Where is all the Clean Water?
The process that occurs when sediment fills a lake or other body of water is known as eutrophication. It is a natural process that evolves over a number of years, until lakes become swamps and marinas become water-front real estate. Accelerated eutrophication occurs when ground cover vegetation is disturbed within the drainage basin. This disturbance occurs during construction projects, farming operations, natural disasters or any other time the ground cover is disturbed.
What is Dredging?
Dredging is an excavation activity usually carried out underwater, in shallow seas or freshwater areas with the purpose of gathering up bottom sediments and disposing of them at a different location. This technique is often used to keep waterways navigable. It is also used as a way to replenish sand on some public beaches, where sand has been lost because of coastal erosion. Fishing dredges are used as a technique for catching certain species of edible clams and crabs.
When is it Time to Dredge?
Think that murky water is just an inconvenience or an eyesore? Think again. The clarity of a lake, marina or pond may have a significant impact on property values in a community. In fact, according to a study conducted by the University of Maine, a one-meter drop in water clarity causes real estate values to plummet by 10 to 15 percent. In some cases, people have purchased water front property and, after a few short years of development upstream, they end up with swamp-front property that cannot be resold.
The first sign of trouble is shallow water near the head of the lake or at the end of the coves where the most drainage enters the water. Boat motors start churning up mud and unsightly weeds begin to emerge. At first this area is limited to a small area. However, over time, this area grows, and the surface water area starts shrinking in size. It becomes more difficult to maneuver boats without damaging expensive motors.
The second warning sign is that the water quality begins to deteriorate. After rainfall or high winds, more sediment is deposited and re-suspended. The water takes longer to clear and it is a murky brown color. The water quality is noticeably lower than in years past. This reflects on the aesthetics of the entire community.
The final sign that it’s time to dredge may show up in the form of diminished irrigation capacity. This might manifest itself as sediments clogging up sprinkler heads or it might show up as a limited quantity of water available in high usage months. As sediments fill the lake, less water capacity is available for irrigation, since the sediments are taking up the space that was once occupied with water.
When a lake disappears and water front property plummets in value, we then truly understand the value of clean water.
What is the Dredging Process?
There are typically three ways to approach the cleaning a body of water:
Drain the Water and Excavate
Draining a lake may be the least expensive option, but will usually have the greatest environmental impact and often take the longest to complete. This method will usually require building a haul road to provide access for heavy trucks to haul the sediment offsite. Damage to shoreline, re-grading the haul road and re-landscaping the area are all inconveniences and additional costs to consider.
In addition, draining the lake is highly susceptible to weather issues. Since water will naturally drain to this area, even a small rain may impact the schedule for an extended period of time. It may take weeks or months for the material to dry enough to excavate and haul offsite. Another consideration is the undetermined amount of time until the lake is restored and filled again with water. Depending on the size of the drainage basin and the amount of rainfall, it may take several months for the lake to fill back to capacity.
Mechanical Dredging from the Shore
This method addresses sediment issues from the shoreline. Heavy excavation equipment is hauled to the shoreline. Surrounding trees and other obstructions are often removed and the shoreline leveled for the equipment to work safely. Haul roads must be constructed for trucks to have access to excavating equipment. Most common mechanical excavators such as dragline and long-reach excavators used in this kind of excavation reach 40 or 50 feet from the shoreline, so candidates for this type of dredging are small ponds or the ends of coves.
Mechanical Excavation from Barges
This method of excavation involves an excavator or crane with a clamshell bucket. The barge is moved into place and excavated material is placed in the barge. When the barge is full, it is towed to shore where another excavator unloads the material onto the shore or into a truck. This method requires several pieces of heavy equipment and handling of the material multiple times.
Portable Hydraulic Dredging
Another option for removing sediment from the lake bottom is using a hydraulic dredge. The dredge floats on the water and pumps the material through a temporary pipeline to an offsite location, often several thousand feet away. The dredge acts like a floating vacuum removing sediment very precisely.
Hydraulic dredges use a discharge line, and possibly a return line, which are the only disturbances to the surrounding environment. These lines can easily be run under roads or sidewalks. Other than this, the dredge, which is not much larger than a small boat, is the only machine to be seen. It is an unobtrusive method that does not require disturbing the shoreline and requires one trip in to put the dredge in the water and one trip out when the project is complete.