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5 CHAPTER 2 FINDINGS LITERATURE SEARCH DP 97 was developed by the FHWA to facilitate tech- nology transfer of instrumentation-related research to the Information Sources highway industry. The objective was to promote at the state and local level the use of instrumentation, both existing and The literature search was completed by investigating as new technology, to measure and monitor scour at bridges. many sources of potential information as possible. This in- The demonstration project was presented over 30 times to cluded searching various computer literature databases, review nearly 1,000 people between 1996 and 1999. Attendees often of results from recent international scanning tours, assem- included inspectors and others responsible for field work, bly of relevant information obtained during the presentation including bridge scour measurements sometimes made with of DP 97, and review of various trade publications. unique and innovative equipment developed in house. To A search of various computer literature data bases did not identify and document such experience and knowledge, the identify many references on portable scour monitoring. Data- course evaluation forms from the various courses were bases searched included the following: reviewed and, in selected instances, requests for additional information or documentation were made to given states. · The TRB TRIS, Trade publications, such as Ocean News and Sea Tech- · The National Science Foundation, nology, also have articles and advertisements for specialized · The American Geophysical Union Water Resources hydrographic survey equipment. Although most of this tech- Research, nologically may not be directly applicable to scour measure- · The American Geophysical Union Earth and Space ments, because of size, complexity, or cost of the equipment, Index, these magazines were reviewed for potential ideas and equip- · ASCE Publications, and ment sources. · USGS Publications. Literature Review Findings Keywords used included instrumentation, scour, bridge scour, and scour monitoring. The most common source of As described by Mueller and Landers (4), any portable literature was various bridge scour symposiums held at scour measuring system includes four components: ASCE Water Resource Engineering Conferences from 1991 1. An instrument for making the measurement, to 1998. These papers have been consolidated into a single ref- 2. A system for deploying the instrument(s), erence, "Stream Stability and Scour at Highway Bridges" (3). 3. A method to identify and record the horizontal position The 1999 symposium is available through the conference of the measurement, and proceedings. 4. A data-storage device. Similarly, not much information was available from recent scanning tours. A 1998 Scanning Review of European Prac- The following sections describe the results of the literature tice for Bridge Scour and Stream Instability Countermeasures review for each of these four components. was undertaken primarily to review and document innovative techniques used to mitigate the effects of scour and stream sta- Instrument for Making the Measurement bility. This tour was organized under the auspices of FHWA's International Outreach Program by AASHTO through TRB. The results of the literature review identified a range of Although the focus of the tour was not scour measurement methods for making a scour depth measurement during a or instrumentation, results were reviewed in that context, flood. In general, these methods can be classified as follows: and several contacts made in Europe were revisited, by e-mail and telephone, to identify any potential portable scour instru- · Physical probing, mentation techniques. · Sonar,
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6 · Geophysical, and vide depth information. Multi-beam systems provide a fan- · Other methods. shaped coverage similar to side scan, but output depths rather than images. Additionally, multi-beam systems are typically Physical Probes. Physical probes refer to any type of attached to the surface vessel, rather than being towed. Scan- device that extends the reach of the inspector, the most com- ning sonar works by rotation of the transducer assembly, or mon being sounding poles and sounding weights (1). Sound- sonar "head," emitting a beam while the head moves in an arc. ing poles are long poles used to probe the bottom. Sounding Because the scanning is accomplished by moving the trans- weights, sometimes referred to as lead lines, are typically a ducer, rather than towing, it can be used from a fixed, sta- torpedo-shaped weight suspended by a measurement cable. tionary position. Scanning sonar is often used as a forward- This category of device can be used from the bridge or from looking sonar for navigation, collision avoidance, and target a boat. An engineer diver with a probe bar is another example delineation. of physical probing. Physical probes only collect discrete data Mellor and Fisher (6) reported on the use of side scan sonar (not a continuous profile) and their use can be limited by at tidal bridges in North Carolina. Their study found good depth and velocity (e.g., during flood flow condition) or debris results with 100- and 500-kHz side scan imaging of large and/or ice accumulation. Physical probes are not affected by bridges, mapping significant areas both upstream and down- air entrainment or high sediment loads and can be effective in stream of the bridges that revealed areas of erosion, deposition, fast, shallow water. and paths of sediment transport. However, they expressed con- cern about collection