Laboratory investigation flow pattern and bed topography in the presence of attracting, repelling and deflecting porous spur dikes in the straight channel with erodible bed

Document Type : Original Research

Authors
Z. badpa
M. Fazli
bu ali sina uni
Abstract
One of the usual methods for river banks protection is using spur dike structures that if properly designed and executed, in addition to controlling erosion, It leads to the rehabilitation of rivers margin valuable lands. Spur dikes affect the streamlines and it make changes in the velocity and direction of the flow, leading to major changes in the bed topography around the spur dike as well as the beaches. Recognizing and directing these changes will lead to the River Affordable Planning in the desired areas. In the present study, the effect of the Deflecting open gabion spur dike, attracting and repelling on the bed topography of the flow path and flow pattern has been investigated. ADV was used to measure flow velocity in different directions. This velocity meter is submitted by transmitting waves of 10 or 16 MHz frequency from a transmitter to a sample size of 6 mm in diameter and 3 mm in diameter at a distance of 5 cm from the transmitter and receiving waves by receiver antennas measures the velocity of particles within the sampling volume. The device has the ability to measure the distance from the floor inside the water. Therefore, taking into account the baseline level, the measured distance at each point was deducted from the base value and the scouring of that point was obtained. In these experiments, the Froude number was fixed at 0.26. Also, the depth of flow in the set of experiments is 14.6 cm, which is extracted according to the discharge rate and the displacement threshold formula. The experiments were carried out in such a way that after the equilibrium of the bedding and scouring harvest, the flow pattern was started using the ADV device.

In this review, the performance of the spur dikes will be compared with the impervious spur dike. The results show that by decreasing the porosity of the spur dikes, the mainstream deviation and the intensity of the secondary flows around the spur dike have increased, which increases the topographic changes of the bed and creates larger cavities around the spur dike. As the erosion rate increases around the spur dike, sedimentation on the edges increases.In all three types of spur dikes, with increasing porosity, the dimensions of the scour hole are reduced. By increasing the porosity of the spur dike, the flow velocity from the pores of the spur dike increases, which reduces the difference in the flow rate from the headland and the flow through the pores of the spur dike and reduces the ability to carry flow sedimentation. In a spur dike with 50% porosity, bed topography changes occurred in a very small area around the spur dike and focused on the nose, while for a spur dike with zero porosity, the topography of the bed, depending on the type of spur dike, is several times the length of the spur dike, in Channel length and width occurred. The attracting spur dike has created much less variation in the flow pattern due to the way it is placed in the path of flow, and therefore the bed topography is less influenced by the presence of the spur dike.

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Modares Civil Engineering journal
Volume 19, Issue 3
September and October 2019
Pages 1-15