Investigation of the effect of the contraction and downstream structures on the flood wave propagation

Floods can cause great socio-economic damages and lead to loss of life and property. For this reason, it is of great importance to determine the characteristics of the unsteady flows that change over time and investigate their propagation during this period. The natural and artificial obstacles faced by flood waves along the rivers interfere the structure of these waves. The purpose of this study is to determine the effect of a contraction element placed in an open channel and a downstream weir located at the end of a laboratory flume on the propagation of the flood wave. Four cases were considered, namely with and without a weir and with and without a contraction. Experimental studies were carried out in the 18 m long and 70 cm wide channel having rectangular cross section in the Hydraulic Laboratory of Ege University Civil Engineering Department. As a contraction element, a piece that will reduce the width of the zone between the points 6.35 m and 11.65 m of the channel to 40 cm was used in the channel. The height of the downstream weir used at the end of the channel is 14.2 cm. Steady flow experiments were conducted by usingwith six different flow rates. The unsteady flow experiments were carried out by using two hydrographs with the same base and peak values in the form of an isosceles triangle and different rise and fall durations generated by a pump speed control unit. The variation of the discharge with time was recorded with the electromagnetic flowmeter on the feed line of the channel. Three dimensional point velocity measurements were made during the hydrograph by using an ultrasonic velocity meter (FlowTracker, SonTek) at different locations and at different elevations in the channel, and velocity time series was obtained. In specific sections of the channel the flow height is measured with the IMP device. The centre-line mean velocity is calculated by using the streamwise instantaneous point velocity time series and flow depth. The generated input hydrograph is compared with the output hydrograph, and information about routing and propagation was obtained. The effects of the existence of the downstream and contraction structures on the variation of flow depth and centre-line mean velocity over time for two hydrographs were investigated and the results were presented.