Remote Sensing of Sediment in Water Bodies

by

Michael Newton

Fall 2007

Sediment off the coast of the Gulf of Mexico
Gulf of Mexico
(Image taken from http://rst.gsfc.nasa.gov/Sect14/Sect14_13.html.)

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The Significance of Sediment

Geologic and other natural processes have always caused sediment run-off to contaminate water sources. Erosion due to earthquakes, tsunamis, volcanic eruptions, mudslides, and wind are just a few examples of these natural processes.

Multispectral image showing suspend sediment after a tsunami receded in Cuddalore, India
Tsunami Aftermath
(Image taken from http://volcanoes.usgs.gov/About/What/Monitor/Hydrologic/SedsMove.html.)

Urbanization and agriculture are both examples of man-made activities that increase sediment erosion. This increase further contaminates water sources and can disrupt the ecological balance in certain water bodies. Water management, biological growth, and potential weather patterns can all be affected by the increased volume of sediment.

The Importance of Remote Sensing

Landsat 7
Landsat 7
(Image taken from http://svs.gsfc.nasa.gov/stories/earth_sci_20040422.)

Attempts to measure the volume of sediment in water (or turbidity) have long been problematic. For example, attempting to gather accurate data in a river using field measurements is complicated. Calculating the volume or cross sectional area of a river, with any accuracey, is fairly complex. Nature rarely exhibits basic shapes with simple calculations, not to mention that over time, the shape and size of a river changes, sometimes dramatically. Once a reasonable volume is found, measurements can be taken, often times using intruments lowered into the river from overhead. These instruments measure the speed and flow of water as well as the amount of sediment at different depths in the river. The measurements are usually taken at specific time intervals. Once a set of data is collected, the volume of sediment discharge can be found using all the information gathered.

Measuring Sediment
Example of measuring sediment
(Image taken from http://volcanoes.usgs.gov/About/What/Monitor/Hydrologic/SedsMove.html.)

This volume, however, is often coupled with a relatively high uncertainty. Attempts to lower the uncertainty have been aided by remote sensing. Until relatively recently, remote sensing has not been a productive way to gather data about water. The high reflectivity of water has prohibited accurate, if any, data to be collected from any body of water. The advent of multispectral and hyperspectral instruments, however, has helped to alleviate this problem. These and other types of measurements are not hindered by the reflective properties of water, and aid dramatically in identifying suspended sediment by doing so spectrally. Accuracey in temporal measurements can also improve using satellite images taken over a variety of time intervals.

However, remote sensing has a set of problems all its own. A cloudy day, for example, can hinder or disrupt data collection. Also, depending on the speed of water and or amount of wind, foam can be generated which could also disrupt the data. Studies have shown that, even with its problems, remote sensing often yields results with a lower uncertainty. Data collection using remote sensing and on site instruments often provides the most accurate results.

Sediment in Rivers

The convergence of the Toutle and Cowlitz River
Sediment in a River
(Image taken from http://volcanoes.usgs.gov/About/What/Monitor/Hydrologic/SedsMove.html.)

One of the first ways to best look at suspended sediment is in river. Rivers are one of the main transorts of sediment and are often the distributor of sediment into other bodies of water. The photograph above shows the abundance of sediment deposited in a river following a volcanic eruption, in this instance, the eruption of Mount St. Helens in 1980. Rivers are an important factor in determining how and where sediment is deposited. By measuring the turbidity at different locations downstream, accurate models can then predict the amount of discharge into different water bodies.

Sediment in Lakes, Bays, and Coastal Waters

One of the bodies of water that rivers deposit sediment in is lakes. Whether the lake is used as a water source, for recreation, or to sustain an ecologic habitat, the amount of sediment in the lake can change its ability to maintain its usefulness. An increase in turbidity, either from natural or anthropogenic causes, can decrease the amount of sunlight that reaches the bottom of the lake, causing plant life to diminish. Data gathered using remote sensing can give scientists, and others concerned, a better understanding of the factors involved, and can aid them in finding ways to better control certain factors.

Inland bays can be similarly affected. The Chesapeake Bay, for example, has shown a decrease in aquatic vegetation. This decrease is due to low light conditions caused by higher amounts of suspended sediment. The sediment for the Bay was found to come from a variety of sources. (See image below)

Chesapeake Bay
Chesapeake Bay
(Image taken from http://geology.er.usgs.gov/eespteam/Atlantic/maps.htm.)

The above image was created using GIS software and remotely sensed data. This type of data analysis can aid dramatically in better understanding the issues of suspended sediment. It has also been used to study the coastal areas around New Caldonia. Located approximately 1500 km west of Austrailia in the South Pacific Ocean, New Caldonia is known for its high biodiversity and coral reefs. Anthroprogenic effects, such as mining, have increased the amount of sediment input into the waters around New Caldonia, specifically, the southwest lagoon area. This increase is having damaging results on the coral and aquatic life of the area. Using remote sensing and GIS software, researchers have been better able to make numerical models that are highly accurate. This process without the use of remote sensing would have taken several years to complete, given the vast size of the research area. However, the addition of remotely sensed data accelerated the data collection, and gave those interested an opportunity to take the necessary steps sooner to prevent further damage.

New Caldonia
New Caldonia
(Image taken from http://www.jsc.nasa.gov/images/eol/2003/noumea.jpg)

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