Spatial and temporal water quality changes during a large scale dredging operation

Stark, Clair (2016) Spatial and temporal water quality changes during a large scale dredging operation. PhD thesis, James Cook University.

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View at Publisher Website: https://doi.org/10.4225/28/5afb76a51fb47
 
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Abstract

Dredging poses an environmental risk by increasing suspended sediment which has a range of effects on sensitive benthic communities, particularly coral reefs. Understanding spatial and temporal sediment related dredging impacts is essential to improve environmental impact assessment (EIA), monitoring and management of dredge operations. Despite the scale of global dredging projects, our understanding of the impacts is limited due to a lack of sufficiently large water quality datasets, the site specific nature of water quality changes during dredging, and the complex response of corals to the various associated suspended sediment pressures (i.e. reduced light, increased sediment deposition). Of particular importance during the EIA phase, and while monitoring dredging impacts, is understanding the distance to dredge effects i.e. how far the dredge related sediment impacts extend to more accurately predict environmental impacts and provide greater protection to coral reefs during dredging operations.

The distance to dredge effects on water quality conditions (i.e. the spatial impacts of dredging) was investigated at Barrow Island, Western Australia, to determine how dredging affects turbidity, submarine light and sediment deposition conditions. Analysis was made possible using the largest water quality dataset ever collected prior to and during a large scale dredging operation. Water quality conditions prior to and during 18 months of dredging at Barrow Island, Western Australia, as well as the distance to dredge effects, were analysed to determine the impacts of dredging on turbidity, submarine light and sediment deposition. A high proportion of water quality sites (10/29) were located within 1.5 km south of dredging, allowing a high resolution of spatial dredging impact analysis close to the dredge zone. During dredging, water quality impacts were primarily confined to sites within 2 – 5 km south of the dredge zone, gradually decreasing to ambient levels at sites north of the dredge zone and sites > 10 km south. Turbidity maximums, means and standard deviations were up to 4 – 6 x higher, median light attenuation coefficients 1.5 x higher, median deposition levels up to 7 x higher and median overburden (dredge related turbidity, calculated using a simple statistical turbidity model which estimates natural turbidity during dredging) were 3 – 4 x higher at sites within 2 – 5 km south of dredging. Sites north of the dredge zone (extending up to 30 km north), sites > 10 km south of the dredge zone (extending up to 30 km south), and 2 dredge disposal perimeter sites were unaffected by dredging. There was also a strong relationship between light attenuation and turbidity at almost all of the 25 Barrow Island sites used to study light levels; 24 of the 25 sites had R² > 0.5 and 17 had R² ≥ 0.50.

Turbidity conditions at Barrow Island were also characterised by using a range of different temporal analysis, including running mean and spectral analysis. By applying running means using increasing window sizes (from 1 hour to 30 days) separately to the baseline and dredge periods, it was revealed that dredging increases both the intensity and the duration of turbidity, with monthly, daily and hourly turbidity conditions higher at sites within 2 km of dredging; monthly averages were up to 25 NTU (compared to ~ 10 NTU at reference sites), daily averages up to 200 NTU (compared to maximum ~ 30 NTU at reference sites) and hourly averages up to 400 NTU (compared to maximum 100 NTU at reference sites). Spectral analysis also revealed the occurrence of horizontal advection during dredging at sites within 2 km of dredging.

The use of a simple, statistical turbidity model to estimate natural turbidity (due to the natural resuspension processes of waves and tides) during dredging, and as a possible turbidity and deposition threshold exceedance monitoring tool, was investigated. The model is designed to be simple – an alternative method to the more complex three dimensional hydrodynamic models which require numerous inputs – and as such has expected limitations. Despite these limitations, the purpose of the model in this study is to decouple the natural turbidity and dredge induced turbidity, and possibly as an exceedance threshold tool. Model performance was tested in 2 different hydrodynamic settings – a clear water environment (Barrow Island) during a dredge operation and a turbid, energetic environment (Hay Point, Queensland) during a baseline water quality monitoring study. The model was successful at estimating daily turbidity at a few of the Hay Point and Barrow Island sites, with R² > 0.5 between modelled and measured turbidity at 83% of sites during the model test phase at Hay Point (although model skill scores were > 0.5 at only 1 site during the test phase), but only 38 % of sites had R² > 0.5 at Barrow Island and , but improvements could be made to both the input data and possibly more sophisticated parameter estimation tools (such as Bayesian analysis).

The impact of dredging on submarine light levels was also investigated. Light attenuation coefficients (k) were analysed in lieu of measured PAR values due to non-uniform sensor depths across the water quality sites (depths ranged from ~ 4 to 14 m), which introduces a depth dependence to the distance to dredge analysis. Median light attenuation coefficients at sites closest to the main dredge zone (within 2 – 5 km) were between 0.4 – 0.55 m⁻¹ compared to all other sites which had levels 0.35 – 0.4 m⁻¹. As well as calculating k (using the Beer-Lambert Law) for the spatial analysis, the strong relationship between midday turbidity and k (R² > 0.5 at 96 % of sites and ≥ 0.7 at 68 %) was used to derive regression models of light attenuation from measured (midday) turbidity. The use of a double exponential method, which is an extension of the Beer Lambert Law developed by Paulson and Simpson (1977), was also investigated for estimating the light attenuation coefficients but was unsuitable for the Barrow Island study sites.

Item ID: 50022
Item Type: Thesis (PhD)
Keywords: Barrow Island, coral reefs, dredge deposition zone, dredging, light attenuation, optical backscatter, photosynthetically active radiation, sediments, spatial analysis, temporal analysis, turbidity, water quality, water turbidity, Western Australia, Western Australian Marine Science Institution (WAMSI)
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Fisher, Rebecca, Stark, Clair, Ridd, Peter, and Jones, Ross (2015) Spatial patterns in water quality changes during dredging in tropical environments. PLoS ONE, 10 (12). pp. 1-22.

Jones, Ross, Fisher, Rebecca, Stark, Clair, and Ridd, Peter (2015) Temporal patterns in seawater quality from dredging in tropical environments. PLoS ONE, 10 (10). pp. 1-25.

Date Deposited: 04 Sep 2017 02:51
FoR Codes: 04 EARTH SCIENCES > 0405 Oceanography > 040503 Physical Oceanography @ 50%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050205 Environmental Management @ 50%
SEO Codes: 96 ENVIRONMENT > 9609 Land and Water Management > 960903 Coastal and Estuarine Water Management @ 50%
96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961104 Physical and Chemical Conditions of Water in Marine Environments @ 50%
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