Numerical modeling of sediment transport over hydraulic structures

Abstract

Hydraulic structures are present in the designs of different Room for the River projects in the Netherlands. Examples are longitudinal weirs, groins, summer dikes and weirs in the inlet of a side channel. Morphological simulations with Delft3D are frequently carried out to investigate the effect of such projects on for example hindrance for shipping and dredging costs. It is important that also the physical processes around hydraulic structures are correctly modeled in these situations. At the upstream slope of a hydraulic structure, the larger depth-averaged velocity causes an increased sediment transport capacity and increased actual bed shear stresses. The latter is reinforced by a change of the velocity distribution over the vertical with respect to uniform flow. Opposite, the gravity component along the slope results in a higher critical bed shear stress than in flat bottom conditions. At steep slopes, (partial) bed-load transport blockage could occur. Delft3D is meant to model flow phenomena of which the horizontal length and time scales are significantly larger than the vertical scales. Near hydraulic structures, this is generally not the case. These structures are parameterized as weirs in a depth-averaged Delft3D model in engineering practice. The only effect of these weirs is an additional energy loss in the momentum equation. The parameterization aims at representing the influence of the weirs on the flow at larger scales. The local flow around the structures (including turbulence, vertical velocity components and actual shear stresses) is not correctly modeled. Moreover, there is no direct influence of the weir on sediment transport (like increased critical shear stresses and bed-load transport blockage). This inaccurate way of modeling could result in errors in the prediction of the morphological effects of hydraulic structures. The objectives of this study are: (1) Assessing the performance of the current way of Delft3D modeling of sediment transport around hydraulic structures in three-dimensional flows. (2) Making recommendations on the modeling of sediment transport around hydraulic structures in hydraulic engineering practice. The performance of Delft3D has been judged by comparing the results with the results of the numerical model FLUENT. FLUENT is an advanced flow modeling system, in which sediment transport can be studied by analyzing the trajectories of discrete particles. Firstly, some laboratory experiments describing flow and transport over structures have been modeled. In this way, the performance of both models has been investigated and mutually compared. The results of FLUENT gave confidence to use FLUENT as an instrument to judge the performance of Delft3D in modeling three-dimensional flow and transport over hydraulic structures. A three-dimensional flow situation has been designed, which resembles the flow over a longitudinal weir. In Delft3D, all bed-load transport and suspended-load transport that reaches the weir also passes the weir. In FLUENT, this is not the case. Suspended-load transport is distributed between the main channel and the zone behind the weir in the same ratio as the discharge. The distribution of bed-load transport strongly depends on the particle diameter. This difference shows that the parameterization of weirs in depth-averaged Delft3D models gives significant errors in the prediction of sediment transport over hydraulic structures, especially when bed-load transport is dominant. The transport magnitude can be reduced by increasing the bed level points near the weir to crest level. In this schematization, nearly all bed-load transport is blocked and suspended-load transport is reduced. A weir without increased bed level points overestimates the sediment transport over the structure. When the bed level points are increased until crest level of the weir, the sediment transport over the weir is underestimated. The sediment transport over the weir can be tuned by an increased bed level somewhere between zero and crest level. The distribution of sediment between the main channel (index 1) and the area behind the weir (index 2) can be described with a relation S2/S1 = C*Q2/Q1: The value of C as given by Delft3D can be judged with the following rules of thumb: (1) Suspended-load transport is distributed between the main channel and the zone behind the weir in the same ratio as the discharge, so C = 1. (2) For bed-load transport in three-dimensional situations with clearly oblique flow over the weir, the coefficient C can be related to the excess shear stress at the upstream slope, in which the actual and critical Shields parameter are adjusted for slope effects. (3) In situations where the flow is directed almost perpendicular to the crest of the structure, the conclusions of Lauchlan (2001) are recommended. Nearly all mobile sediment is transported over the structure in these situations. The coefficient C in Delft3D can be influenced by giving the bed level points near the weir the right height.