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Field Manual - Scour Critical Bridges: High-Flow Monitoring and Emergency Procedures Idaho Transportation Department
Appendix A Triggers for High-Flow Monitoring or Bridge Closure
The Plan of Action should identify some trigger event that signals the start of high-flow monitoring, or in the case of a Category B bridge, the need to close the structure. This could be the water surface elevation reaching some pre-determined elevation associated with a certain recurrence interval, such as the 25-year flood. If a computed water surface elevation for a moderate flood cannot be determined, then other types of events can be used as triggers.
Practical definitions of high-flow events as triggers for monitoring activities are listed below in descending order of preference. The operative definition should be chosen based on availability of information.
Many of the scour critical bridges in Idaho cross steep streams subject to flashy, short duration flooding. The flowrate in such streams can increase from negligible to severely high levels in a matter of hours. If a bridge over a flashy stream is to be monitored, the crew and equipment must be mobilized to the bridge as quickly as possible. For such bridges, a crew should be placed on high alert for mobilization, or even sent to the most at-risk bridges, as soon as high rainfall for the watershed is forecast or the National Weather Service begins issuing flood warnings for the area.
Appendix B Water Surface Elevation Based Monitoring
Direct streambed elevation monitoring during high-flow conditions is the most accurate method for determining whether a bridge foundation element is approaching a scour closure streambed elevation. However, if direct streambed sounding is not feasible, then a scour monitoring and a scour closure trigger water surface elevation (WSEL) may be determined using hydraulic, structural, geotechnical, and scour analyses. For bridges using WSEL-based scour monitoring, the monitoring and closure WSEL's should ideally be marked at the upstream side of the bridge, using reflective signage visible from the roadway. The signage should be installed on the pier(s), and/or wing-walls or abutment ends. Figure B.1 illustrates typical monitoring and closure WSEL signs. For additional monitoring guidance, measuredown distances to the monitoring and closure WSEL's from the bridge rail or other fixed datum are tabulated in the Plan of Action, similar to the template shown in Table B.1.
The WSEL may be monitored visually, using the WSEL signage affixed to the bridge or other visible location. If signs denoting the relevant WSELs are not visible or have not been placed, the monitoring crew should measure the vertical distance from the reference line (identified in the table from the Plan of Action) to the actual WSEL, and compare this to the monitoring and/or closure measuredown heights listed on the table. This measurement is best performed with a weighted tape, as a surveying rod or other rigid measurement device is likely to break or be ripped from the monitor's grasp by high-velocity high flows.
Notes on Table B.1:
Appendix C Streambed Level Monitoring Equipment
At some bridges, the Plan of Action may call for direct monitoring of the streambed during high flows. Table C.1 below is a template of the Table of Scour Critical Bed Elevations that will be provided in the Plan of Action if streambed monitoring is required.
Notes on Table C.1:
Many different types of devices have been used for monitoring scour at bridges. The two basic categories of scour monitoring devices are portable instruments and fixed instruments. A single portable instrument can be used to monitor scour at several different locations at a given bridge and can be moved from one bridge to another. A fixed instrument is kept stationary at a particular location, such as at the end of an at-risk pier. A fixed instrument can be used to monitor scour over long periods of time without an inspector or operator present.
The focus of this manual is on high-flow monitoring. Fixed monitors can be valuable during high-flow monitoring under certain circumstances. In most flood monitoring situations, however, flexibility and rapid mobility among multiple bridge sites is imperative. This section, therefore, will focus on portable monitoring devices. Two types of portable streambed level monitoring equipment are commonly used for high-flow scour monitoring: physical probes and sonar instruments.
A physical probe is any type of device that extends the reach of the inspector. The most common types of physical probes are sounding poles and sounding weights.
Sounding poles are long poles used to probe the bottom (Figure C.2). Sounding weights, sometimes referred to as lead lines, are typically torpedo-shaped weights suspended by measurement cables (Figure C.3).
Both sounding poles and sounding weights can be deployed from the bridge deck. Sounding weight cables can be lowered by hand or from a reel mounted on a truck bed or portable frame. The United States Geologic Survey (USGS) has used sounding weights and similar instruments in their streamflow monitoring program for years. USGS stream-gaging techniques are documented in Water Supply Paper 2175 (can be downloaded from http://pubs.usgs.gov/wsp/wsp2175/). Information in Volume 1, Chapter 5 of this reference, related to sounding methods, is valuable for scour monitoring work. A visit to the local USGS office may also allow inspection of the equipment used for stream-gaging and guidance on the successful use of sounding weights under high flow conditions.
Sounding weights can also be deployed from a manned boat. During flood conditions, manned-boat deployment should be attempted only if the equipment and circumstances allow for the safety of the boat crew. The boat must have adequate size and power to prevent being swept downstream by the flow or being hung up on a bridge pier. The bridge must provide adequate vertical clearance and ample clear spans to allow a boat to safely pass under the bridge.
Physical probing can be difficult or impossible when the flow is both deep and fast, especially if the bridge deck is very far above the water. Under such circumstances, heavy sounding weights, properly deployed with sounding reels and cranes, have proven easier to use than sounding poles.
Apart from the challenge of high-flow deployment, physical probing is relatively simple to understand and implement. It is not affected by air entrainment or high sediment loads. It is well suited to lower-profile bridges over relatively small channels. To convert the sounding information to elevation data, the measurement should be referenced to the pavement, curbline, or guard rail or other location with a known elevation (e.g., from the bridge plans).
Sonar instruments have also been successfully used to monitor streambed elevations under high-flow conditions at bridges. Applications of single beam sonar range from fish finders to precision survey-grade fathometers. Low-cost fish-finder type sonar instruments have been widely used for bridge scour investigations (Figure C.4).
A sonar instrument can be deployed from the bridge deck, by attaching it to a probe or a mechanical arm, or by tethering it to a float. Under the mechanical arm category, a truck mounted articulating crane has been developed (Figure C.5) and has proven successful in testing under actual flood conditions with velocities in excess of 10 fps. Float platforms have included kneeboards (Figure C.6) and pontoon-style floats (Figure C.7). Tethered float platforms can be suspended from a pole or a bridge inspection truck to help control the movement of the float under the bridge.
Sonar instruments do not have to be positioned at the streambed to measure the streambed elevation, but the sonar transducer must be in the water. Sonar monitoring is more viable than physical probing, therefore, under high-flow conditions that are deep and fast. A disadvantage of sonar monitoring is that its accuracy is negatively affected when large quantities of debris are present, under high turbulence conditions, when significant air entrainment occurs, and when the flow carries a high sediment load. To convert the sonar data to elevation data, the elevation of the water surface and the distance the transducer is underwater typically must be known. The water surface elevation can be defined by a drop-line measurement from a known point on the bridge deck.
Appendix D Equipment List for Field Monitoring Crews