Flume experiments were done to clarify the degree an open check dam lowers peak discharge of a muddy debris flow and a method of estimating the necessary conditions and anticipated discharge rate reduction were proposed. The mixture of sediment and water was set so the simulated muddy debris flow would maintain Froude similarity with the May 1998 debris flow near Sarno, Italy. The flume was 20 cm wide, 40 cm deep, and 500 cm long, and it was set at two different slopes of 5° and 10° . The wooden dam models included three slit types with widths of 5 cm, 10 cm, and 15 cm and three square conduit types with sides of 5 cm, 10 cm, and 15 cm. After the sediment and water were mixed for 6 minutes with two electric drills equipped with a special propeller in the upstream space from a gate, the gate was opened manually to create a muddy debris flow in the flume. Sampling boxes on a roller at the bottom end continuously collected the muddy debris flowing from the dam models. Both the slit and conduit dams lowered the peak flow volume. The smaller the dam dimensions, the lower the peak discharge rate. Surges were observed forming upstream from the dam models. Equation of continuity, momentum equation, and energy equation for positive surge obtained equations representing the surge formation conditions and discharge rate. Their application to the experimental results showed that the calculated values generally reproduced the test results.