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Wetland Mitigation Strategies For Success

by George Milner, V3 Companies

Unvegetated mitigation area due to long-term, deep flooding.

Four things are required to establish desirable wetland vegetation for mitigation purposes – a thorough understanding of the natural environment (e.g. soils and hydrology), a well thought-out design, proper plan execution and follow-up stewardship. If these are not achieved, the probability of success will be low - even for a quality wetland mitigation plan.

Despite the difficulty of anticipating and controlling wetland hydrology and soils, both of which strongly influence wetland mitigation results, the negative impact these may cause on the success of a project can be alleviated through quality design, implementation and management strategies. These strategies include establishment of suitable soils that retain much of their natural structure and condition, accurate prediction of the anticipated hydrologic condition, installation of water control structures to facilitate hydrologic adjustments, appropriate species selection, meticulous plant installation, and excellent stewardship during the management and monitoring period. This article addresses specific wetland strategies for the upper Midwest region of the United States.

Construction Equipment Problems and Solutions. Establishment of suitable soils in a wetland mitigation area is paramount to successful generation of native vegetation. This can be very challenging, due to the detrimental effects conventional excavation and grading equipment can have on soil structure.

To improve soil conditions, specific construction and topdressing specifications for every mitigation project are needed to minimize soil compaction. Through proper construction the soil structure retains more of its natural structure and is more suitable as a plant-growth-medium.

To maintain soil structure during excavation and grading activities, scrapers and wheel-based equipment use should be minimized. Instead, low impact track equipment (e.g., D-6 wide track dozer, long reach backhoe, etc.) should be used where feasible to minimize compaction.

Execution and Topdress Material Tips. Mitigation areas should be over-excavated a minimum of one foot to facilitate placement of one foot of topsoil material. One foot of topdressing has been specified for years; however, execution is sometimes poor and topdress soils unsuitable. Topdress soils are often high in clay, low in organic matter, contain plant propagules (i.e., roots, stems and seeds) of aggressive wetland weeds, and have been stockpiled for an extensive period of time. These factors reduce the quality of the topdress materials and can encourage establishment of undesirable plants.

According to V3 staff soil scientists, quality topdress material should contain an organic matter content of 3 percent or more and a clay content of 27 percent or less. This clay content will minimize compaction during handling and provide adequate nutrients for plants. Minimizing stockpile residence time or avoiding it entirely also reduces compaction. Whenever feasible, topdress soils should be placed in the mitigation area immediately following their excavation. It also is important to ensure that the topdress material is free of weedy, wetland plant material. This can be accomplished by scraping the upper six inches of topdress soils to remove the “weed contaminated” portion. Using suitable upland topsoil for topdressing should be considered in cases where wetland soils are highly contaminated with weeds, such as reed canary grass and common reed.

Disking and deep chisel-plowing, worth the effort. Because soil compaction still can occur even though heavy equipment use is minimized, soils should be disked to further reduce compaction. Because subsoil in the Chicago region is usually high in clay, these soils can be significantly compacted, resulting in a “bath tub” effect that can cap the ground water table. To avoid that, the surface at the subgrade elevation (i.e., one foot below final grade) should be disked prior to topdressing. Disking the subsoil will increase infiltration and promote potential ground water movement into the root zone, creating a more natural, stable hydroperiod. Additionally, establishment of wet prairie and sedge meadow vegetation, which is difficult but very desired, needs seasonal ground water hydrology to be successful.

Plant Stress and Duration of Flooding. Regardless of the physical adaptations that wetland plants posses, allowing them to survive anaerobic conditions, these survival mechanisms are effective for their natural growing conditions. Plant stress occurs when adaptations cannot handle site conditions. This often occurs during long term, deep flooding. In general, longer flooding events result in higher plant stress. Because only a few native wetland species can tolerate an extensive period of deep flooding, many mitigation projects wind up with low species diversity. Minimizing flood duration and being familiar with flood tolerance of wetland species are crucial to mitigation success.

Various models (e.g., HEC-1, TR20, FEQ) are available to predict depth and duration of flooding following particular storm events. Even the best models are severely limited, in that they contain many assumptions and only generate data for a hypothetical storm event. Still, they are useful because they can predict the time in which water levels are above normal (i.e., flood duration), which can be beneficial in assessing the potential hydrologic stress on vegetation and can guide the selection of appropriate mitigation plant mixes.

