# Experimental Study on the Near-Red Flow Characteristics of Alluvial Channel with Seepage
## 1 Introduction  
The flow interaction with the sediment in a river might cause bed aggradation and degradation processes, which are essential in river control engineering. One of the most important characteristics of a fluvial bed is the movement of the water through the soil in terms of downward seepage because of soil permeability. The channel bed distortions and the turbulent flow features are altered in the presence of downward seepage flow (seepage through the channel in the downward direction) [1].The seepage and related turbulent dynamics play an important role in the solute transport across the hyporheic area in rivers [2].This aspect could have important implications for groundwater quality. Further, [PERSON] et al. [3] emphasized the seepage influence on the thermodynamic nature of a sediment bed channel, with the deformation of bed materials and an exacerbation of the sediment transport rate. Hence, seepage processes might be crucial for the sustainability of the hydraulic structures because of their interaction with groundwater. [PERSON] et al. [4] argued that sediment transport rates can be modulated by the seepage processes. Consequently, seepage might have an impact on river channels and bed morphology, particularly at local scales [4]. Moreover, downward seepage would reduce the conveyance efficiency of a channel [5]. Field studies [6, 7] evaluated the amount of downward seepage in natural rivers and detected that the seepage flow may vary from 10% to 45% of the main flow discharge delivered at the canal head. Downward seepage increases the bed shear stresses as well as the rate of sediment transport [1, 4]. Moreover, the turbulence intensities are decreased in flows over seepage beds [8]. Many previous studies [9, 10] investigated thenature of the velocity distribution under the existence of downward seepage. A tendency towards uniform velocity profiles in the near-bed flow and a higher level of anisotropy in turbulent flow were observed [10]. In addition, the sediment particles that form the riverbeds are mostly non-uniform (i.e., sediment mixtures) and the finer grains are usually transported on the bed surface [11]. Turbulent flows causing the erosion and deposition of riverbeds are important to fluvial hydraulics. The relation between flow velocity and suspended sediment was studied by [PERSON] and [PERSON] [12] and they pointed out that the flow velocity and the turbulence intensities in near-bed flow are directly related to the suspension of sediment. [PERSON] et al. [13] considered the sediment movement in a channel with coarse sand and they detected low values of mean longitudinal velocities and turbulence intensity because of sediment transport. Moreover, the mean streamwise velocity is inversely proportional to the friction factor connected to the apparent roughness in the reflection of bed material movement [14]. The flow turbulence in an alluvial channel for the discharge corresponding to incipient motion was inspected by numerous investigators [15; 16]. Moreover, previous studies [17; 18; 19] examined the flow turbulence in sandy bed channels for the mobile bed condition. [PERSON] et al. [20] analyzed the turbulent flow characteristics at high Reynolds number in a vegetated channel and compared them to the turbulent coherent structure in an alluvial channel without vegetation. Recently, [PERSON] and [PERSON] [21] analyzed the flow turbulence at high Reynolds number in a rough bed channel and correlated the sediment transport with flow turbulence. From the literature review, it emerges that most of the studies focus on the flow characteristics in sand/gravel bed channels at high Reynolds number. Moreover, riverbeds are typically composed of non-uniform sediments, and seepage is an explicit parameter, which influences the turbulent parameters and initiates the sediment movement. The resulting sediment movement alters the channel morphology [1; 2], which might have an effect on turbulent parameters along with seepage.