of side scan data during storm events and Sonar. Sonar instruments (also called echo sounders, fath- commented on the laborious effort required to deploy and con- ometers, and acoustic depth sounders) measure the elapsed trol the towfish. time that an acoustic pulse takes to travel from a generating The Sonar Scour VisionTM system was developed by Amer- transducer to the channel bottom and back (1). Sonar is an ican Inland Divers, Inc (AIDI) using a rotating and sweeping acronym for SOund NAvigation and Ranging that was 675-Khz high-resolution sonar (7). The transducer is mounted developed largely during World War II. However, early sonar in a relatively large hydrodynamic submersible, or "fish," that systems were used during World War I to find both sub- creates a downward force adequate to submerge the transducer marines and icebergs and called ASDICs (named for the in velocities exceeding 20 fps (6 m/s). Given the forces created, Antisubmarine Detection Investigation Committee). As the fish must be suspended from a crane or boom truck on the technology has improved in recent years, better methods of bridge. From a single point of survey, the system can survey transmitting and receiving sonar and processing the signal up to 325 ft (100 m) radially. Data collected along the face of the bridge can be merged into a real-time 3-dimensional have developed, including the use of digital signal processing image with a range of 300 ft (90 m) both upstream and down- (DSP). The issues of transducer frequency (typically around stream of the bridge. 200 kHz) and beam width are important considerations in the A low-cost scanning sonar was recently developed by Inter- use of sonar for scour monitoring work. phase Technology, Inc. The Interphase TwinscopeTM uses a Applications of single-beam sonar range from "fish finders" phased array scanning technology to provide an affordable to precision survey-grade hydrographic survey fathometers. navigational sonar with the ability to look far forward and to This type of instrument has been widely used for bridge scour the sides of the vessel. investigations, particularly the use of low-cost fish-finder type sonar instruments. Based on work completed in part for the Geophysical. Surface geophysical instruments are based National Scour Study, Mueller and Landers (4) recommended on wave propagation and reflection measurements. A signal a low-cost echo sounder (preferably a paper chart, and if not, transmitted into the water is reflected back by interfaces a graphical display), and a tethered kneeboard to deploy the between materials with different physical properties. A pri- transducer for use in bridge inspection work. Their work also mary difference between sonar and geophysical techniques included development of a prototype remote-control boat for is that geophysical methods provide subbottom, while sonar unmanned data collection. In 1994, Schall et al. (5) reported can only "see" the water-soil interface and is not able to pen- on the use of a similar device based on a fish finder and several etrate the sediment layer. The main differences between different float designs for deploying the transducer, including a different geophysical techniques are the types of signals pontoon-style float and a foam board. transmitted and the physical property changes that cause Other types of sonar, such as side scan, multi-beam, and reflections. A seismic instrument uses acoustic signals, sim- scanning sonar, are specialized applications of basic sonar ilar to sonar, but at a lower frequency (typically 216 kHz). theory. Side scan sonar transmits a specially shaped acoustic Like sonar, seismic signals can be scattered by air bubbles and beam to either side of the support craft. These applications high sediment concentrations. A ground penetrating radar often deploy the transducer in a towfish, normally positioned (GPR) instrument uses electromagnetic signals (typically behind and below the surface vessel. Although side scan sonar 60300 mHz), and reflections are caused by interfaces between is one of the most accurate systems for imaging large areas materials with different electrical properties. In general, GPR of channel or ocean floor, most side scan systems do not pro- will penetrate resistive materials and not conductive materi-
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7 als. Therefore, it does not work well in dense, moist clays or System for Deploying the Instrument saltwater conditions. A comprehensive investigation on the use of geophysical The system for deploying the scour instrument is a critical techniques in bridge scour was completed by Placzek and component in a successful portable scour measurement sys- Haeni (8). In this investigation, measurements were typically tem. In practical application, particularly under flood flow taken at lower flow conditions and used to locate scour holes conditions, the inability to position the instrument properly is and areas of infilling. Results of this study demonstrated that often the limiting factor in making a good measurement. The geophysical techniques can be successfully used for bridge use of different measurement technologies from different scour investigations. At the time of this investigation the cost deployment platforms can produce a wide variety of alterna- and complexity of the equipment and interpretation of the tive measurement approaches. data appeared to be limiting factors for widespread use and Deployment methods for portable instruments can be application. These issues have moderated as newer, lower- divided into two primary categories: cost GPR devices with computerized data