HI. Sediment Transport
95
2.2.3
Concentrations in sheet flow layer
Observations have shown that bedforms are washed out when the mobility
parameter • is larger than about 200 to 250. In that case a thin (= 0.03 m or
0.096 ft.) layer of moving sediment particles with high concentrations close to the
bed is generated. This is called the sheet flow layer. Time-averaged
concentrations in the sheet flow layer have only been measured in wave tunnel
experiments (Horikawa et aL, 1982 ; Staub et al., 1984 and Ribberink, 1989).
Field data are not available. Horikawa et al. (1982) used an electroresistance
probe to measure the concentrations over a sand bed of 200 #m. Staub et al.
(1984) and Ribberink (1989) used a pump sampling instrument. A regular
sinusoidal oscillatory motion was generated by Horikawa et al and by Staub et al.,
while irregular oscillatory flow (Jonswap spectrum) was generated by Ribberink.
Based on their results, the following characteristic concentration ranges at 0.01 m
(0.03 ft.) and at 0.02 m (0.06 ft.) above the initial bed level (measured before the
test) are given :
Height
Sediment concentrations"
Staub etaL, 1984
Ribberink, 1989
above
Horikawa
the bed
et al., 1982
200 txm
(m)
ft = 1.08 m/s
T=4s
0.01
3 kg/m ~"
0.02
1 kg/m ~
190 Ima
~= 1.3 rn/s
T=9.1 s
10-25 kg/m ~
3-10 kg/m ~
190 Ima
ft = 1.68 m/s
T=6.8s
50 kg/m ~
20 kg/m ~
380 lain
ii= 1.9 m/s
T=6.8 s
I0-30 kg/m ~
3-15 kg/m ~
200 Inn
fi,~ = 1.13 m/s
Tp=SS
15-20 kg/m 3
4-8 kg/m 3
The results of Staub et al and Ribberink are in good agreement for peak velocities
in the range of 1.1 to 1.3 m/s (3.52 to 4.16 ft./s) and sediment material of about
200 #m. The concentrations measured by Horikawa et al are a factor 5 smaller at
a peak velocity of 1.08 m/s. (3.46 ft./s).
2.2.4
Summary
Summarizing all available concentration data for non-breaking and breaking
waves in water depths of 0.5 to 2 m (1.6 to 6.4 ft.) and bed material sizes from
100 to 300 #m, the following characteristics can be observed :
• two-layer concentration profile with rather large concentration gradients in
the near-bed layer (~ 3 ripple heights) and smaller concentration gradients in
the upper layer, in case of non-breaking waves over a rippled bed,
m/s--, ft/s : 1.08 ~ 3.46 ; 1.3 --,4.16 ; 1.68->5.12 ; 1.9--,4.16 ; 1.13 --,3.62
kg/m 3 --, lb./ft 3 : multiply by 0.062
95
2.2.3
Concentrations in sheet flow layer
Observations have shown that bedforms are washed out when the mobility
parameter • is larger than about 200 to 250. In that case a thin (= 0.03 m or
0.096 ft.) layer of moving sediment particles with high concentrations close to the
bed is generated. This is called the sheet flow layer. Time-averaged
concentrations in the sheet flow layer have only been measured in wave tunnel
experiments (Horikawa et aL, 1982 ; Staub et al., 1984 and Ribberink, 1989).
Field data are not available. Horikawa et al. (1982) used an electroresistance
probe to measure the concentrations over a sand bed of 200 #m. Staub et al.
(1984) and Ribberink (1989) used a pump sampling instrument. A regular
sinusoidal oscillatory motion was generated by Horikawa et al and by Staub et al.,
while irregular oscillatory flow (Jonswap spectrum) was generated by Ribberink.
Based on their results, the following characteristic concentration ranges at 0.01 m
(0.03 ft.) and at 0.02 m (0.06 ft.) above the initial bed level (measured before the
test) are given :
Height
Sediment concentrations"
Staub etaL, 1984
Ribberink, 1989
above
Horikawa
the bed
et al., 1982
200 txm
(m)
ft = 1.08 m/s
T=4s
0.01
3 kg/m ~"
0.02
1 kg/m ~
190 Ima
~= 1.3 rn/s
T=9.1 s
10-25 kg/m ~
3-10 kg/m ~
190 Ima
ft = 1.68 m/s
T=6.8s
50 kg/m ~
20 kg/m ~
380 lain
ii= 1.9 m/s
T=6.8 s
I0-30 kg/m ~
3-15 kg/m ~
200 Inn
fi,~ = 1.13 m/s
Tp=SS
15-20 kg/m 3
4-8 kg/m 3
The results of Staub et al and Ribberink are in good agreement for peak velocities
in the range of 1.1 to 1.3 m/s (3.52 to 4.16 ft./s) and sediment material of about
200 #m. The concentrations measured by Horikawa et al are a factor 5 smaller at
a peak velocity of 1.08 m/s. (3.46 ft./s).
2.2.4
Summary
Summarizing all available concentration data for non-breaking and breaking
waves in water depths of 0.5 to 2 m (1.6 to 6.4 ft.) and bed material sizes from
100 to 300 #m, the following characteristics can be observed :
• two-layer concentration profile with rather large concentration gradients in
the near-bed layer (~ 3 ripple heights) and smaller concentration gradients in
the upper layer, in case of non-breaking waves over a rippled bed,
m/s--, ft/s : 1.08 ~ 3.46 ; 1.3 --,4.16 ; 1.68->5.12 ; 1.9--,4.16 ; 1.13 --,3.62
kg/m 3 --, lb./ft 3 : multiply by 0.062