Bridge safety is severely affected by the pier scour during rainfall events in Taiwan. This study focus on scour problems of two highway bridges across Choushui River. The first highway bridge is located in the relative upstream composed of sand and gravel and realized with great localized sour due to the flow constriction by bridge piers. The other one is located in the downstream with sand and silt riverbed materials, and issued with interference among downstream groundsill works, the bridge itself, and the oil pipeline bridge. To effectively obtain the scour depth around the bridge piers, the float-out device is suggested for field implementation. The embedded float-out devices, a kind of sacrificed sensor for once time measurement, is employed as real-time scour monitoring during flood events. This method is comparatively cost-down and highly feasible in practice. Therefore, 10 float-out devices for each bridge were installed at 5 to 6 m depth of the upstream side next to the bridge piers in 2017. The spacing between each device is set 0.5 m, presenting the spatial resolution of the scour depth. In addition, more two sets of float-out devices were added under the groundsill works downstream in the second case. The monitoring result of the second one revealed the maximum scour depth next to the pier equals 4 m after the peak flow and 3 m under the groundsill works downstream during water level rising of the rainfall event in June 2017. These pieces of scour information enhance the calibration of the related numerical simulation. Although there is no obviously scour in the first case in 2017, a false alarm of the float-out device was found at the shallow depth just because of accidental excavation of the new groundsill work construction. This message proofs the robust real-time monitoring of the float-out device applied in field.