processing capa- bilities have been developed. However, GPR may still be · From the bridge deck and limited by cost and complexity and, often, the need for bore- · From the water surface. hole data and accurate bridge plan information to calibrate and interpret the results properly. Bridge Deck Deployment. Bridge deck deployment fell Horne (9) reported on the results of a GPR investigation of into two categories: non-floating and floating (4). Non-floating bridges in New York. A 300-Mhz antenna floating on the systems generally involved standard USGS stream-gaging water was most successful in penetrating silty, granular ma- equipment and procedures, including the use of various equip- terial in less than 2 meters of water. The availability of soil ment cranes and sounding weights for positioning a sensor in boring and samples provided both ground truth data and, the water. This category could also include devices that use more important, information to calibrate the natural material a probe or arm with the scour measurement device attached to properties, such as density and gradation, with the radar wave the end. Probes or arms include things as simple as an extend- travel time. able pole or rod (such as a painter's pole) to a remotely con- A related investigation used GPR for high flow measure- trolled articulated arm. Hand-held probes or arms are not ments by positioning the antenna above the water, which generally useable at flood flow conditions. avoids the problems of placing instruments in the water at The Minnesota DOT has developed an innovative boom flood flow conditions (10). Results indicated that it was and sounding weight setup using a boom with a standard four- possible to estimate channel cross sectional area to within wheel drive winch for raising and lowering the sounding +/- 20 percent, or better; and, if the radar signal is calibrated weight and a standard rope/wire rope counter for measuring to the specific site and time of measurement, the estimate is the sounding distance. The boom, built by their maintenance within +/- 10 percent. shop at a very reasonable cost, is long enough to allow posi- tioning the weight under various flow and bridge conditions. Other. Based on the results of the literature review, two A prototype articulated arm to position a sonar transducer additional methods were identified that might have applica- was developed under an FHWA research project (11). An tion to portable scour measurements. One alternative is a small onboard computer calculated the position of the transducer underwater camera, either alone or in combination with a sen- based on the angle of the boom and the distance between the sor making a scour measurement. Such cameras are typically boom pivot and transducer. Additionally, the system could used for video inspection of pipe systems, boreholes, water calculate the position of the boom pivot relative to a known wells, and tanks. Other applications include navigation cam- position on the bridge deck. The system was mounted on a eras for remotely operated vehicles (ROVs) working offshore. trailer for transport and could be used on bridge decks from Offshore cameras are often designed for either low-light or 15 to 50 ft (515 m) above the water surface. Field testing dur- long-range viewing. Prices for combined video and lighting ing the 1994 floods in Georgia indicated that a truck-mounted units start at about $3,000. The primary complications for their system would provide better maneuverability and that a sub- use in scour monitoring include the ability to position the cam- mersible head or the ability to raise the boom pivot was nec- era close enough to see the scour hole in highly turbid water essary to allow data collection at bridges with low clearance and to maintain adequate stability in the extremely turbulent (i.e., less than 15 ft or 5 m). conditions that exist in or near the scourhole. Other sources of articulated arm technology include con- Another alternative is the potential application of a green crete pumper trucks and other devices built primarily for the laser as a sensor. Green lasers can operate in water up to depths construction industry, such as those used to handle drywall of 70 ft (21 m); however, because these lasers are not yet devel- sheets and concrete block. The latter type of devices might be oped as a distance-measuring device and are still proprietary to a lower cost, commercially available option to a custom-built the Department of Defense, they are not currently an option. articulated arm. They may become viable in the future with more research and Another non-floating approach that might have application development. in scour monitoring is the use of the towed data acquisition
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8 vehicles used in the offshore industry. These tethered vehicles sels has been in the offshore industry where ROVs have been submerge some distance below the water surface based on a widely used for oceanographic research. Recent improve- servo-control elevator. Smaller, lower cost towed vehicles ments in technology for electrically powered systems lead to are available (about $5,000), with a cargo body in the 20-in. more low-cost ROVs (LCROVs) that have been developed (50-cm)-wide range that can carry several pieces of equip- primarily for use in inspection and observation. LCROV ment. Most are equipped with onboard electronics to measure prices start at about $10,000, and their application to inland and record depth and pitch data. operations, typically in shallow water, has been growing Float-based systems permit measurement beneath the bridge