A wind tunnel study of aeolian sand transport on a wetted sand surface using sands from tropical humid coastal southern China

Abstract

This article reported a wind tunnel test of sediment transport related to surface moisture content and wind velocity using sands from tropical humid coastal area.

A fully predictive model for aeolian sand transport

“…The impact threshold is smaller than the fluid threshold because the transfer of momentum to the surface through particle impacts is more efficient than through drag (Kok et al, 2012). Herein, only the dynamic threshold shear velocity based on Equation (2.2) is used with  th = 0.11 based on data of Shao-Lu (2000) and Han et al (2011) whereas  th 0.1 based on Bagnold (1941). Experimental data (Kok et al, 2012) show that the measured threshold shear velocities of very fine sediments between 20 and 100 m are scattered with values between 0.15 and 0.25 m/s.…”

“…Experiments in wind tunnels with dry, loose sand surfaces have shown that the transport layer including the saltation layer and the suspended layer is of the order of 0.1 to 0.5 m for low to high wind velocities and grain diameters between 0.2 and 0.45 mm. Most of the transport takes place in a layer of about 0.05 m (Han et al, 2011;Yang et al 2019). The adjustment of the boundary layer flow over a rough surface can be described by 0.2x 0.6 (Granger et al, 2006).…”

“…Saturated/equilibrium transport: q s,eq =  B  ad  shell ( with: q s,eq = mass flux of sediment at equilibrium conditions (saturated transport, kg/m/s); d 50 = particle size (m); d 50,ref =reference particle size = (250 10 -6 m; 250 m);  air = density of air (1.2 kg/m 3 );  s = density of sediment (2650 kg/m 3 ); s= s / air = relative density, =kinematic viscosity coefficient (m 2 /s), g = acceleration of gravity (m/s 2 ); u * = surface shear velocity due to wind forces (m/s); u *,th =surface shear velocity at initiation of motion; threshold shear velocity (m/s); k s = equivalent roughness length scale of Nikuradse (m); u w = wind velocity at height z wind above the surface (m/s); = constant of Von Karman (=0.4 for conditions without transport);  B = Bagnold-coefficient for dry sand  1.5 for uniform sand; 1.8 for naturally graded sand; 2.8 for widely graded sand (Bagnold, 1941); herein the value of 2 is used as default value:  B = 2;  ad = adjustment coefficient related to fetch (maximum 1);  shell = reduction coefficient related to the presence of shells;  th = 0.11 based on data of Shao-Lu (2000) and Han et al (2011);  0.1 based on Bagnold (1941);  slope = coefficient for sand grains at a sloping surface;  w = moisture coefficient (=1 for dry sand);  veg = vegetation coefficient (=1 for conditions without vegetation). The effects of moisture ( w ), vegetation ( veg ), shells ( shell ) are discussed in Section 5.…”

A Rational Method for the Design of Sand Dike/Dune Systems at Sheltered Sites; Wadden Sea Coast of Texel, The Netherlands

“…The aeolian sand transport model of Bagnold [20], which is only valid for loose and dry sand has been modified to include the effects of moisture and vegetation, as follows (Eqautions (2)-(4)): ; α sh = sheltering coefficient (α sh < 1 for sheltered sites; α sh = 1 for exposed sites); α site = coefficient for locations higher than the beach (more exposed; giving higher wind speeds due to local contraction) = 1 + 0.03h e ; h e = exposed level (crest level) above beach (m); h e = 0 m for sand transport at beach; L fetch = fetch length at beach (input; about 10 to 100 m normal at beach); L adjustment = adjustment length scale of sand transport to attain equilibrium transport (input; about 100 to 200 m). The saltation characteristics (length and height) have been derived from the work of Han et al 2011 [40] and Kok et al 2012 [38] and read as follows: L saltation = 0.0001(u * ) 1.7 /(d 50 ) 1.2 and h saltation = 0.04 L saltation (5) Equations (2)- (5) have been used to compute various basic parameters of aeolian sand transport. Figure 12 (upper panel) shows the wind-driven transport as function of the wind speed at the beach level and at dune crest level.…”

“…It is noted that the presence of steep dune slopes and vegetation (maram grass) will strongly reduce the wind-driven transport upslope. Figure 12 (lower panel) shows the saltation length and height of the sand grains for various wind strength classes based on the work of Han et al 2011 [40] and Kok et al 2012 [38]. This information was used to determine the height and locations of wind screens to prevent the transportation of sand to the hinterland as much as possible.…”