This initial step is critical so that wetland construction can be properly adapted to the site.
Before anything else can be done, there are several important questions to answer:
The answers to these questions will serve as the basis for the remainder of your planning operations. From the answers to the above questions, you should be able to arrive at several conclusions with the help of an expert as you begin the decision making process.
There are a few important notes that should be considered:
After you have answered the questions from Step One or if you need help answering some of them, contact Sarah White email@example.com or another wetland expert. They will be happy to answer any remaining questions you may have while walking you through the remaining steps necessary for constructed wetland construction.
You should be able to:
Time spent in careful site selection and wetland orientation will significantly lower
construction costs. Land availability, topography, and soil types should all be
considered. The wetland should be close to the source of the wastewater
to minimize the need for long ditches and pipes. Additionally, it should be
downhill from the source whenever possible to avoid pumping. Large
amounts of grading and earthmoving can significantly raise construction
Well suited sites have the following characteristics:
Also, consider the following items as you select a site:
Wetlands should be sited so that gravity will drive water flow. Typically they should not be located near houses. Areas should be easily accessible to earth moving equipment and maintenance personnel. Be sure to include a large buffer zone surrounding the wetland to preemptively alleviate any problems that might result from the opinions of nearby landholders.
Since retention time (the amount of time that water is in a wetland) is critical for treatment efficacy, wetlands must be large enough to allow for adequate retention times. Sites should be selected to be large enough for current needs plus any future expansion.
Sites that are gently sloping and can be readily modified to hold water allow for least costly construction.
Always make sure that the area is not already a wetland. The Natural Resources Conservation Service or the US Fish and Wildlife Service can help you determine if wetlands are already present.
Permitting requirements and water regulations will be specific to individual areas since state and municipal water standards vary in both existence and enforcement. Now is a good time to make contact with state agencies to gather information on any requirements regarding wetland construction and water discharge. Typically, discharging runoff into a natural water source will require a permit.
Most states and many local ordinances have storm water regulations that will need to be followed. A discussion with a state agency may be necessary prior to permitting to evaluate the relationship between your wetland and all other nearby bodies of water.
By the time you have sorted out all of the answers and Dr. Sarah White or another expert has reviewed your calculations and helped you finalize a preliminary plan, you are ready to speak with a civil engineer regarding the next phase of the project. An engineer will be able to help you refine the initial plan by looking at how water flows and determining specifically:
With answers to these questions, a plan for the wetland can be drawn. The plan will take into account many factors which are important to the future efficacy of the wetland, including a series of structures:
These are the units of constructed wetlands and are constructed by digging to create basins or by constructing dikes. Frequently, both digging and dike construction are utilized. When constructing a dike, the soils used must have small particle sizes that can be compacted to create a bank that will not allow water penetration. The height of the dikes should take into consideration the intended normal water volume, buildup of detritus, sediment deposition, and potential high water flows.
Dike sides should be sloped, and the vertical dimension should never be less than twice the height. Typically the bottom of the cell should be flat.
Some precaution must be taken to ensure that water from the wetland does not mix with water in the environment. Clay soils can often be compacted to create a seal that is sufficient to prevent water from escaping. Sandy soils or others with large amounts of pore space will require addition methods. Lab analysis of the soil may be required before a sealing method can be selected. If permeability is less than 10^-6 cm/second then compaction alone should be adequate. Additionally, soils that contain at least 15% clay can typically be compacted sufficiently. In cases where additional lining is required, there are several options including asphalt, synthetic butyl rubber, and plastic membranes. Special precautions must be taken when using pliable synthetic liners to prevent them from being damaged by stones and plant roots.
Flow control structures are used to control water levels in the wetland and typically include inlets and outlets. Their construction should allow for easy manipulation of water levels so that the wetland's efficacy can be maximized. All of the inlets should be adjustable independently to allow control of flow distribution. Insure that all flow control structures are protected from possible animal damage or inadvertent human sabotage.
Inlets are pipes or channels that direct water into the wetland. As the length to width ratio of the wetland gets smaller, flow distribution becomes more critical. Inlets serve as the point of distribution control in wetlands with smaller length to width ratios. Inlets are generally 12-24 inches above the water surface with gravel or coarse rock directly underneath to prevent ponding and allow for quick infiltration. Avoid open water near inlets to prevent algal growth. This area should be shaded.
Outlets are responsible for controlling water levels in wetlands. The responsible structure is typically a weir, spillway or riser pipe. Weirs can be adjustable as can riser pipes while spillways are not adjustable. Water height is critical so adjustable height outlets can be easier to use. Outlet structures must be set up so as to allow for the maximal flow. Slopes for spillways should be the same as those of normal dikes and should be lined with fabric or riprap to prevent gradual erosion of the spillway, which might lower water levels. Ensure that the discharge is high enough above any water levels to avoid system back up in the event that the river or stream is at flood stage.