MORPHOLOGICAL PARAMETERS IN THE UPSTREAM REACH OF A FLOW CONSTRICTED BY A SPUR DIKE

This paper presents the results of an experimental and theoretical study of the morphological parameters of flow in the upstream reach constricted by a spur dike (solid spur). The experiments were carried out in three flumes measuring40×60×1500 cm,40×60×800 cm, 100×50×800 cm. The first flume had glass walls and a wooden bed, the second had concrete walls and a wooden bed, and the third had concrete walls and a concrete bed. Experiments were carried out under the following flow and spur dike conditions: degree of flow constriction nb=lb sinθ /B; 0.125, 0.2, 0.3, 0.375. Here lb is the length of the spur dike; θ is the installation angle, 60–90°; water discharge from 5 to 25 l/s i_d=0.0001-0,0002. Fr_δ=0.02"–" 0.35 The influence of these factors on the formation of the section of maximum backwater and on the length of the upstream recirculation zone was examined. It was found that as the degree of flow constriction increases, the relative distance to the section of maximum backwater S_1/(B-b_0 )decreases from 2.0 to 0.96, while the relative length of the upstream recirculation zone l_b/(B-b_0 )increases from 3.0 to 4.55. An increase in the Froude number leads to an expansion of the zone of hydrodynamic influence of the structure and a shift of the maximum-backwater zone upstream. It was experimentally confirmed that as the spur dike installation angle increases, the processes of flow separation and circulatory flow in the upstream reach intensify. To provide a theoretical description of backwater propagation, the momentum equation was used, accounting for the influence of hydrostatic pressure forces, bed and bank resistance, and the redistribution of flow momentum in the zone of local constriction. A comparison of calculated results with the experimental data of K.Sh. Sharapov showed satisfactory agreement, including at a substantial bed slope of 0.004: the calculated backwater length was L_p=0.86"–" 0.94 m against an experimental value of L_p=1.0 m. The relationships and calculation approaches obtained can be used in the design of channel-regulating and bank-protection structures, as well as in assessing the hydrodynamic conditions governing the formation of local backwater and recirculation zones in river channels.

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