Bridge scour is the number one cause of bridge failures. Erosion of soil around the bridge foundation could lead to problems from instability of substructure to complete bridge failures. An effective scour risk countermeasure is to deploy bridge scour monitoring system. Besides preventing the risks of catastrophic failures, field scour data can also be used to calibrate and refine the existing bridge scour simulation models. This paper describes the development and field evaluation of a TDR (Time Domain Reflectometry) based bridge scour monitoring system. TDR is a guided electromagnetic wave that determines the interface between materials layers based on the contrast in the dielectric properties. It is capable of determined the interface between water and sediment, from which the extent of erosion can be determined. This paper will discuss the technical basis, the sensor fabrication, the procedures for field installation, and TDR scour sensor performance in a long term bridge scour monitoring program.