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One of the natural hazards is the flood occurrence. This natural phenomenon can lead to more risks on the riversides, especially at the downstream of dams and floodgates and near the bridges. Rivers that pass through forest areas on their route can carry a lot of wooden pieces. One of the issues that increases the flood damages, especially in mountainous and forest areas, is the entry of trees and wood chunks and trees branches into the rivers and their movement to the downstream of flow. Generally, these floating tree branches and wooden pieces are called driftwood. Rivers are fast in mountain slopes and can carry more driftwood. This will carry more material and objects floating along the river flow and causes the timber or driftwood accumulation in the bridge spans and river flow obstruction, thereby reducing the capacity of flood flow. Reducing the capacity of intersecting structures provides the conditions for entering the flood into the marginal waters of the river and causing a rise in water flow and secondary damages. Due to the potential risks, it is very difficult to study such a mechanism in nature, so it is necessary to examine these conditions by experimental studies and these condition and hydraulic flow changes are measured with high precision. Parameters such as the rate of uplift flow, the probability of obstruction by the driftwood, the type and dimensions of the trapped driftwood, etc., are among these. The present study, with laboratory modeling, examines the process of river flow obstruction due to the movement of floating driftwood in Mountain Rivers. A bridge was built with protective railings in a rectangular channel for study. Each of the experiments was carried out with different conditions, including the size and the number of driftwood, and the presence or absence of branches in several times. Experiments was carried out with two different modes of existence and lack of bridge pier in three levels of water flow. Also, the type of flow of experiments was modeled in two sub-critical and one super-critical state, 3 different slopes and 9 different discharges. Each of the tests is repeated ten times with similar conditions. According to the definition given in this study, an obstruction occurs when at least one of logs or driftwoods stops for at least 30 seconds at the bridge section. The results of the tests showed that the effect of branches on the flow level increase is more than twice that of driftwoods without branches, and that with increasing initial height of the stream and passing from the surface below the bridge deck, the growth rate of the water level decreases compared to the initial height. Also, the correlation of the effect of changes in the number of driftwood with branches is greater than the same value in driftwood without branches and is closer to number one. Another important result in this paper is that the effect of the initial height of the flow on the increasing the flow level is less than the driftwood branches. This is the result of a comparison of the gradient of Changes in water flow height in two states.

Keywords: Drift Wood, Flow Hydraulic, River Obstruction, Bridge, Backflow

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