Consideration on the use of Delft-3D turbulence modelling for the design of settling basin

Settling Basins are used on irrigation and hydropower channels to removeobjectionable sediment of a specified size and quantity and are typically designedbased on ideal settling theory, with a factor to account for turbulence. Theadvantage of this approach is that it is simple to use, and will often yield areasonable approximation to actual sediment removal efficiency. However, idealsettling theory does not quantify turbulence, and various empirical andanalytical methods have previously been proposed to estimate settling basinperformance.Settling basin is generally accomplished by reducing the velocity to the lowestpermissible magnitude for the longest possible period. An expansion transitionby widening the approach channel and lowering its floor are possible to reducethe flow velocity and make it possible for sediment to settle out of suspensiondue to gravity. However, the movement of sediment is vertical direction is notonly due to gravity alone, but also under the combined action of gravity andvertical component of turbulence.Effective design of settling basin therefore depends upon an analysis of the effectof turbulence on deposition. The description of the deposition profiles in settlingbasin is studied by applying Delft-3D k-""Ø turbulence model.This study reports that the result for sedimentation process in the settling basinobtained by Delft-3D k-""Ø turbulence model fits the field measurement ratherbetter than that computed using 1D, Quasi-2D and 2DH mathematical models.The recommendation for the design of settling basins then can be derivedaccording to Delft-3D simulation results.Implementation of transition zone by widening its width has been successfullyachieved the 90% removal efficiency. The comparison indicates that when basinsare modified using 1 : 4 gradual expansion transition, the head losses created arebeing very small and the optimum bed shear stress are achieved at the entranceof settling basin. Maintaining this angle, the deposition of suspended sedimenttransport in the basin is preserved without re-suspension if the width of settlingbasin is designed ranging from 1.75 up to 2.0 times wider than the length oftransition zone. Consequently, the length of settling basin can then be reduced upto 50% and leading to more economic design consideration compared to the onewithout transition zone.In addition, this study confirms that the proposed model represents the headlosses in the transition zone with strong correlation compared with the oneobtained by experimental study.For purpose of justification, the research recommends the implementation ofphysical model, a number of additional tests and further validation againstseveral real-life cases