Wetland Construction and Management (Part 1)

Those of you who have spent a little time in the field with me know that wetlands and waterfowl are my favorite habitats and wildlife species to talk about. That’s why I was excited to see the comments about wetlands made by Richard and Dan Dawson on the Harvey WMA blog last week. Since other Harvey members could probably benefit from information on wetland construction and management, I’ve dedicated to post discussing the basics. All of the following information can be found in a Texas AgriLife Extension publication titled “Techniques for Wetland Construction and Management”. As always, if you would like additional information or a copy of the publication, please contact me.

Many people see wetlands as “wastelands” or as areas suitable only for snakes, alligators, and mosquitoes. Wetlands, however, provide many important ecological functions as well as recreational and aesthetic functions to those who learn to recognize them. Wetlands help regulate the climate, provide flood and erosion control, store and recycle nutrients, protect water quality, provide wildlife habitat, and provide recreational hunting and wildlife viewing opportunities. Many of these functions are enhanced on wetlands that are closely tied to a river system and waterfowl travel corridor like those of the Harvey WMA. Many of you have the opportunity to create, restore, or enhance wetlands on your property, and I hope the remainder of this post will provide you with the basic tools needed to begin.

The first step to a wetland development project is to assess the soil characteristics, seasonal availability of water, abundance of wetland adapted plants in the soil, and existing plant communities. Impermeable soils, adequate surface water during the fall and winter, and a community of annual seed producing plants indicate an area suitable for wetland development. After a suitable site has been identified, contact a private land surveyor or engineering personal with NRCS to supply an elevation survey accurate to one-foot contours. This level of accuracy is necessary for best placement of levees and control structures. Waterfowl and other wetland wildlife are attracted to plants that grow in shallow (<3ft).

Before beginning levee construction, contact a qualified contractor or NRCS personnel to provide soil sampling and engineering design to ensure that levees (1) will not leak, (2) will impound the desired amount of water, and (3) will withstand extreme flood events. Levees should not be more than 4-5 feet tall. The distance from the water level to the top of the levee should be between 18 and 30 inches. Levee side slopes should be no steeper than 3:1, but may be as gradual at 10:1. Levee elevations should be uniform throughout to prevent erosion at lower areas where floodwaters would overtop the levee. The levee should, however, be at its highest point near the control structure and discharge pipe to prevent washouts at this important location. A large and well-designed spillway should be set about 1-6 inches above the normal water level so that flood waters will immediately pass through the wetland and avoid placing unwanted stress on the levee system. Levee material should be borrowed from inside the constructed wetland area. This will create a narrow barrow ditch which may occasionally be more than 3 feet deep along the inside of the levee. If a levee will completely surround the wetland, remember to leave an area without a borrow ditch to allow access by hunters and/or equipment. After levee construction, immediately re-vegetate exposed soil on levees and borrow areas. Common or coastal Bermuda grass offers the best option for sod-forming grasses to hold soil and reduce erosion on these areas. Roots from woody plants and trees can penetrate the levee embankment and cause a washout. Shredding or herbicide treatments should be used to remove trees from levees.

Water control structures provide the ability to control water levels in the wetland. The most common types of structures are flashboard risers, screw gates, flap gates, and drop pipes. The flashboard riser is the most versatile and desirable. A flashboard riser functions by using a series of wooden, aluminum, or PVC “boards” fitted in slots to form one side of the riser box. Boards are added or removed to regulate water levels in the impoundment. The riser box is coupled to a culvert pipe that passes through the levee to discharge water. Install flashboard riser(s) near the lowest point(s) of the wetland to provide for maximum drainage capability. When placing risers in the levee, be sure that the top board in the riser box is the same elevation as the normal operating water level will be.

Check back next week for Part 2 on how to manage your wetland once it is built.