There’s more than one way to dredge a body of water, but each one has pros and cons. The type of dredging you require will really depend on the body of water you’re working in, and what your goals are.
In many cases, both types of dredging are effective, but depending on the cost restraints, equipment access, and how fragile the environment is, one method of dredging may be better than the other. The first thing is to fully understand the difference between mechanical dredging and hydraulic dredging. We’ll take a look and each type, and why each method may or may not work for you.
Although mechanical and hydraulic are the two main groups of dredges, each group can be broken down even further into specific dredging equipment and methods, we’re just going to explore the main groups in this article. The exact equipment that’s right for your job will depend on a lot of factors.
One Major issue that factors into all types of dredging is resuspension. To sum this up neatly, it’s basically when you start poking around the bottom of a body of water, you’re likely to kick up all kinds of material – some organic, some man-made, and some of which isn’t very good. Usually, contaminants are contained within a layer of silt, which gets kicked up with ship and dredging activity. Generally this problem is worse in harbors and areas where industry is present, however, this problem can appear anywhere.
Uncontrolled resuspension could remobilize weakly bound heavy metals into overlying water and pose a potential risk to aquatic ecosystem.
Whenever resuspension is an issue, you’ll need to put more thought into how disruptive the dredging technique is.
What is Mechanical Dredging?
Mechanical dredging is using heavy equipment to “dig” the bottom up and remove it. This equipment is usually brought in on a barge, or works while the body of water is drained. The equipment used can be very similar to that of residential construction equipment where a bucket is sent down to haul up dirt and remove it.
There are also many specialized pieces of equipment that are used to drill or dig up the dirt on a more continual basis. Think of a mining operation where minerals are constantly being cut from the wall and conveyed to a holding area. It’s a very similar process.
The type of mechanical dredging that is prefered really depends a lot on the size of the water, and the access you have to it.
One major issue with mechanical dredging is the inability for it to ensure all the sediment or contaminant.
Resuspension and contaminant release by mechanical dredges may result from dynamic impact of the bucket with the bottom sediment (for wire supported buckets), sloughing of material into the cut, washing of sediment from the bucket exterior as the bucket is raised through the water column, and leakage from the bucket (either from the top of an open bucket or from the lips of the bucket if closure is not complete due to debris). All these mechanisms result in a pattern of resuspension and contaminant release that may be exhibited both near bottom and in the full depth of the water column.
Basically, when you’re digging down with a bucket, you can’t ensure that you’re getting everything off the bottom that shouldn’t be there, or how much you’re kicking up by just digging around. While you’re brining the bucket up, you contaminant can fall out, off, or through the bucket. If your dredging goal is to remove something specific, this type of dredging may not work well.
What is Hydraulic Dredging?
Unlike mechanical dredging, hydraulic dredging doesn’t use a “scoop” to remove the silt. It works more like a vacuum cleaner to suck up and filter the bottom to remove contaminant and create depth. It’s a little more complicated than that, and there are a few different methods you can use, but that’s the basic way that it works.
Studies have shown that when environmental sensitivity is a major concern, Hydraulic dredging seems to be the more effective method.
“Dredging methods can be assessed and ranked with regard to their environmental effectiveness (Van der Veen, 1993). Purely mechanical approaches such as grab cranes and digger buckets have the lowest ranking of the existing methods. The highest scores can be assigned to the combined mechanical/hydraulic techniques and these can be the most effective in dredging contaminated soils.”
The Government of Texas also found that hydraulic dredging was the preferred method as it produced among the lowest resuspension rates of common dredge types.
It’s not all roses though. Hydraulic dredging isn’t meant to dredge large or heavy bottom materials like rocks. Although smaller stones and objects aren’t a problem, it works best with fine silt and sediment. if you’re removing an old rocky bottom, mechanical would likely be the only solution.
Quite often a combination of dredging techniques can be used to achieve the greatest results in terms of outcome and efficiency. The only way to know for sure is to contact a professional to help you determine exactly what type of dredging is appropriate for you.
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