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

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, ﬂ ow 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 ﬂ ow characteristics, which are similar to ﬂ ows over a ﬂ at bed. At the boulder centerline, the ﬂ ow is largely de ﬂ ected 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 – rif ﬂ e sequence. An analysis shows that the length scale ( L/D ) of geometric features associated with pool depth, rif ﬂ e height and pool – rif ﬂ e distance ( S/D ) are positively related to the boulder-related Froude number ( Fr b ): L/D ¼ 1.18 Fr b (cid:2) 0.11 and S/D ¼ 12.5 Fr b þ 0.6; and the erosion volume ( V e ) for ﬂ at bed and boulder bed is positively and negatively related to the averaged Froude number ( Fr ): V e / D 3 ¼ 37.1 Fr (cid:2) 21.3 and V e / D 3 ¼ (cid:2) 44.8 Fr (cid:2) 38.6, where D is the boulder diameter.


INTRODUCTION
Bed configurations in mountain rivers are much more complex than in plain rivers, owing to steep slopes, poorly sorted surface grains (Rickenmann ; Papanicolaou et al. ), wide grain size distributions, heterogeneity in bed topography, large and immobile boulders, pebble clusters, sometimes bedrock, and so on. In addition, large-size boulders are likely to be introduced onto river beds by extreme events such as torrential floods, landslides and debris flows, leading to high obstructive roughness (Zimmermann & Church ; Galia & Hradecký ).
The above factors enable hydromorphodynamics in mountain rivers to largely differ from those in plain rivers (Shamloo et al. ).

IMPACTS OF BOULDERS ON HYDRODYNAMICS
In recent decades, numerous laboratory studies and field   Measured data were filtered out when the correlation coefficient was less than 70%, the noise decibels were less than 15 dB, and overall confidence intervals were higher than 90% (Lacey & Roy ).

IMPACTS OF BOULDERS ON BED MORPHODYNAMICS
By fitting the coordinate with the flume, u, v and w denote time-averaged velocities in x, y and z directions.
The coordinate system is nondimensionalized by effective diameter D ¼ 15.7 cm for x þ ¼ x/D, y þ ¼ y/D, and by con-  Table 1.

Experimental measurement of bed morphological characteristics over a boulder bed
To investigate the impact of a boulder on bed morphological adjustment, water scour experiments were carried out in an outdoor cement flume with a steep bed slope of 1.25% as Overall, initial gravel bed constitutuents, grain size distribution and initial bed surface slope were constant. Three water discharges (Q) were adopted for each group.
According to the particle incipient motion velocity (U e ) calculated by the Shamov formula and the measured flow velocity pre-experiment, Q ¼ 15 l/s, 20 l/s, 25 l/s   Table 2).
In the case of a boulder bed, a boulder with an effective diameter of D ¼ 12.3 cm was embedded in the sediment layer 4-5 cm below the sediment surface.
Consequently, the ratio of boulder size to gravel size reached ∼23.06, which was close to the geometric ratio from the survey. The size of fed sediment ranged from 4 mm to 8 mm. For the three discharges, sediment of 30 kg was used and the feeding rates were 1 kg, 1.5 kg and 2 kg per two minutes, respectively. When the equilibrium bed morphology was reached, the water level and bed elevation at monitored cross-sections were measured in the following strategy. An electronic total station (ETS) in a local coordinate system was firstly used to measure several locations along the entire reach, referred to as controlling points. Then, the software Agisoft PhotoScan  This effect is more pronounced for regions at the downstream of the boulder. Especially, the streamwise velocity tends to increase vertically above the boulder top level (z þ ¼ 1), assembling an inflection in the velocity profile.  Moreover, below the boulder top level, the streamwise velocity is dramatically deflected compared with its counterpart in the upstream, indicating potential sediment deposition in the wake region of the boulder.
For the vertical velocity (W/u * ) (see Figure 3   An increasing discharge tends to result in greater degradation. In the presence of a boulder, the bed profile can be divided into several segments with constant slopes represented by i wb and i wbs in the cases of no sediment feeding and sediment feeding. A scour pool can be clearly observed around the boulder, following which a high-level riffle exists downstream. Two such bed morphological features constitute a pool-riffle sequence. However, with upstream sediment feeding, the bed topography differs a lot from its counterpart without upstream sediment feeding. When sediment is supplied at the upstream, moving sediment can partially compensate for the original bed materials which are eroded downstream. Consequently, the bed topography in the presence of sediment feeding is generally higher than that in the absence of sediment feeding. Therefore, the pool region becomes shallower and the riffle region becomes higher in the presence of sediment feeding. In addition, this tendency is stronger when the water discharge is smaller (Q ¼ 15 l/s and 20 l/s). However, an exception occurs for the highest water discharge, under which the bed topography in the presence of sediment feeding is slightly lower than that in the absence of sediment feeding.
As indicated by Figure 6, it is rationally expected that the characteristic bed morphologies (i.e., pool and riffle) might be substantially related to the flow condition and the geometry boundary. Thus two length scales (L 1 and L 2 ) which are associated with the geometries of pool and riffle are defined as shown in Figure 6(a). In addition, the distance (S) between the pool and riffle is regarded as a characteristic geometric scale. Two scenarios are proposed according to flume observations. It is simple to know that the riffle is below the initial bed elevation when L 1 > L 2 and above the initial bed elevation when L 1 > L 2 . Figure 6 increase in pool length scale with the increasing boulderrelated Froude number (see Figure 6(b)). In fact, more scour also results in more downstream deposition unless the stream power is large enough to erode all downstream bed materials. Therefore, it is reasonable that net bed erosion can decrease with increasing mean stream power.
Interestingly, the biased effect observed in Figure 6(a) and 6(b) disappears in this relation. scenarios V e /D 3 ¼ À44.8FrÀ38.6, differing from the high bed degradation with high Fr in the absence of the boulder V e /D 3 ¼ 37.1FrÀ21.3.