Frequently Asked Questions

1. What is STA?
2. Why would STA be useful?
3. What are the data required for STA?
4. How many samples are required for an STA?
5. Is any other information needed such as current, tide, or wave data?
6. What is meant by the term "rules of transport", which is often referred to in STA literature?
7. Is STA a form of modeling?
8. How are the sediment pathways and their dynamic behaviour obtained?
9. Does the theory underlying STA hold true for all sizes of sediment?
10. Surely cohesive sediments will not obey the rules of transport?
11. Won't the results of STA be completely changed by the next storm?
12. Is there any point in doing STA in a river where the direction of transport is known?
13. How much time is represented in the results of STA?
14. Does STA give any quantitative information about rates of transport or rates of deposition?
15. My study area is extremely complex and I'm really not sure if STA will work in this particular situation.
16. Can STA be used as a predictive tool?
17. Have the results of STA ever been verified?
18. What materials do you provide once STA process is complete?
19. Are there any situations where STA has failed?


1. What is STA?
STA (Sediment Trend Analysis) is the acronym given to the technique founded by Dr. Patrick McLaren of SedTrend. The technique uses relative changes in grain-size distributions of sediments to determine net sediment transport pathways. The analysis also tracks the dynamic behavior of the substrate to understand sediment stability.

Return to Top
2. Why would STA be useful?
The mobility of sediment is a crucial factor concerning virtually all marine, coastal, estuary, river and lacustrine environmental management decisions.  Its understanding is essential for environmental risk assessments associated with dredging, dredged material disposal, remediation of contaminated sites, habitat restoration, coastal erosion and beach replenishment.

Return to Top
3. What are the data required for STA?
STA uses the complete grain-size distributions that are taken from sediment grab samples.  These distributions are determined with a laser-based particle-size analyzer. Samples are collected over the entire area of interest on a triangular grid with spacing that may range from greater than 500m to less than 100m, depending on the project requirements.  The minimum number of samples required in an STA is about 200.  Large projects, such as the Wadden Sea (Dutch Waddenzee) or the mouth of the Columbia River, have required more than 2000 samples.

Return to Top
4. How many samples are required for an STA?
For statistical reasons, STA requires about 9 samples in any one sequence to evaluate sediments for transport direction.  A 9X9 grid (81 samples) is the minimum requirement in order for us to perform STA.  The study design must take into account budgetary constraints, the aims of the project, and an assurance that all relevant environments have been adequately sampled.  As a general rule, the patterns of transport can be determined at a scale of 2X the sample spacing.  For example, at 100m spacing, pathways with a minimum spatial scale of 200m can be found. 

Return to Top
5. Is any other information needed such as current, tide, or wave data?
It is vital that STA is performed without any other information related to the processes that may result in sediment transport.  The fundamental theory of STA dictates that the patterns of net transport represent an integration of all processes that may be responsible for sediment movement, regardless of which process may dominate.  Preconceived ideas about processes only interfere with the exploration for trends and detract from the advantage of impartiality.  It is only after the completion of STA that it may (and often does) become apparent which processes are likely responsible for the net transport of sediment (e.g., a dominant flood or ebb current, a tidal residual, waves from the northwest, or extreme flood events etc.). In performing STA, it is useful, though not essential, to have the sample locations superimposed on a bathymetric chart.  It may also be helpful to be aware of coastal features, the presence of dredged channels, and disposal sites.

Return to Top
6. What is meant by the term "rules of transport", which is often referred to in STA literature?
The rules of transport are derived from the McLaren and Bowles theory, which demonstrates how grain size distributions must change in the direction of transport and describes a simple statistical technique to infer transport direction from these changes.

There are only 2 rules:

(1) If the mean grain-size of Deposit B is coarser than that of Deposit A, and the variance (or sorting) of its distribution is smaller, and the skewness more positive, then it is possible that sediment transport has occurred from A to B;

(2) If the mean grain-size of Deposit B is finer than that of Deposit A, and the variance (or sorting) of the distribution is smaller, and the skewness more negative, then it is also possible that sediment transport has occurred from A to B.

If any other combination of mean, sorting and skewness is observed (e.g., finer, smaller and more positive) then the premise that transport has occurred from A to B must be rejected.

Return to Top
7. Is STA a form of modeling?
No, STA is not a model.  It is an empirical technique that requires a number of observations (the grain-size data) from which transport pathways and dynamic behaviour are inferred. An assumption is made that the sediment is not “there by accident” nor is its grain-size distribution merely random; rather any deposit has been derived from elsewhere (a source, or as part of a continuum in transport), and is very likely going someplace else. As a result of this transport, a predictable signature is left in the grain-size distributions; the discovery of which forms the basis for STA.

Return to Top
8. How are the sediment pathways and their dynamic behaviour obtained?
In practice, the patterns of net sediment transport are obtained by finding sample sequences that change according to the McLaren and Bowles’ theory in a statistically acceptable way.  Once a pattern of transport is established for the same sample type, the distributions of the sediments may be used to assess the probability of each particle size moving along the pathway.  This, in turn, establishes the dynamic behaviour of the bottom sediment (i.e., net erosion, accretion, dynamic equilibrium, etc.).

