Experimental and CFD Studies on the Flow Field and Bed-Morphology in the vicinity of a Sediment Mining Pit
Authors: Abdullahi Nejib Hassen , Ahmad Zulfequar 
Published: February 2023

Most of the alluvial rivers in the globe are suffering and necessitate critical management regarding sand and gravel extraction and other related river ecosystem protection. Sediment mining is the activity of extracting useful materials such as sand, gravel, and aggregate from a river bed, banks, and flood plains. Sediment mining activity causes hydro-morphodynamic variations on the river bed, impacting the river plan and hydraulic structures, and also can harm flora and fauna within the river ecosystem. The period and magnitude of the mining rate with the discharge and channel bed material properties directly impact channel bed mobility and bed-level equilibrium conditions. The flow field, bedload transport, and morphological evolution in the pit's vicinity are varied in space-time, multifaceted, and three-dimensional. Most of the preceding researchers on sediment mining characteristics were concerned with the physical aspects, scouring study, and degradation rate. Very few researches are available on the flow regions and morphological bathymetry of a river under sediment mining. On the bases of this area of gaps, the present study aimed to understand the flow field and bathymetry of the channel under sediment mining. The experimental work is conducted at the Hydraulics Laboratory of the Civil Engineering department, IIT Roorkee, India, on a trapezoidal sediment mining pit constructed of uniform cohesionless bed material in an open channel flow. The average streamwise velocity was measured at five various sections near the sediment mining pit using a three-dimensional acoustic doppler velocimeter. The numerical simulation was conducted in a Flow-3D solver using the Reynolds Averaged Navier&#8211;Stokes equations (RANS), standard k-&#603;, the volume of fluid, and FAVOR, and the results were compared to the experimental observations. Both approaches depict an increase in the average longitudinal velocity at the upstream and downstream nick points. The observed longitudinal velocity from the experiment at the downstream nick point is higher than the velocity simulated using CFD at the downstream nick point. Both approaches indicated a significant increase in the longitudinal velocity downstream of the pit center, especially in the flume center. CFD simulations depicted the decreasing velocity at the upstream nick point due to more degradation and increment of flow depth at the section. The detachment and degradation of the upstream nick were observed in the initial stage of the experimental work but not at the downstream nick point. Because the pit is used as a bedload trap, the likely sediment-free water compelled out of the pit causes bed erosion and flattening in the downstream section of the sediment mining pit. The degradation at the upstream nick was 4.6% and 9% for the experimental observations and the CFD simulations, while it was 21.3% and 3.1% for the downstream nick, respectively. The findings of this study can help authorities and experts in the effective maintenance and supervision of river ecosystem balance by supplying cost-effective sediment resources.