Temporary cofferdam used in river improvement work causes the backwater. In flood time, this backwater increases the risk of overflow over the bank. To diminishthis backwater there are two method. One is kinematic method which is to diminish the flow resistance by using smooth shutterings in shape and rough-ness. The other is dynamic method which is to diminish this backwater by adding momentum ofaccelerator. We carried the mock-up test of dynamic method using the propellers of work-ship as accelerator, and as the result the water level fall down 10 cm. The result exhibited good agreement with the momen-tum principle. This paper presents the result of mockup test.