Return to Top
9. Does the theory underlying STA hold true for all sizes of sediment?
Yes, STA has been carried out successfully in marine, coastal, estuarine, fjord, river and lacustrine environments, encompassing sediments ranging from gravel to clay-sized materials.

Return to Top
10. Surely cohesive sediments will not obey the rules of transport?
On the contrary, the “rules of transport” apply equally well for all grain-sizes of sediment.  Because fine sediment (silt and clay) is cohesive, once deposited it may be difficult to re-suspend it and the dynamic behaviour of “Total Deposition” is frequently observed.  However if re-suspension does occur (processes include bioturbation, storms and, often in dock areas, propeller wash), further size sorting can take place and a transport pattern will be derived.

Return to Top
11. Won't the results of STA be completely changed by the next storm?
The product of all processes responsible for the erosion, transport and deposition of sediment is the formation of an actual deposit, or geological facies.  It is almost impossible for a single event, such as a storm, to change, in its entirety, the facies that characterizes the specific sedimentary environments that are sampled for STA.

Experiments have shown grain-size distributions remain essentially identical, even in highly dynamic environments despite the lapse of considerable time between sampling (several years).  For example, samples collected in 1993 from the bed in the Fraser River, British Columbia, in pre-freshet conditions were more than 95% similar to samples collected in post-freshet conditions three years later.  The answer, therefore, is no: such events will not change the interpretation of a Sediment Trend Analysis.  The results of STA, however, will provide an understanding of the probable processes responsible for the sampled facies, which may demonstrate that storm events rather than the “normal” day-to-day processes are predominantly responsible for the derived patterns of transport.

Return to Top
12. Is there any point in doing STA in a river where the direction of transport is known?
STA has been performed in many rivers including the Snake, the Ems, the Anacostia, the Willamette, and the Columbia, to name a few.  Despite the fact that the direction of transport is already known, STA is able to divide the river into its separate transport environments based on changing sediment sources and dynamic behaviour (stability). Such information is used, for example, to identify contaminant sources, together with their transport and fate.  Appropriate remediation options can then be clearly evaluated.

Return to Top
13. How much time is represented in the results of STA?
STA represents sediment transport patterns and behaviour over the period of time represented by the thickness of the geological deposits being sampled. For example, in an energetic tidal channel a sample may represent one or several tide cycles, whereas the same depth of sample from an adjacent mud flat may have taken several or many years to form.  The time difference encompassing the two samples does not, however, preclude a sedimentological relationship between the two environments, and a transport pathway may quite easily exist between the two facies.

Return to Top
14. Does STA give any quantitative information about rates of transport or rates of deposition?
No.  STA is qualitative and empirical.  Often the derived understanding following STA is sufficient without further quantification.  Where it is desirable to determine rates, STA will provide an understanding of how best to obtain such information.  For example, STA will assist in deciding where best to place wave data buoys, current meters, or sediment traps, or in determining optimum locations to take cores suitable for dating sediments or to develop a history of contaminant inputs.

Return to Top
15. My study area is extremely complex and I'm really not sure if STA will work in this particular situation.
This is a common concern, as it is often believed that one’s own area of study is more complex than others.  There is also occasional confusion between the evident complexities of sediment movement while actually in transport compared to the single net movement as determined by the deposits.  While a particle in transport may well be subject to the vagaries of many processes acting on it, the size relationship of particles actually in the deposits can represent only one net direction. 

STA has performed successfully in what could arguably be deemed as some of the most complex environments in the world, notably the Bristol Channel, a huge estuary in the UK with a tidal range in excess of 12 meters.  Regardless of the complexity of the environment, STA works as long as there is sediment on the bottom available to be sampled.

Return to Top
16. Can STA be used as a predictive tool?
The derived understanding of transport patterns and dynamic behaviour may easily be used to predict qualitatively the likely changes that could be expected when an environment is altered, or a development such a breakwater or port is planned.  For example, STA has been particularly useful in demonstrating the consequences of dredging channels transverse to the transport pathways (they rapidly infill), choosing disposal sites, or predicting the effects of different remediation options at contaminated sites.

Return to Top
17. Have the results of STA ever been verified?
STA is the only technique that is self-validating. The grain-size distributions can be considered as observations and the derived patterns of transport provide an explanation for those observations. The pattern, therefore, provides and interpretation that can be treated with 100% confidence that it is correct. (This is why in over 25 years of STA projects around the world, the results have always been proven correct).

In addition to the patterns of transport,  STA, verification is often apparent from the large number of other observations that can now be explained.  Examples include a realization of how contaminant concentrations occur where they do, why a spit developed in the “wrong” direction following the construction of a port, or why a beach is eroding even faster following the placement of a breakwater designed to protect it.  In other examples, the understanding of which processes must be responsible for the derived patterns of transport are verified by correlating the results with existing process data, or from process data collected later in order to verify the STA.  

Return to Top
18. What materials do you provide once STA process is complete?
Every STA includes a variety of maps that can be generated from the data base. These include maps of sample observations taken at the time of the sediment sampling, sediment types, the patterns of transport and their dynamic behaviour, and any sediment attribute that might be useful for benthic habitat studies or numerical modeling. Such maps provide baseline information from which all future changes can be compared.

Return to Top
19. Are there any situations where STA has failed?
No, not when the technique has been applied under the procedures represented under the registered trade mark STA.


Return to Top