Most systems are boat mounted and require forward progress of the boat to advance the position of the send/receive signal. Operating frequencies usually range between 0.7 MHz and 1.8 MHz. Other multi-beam swath systems are set up with extremely low frequencies for sub-bottom profiling applications. Multi-beam sonar systems function similarly to single-beam echo sounders except that they have multiple sonar beams acting simultaneously allowing for much denser data coverage in a shorter period of time. This type of system uses a fanned array of sound beams that typically give near 100% coverage of the seabed or channel bottom. For instance, a typical multi-beam survey may have a fanned array that is capable of a ‘swath width’ of seven times the water depth. This means that if the water is 30m deep, bathymetric data can be obtained up to a swath of 210m wide. [caption id="attachment_17678" align="aligncenter" width="640"] Figure 2: 3-D acoustic image of Winona Bridge, Winona, Minnesota, USA showing the undermined bridge pier with piles exposed[/caption] For vertical imaging applications, the same theory applies but the swath width is dependent upon the distance between the transducer and the pier face. Another form of multi-beam sonar is three-dimensional mechanical scanning sonar, which is essentially a multi-beam sonar unit fitted with a mechanical stepping motor. The sonar needs to remain stationary while performing scans. Stationary deployment is often achieved with a stable pole/tower or a well-anchored tripod. The primary benefit of multi-beam swath sonar is the ability to quickly obtain large quantities of three-dimensional data. Multi-beam swath sonar produces a three-dimensional still image that is often referred to as a point cloud. For bridge inspection applications, the production of three-dimensional data would allow an inspector to document and assess the depth of spalling and foundation undermining. By using multiple or overlapping passes, the sonar operator is able to obtain greater data density and 100% bottom coverage of the area. The primary limitation of multi-beam swath sonar is that the vast quantities of data produced can be time consuming to post-process. Both field operation and data post-processing requires a great deal of training and skill. The multi-beam sonar, as it pertains to bridge scour inspection, should be lowered to the channel bottom on a tripod to allow it to smoothly transition data from the channel to the vertical face of a bridge support. In the hands of a skilled operator, multi-beam swath sonar can yield high quality surveys. It should also be noted that multi-beam systems would be a poor choice for shallow waterways with relatively simple bottom topography. However, in areas with certain environmental characteristics (e.g. deeper water, complex bottom topography, limited visibility, strong currents), multi-beam surveying offers a number of unique benefits.

Sector-scanning sonar

Two-dimensional imaging sonar systems have oblong, fan-shaped beams. They essentially work by recording the full range of returns from the wide dimension of the cone angle and plotting them on a two-dimensional drawing. The sonar unit cannot distinguish which portion of the wide cone angle a return came from, but it can tell if an echo returns from more than one distance. Sector-scanning sonar is the most common 2-D underwater imaging device used during floods. Scanning sonar works similarly to side-scan sonar in that the transducer emits fan-shaped acoustic pulses through the water; however, unlike side-scan sonar, which requires vessel movement to develop an image, scanning sonar works best if the transducer remains stationary while the head is mechanically rotating. [caption id="attachment_17679" align="aligncenter" width="640"] Figure 3: 2-D acoustic image of Charlemont Bridge Pier, Moy, Co. Tyrone, showing the exposed footing[/caption] The acoustic images are recorded in a series of ‘slices’ generated by a ping after each rotation of the transducer. Scanning sonar operating frequencies usually range between 330 kHz and 2.25 MHz, with a common frequency used for channel bottom and structural imaging of 675 kHz (Atherton, 2011). The primary benefit of scanning sonar is the ability to produce detailed images of the channel bottom and vertical components of submerged structures that extend from the channel bottom to the water surface. Scanning sonar can also be used prior to and during diving operations to direct the underwater inspector to potential deficiencies, as well as direct the inspector around potential underwater hazards. Sector scanning of vertical structure surfaces typically does not require geo-referencing, thus simplifying the process. Due to limited range and the need for the sonar head to be located in a stable mounting position, the primary limitation of scanning sonar is that stationary set-ups require greater time to obtain. Additionally, developing highly detailed images using scanning sonar is heavily dependent on sonar positioning and stability.

Conclusions

Underwater acoustic imaging has been deemed extremely valuable during emergency bridge inspections during floods when an engineer-diver could not enter the water. However, there are often challenges to determining the most appropriate equipment for an individual site. With the appropriate equipment, which typically for bridge inspections utilises sector-scanning sonar or some type of multi-beam sonar, quality, high-definition images are often possible. In some circumstances, environmental conditions, material attributes, or geometric configuration may prevent the highest quality of image, although the skill of the operator is often a major factor. Depending on a number of factors, it is generally best to limit acoustic technologies to sector scanning sonar for 2-D images and multi-beam sonar for 3-D images. Also, it is recommended the imaging operator has formalised training in both bridge inspection and sonar imaging. Detailed guidance on underwater imaging, as compared to that for an underwater inspection, is not commonly found in many manuals for bridge evaluations. However, more and more publications are stating that acoustic imaging can be used to some degree when unsafe conditions exist for the engineer-diver. Since there is a variety of sonar equipment with varying degrees of resolution and range, it is important that bridge owners provide performance specifications. With thanks to CEI Collins Engineers Ltd.