Bridge scour is a major factor causing instability of bridges crossing waterways. Excessive scour contributes to their high construction and maintenance costs. Design of innovative scour-monitoring instrumentation is essential to ensure the safety of scour-critical bridges. The ability of real-time surveillance is important since the most severe scour typically happens near the peak flood discharge. A new scour-monitoring instrument based on the Time Domain Reflectometry (TDR) principle has been developed to provide real-time monitoring of scour evolution. A framework based on dielectric mixing model has been developed, which can be easily incorporated into an automatic analysis algorithm. This paper introduces a comparative study of TDR method and ultrasonic method for scour measurements. The results indicate that both TDR and ultrasonic methods can accurately estimate scour depth. TDR method, with the developed analysis algorithm, yields information on the river properties such as the electrical conductivity of river water and the density of sediments. TDR methods are also found less influenced by turbulence and air bubbles, both likely to occur during flood events. 1. Introduction Bridge scour or bridge sediments scour refers to the lowering of the streambed around bridge piers or abutments. Typical types of scour include: long-term degradation of the river bed, general scour including contraction scour, and local scour at the piers or abutments [4]. Among these types of scour, local scour is the most critical (Figure 1). Local scour is caused by the interference of piers and abutments with stream flow. It is characterized by the formation of scour holes surrounding bridge piers or abutments, and as a result, the support provided by the surrounding soils will be reduced. Figure 1: Schematic of local scour [ 1]. Bridge scour poses a severe threat to bridge safety. Since 1970 over 1,000 bridges have collapsed with associated deaths due to bridge scour. Bridge scour is also a major factor contributing to the high construction and maintenance cost of bridges. Proper scour prediction is essential for an economical and safe design of bridge piers and abutments. Current scour design relies on the empirical scour prediction equations developed from laboratory data, which generally do not accurately predict the scour under field conditions [5]. This necessitates collecting scour data from the field to improve current scour prediction theory. Because scour can cause catastrophic failure of bridges without any prior warning such as signs of distress in the
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