||This project focuses on two fundamental questions: 1) How does
flow and sediment routing through a lowland floodplain system moderate
short and longer-term variations in sediment delivery towards offshore
depositional environments? and 2) What controls the proportion of
a river's sediment load that is deposited on its floodplain? It
is hypothesized that net sediment loss to the floodplains was highest
during Holocene sea-level rise and, after near stabilization of
sea level, the proportion of the sediment load deposited in the
floodplain has progressively declined.
These two basic question are being addressed by comparing the
flow and sediment routing processes, and the proportion of sediment
lost to floodplains on the middle Fly and lower Strickland Rivers.
Historically, the Strickland carried about 7 times the load of
the Fly and it is hypothesize that this larger load has led to
greater channel dynamics, steeper slopes, coarser bed, more elevated
floodplain and possibly a lower trapping efficiency than the Fly.
A numerical model is being developed that is sufficiently mechanistic
that it can address the questions of damping and trapping efficiency
on the event and seasonal scale, yet simplified enough that it
can also model the co-evolving bed grain size, channel slope and
floodplain topography, a crucial capability for understanding
the time evolution of trapping efficiency. Though ambitious, many
parts of the model have been assembled in previous research. The
hydrodynamic model accounts for the effects of floodplain morphology
and hydrology. The sediment routing includes effects of channel
migration and accounts for the deposition and erosion of sediment
by grain size.
A field program is being conducted to document the flood wave
damping rates, the sediment trapping efficiency of the Strickland
and Fly Rivers, and to parameterize the numerical model. Quantification
of floodplain morphology is being done through topographic surveys
and analysis of remote sensing imagery. Intensive surveys of the
velocity and suspended sediment fields are used to motivate and
parameterize the hydraulics and sediment transport model. Short
and longer-term (less than 100 years) rate of floodplain deposition
is being documented from shallow cores. Extensive flow and sediment
monitoring data are being provided by Ok Tedi Mining, Ltd. on
the Middle Fly and by Porgera Joint Venture on the Strickland.
Previous research on the Fly River enables the research focus
to be on the Strickland River.