Sediment Trend Analysis Looks at In-Place Material to Find How Sediment is Moving
This took place in the 1950’s and Hans’s bedload equation is still frequently referenced. Also, Albert knew what he was talking about as he himself, dabbled in fluvial geomorphology, and decided it really was too complicated.
Patrick McLaren tells this story to illustrate his quest for a basic method of studying sediment transport in dynamic situations – something that has eluded planners throughout history.
Twenty-five years ago he came up with the idea of observing and analyzing the sediment already on the bottom, rather than guessing and measuring processes that may or may not be responsible for net sediment transport.
His method has been proven in projects around the world, including the United States, and, if used for all aspects of coastal and marine planning and management, can save governments substantial amounts of money.
His Sediment Trend Analysis (STA®) determines the patterns of net sediment transport and their dynamic behavior. Samples are simply observations, and the derived patterns of transport explain the observations, McLaren says.
“Because only one pattern can ever be found linking the sample distributions, I’m 100 percent confident that the derived understanding of how the environment is working is correct,” he says.
It is impossible to foresee all possible variables in a sediment transport situation in order to include them in a numerical model, which limits the value of that model for predicting sediment movement and deposition.
The same with using tracers or in situ measurements. Tracers will only show movement for the time and conditions existing during the experiment. Measurements rely on previous assumptions in order to decide what to observe, and so don’t allow for unkowns.
The strength of STA is that it observes what is already there; it does not require any preconceived assumptions that can frequently lead to expensive mistakes.
McLaren has devised a Webinar that he gives at no charge to groups around the world. After explaining the technique and giving sometimes startling examples of failures of sediment prediction, and his after-the-fact analysis of the failure, the Webinar concludes that the strength of the technique lies in the facts that:
• No prior assumptions have been necessary.
• Stakeholders find the concepts are easily acceptable in that sediments do move about and that grain size distributions will be changed by that movement.
Given a situation where a project owner would like to see if dredged material deposited in a littoral zone will move onto the beach, McLaren described his thinking process:
“First we look at the problem such as ‘what will happen to material placed at a certain location.’ The concept is that we must understand how the present environment is working before we can have any idea of what will happen when the material is placed there.
“In assessing the area to be sampled, I look for all the possible environments that can be affecting the site – e.g. the beaches, the incoming river, the bathymetry to assess how far to the offshore we should go so we can understand the onshore-offshore transport regime. In assessing the area I try to get a feel of how ‘noisy’ the sediments are likely to be.
“For STA, grain-size distributions of the bottom sediments are the signal, and the pattern of transport that is derived is the message. Noise is simply the data that have no meaning and are therefore unwanted. For me, noise is usually human-based. Has the area been dredged? Are there other known disposal sites in the area? Has coastal development blocked the natural sediment transport regime?
“Obviously, as distance between samples increases, the possibility of noise increases as does the possibility of picking up samples that have no relationship with each other anyway. Generally, the patterns of transport you get are as good as two times the sample distance. So if we sample at 100 meter spacing, we can pick up transport features within a scale of 200 meters. The more detailed the problem (e.g., deposition inside a single marina) the smaller the distance between the samples.
“In open coastal environments, 500 meter spacing is usually pretty good (based on experience rather than a mathematical calculation – although a group of Frenchmen recently published The application of geostatistics in defining the characteristic distance for grain size trend analysis).”
To take the samples, McLaren uses a small grab that can hold the top 15 to 20 cm of the bottom sediment.
His small grab is good for about 100 meter depth.
“Deeper than that you need a larger winch to hold more cable, and you are usually in larger waves requiring more stability and a heavier grab that is not subject to interference by rapid vertical movements and currents,” he explains.
In the rig shown in the picture, two people are necessary – a driver and a sampler.
“We have specialized navigation programming that has the proposed sample grid on a screen (shown as green dots). When you touch a dot with the stylus, the range and bearing are given to get to that site. On approaching the site, the screen turns into a bulls-eye with the boat’s position showing as well – so you drive into the center of the bulls-eye and drop the grab.
“At the instant the grab hits, an actual position is taken and now the dot turns red on the screen. The program allows the input of observational data of the grab sample, which often proves to be very useful for both the STA and client alike.
“We now have a depth-integrated sample of all the processes responsible for the formation of the full thickness of the geological deposit at that location,” McLaren explains.
Projects typically are from 500 to 1000 samples in size.
Fifteen hundred samples is considered very large. Costs for a full STA, including field work, sample analyses and report range from $190 to $220 per sample. Regardless of the project size it can usually be completed in less than four months.
Laser Particle Size Analyzer
The full grain-size distributions are analyzed with a Malvern laser particle size analyzer (Mastersizer 2000). The instrument can handle a size range from 2 mm down to 1 micron.
McLaren puts the sample into the water bath of the machine through a 1 mm sieve. If there is any material left on the sieve, that sample gets dried in an oven and is sieved through a nest of sieves going from 4 mm to 710 microns (middle of the coarse sand size). This ensures that there is a substantial overlap in the sizes determined by the two methods. The sieve data are then merged with the laser data to form a single distribution.”
What the Arrows Mean
The final product is a map of the sample area containing arrows of different colors illustrating the movement of different sediments. The accompanying chart is from a project at Aguadilla on the west coast of Puerto Rico.
The yellow arrows are Dynamic Equilibrium. This means that for every particle of a certain size coming into a deposit, there is an equal probability for a same-sized particle to be eroded from the deposit. Therefore the bed is neither undergoing net erosion nor net accretion even though the sediments are moving down the transport pathway. This is significant if, for example, there happens to be a contaminated hot spot present. The hotspot will tend to move down the pathway as a hotspot rather than being dispersed altogether.
