Monitoring Dredging Plumes from Space

August 3, 2020, posted by savannahweeda

Emily Twiggs, Senior Project Scientist, EOMAP Australia

EOMAP applies its
satellite-derived turbidity monitoring capabilities to dredging programs run by
the North Queensland Bulk Ports Corporation (NQBP).

Turbidity plumes are stirred-up sediment which can
be transported long distances by currents. Understanding the full spatial and
temporal variability of turbidity, whether from natural causes such as tidal
currents and waves or from anthropogenic activity such as dredging, can be
costly if done with in-situ monitoring. Typically, in-situ monitoring, although
generally more precise, will be limited both temporally (from the time of
installation and onwards) and spatially (point measurements). Furthermore, it
requires mobilisation and ongoing maintenance of the equipment.

Satellite-derived turbidity data on the other hand,
provide a reliable and cost-effective synoptic overview of the variability of
turbidity across both space and time, without the need for field deployment. By
accessing image archives, satellite-based mapping can also retrieve historical
turbidity levels and extents going back a decade or more. These attributes can
offer a considerable advantage when trying to understand sediment transport and
the potential impacts on benthic communities, especially over large and/or
dynamic areas.

Recognising the essential value in this capability
NQBP, through a project led by Port and Coastal Solutions (PCS), contracted
EOMAP in 2019 to undertake satellite-derived turbidity monitoring before,
during and after annual maintenance dredging works at the Ports of Weipa and
Hay Point in Queensland.

Andrew
Symonds, Director of PCS commented, ‘‘We have utilised satellite-derived
turbidity data in several projects and have found it to be a valuable resource.
The satellite data can be used to provide an understanding of the natural
turbidity and how this varies spatially and temporally depending on the
metocean conditions. In addition, the data can also be used to determine
the spatial and temporal variability in turbidity plumes resulting from activities
such as dredging or from the propeller wash erosion of large vessels.”

NQBP
was granted a 10-year Commonwealth permit, issued by the Great Barrier Reef
Marine Park Authority (GBRMPA), to undertake sustainable
management of maintenance
dredging at the Port of Hay Point
in existing navigational channels. The approved Dredge Material Placement Area
(DMPA) for relocated sediment was 6 km north of port operations. The
Trailing Suction Hopper Dredge (TSHD) Brisbane completed the program during a
33-day period between March and May 2019.

To maintain safe and navigable depths for
port users, the TSHD Brisbane and Oranje undertook the dredging program within
the Port of Weipa with placement at the Albatross Bay DMPA, over 40 days
between June and July 2019. It was a much more extensive operation than
previous years due to several significant weather events over the preceding wet
season, including three tropical cyclones and a prolonged tropical low, that
resulted in widespread natural resuspension of sediment and three times the
highest annual sedimentation volume measured previously. This made it
especially important to try and capture the complete picture of sediment
dynamics in the area, something which the three turbidity loggers deployed
could likely not provide through the point measurement time-series.

For the retrieval of satellite-derived water quality
data, the physics-based Modular Inversion Processing System (MIP) developed by
EOMAP, is applied to multispectral satellite imagery at varying pixel
resolutions. This sensor-independent approach includes all the relevant steps
to guarantee a robust, standardised and operational retrieval of water quality
parameters such as turbidity, chlorophyll-a, harmful algal blooms and trophic
states, which in turn provide insights into the dynamics of inland and marine
waters, at both local and global scales.

To better understand the spatial distribution
and the temporal variability of the plumes resulting from the maintenance
dredging and placement of dredged sediment at the DMPA’s, three different
satellite systems were used, including high-resolution sensors Sentinel-2 (10m
pixel resolution, 5 days revisit time) and Landsat-8 (30m pixel resolution, 16
days revisit time), and the medium-resolution but high revisit frequency
Sentinel-3 (300m, 1-2 days revisit time). Automatic downloads of newly acquired
true-colour satellite imagery and the resulting processed turbidity maps were
uploaded to a customised version of EOMAP’s user-friendly web application
eoApp© which allows rapid and easy visualisation of turbidity products and the
generation of turbidity time-series plots.

Both dredging programs also included
extensive in-situ monitoring data, including real-time water quality loggers
and benthic habitat and infauna surveys. “When the satellite data is
combined with in-situ water quality measurements it can be possible to identify
when/if the turbidity at specific measurement sites is elevated due to
anthropogenic activities such as dredging. Satellite-derived turbidity
data can therefore be used to provide extremely useful data to support a range
of different projects including natural sediment transport assessments and
monitoring associated with dredging”, explained Andrew Symonds.

The Hay Point program was completed
successfully in trying weather conditions, with the dredging and relocation of
~353,740 m³ of sediment. Satellite data showed that turbidity over
the area, pre, during and post dredging, was predominantly due to the natural
resuspension of bed sediment attributed to strong tidal currents and energetic wind/wave
conditions.

At Weipa, where more than 2.4 million m³
of sediment was dredged and relocated, satellite-mapped plumes were visible and
remained for multiple dredge cycles but remained close to where they were
generated (dredged channels and DMPA sites). The turbidity map series also
showed that some of the apparently elevated levels of turbidity in the wider
area were not originating from the dredging activities, and instead attributed
to natural processes. The satellite data furthermore provided enough detail to
show that propeller wash from the dredgers resulted in the resuspension of some
fine-grained bed sediment when sailing to and from the dredging and DMPA sites.

In summary, the synoptic information gained from
the satellite-based monitoring offered ongoing spatio-temporal dynamics
of turbidity dredge plumes and
gave further insights into natural sediment transport mechanisms for both
areas. Satellite-based turbidity monitoring is a cost-efficient way for
both retrospective baseline analysis and continuous, spatially coherent dredge
monitoring, requiring no mobilisation effort.

Figs/video: Turbidity maps from each site and video of TSHD Oranje in action at Weipa.