wAbstract
@A month-long comprehensive field experiment during the beginning
of the flood season of a small mountainous river in southern Taiwan
was carried out in 2004. The major goal of the study was to investigate
the effect of hydrodynamic sorting related to the salt-water intrusion
on the spatial variability of suspended and riverbed sediments
and their coupling of different sizes. The experiment included
the deployment of an instrumented tetrapod near the river mouth
with an upward-looking ADCP and two CTDs mounted at 50 and 100
cm above the bed (cmab), respectively. On three different days
along the river, turbidity, salinity and temperature of the water
column were profiled; and water samples were taken from the surface
and near the bed at different stations. Additionally, one sediment
sample was also taken from the riverbed. Suspended sediment concentrations
(SSC) were analyzed for five different sizes, i.e.500, 250-500,
63-250, 10-63 and 1.2-10Κm by filtration. For each riverbed sample
the size-composition was analyzed for the subsample that contained
lithogenic and nonlithogenic components; and the subsample having
major nonlithogenic components (organic matter, carbonates, and
biogenic opal) was removed. The grain-size frequency distributions
of the riverbed samples were analyzed using a laser particle analyzer.
The results were grouped into the following size classes: 473,
249-473, 62-249, 10-62, and 10Κm for comparison with those of
the suspended sediments near the riverbed. Some suspended sediment
samples were analyzed for POC and PON. Some riverbed samples were
analyzed for TOC. Statistical methods of linear regression and
Empirical Orthogonal (Eigen) Function (EOF) were used in the data
analysis.
@Two-layered estuarine circulation pattern was observed at the
tetrapod site. The tidally-driven salt-water intrusion is the
major factor influencing the hydrodynamics of the Gaoping River,
which in turn, affect the longitudinal and vertical distribution
of the suspended sediments and the longitudinal distribution of
the riverbed sediments. During the flood, the intrusion front
of the salt-wedge creates a dynamic barrier. Upriver from this
barrier, the riverbed substrate is coarser, composed of sediments
mostly coarser than 249Κm. Within the salt-wedge the riverbed
substrate is finer, consisting of mostly mud (62Κm). The barrier
creates a trap on the riverbed immediately seaward from the intrusion
front, retaining higher percentages of clay-sized sediments and
TOC.
@The barrier also creates partition in the terrestrial and marine
sources of organic matter in the suspended and riverbed sediments.
Within the salt-wedge the major contributor of riverbed TOC is
the clay-sized marine sediment transport upriver by the intruding
seawater. The terrestrial POC is a minor contributor to the riverbed
TOC.
@The riverbed and suspended sediments are coupled. Most size-classes
in corresponding suspended and riverbed sediments have a reciprocal
relationship (negative feedback) through resuspension and deposition
on the water-bed interface. The only size-class of 62-473Κm on
the riverbed and 63-250Κm in the suspension are co-varying (positive
feedback). This size class contains largely a transient floc population
that is formed and disintegrated in-situ both on the riverbed
and in the suspension in the course of a tidal cycle.
Keywords: salt-wedge; dynamic barrier; hydrodynamic sorting; particle
size; flocculation; carbon-particle relationshipx
1. Introduction
2. Study area and background
3. Field experiments
@3.1. Tetrapod deployment
@3.2. Hydrographic profiling and water and substrate sampling
along the river
4. Laboratory analyses
@4.1. analyses of water samples
@4.2. Analyses of riverbed sediment samples
@@4.2.1. Grain-size analysis of subsamples having all components
@@4.2.2. Grain-size analysis of subsamples having only lithogenic
components
@@4.2.3. Total organic carbon (TOC) analysis
5. Results
@5.1. Time series measurements on the tetrapod
@@5.1.1. ADCP measurements
@@5.1.2. Sea surface fluctuation and salinity variations with
respect to the river discharge and tide
@5.2. Spatial observations along the river
@@5.2.1. Longitudinal variability of water-born constituents
@@5.2.2. Analysis results of riverbed samples
6. Statistical analyses
@6.1. Empirical Orthogonal (eigen) Analysis (EOF)
@@6.1.1. The water column subset
@@6.1.2. The interface subset
@6.2. Linear regression analysis
7. Discussion
@7.1. Grain-size partition related to the salt-wedge
@7.2. Terrestrial versus marine source partition related to the
salt-wedge
@7.3. A transient floc population related to the salt-wedge
@7.4. Hydrodynamic sorting and source-to-sink implication related
to the salt-wedge
8. Conclusions
Acknowledgements
References