rapidly. Such vehicles have been used for bridge footing and beside the bridge piers. Tethered floats are a low-cost inspections, dam and penstock inspections, boat salvage, approach that has been used with some success during flood intake and outlet inspections, and so forth. It is unknown if any flow conditions. Various float designs have been proposed and of these systems would have enough power to operate in a used to varying degrees for scour measurements, typically to river at flood flow conditions. deploy a sonar transducer. Common designs include foam Investigation of remote control boats for bridge scour data boards, PVC pontoon configurations, spherical floats, water collection was completed by Skinner (12) and Mueller and skis, and kneeboards (1). The size of the float is important Landers (4). Remotely controlled boats have been designed to stability in fast-moving, turbulent water. Mueller and for hydrographic surveying in reservoirs, but such boats typ- Landers (4) suggested using a conventional kneeboard with a ically do not have enough power to perform well in a river, nylon-covered Kevlar braid around the transducer cable to particularly at flood flow. Skinner's design objectives focused provide a single tether with adequate strength and abrasion on a relatively small, portable system that could be readily resistance. They also reported good results under limited flood deployed from the bridge deck. He concluded that a small, flow testing with a PVC style pontoon float. flat-bottom, propeller-driven boat using an electric motor, Floating or non-floating systems can be also be deployed and either a tether or acceptance of a lower design velocity, from a bridge inspection truck, an approach that is particularly might be feasible for bridge scour applications. useful when the bridge is high above the water. For example, Mueller and Landers (4) developed a prototype unmanned bridges that are higher than 50 ft (15 m) above the water typ- boat using a small flat-bottom jon boat and an 8-hp outboard ically are not accessible from the bridge deck without using motor with remote controls. The boat included a wet-well this approach. to deploy instruments through the hull and was successfully tested during six flood events. Water Surface Deployment. Water surface deployment Large-scale remotely controlled power boats might be an typically involves a manned boat; however, because of safety alternative for a remotely controlled, unmanned vessel. These issues related to flood conditions, the use of unmanned vessels boats range from 3 to 5 ft (1 to 1.5 m) in length and use con- has been suggested. The use of manned boats generally requires ventional gas engines, often a modified gas engine from a line adequate clearance under the bridge and nearby launch facili- trimmer, which might have the power to operate in a river. ties. This can be a problem at flood conditions when the river This style of boat was first developed in the early 1980s and stage may approach or submerge the bridge low chord and/or has become quite popular because of its ease of operation and boat ramps may be underwater. Smaller boats may be easier to low maintenance. These boats are commercially available, launch, but safety at high flow conditions may dictate use of a ready-to-run, for under $1,000. Given the size of the hull, the larger boat, further complicating these problems. payload should be fairly large and yet, the boat could still be As an alternative to a full-sized survey boat, some compa- deployed from the bridge deck, and the motor would be power- nies have developed a personal watercraft completely equipped ful enough to work at flood flow conditions. Alternatively, the for hydrographic surveying. These systems typically include a engine and controls could be purchased and used on a differ- survey grade fathometer, a survey grade global positioning ent style hull, such as a small, flat-bottom design. This type of system (GPS), and radio communications to transmit data to a hull might have more payload and stability, at the expense of shore computer. top speed, which is not a concern for this application. When clearance is not an issue, the current and turbulence in the bridge opening may be avoided using one of the tethered floating or nonfloating methods described above from a boat Method to Identify and Record the Horizontal positioned upstream of the bridge. For example, a pontoon or Positions of the Measurement kneeboard float with a sonar transducer could be maneuvered into position from a boat holding position upstream of the In order to evaluate the potential risk associated with a mea- bridge, thereby avoiding the current and turbulence problems sured scour depth, it is necessary to know the location of the at the bridge itself. measurement, particularly relative to the bridge foundation. Because of the safety, launching, and clearance issues, that Location measurements can range from approximate methods, an unmanned or remote control boat might be a viable alter- such as "3 ft (1 m) upstream of Pier 3," to precise locations native has been suggested. A common use of unmanned ves- based on standard land and hydrographic surveying technol-
