Construction generates waste; that is a fact. And one of the worst wastes is sediment left in a basin. An unlikely scenario is for sediment to be left onsite as part of an aquatic feature. More likely, plans call for its removal; and in the worst case, plans make no mention of sediment after construction is completed. In most cases, the owner does not want it in its present state, a perpetual unusable nuisance. Owners look to you for a cost-effective answer. If removal is not on the plan, what will you do with it? The solution may not be yours alone.
Who Is Involved?
The removal of sediment from a basin is a permitted function by local regulatory authorities and may require approval by the US Army Corps of Engineers. Such approvals are based on the size of the operation, and the impact on the existing and final environmental settings. Each appropriate regulatory authority will have specific requirements before approval can be granted. In most cases, the regulatory authority will require the contractor to submit a plan for the removal process. The plan will include an evaluation of the existing condition, a description of the removal process, and an assessment of the expected post-removal condition. The contractor will also need to address how the removed material will be treated and stabilized. It is important that the contractor make contact with the approval-authorities.
Another participant in the process is the owner. If the owner has a specific use for the sediment, then sediment has value and can stay on site. If the owner does not have a use for the sediment, then removal must include the cost of transport to another site. The process must also address the impact to adjoining property owners. If they have a specific water-use, it is imperative that their supply not be diminished in quantity or quality. In order to facilitate the development of a sediment plan, all parties involved in the process to be familiar with and agree to the conditions of the plan.
Are contaminants present? Contaminants occur in many forms, each contributing an undesirable property to sediment. In most cases, it is extremely difficult to remove contaminants. Contaminants are not usually confined to a specific location, but instead are spread throughout the sediment. Separating and removing contaminated sediment is often extremely difficult and expensive. Instead, the entire volume of water and solids is usually treated as being contaminated. If there is any doubt, contractors should consider leaving treatment and removal to professionals who have the appropriate skills.
The Benefits of Keeping the Sediment On Site
Does the sediment have value to the owner? Currently, there is a growing trend to use controlled placement of slurry as part of a landscape feature by pumping sediment/slurry into porous containers known as dewatering cells or dewatering sock geotextile tubes, made of woven and nonwoven geotextile fabrics. These containers confine most of the solids and, at the same time, allow more water to seep out. The containers are available in various sizes or as tubes upwards of 6- to 8-feet in diameter and 200 or more feet in length. Every vender will caution the operator that filing the tubes must be monitored. Excess fill can rupture seams and total uncontrolled spillage can occur.
These tubes can be used as a shoreline protection or as land fill extensions. Once dried out, the bags can remain in place, or be slit open and the dry contents can be removed from the site. One vender claims that his bags can be reused until such time as the fabric pores are sealed with sediment. Other practices involve spreading grass and other vegetative species on the surfaces of the tubes. Roots penetrate the fabric pores and permeate the sediment.
If a filled geotextile bag is placed on a hard surface, such as the ground, only the pores on the top can release water, which greatly reduces the dewatering rate. However, if the filled bags are place on a porous surface, such as hay bales or crushed stone, the water can drain out of the bags quickly.
Although using these geotextile containers can be an effective solution, it will require experience and safety. Few local contractors can rent a pump, attach it to a bag, and start the process without creating serious problems.
Properties of The Sediment
Particle size of sediment can range from gravel to colloids. The coarse fraction usually drops out quickly in a forebay nearest the inlet. If the pond can be dewatered, the coarse fraction can be removed physically by land-based equipment and possibly used for other purposes. In basins that experience resuspension of previous sediment by the new inflow, the total sediment load must be treated as a single unit. For fine-grain sediment, other methods are required. It is desirable to convert this soupy liquid to a semi-solid that is more easily managed. Chemical additives such as flocculants, polymers, polyacrylamides, chitosan, and other chemicals that bond individual particles together to promote settling scan be used. Maximum benefits are achieved when these products are thoroughly mixed throughout the sediment. The particles settle to the bottom and the clear supernatant can be decanted from the top.
Some venders have developed equipment that can inject the chemical in powder or liquid form. If there is no current in the liquid, then other methods to distribute the chemical throughout the basin will be needed. For example, a closed-loop pumping system can introduce the chemical into the basin. Again, mixing is critical.
If one portion of polymer is good, are two portions better? If too much polymer is introduced, the sediment becomes very viscous and difficult to handle. Venders and consultants can test to determine just how much to use.
As the sediment in the basin continues to consolidate, it can become a suitable medium to support vegetation, turning a pond into a wetland. This process must be identified in the approved plan; otherwise, it can be removed and used as a topsoil substitute.
How Is Sediment Removed from a Basin?
Treating sediment as a liquid can be achieved with land-based equipment such as draglines—long-sticks with buckets on the end. Transport off site can require a large number of trucks equipped to prevent spillage. The material can be hauled to offsite locations such as municipal solid waste landfills (MSWLFs) or construction and demolition (C&D) debris landfills.
More effective in terms of clean out is the use of vacuum pumps. Slurry is collected under negative pressure and transported through large, flexible hoses to tanker trucks that transport them to the disposal site and then are emptied. The rate of removal depends on the weight and viscosity of the slurry.
A third method is the use of floating mini-dredges equipped with subsurface augers that extract viscous mud and dense aquatic vegetation in larger basins. The sediment can be pushed long distances through pipes and discharged into a controlled area. The geotextile tubes mentioned earlier are a common and useful destination. With this method, the risk of spillage is minimal.
Summary
Most plans to not address the removal of sediment from a basin, leaving the problem for the contractor to resolve. To handle the sediment safely and economically, the contractor should consider the following:
- Does the sediment need to be removed?
- Does the sediment have value?
- Is the sediment contaminated?
- Should the sediment be extracted as slurry or treated and removed as a semi-solid?
Considering these factors will help contractors decide how best to deal with sediments left behind after a construction project is finished. While contaminated sediment should be disposed of carefully, other sediments may offer onsite benefits. Whatever the case, finding the best and most cost-effective way to handle sediment is possible.
About the Author
James Spotts
James Spotts, PhD, is president of Southeast Environmental Consultants. He is a certified professional soil scientist (CPSS) and a certified professional in erosion and sediment control (CPESC). His specialty is the construction and management of sediment ponds on construction sites. He has been an active member of IECA for many years.