13.2 Transport and Mixing in Estuaries
and a, respectively. The power W takes the form:
WS
W=-j3,,"u. '
395
(13.6)
where Ws is the falling velocity of the suspended sediment, j3 is a constant, and
"" is the Karman constant equal to 0.4.
When the time series of concentration, c, at a given point is known and relationships for friction velocities are established, the erosion and deposition terms
can be applied in Eq. (13.1). The comparison predictions with observations
showed that both the hydrodynamics and the sediment load were satisfactory
reproduced. The model confirmed that in the Forth Estuary, the horizontal
processes are unimportant in comparison to the effects of erosion/resuspension
and settling. In Fig. 13.1, a comparison is shown for observations and simulations using a full two-dimensional model (Eq. 13.1) and a simplified model
in which horizontal advection and diffusion were neglected. The character of
variation in time of the concentrations suggests that horizontal processes are
relatively unimportant due to the strong quarter-diurnal signal that is characteristic of local resuspension and deposition.
Extensive data on fine sediment dynamics were obtained in the extremely
turbid Jiaojiang River Estuary, China (Guan et al., 1998). These data have
been used to calibrate a two-dimensional width-integrated model. The Jiaojiang River Estuary is very shallow with depth 1-3 m at low tide. Semi-diurnal
tides have a maximum tidal range of 6.3 m and a maximum vertically-averaged
tidal current of 2.0 m/s. At the estuary bed, a cohesive sediment (clay and
fine silt with a diameter less than 8 J.Lm) prevails, and the suspended sediment
concentration exceeds 40 kg/m- 3 during spring tides in calm weather.
The governing equations of the model include the mass conservation and momentum equation for water, and conservation equations for salinity and sediments. Horizontal and vertical diffusion and the mass exchange of suspended
sediment with the bottom have been parameterized using relationships known
in literature. A comparison of measured currents and sediment concentration
at three levels above the sea bottom (1.67 m, 0.55 m and 0.35 m) with model
predictions is shown in Fig. 13.2. This figure demonstrates cycles of resuspension and settling in the bottom layer where concentration only rarely reaches
10 kg/m 3 . A comparison of concentrations at 1.67 m and 0.55 m levels shows
the occasional presence of a strong vertical gradient of concentration.
The field data also indicate that sediment concentration during a flood tide is
30% larger than during ebb tide. This asymmetry is responsible for an infilling
rate of the estuary of about 0.1 m/year, while measured sediment load from the
riverine inflow is too small to be a substantial contributor to the siltation of the
estuary. Presumably the origin of such a large amount of sediment, which is
infilling the estuary, is the Yangtze River, which has its mouth located 200 km
further north (Guan et al., 1998). The Yangtze River, the third largest river in
the world, transports 9.3 x lO lD m 3 /year of water and 4.7 x 10 7 tonnes/year
and a, respectively. The power W takes the form:
WS
W=-j3,,"u. '
395
(13.6)
where Ws is the falling velocity of the suspended sediment, j3 is a constant, and
"" is the Karman constant equal to 0.4.
When the time series of concentration, c, at a given point is known and relationships for friction velocities are established, the erosion and deposition terms
can be applied in Eq. (13.1). The comparison predictions with observations
showed that both the hydrodynamics and the sediment load were satisfactory
reproduced. The model confirmed that in the Forth Estuary, the horizontal
processes are unimportant in comparison to the effects of erosion/resuspension
and settling. In Fig. 13.1, a comparison is shown for observations and simulations using a full two-dimensional model (Eq. 13.1) and a simplified model
in which horizontal advection and diffusion were neglected. The character of
variation in time of the concentrations suggests that horizontal processes are
relatively unimportant due to the strong quarter-diurnal signal that is characteristic of local resuspension and deposition.
Extensive data on fine sediment dynamics were obtained in the extremely
turbid Jiaojiang River Estuary, China (Guan et al., 1998). These data have
been used to calibrate a two-dimensional width-integrated model. The Jiaojiang River Estuary is very shallow with depth 1-3 m at low tide. Semi-diurnal
tides have a maximum tidal range of 6.3 m and a maximum vertically-averaged
tidal current of 2.0 m/s. At the estuary bed, a cohesive sediment (clay and
fine silt with a diameter less than 8 J.Lm) prevails, and the suspended sediment
concentration exceeds 40 kg/m- 3 during spring tides in calm weather.
The governing equations of the model include the mass conservation and momentum equation for water, and conservation equations for salinity and sediments. Horizontal and vertical diffusion and the mass exchange of suspended
sediment with the bottom have been parameterized using relationships known
in literature. A comparison of measured currents and sediment concentration
at three levels above the sea bottom (1.67 m, 0.55 m and 0.35 m) with model
predictions is shown in Fig. 13.2. This figure demonstrates cycles of resuspension and settling in the bottom layer where concentration only rarely reaches
10 kg/m 3 . A comparison of concentrations at 1.67 m and 0.55 m levels shows
the occasional presence of a strong vertical gradient of concentration.
The field data also indicate that sediment concentration during a flood tide is
30% larger than during ebb tide. This asymmetry is responsible for an infilling
rate of the estuary of about 0.1 m/year, while measured sediment load from the
riverine inflow is too small to be a substantial contributor to the siltation of the
estuary. Presumably the origin of such a large amount of sediment, which is
infilling the estuary, is the Yangtze River, which has its mouth located 200 km
further north (Guan et al., 1998). The Yangtze River, the third largest river in
the world, transports 9.3 x lO lD m 3 /year of water and 4.7 x 10 7 tonnes/year