Hydrodynamics and bed morphological characteristics around a boulder in a gravel stream

Abstract

This paper presents experimental studies on hydrodynamics and bed morphological characteristics under varying water and sediment discharges over a gravel channel bed with a boulder. Firstly, flow characteristics over a non-eroded bed with a mild slope were investigated. Results show that along the transect line located one diameter away from the boulder centerline, the existence of the boulder has negligible impact on the mean flow characteristics, which are similar to flows over a flat bed. At the boulder centerline, the flow is largely deflected by the boulder and turbulence characteristics in the horizontal plane are largely enhanced in the wake of the boulder. Secondly, water scour experiments were carried out over a steep slope. It could be observed that scour occurred around the boulder and bedloads were deposited downstream, forming a typical pool–riffle sequence. An analysis shows that the length scale (L/D) of geometric features associated with pool depth, riffle height and pool–riffle distance (S/D) are positively related to the boulder-related Froude number (Frb): L/D = 1.18Frb − 0.11 and S/D = 12.5Frb + 0.6; and the erosion volume (Ve) for flat bed and boulder bed is positively and negatively related to the averaged Froude number (Fr): Ve/D3 = 37.1Fr − 21.3 and Ve/D3 = −44.8Fr − 38.6, where D is the boulder diameter.

Determination of River Hydromorphological Features in Low-Land Rivers from Aerial Imagery and Direct Measurements Using Machine Learning Algorithms

“…Boulders are one of the most widespread natural obstacles that cause the turbulence distribution across the river flow. The large-size boulders affect hydrodynamics and morphodynamics of the surrounding area [33]. The effects are especially highly expressed along both sides of the boulder where the acceleration of the flow is induced [34] and behind the boulders or their groups [35].…”

The influence of different morphological units on the turbulent flow characteristics in step-pool mountain streams

“…Accordingly, pools are dominated by wake turbulence and the steps, treads and runs are governed by form or bed-generated turbulence. The wake turbulence in pools is characterized by recirculation eddies and its strength diminishes with increase in distance from the impingement point 25 , 26 . Incidentally, the variations in hydrodynamics within step-pool systems do not furnish considerable differences in the sediment transport estimation since the measured and computed magnitudes differ up to an order of three because of the limited sediment availability in mountain streams 27 , 28 .…”