Optical sensors to measure the concentration of suspended solids (SS) were made as a trial using infrared emitting diodes (IRED), a laser diode (LD) and photo-tranSistors (PT). The sensors have been utilized in our experiments and field surveys to study the erosional process of cohesive bottom sediments. Three types of optical sensor are developed and applied to measure SS in laboratory and field. The first two types of sensor are for laboratory use and consisted of IRED and LD, respctively. They realized high emitting power, sharp directivity of emission and high sampling frequency of measurement. The sensors for field surveys built up of IRED could measure vertical profiles of SS with high sampling frequency. These sensors attained our objects of each measurement with good performance. Measurements indicated that turbulences near bottom were cIosely related to erosions of cohesive bottom sediments.
