The present invention develops a sensing assembly using fiber optical sensors and waters wellable polymers to detect or monitor the scour damage or water presence. The device is assembled with two types of units, the standard unit and the adjustable unit. The standard unit consists of the fiber optic sensing element, the water-swellable polymer cube, the fastening part, and the transition part filled with dehydrant. The adjustable unit is a cylindrical housing used to connect between standard units and to protect the connecting fiber cables. The configuration of the standard unit is fixed while the adjustable unit can be a stiff steel pipe or a hose that is varied in length. Therefore, the device can be designed to any required length and different accuracy by assembling numerous standard units with varied long adjustable units. The device is inserted vertically into the soil along the bridge foundation or laid down besides the subsea pipeline. In the presence of water, the polymer cube expands in volume and bends the fiber optic sensor. The wavelength shifting of the sensor caused by the bending is then transduced into a measured signal, which indicates this sensor is no longer covered with soil, i.e. this unit has exposed to water. The scour depth or location can be identified accordingly. In addition, a standard unit can work independently to detect the presence of water or water leaking. And by interchanging the polymer into some material sensitive to other liquid, the device can be used for other types of chemical sensing.