Marina Dredging Overview
What is Marina Dredging
From keeping the docks and ports safe to monitoring the state of the water, there’s nothing about a marina that isn’t crucial to daily operations.
Marina dredging is a major part of that regular maintenance. It’s important to every marina worker to understand what it is, how it’s done, the risks involved, and why it’s important.
Water currents naturally carry around small pieces of rock, sand, silt, and other debris. This is called sediment, and its depositing into banks at the bottom of a body of water is called sedimentation. Sedimentation can be the cause of a lot of problems; it can make bodies of water too shallow for ships to pass through safely or threaten the habitats of wildlife.
This is where dredging comes in. Dredging is the process of removing sediment from the bottom of a body of water to solve the issues it might cause. This is especially important in marinas, where the water is significantly calmer and more prone to settling and sedimentation.
What Problems It Solves:
Advantages & Disadvantages of Marina Dredging
There are plenty of advantages to marina dredging.
- Ships are unlikely to run aground and become stuck
- Keeps flow of traffic in a marina constant and safe
- Allows marinas to accommodate larger and more economically viable ship types
- Protects local wildlife from dangerous byproducts that can be left in the sediment from manufacturing and other human activity.
Unfortunately, there are also both economic and environmental disadvantages to dredging.
- It’s expensive
- Can be as environmentally hazardous as it is helpful
- Often requires clearing of the marina of most if not all ships and other crafts, meaning that business is regularly interrupted for the process
- Potentially ruins the natural ecosystems inside the marina if done carelessly by releasing previously sediment-trapped pollutants into the water
- May lead to the breaking up of natural breeding grounds.
Marina Dredging Process
Dredging is a relatively basic process.
- The dredging vessel is positioned above the designated work area.
- The dredge itself, usually a large, flat blade or panel but in some cases a hydraulic system, is lowered to the sediment layer.
- The vessel moves at a steady pace to scrape the dredge along the bottom of the marina, loosening sediment.
- The sediment is collected in containers for transportation or is directly transported to the filling area.
- In the case that the sediment is collected, it is then transported to the filling area and deposited.
The equipment involved in dredging is, of course, the main dredging vessel, but also includes workboats such as a navigational tug, crew boats, and a fuel barge. A survey boat to assess the size and shape of the area to be dredged is also a vital part of the process.
One important aspect of the process is “filling,” or the depositing of sediment elsewhere. Transport units or systems for the sediment are additional required materials, as is a designated fill site for the material to go into. These materials and land areas may have an added cost outside of the cost of dredging.
Because of its importance, this process must be overseen by the proper authority to be done legally. In the United States, those authorities are the Environmental Protection Agency and Army Corps of Engineers. In the UK, the Marine Management Organisation oversees dredging and filling.
Different Types of Marina Dredging
There are a number of different types of marina dredging that are in common use today. They fall into two basic categories, which are capital and maintenance dredging. Capital dredging is for new channels or berths and construction while maintenance dredging is to keep existing channels and berths functioning properly.
- Navigational dredging is done to deepen bodies of water to allow for the safe and functional navigation of crafts through the marina. This is considered maintenance dredging and is done regularly on all marinas. It usually requires a license but can be exempt in low-volume areas under very specific circumstances.
- Clearance dredging focuses on removing a specific material or sediment amount for reasons other than navigation, such as for environmental protection. This is also maintenance dredging, though it’s done slightly less often. Once again, a license is required for this kind of dredging.
- Aggregate dredging is dredging done for construction purposes, usually removing sand and gravel. This is capital dredging. There is no mention of the need for a specific kind of license for aggregate dredging.
Alternative Solutions to Marina Dredging
Though dredging is still one of the most common sediment management solutions used for marinas around the world, there are many alternatives to it that might be more environmentally sustainable.
- Anti-sedimentation infrastructure. This is a method of focusing on the physical construction of a marina to reduce the amount of sediment that accumulates at all rather than having to remove it once it builds up.
- Remobilizing sediment systems. Similar to anti-sedimentation infrastructure, remobilization efforts aim to introduce movement to areas of a marina or other body of water that naturally accrue sediment.
- Sand bypassing plant systems. This is the introduction of naturally filtering plantlife that processes sediment as a part of its biology, meaning that it wouldn’t build up. This essentially acts as a natural dredge.
Costs of Marina Dredging
According to the South Carolina Department of Health and Environmental Control, dredging can cost less than $20 up to more than $40 per cubic yard to be dredged (depending on the method chosen and the manpower required) and more than $100,000 per year in disposal costs, depending on the dredging method.
In the UK, costs start with licensing, which can be between £161 and £645 per application, depending on the tier, and between £2,500 and £7,500 for various legally required assessments. The cost of the actual dredging is cumulatively between £13 and £50 per cubic metre of area dredged, according to industry reports.
For a better idea of total costs, look at a sample estimate for the cost of dredging a marina from the American Society of Professional Estimators.
The channel being dredged in the example is 5000 feet long and 600 feet wide. They estimate a total volume to be dredged at 3000 cubic yards, which will be done with a clamshell dredge that can move 16 cubic yards at a time. They are also assuming in-water sediment filling rather than land disposal. They charge per item per month for a comprehensive breakdown.
|Item||Cost (Per Month of Operation)|
|Clamshell Dredge||$80,000 (£57,522)|
|Work Tug||$10,000 (£7,190)|
|Crew Boat||$8,000 (£5,752)|
|Survey Boat||$12,000 (£8,628)|
|Fuel and Water Barge||$2,000 (£1,438)|
|Towing Vessel||$100,000 (£71,903)|
|Scrows and bottom dump||$50,000 (£35,951)|
This puts the average monthly cost for dredging a marina of this size at $262,000, or about £188,340. This doesn’t include surveying costs.
Drawing up a report of the marina’s specifications before moving forward is a must. Allocate an ideal budget and flex room based on industry averages. Then, get comparable estimates from multiple, reputable dredging services at the same time. This is the best way to ensure that you are getting the best price for the quality of work available.
Frequently Asked Questions
While there will always be some disruption due to the nature of dredging, modern methods don’t usually require the entire marina to shut down. Most dredging machinery can operate around vessels in small areas at a time to make sure there is as little disruption in day-to-day activities as possible.
Because it is done slowly in small portions over a large area, dredging takes quite a long time. Dredging projects can last several months, even when running continuously.
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