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9 ogy. A good review of these methods as applied to portable developed by a seven-agency partnership under a DOT ini- scour measurement is provided in the DP-97 class notes (1) tiative to provide DGPS for public safety services. This net- and by Mueller and Landers (4). work will extend throughout the country, not just the coastal The most significant improvement in the approach to loca- areas. In other places, state agencies, sometimes in coopera- tion measurement for portable scour applications may be in tion with universities, have established local DGPS networks. the use of GPS. Over the last 10 years, GPS has revolution- The use of DGPS with lower cost GPS receivers, such as ized the way surveyors perform geodetic and control surveys those developed primarily for precision farming applications, on land. GPS is a positioning system that makes use of a "con- may be a viable alternative for bridge scour monitoring. This stellation" of satellites orbiting the earth. Perhaps one of the approach is capable of sub-meter accuracy. An even lower cost greatest advantages that GPS provides over traditional land- GPS option is the recreational variety designed for sportsmen. based surveying techniques is that line-of-sight between con- The accuracy of these units may limit their application; how- trol points is not necessary. A GPS survey can be completed ever, as technology continues to improve, these could become between control points (even on opposite sides of a mountain) a viable low-cost alternative when sub-meter accuracy is not without having to traverse or even see the other point. GPS necessary. Currently, such units cost less than $500 and typi- also works at night and during inclement weather, which could cally provide position information within 30 to 65 ft (10 to be a real advantage for scour monitoring during flood con- 20 m) accuracy. With DPGS, accuracy of 3 to 16 ft (1 to 5 m) ditions. The most significant disadvantage of GPS is the is possible even with these low-cost GPS receivers. inability to get a measurement in locations where overhead obstructions exist, such as tree canopy and, in the case of scour Data Storage Devices monitoring, bridge decks. The use of GPS technology has similarly changed the way Data storage devices include hydrometeorological data hydrographic surveys are performed. The rapid changes and loggers, laptop computers, and, more recently, Personal Dig- improvements in GPS technology, along with reduced cost, ital Assistants (PDAs). Data loggers provide compact stor- make this technology increasingly viable for portable scour age; however, they are generally not very user-friendly with measurements. For example, various manufacturers have a each company typically having a unique programming lan- survey-grade, waterproof GPS receiver with a built-in antenna, guage and approach. memory, battery, and radio. These models are typically quite In field applications, laptop computers are bulky and need compact and could be used on a boat and possibly even on a to be ruggedized to survive the rain, dirt, and dust of a field floating or non-floating deployment system to track the posi- environment. PDAs may prove useful as their capability and tion of the scour measurement continuously. Data collection user-friendliness continue to improve. The advantage to these under a bridge deck is still restricted, but it is often possible to approaches is the ability to integrate data reduction software, get some distance under the bridge before all satellite cover- such as plotting or topographic mapping programs, to visual- age is lost, particularly on bridges that are relatively high above ize the results, often in real time while the data collection the water. This type of technology is rapidly making most other occurs. traditional hydrographic surveying positioning methods (such as automated range-azimuth systems) obsolete. Questionnaire The accuracy necessary for survey operations requires the use of differential global positioning systems (DGPS). In To document recent information and experience of state DGPS, two or more receivers are used: one or more receivers transportation agencies, a survey was sent by mail to every are set up over known points (base stations), and a remote (or State Hydraulic Engineer. In order to keep the survey short and rover) GPS receiver is used to locate unknown points. Any to ensure a higher return rate, the primary objective of the sur- errors in the system are defined by the base stations and a dif- vey was simply to identify what states use portable techniques, ferential correction is applied to the rover. Differential pro- what those techniques consist of, and what are their common cessing can be accomplished through an existing network of problems and limitations in field application. A total of 31 sur- base stations operated and maintained by various governmen- veys were returned. tal or private agencies. Alternatively, base stations in the sur- The type of equipment or procedures identified included vey area can be established over known points to provide the the usual probing, weighted lines, fish-finder sonar, and div- differential corrections. Given that the accuracy of the survey ing. The most common response was sonar, particularly for is a function of the rover's distance from the base stations, flood monitoring. Not surprisingly, success was considered greater accuracy is often achieved using the latter technique. better during routine inspection work rather than during flood With more widespread use of GPS, more base station net- monitoring. works have been established. The Coast Guard created one of Of the 25 general comments and observations received in the first networks of base station data along coastal areas that Part C of the survey, 10 specifically mentioned that the equip- transmitted corrections on a marine radio frequency. More ment should be easy to deploy and use. These comments recently, a Nationwide DGPS (NDGPS) network is being included phrases such as easily attached to existing vehicles;