So if you dredge out the hotspot you want to be sure there are no other hotspots coming down the pathway – or you might want to cap it to hold it in place because you don’t want the contaminants to get to wherever they are going. And if you do that, you want to know where and what you should cap it with, both of which might be easily answered by the STA that you have already done to discover the behavior of the hotspots in the first place.
Green arrows are Net Accretion. In this case, the sediment is becoming finer down the transport pathway, and more particles are coming into the deposit than are being removed. Contaminant levels in the sediments will tend to increase down the pathway, again providing information on what the effects of various cleanup options might be. (This becomes interesting in litigation issues because the environment is actually being responsible for concentrating contaminants in certain areas, and it may not be easy to assign their specific sources to the PRP’s).
Red arrows are Net Erosion. Here there are more particles leaving the deposit than are coming into the deposit and there is net erosion down the pathway. Contaminants will be quickly dispersed out of the sediments altogether in such environments and you will see a steady decline in levels as you move down the transport path.
Blue arrows are Total Deposition I. Once a particle hits the bottom it doesn’t want to move again. It will, of course, but only in extreme events. If a particular particle picks up a contaminant from elsewhere, then a hot spot will tend to form at a reasonably specific location relative to the contaminant source.
Again, if you choose to dredge out the hotspots, you must be sure that the sources have been adequately cleaned up. Or you could cap the hot spot to remove it from biotic processes. There is a Total Deposition II, which happens only when the particles are so fine that there is no longer a preferred location for their deposition. Hot spots can’t form in such an environment, but contaminants will tend to be more or less equally distributed over the entire area of deposition. In this case, it is very hard to know what to do as far as clean up is concerned.
McLaren has presented his Webinar to many groups, in hopes that clients will seek his services in the planning stage, rather than after the fact when trying to discover why a project did not go as planned. He sees an opportunity to save governments millions of dollars by ensuring that planning mistakes are not made.
A case in point is a small boat harbor designed and built at Aguadilla, on the northwest coast of Puerto Rico as illustrated in the accompanying figures.
Planners perceived a littoral drift transport regime moving sediment from north to south down the west coast of the island. The breakwater was therefore designed to open southwards to protect the inlet from acting as a sediment trap. Shortly after the construction of the $1.2 million breakwater, the entire inlet had filled with sand to above the water level.
Finding the Error
The Corps Jacksonville District hired McLaren to analyze the deposition patterns, and he found that sediments acted, not in response to a southward littoral drift system, but rather in response to oceanographic currents in which a strong southward current in the deep water was causing a coastal eddy to transport sediment northwards and into the harbor.
The result was to infill the harbor with trapped sand, an event the earlier numerical models had failed to perceive. The understanding of the situation provided by the STA suggested that large culverts could be placed beneath the breakwater to help re-establish the circulation of sand back out to the deeper water.
An alternative could be to use the harbor as a sustainable sand mine by dredging and selling the aggregate to Puerto Rico industry. The island not only has a shortage of aggregates, but removing the sand from behind the breakwater could have no deleterious affects on coastal erosion elsewhere.
Among the groups who have seen the Webinar are: the Corps of Engineers Portland, Galveston, New Orleans, and Los Angeles Districts, and the Seattle District Inlet Program;
Coos Bay Harbor, Oregon; Dupont Chemical; State of Georgia Coastal Resources Division; State of Maine Geological Survey; State of New York Department of Environmental Conservation;
State of Michigan Office of the Great Lakes, Department of Natural Resources and Environment; State of Ohio; State of Oregon Department of Environmental Quality; State of Rhode Island; State of Louisiana; Department of Sustainable Development of the Organization of American States;
Municipality of Tofino, BC; Environment Canada; Public Works, Canada; Tortola, British Virgin Islands; National Environmental and Planning Agency, Jamaica; Anguilla National Trust, British West Indies; a number of ports in Australia and New Zealand, as well as many others.
Interested persons can book a complimentary Webinar on Sediment Trend Analysis through the company Web site: www.sedtrend.com
SedTrend Corps of Engineers Projects
• 1992: Sediment transport pathways and dispersal of dredged material in Mississippi River, mile 26.5 to mile 29.5. Rob Davinroy, U.S. Army Corps of Engineers, St. Louis District.
• 1994: Sediment transport in Elliott Bay and Duwamish River, Seattle: Implications to estuarine management, Seattle District (as part of the State of Washington: Department of Ecology: Toxics Cleanup Program)
• 2001: A sediment trend analysis (STA) and an Acoustic Bottom Classification (ABC) in the mouth of the Columbia River: Implications to dredge disposal operations and coastal erosion. Mr. M. Siipola, US Army Corps of Engineers, Portland District.
• 2002: A Sediment Trend Analysis in San Juan Harbor and vicinity, Puerto Rico. Dr. Tom Smith, US Army Corps of Engineers, Jacksonville District
• 2003: A sediment trend analysis in Maumee Bay, Lake Erie. Craig Forgette, US Army Corps of Engineers, Buffalo District.
• 2003: A sediment trend analysis in the vicinity of the Aguadilla Breakwater, Puerto Rico. Dr. Tom Smith, US Army Corps of Engineers, Jacksonville District
• 2010: The results of a Sediment Trend Analysis in western Lake Erie. Craig Forgette, U.S. Army Corps of Engineers, Buffalo District.