Overland flow hydrodynamic characteristics at low Reynolds number with
roughness bed
Abstract
Overland flow is the major contributor to soil erosion. To clarify the
hydrodynamic characteristics of overland flow at small Reynolds number,
indoor experiments with fifteen unit-width flow discharges from 0.069 ×
10-3 m2·s-1 to 2.5 × 10-3 m2·s-1, five slope gradients from 5.23% to
25.88%, three surface roughnesses and two kinds of flow (80% glycerol
and water mixed flow and water flow) were systematically investigated.
Results showed that mean depth and mean flow velocity can be good
predicted by unit-width flow discharge, slope gradient and surface
roughness. Based on flow regime criterion of parameter m, for 80%
glycerol and water mixed flow, the flow regime was laminar flow. For
water flow, it was between laminar flow and turbulent flow. According to
the transitional Fr of 1, the experimental flow state tended to
subcritical laminar flow with the increase of surface roughness. For
80% glycerol and water mixed flow, parameter K was 57. For water flow,
parameter K was increased with the increase of surface roughness and
fluctuated as slope gradient increased. The resistance law of open
channel hydraulic for laminar flow (f = 96/Re) is not suitable for
overland flow. In general, resistance coefficient had a good power
function with Re. Meanwhile, there was a high significant correlation
between resistance coefficient and inundation ratio and slope gradient.
Resistance coefficient decreased as inundation ratio and slope gradient
increased. For all flow regime in this study, a more accurate resistance
coefficient prediction model was established by multiple regression
analysis. As for hydrodynamic parameters, shear stress had a positive
correlation with surface roughness. Meanwhile, stream power is not
affected by increasing surface roughness, while unit stream power was
negative with surface roughness. The slope gradient played a more
important role in increasing the flow energy.