Author(s): S. Nassrullah; B. Bockelmann-Evans; T. Stoesser

Linked Author(s): Thorsten Stoesser, Shahla Nassrullah

Keywords: Colmation; Decolmation; Gravel bed; Velocity profiles; Acoustic Doppler Velocimeter

Abstract: Two different processes occurring in riverbeds and affecting the aquatic habitat were investigated in this study: colmation and decolmation of suspended sediment. Flume laboratory experiments were carried out and analysed to study velocity profiles above smooth and gravel beds. Riverbeds provide a habitat for aquatic organisms, such as fish and macroinvertebrates, and require high oxygen concentrations. Normally, riverbank erosion is caused by flood events, while human activities, such as agriculture, cause further water pollution. These processes result in sediment and organic matter being suspended in the rivers. However, low velocity flows cause the colmation of sediment leading to reduced hydraulic conductivity through the gravel bed, which in turn results in reduced oxygen levels. Moreover, the colmation occurs by straining and settling of suspended sediment as water moves through the river bed material and other causes such as microbial transport. This phenomenon has multiple effects on the supply of oxygenated water to the incubation zone of salmon embryos for example. The impacts are threefold; firstly, it reduces oxygen in the intragravel volume. Secondly, the fine clay particles have a negative effect on the exchange of O2 through the egg membrane. Finally, accumulation of inorganic and organic sediment affects interstitial flow velocities and gravel permeability. In other words, high velocity flows increase hydraulic conductivity in this area, where fish eggs remain until hatching, and are dependent on high oxygen levels being present. During these highly turbulent events the sediment transport process of decolmation occurs. Furthermore, decolmation is the resuspension of deposited fine particles and increases the hydraulic conductivity. Recently, many techniques have been used for estimating velocity and turbulence properties over gravel beds. For instance, Precht et al. compared an Acoustic Doppler Velocimeter (ADV) system with Laser Doppler Anemometer (LDA) in a flume near the bottom. Precht et al. illustrates that more surveys are needed close to bed with ADV configurations, and experimental set-ups based on the data quality. This study aims to test velocity profiles above the gravel bed using a Nortek Acoustic Doppler Velocimeter (Nortek ADV) SNR>15db, with the aim being to better understand the flow processes over the gravel bed and the exchange water velocity between surface and interstitial of gravel bed for different flow processes and identify when colmation and decolmation occur. These experiments were carried out in the Hydraulics Laboratory of the School of Engineering at Cardiff University in a narrow flume, 10 m long, 0.3 m wide and 0.3 m deep. At 6 m from the flume’s inlet the flume bed is covered in gravel with an average diameter of 40 to 35 mm. Data was analysed using winADV to find the average velocities in X, Y, Z directions.

DOI: https://doi.org/10.3850/978-981-11-2731-1_164-cd

Year: 2018