Simulation of Flow and Sediment Transport in an Open Channel With Obstacle using IRIC NAYS2DH

Abstract

Hydraulic structures such as barbs, groins and spur dikes have been constructed in river bank in order to deflect the flowing water away from vulnerable zone. They are made of stone, gravel, rock, earth or piles, beginning at the river bank with a root and ending the regulation line with a head. River flow mechanisms with the hydraulic structures need to be intensively studied to protect the river bank from erosion, increase the bank stability, improve navigation and flood control. Such structure also plays an important role to enrich the biodiversity of different aquatic species by providing shelter them. The main objective of this thesis is to simulate the straight channel with the presence of several types of such structures by using 2D numerical simulation model, iRIC Nays2DH. The groin like structures can be oriented perpendicular or be inclined either upstream or downstream. Each orientation affects the stream in a different way and shows different result in the vicinity of the groin. In chapter, simulation of flow around groin, details of streamline variation, velocity vector and computed recirculation around groin are shown for 45º, 90º and 135º groin. The length of recirculation zone is found 1.2m, 1.04m and 1m, respectively. The simulated present study of velocity profiles and shear stress profiles are compared with previous experimental and numerical results. Good agreement is found for all of the cases. It is observed that predicted velocity profiles and predicted bed shear stress profiles for 45º, 90º and 135º, the position of maximum velocity and shear stress are found to be shifted towards downstream with increasing y/l = 1, 1.5, 2, 3and 4, respectively. For all the cases, peak of the velocity and shear stress are also deviated from upstream to downstream, 45º to 135º angled groin. From the velocity contour and shear stress contour, it is found that left bank is higher velocity and higher shear stress zone and right bank is lower velocity and lower shear stress zone. In chapter, local scour around groin, to determine the variation of scour depth and sediment deposition near the head of the groin, groins are orientated 45º, 90º and 135º at the upstream of the channel. The comparison of velocity field, bed level contour results, Elevation of bed profiles of 45º, 90º and 135º groin are given. Simulated present study are compared with previous numerical results and good agreement are found. 45º, 90º and 135º groin are simulated for 1 hour and 5 hours, respectively. After 5 hours, equilibrium scour is attended and maximum scour depth for 45º, 90º and 135º are found as 0.065m, 0.062m and 0.054m, respectively. In case of multiple groin, maximum scour are found 0.145m for discharge 0.035 m3/s and 0.24m for discharge 0.046 m3/s.. Bed deformation contour lines, velocity vectors, transverse and longitudinal bed profiles are depict in this chapter. Computed scouring are compared with previous experimental and numerical results. For discharge, 0.035 m3/s, scour at first spur dikes to third spur dikes are 0.123m, 0.005m and 0.005m, respectively. Similarly, when discharge 0.046 m3/s, scour at first to third spur dikes are 0.16m, 0.01m and 0.012m, respectively. The model is also used to evaluate and analyze flow patterns, streamline variation, velocity vector contour, vorticity, bed shear stress contour, scour and sediment deposition around a groin like structure on a straight channel. Most of the natural flow are three dimensional; for example, in case of meandering channel, the flow passing through the bend is obviously of a three-dimensional (3D) nature because of the generation of secondary current due to the centrifugal force. When a flow in an open channel passes an obstacle ( such as groin), the deviation of flow causes the generation of secondary current and flow become highly 3D. Therefore, a 3D hydrodynamic model is necessary to accurately simulate the flows in a meandering channel. However, dealing with the practical engineering problems, such as alluvial geomorphic process, it is not computationally efficient to use 3D models. In such problems, a two-dimensional (2D) model is generally used. To predict the applicability of iRIC software in a flow field with strong secondary current, the model is also applied for 180º sharp and mildly bend. Water surface elevation contour and velocity vector are evaluated from the simulation. For both cases, the velocity of flow is greater in the inner section than the outer section. The results are compared with previous experiment result and found well agreed with previous experiments.