Measurements of high concentration sediment plume in the
estuary with strong tidal currents
Hwa CHIEN*a
Assistant Professor
Institute of Hydrological and Oceanic Sciences, National Central University, Taiwan,
hchien@ncu.edu.tw
No.300 Jhongda Rd., Jhongli, Taoyuan, Taiwan 320
Fax:+886 3 422-2894
Wen-son CHIANGb
Associate research scientist, Tainan Hydraulic Labs, National Cheng Kung University, Taiwan
chws@mail.ncku.edu.tw
Philip L.-F. LIUc
Professor, Dept. Civil Engineering, Cornell University, USA
pll3@cornell.edu
Kon-Kee LIU
Institute of Hydrological and Oceanic Sciences, National Central University, Taiwan,
kkliu@ncu.edu.tw
No.300 Jhongda Rd., Jhongli, Taoyuan, Taiwan 320
Hyperpycnal event is defined as the density of sediment-laden riverine outflow exceeds that
of the ambient sea water. Typically the criterion requires sediment concentration of fresh river
outflows approaching 40g/l. The hyperpycnal flow can plunge and form turbidity plume into
the sea, travel significant distances and sustained for hours to weeks. The dispersal of
hyperpycnal sediment is dominated by diversified processes including the buoyant fresh water
plumes, dilute suspended transport, negatively buoyant sediment gravity currents and the
waves, currents and shelf morphology. The hyperpycnal flows have profound impacts on the
seabed and coastal morphology and they are considered to be the major processes that
transport the terrestrial materials and fluvial sedimentation to the deep ocean. Moreover,
through the oceanic sedimentary deposition, the stratifications of ocean water might be
disturbed accordingly and affect the Navy acoustic/optical applications.
With the facts of high erodibility (3-6mm/yr) resulting from rapid plate convergence (80
mm/yr) accompanied by frequent large earthquakes and the intensified typhoon precipitation
intensity and floods, the occurrence frequency of hyperpycnal events of Taiwanese rivers can
reach to more than 1 event/yr, which is O(2) magnitude larger than the worldwide average.
Taiwan is an ideal place for hyperpycnal flow study for the better chance to obtain the
observational evidence. In present study, the flood induced high sediment concentration
fluvial discharge are focused, data obtained from field surveys at the estuary of Chuoshui
River, the second largest river in Taiwan, are investigated.
In 2008, the extremely torrential rainfall in the Choushui watershed during the landfall of
typhoon Kalmaegi incurred hazardous inundation in the southwestern coast of Taiwan. During
the passage of the typhoon, field surveys of fluvial discharge and suspended sediment
concentration on the bridges were carried out and demonstrated the onset of high density
riverine flow (>200 g/l). The sediment yield exceeded 36 MTon within 48 hours. Field
observations in the estuary were implemented immediately right after the passage of typhoons
(48 hrs after the peak fluvial discharge). A bottom-mount station, equipped with optical
backscatter sensors array and ADCP was setup to measurement the suspended sediment
concentration profile, waves and tidal current. Synchronized measurements by towed ADCP,
water sampling at various depths, OBS and CTD probing were preformed on vessels along
the 13km transaction track from the river mouth. Satellite water color images from Formosa II
were also retrieved for analysis.
River plume with sharp frontlines between turbid and fresh water were observed. Due to the
oscillation of strong alongshore tidal current (up to 2 m/s), secondary and the third frontline
could be identified from the satellite images. The Maximum SSC that measured at the vicinity
(2 km) of the river mouth bed was around 2600mg/l and the value decreases with the
elevation; decreased along the transect to the offshore. The mean grain size was around 10
micron for suspended-load and 0.5mm for bed-load. These observations indicated the rapid
formation and deposition of hyperpycnal event. The negative buoyant sediment current was
not yet able to be observed in time. To assess the sediment deposition yields, the
discrepancies of the estuary bathymetric contours of 2007 and 2008 were verified.
In order to estimate the transport flux of suspended sediments, two approaches, i.e. the
random phase acoustic backscatter model and the bottom boundary layer model-dependent
inversion method, are adopted and applied to the acoustic and optical backscatter data to infer
the suspended sediment concentration (SSC) profile. The results are inter-compared. It is
shown that the strong tidal current and associated eddy diffusivity of momentum bring
difficulties to the accurate determination of sediment mass diffusivity and thus the sediment
flux can hardly be estimated by the inversion method.