Impact Of Submerged Spur Dike On Flow Structure

In projects, especially channel improvement, submerged spur dikes have been widely used. The impact of spur dike on flow structure has changed with overtopping ratio. Spur dikes in engineering normally have upstream slope, downstream slope and head slope, which lead to great difference compared to vertical-wall ones. Flume experiment and 3D mathematical model are used together to study on the impact of head slope coefficient m=0,3,5 and 7 and overtopping ratio△H/H=0.17,0.29,0.38 and 0.44 on local flow structure.1) A 3D shallow water turbulence model with Roe flux format has been established, based on plane unstructured triangle grid and verticalσcoordinate. A conservative diffusion model has been deduced. Computational roughness, water level integral balance method,3D cascade flow hydraulic model and partial slip coefficient are used to solve problems of 3D movable boundary, steep topography, discontinuous elevation and side wall friction simulation, respectively. Consideration of side wall friction is crucial to simulate two small recirculation zones at upstream and downstream near the root of spur dike.2) Downstream recirculation zone has also existed in condition of△H/H=0.17, and disappeared in condition of△H/H>0.17. Transverse flow velocity near spur tip is weaker in submerged condition than in non-submerged condition. In section blocked by submerged spur dike, surface longitudinal velocity is 1.1～1.7V0 (mean velocity), and a downstream horizontal axis circulation occurs.Submerged spur dike has also had adjustment effect in some extent on flow lower than crest, decreasing with increase of△H/H. Deflection angle and transverse flow velocity above crest and next to spur tip have decreased with increase of△H/H. In condition of m=0, a empirical formula between influence extension of transverse flow bt/L and△H/H is as follows:bt/L=-5.80△H/H+2.96.3) A formula has been deduced to compute discharge blocked by spur dike with m in condition of submerged and non-submerged. In consideration of impact of head slope on local head loss, a formula has been proposed to compute downstream recirculation length in condition of non-submerged. And together with impact of overtopping flow on mean velocity in main flume of spur dike axis section, the similar formula has been proposed for small△H/H.In condition of non-submerged, the increase of m has lead to decrease of relative recirculation length l/L and relative recirculation width b/L. The isoline extensions of relative plane velocity V/V0≥1.40 near river bed and relative river bed shear stress have both obviously decreased. The maximum of relative river bed shear stressτbmax/τ0 has decreased from 4.40 at m=0 to 3.68 at m=7.In condition of△H/H=0.17, l/L has increased form 7.81 at m=0 to 9.56 at m=1, and then gradually decreased to 8.16 at m=7, which is different with non-submerged condition. The isoline extensions of river bed plane V/V0≥1.30 andτb/τb0≥2.50 both have decreased with increase of m.τbmax/τ0 has stabilized with 2.90 at m=1.4) Relative unit-width discharge q/qin in spur dike axis section has significantly concentrated at m=0, and has gradually decreased to 1.18, which equals to that in main flume, at m=7. Adjustment effect of head slope has been shown decrease of intension in isoline of q/qin=1.35 from m=0 to m=3, with not obvious increase, even decrease of isoline extension; and decrease of isoline extension of q/qin=1.15 when m>3.The increase of△H/H has brought about increase of q/qin above crest and decrease in main flow zone, weakening adjustment effect by spur dike, and unifying distribution of unit-width discharge.5) The isoline extensions of river bed plane V/V0≥1.30 andτb/τb0≥2.50 and concentration degree ofτbmax/τb0 and q/qin has greatly shrinked with head slope compared to without head slope. In addition, head slope limits down fall flow, and prevents down fall flow and eddy system to directly effect on river bed. All above are beneficial to weaken local scour and transform more discharge to main flume.