Ring net barrier is originally developed as a protection from rockfalls, but some countries have proven that this type is also effective in trapping debris flow sediment. In Indonesia, ring net barrier as sediment control is applied by Sabo Research and Development Center in Lumajang, East Java on a slope of 3.48o and is placed 0.8 m above the river bed surface. To identify the optimal position of the basal opening, a hydraulic flume test is performed with a scale of 1:60, 0.6 cm of diameter of ring, and a variety of discharge (11 liters/sec, 8 liters/sec, and 1.8 liters/sec). These numbers are based on the scaling with 10% sediment concentration and a flow time of 144 seconds. The basal opening position is varied from the same elevation as river bed, placed as deep as 1.66 cm and 3.2 cm The results show that ring net can retain more sediment if the net is placed 3.2 cm deep. Therefore, the sediment trap capacity of ring net barrier that is applied in Lumajang can be optimized by shifting the basal opening’s position to 1.92 meters deep from the river bed.