Establishing Appropriate Hydrology. Establishment of appropriate hydrology begins with minimizing duration of flooding and “flashy” hydrology conditions typical of wetland mitigation areas. This is difficult to accomplish because most mitigation sites also provide stormwater management. An obvious solution is to construct wetland mitigation areas separately from stormwater basins. However, this is unrealistic in many urban settings due to site constraints, city or village requirements, and cost. Furthermore, providing adequate wetland hydrology can be difficult in areas removed from stormwater management systems due to the unpredictability of other hydrologic sources including ground water. Too many mitigation projects designed with a dependence on ground water hydrology have failed. This high failure rate is the result of the limited time and resources available to conduct appropriate ground water investigations. In addition, the ground water table is often altered by the very construction being mitigated (see soils section above). Typically, proposed “ground water wetlands” develop upland communities or low quality communities that consist of weedy hydrophytes.

Under appropriate flood durations, diverse, non-cattail dominated emergent vegetation can be established in surface water mitigation areas. The duration of flooding predicted for various flood events provides insight into whether the mitigation area can support a diverse emergent plant community.

There is limited data available regarding the flood tolerances of wetland species. However, flood durations of approximately three days during a 10-year event have resulted in the establishment of non-cattail dominated emergent communities in a number of successful wetland mitigation projects.

Installing a different mix of emergent plants can create a non-cattail dominated wetland for longer flood durations during a 10-year event. Other strategies must be used to increase mitigation success under those conditions including the use of water control structures and improved planting techniques.

The most difficult communities to create are the sedge meadow and wet prairie because they are not adapted to long periods of deep flooding. These communities rely upon a stable source of subsurface hydrology provided by an undisturbed ground water table. Careful consideration in the planning stage is needed when these communities are desired, which at a minimum should include a thorough ground water and soils investigation. A “modified sedge meadow mix” or for a better term “wet meadow mix” can be established in a surface water mitigation area with short periods of flooding (e.g., dry detention basin). This “modified seed mix” is composed of common, native hydrophytes with very few conservative species. Typical species of this mix include dark green rush (Scirpus atrovirens), red bulrush (Scirpus pendulous), fox sedge (Carex vulpinoidea), sneezeweed (Helenium autumnale), red-rooted spike rush (Eleocharis erythropoda), Torrey’s rush (Juncus torreyi), blue vervain (Verbena hastata), and New England aster (Aster novae-angliae).

Successful wetland mitigation showing desireable vegetation due to good hydrology.

Water Control Structure Features & Design. A typical water control structure consists of several adjustable stoplogs or boards of various heights, and is installed in or slightly downstream from the mitigation area’s outlet structure. The height of the individual boards typically ranges from three to seven inches forming a solid weir when placed together. Water level manipulation is accomplished by adjusting the boards in the structure to establish an appropriate weir elevation. Adding boards in the structure increases the weir height and raises the water elevation, while reducing water levels is accomplished by board removal.

A water control structure requires proper design to maximize effectiveness. It is very important to design the elevation of the bottom of the water control structure and the invert of the outlet’s flared end section at or slightly below the ground elevation of the lowest planting area. This will allow the weir to establish the desired water level (i.e., Normal Water Level), not the invert elevation of the flared end section. It also maximizes the range of water level adjustment, improving seeding, planting and management.

Water Control Structure Utilization and Benefits. Lowering water levels to establish a mud flat or shallow inundation can significantly improve seeding and planting conditions in wetland mitigation areas. Effective seed incorporation in a wetland can be accomplished under mud flat conditions. In contrast, installation of wetland plugs in deep inundation can cause significant stress on immature plants, which results in high plant mortality. An adjustable weir is key in establishing mud flat conditions or shallow inundation in wetlands during the proper planting/seeding time period. For instance, lowering the weir can minimize the potential loss of seed downstream and also can provide plants a “recovery period” following a heavy storm event. Greater wetland seeding and planting success are realized with appropriate water control structure utilization.

Lowering water levels also stimulates seed germination, leading to increased vegetative coverage in wetlands. The seed of most wetland species needs to be dry prior to germination and will lay dormant until such time occurs. In some cases, vegetative coverage has increased by simply lowering water levels to expose the ground.

Effective cattail management involves raising water levels out of the growing season and using the “cut and flood” method. Other wetland species are unaffected by this procedure. To maximize results, the wetland should be fully dewatered so cattail stems can be cut as close as possible to the ground. This low cutting will ensure cattail stem submergence during the flooding period and reduce the required water level elevation. After cutting, water levels should be raised a minimum of six inches above the cut stem for a period of one to two months to effectively drown the plant. If cutting is not practical, extended flooding alone during the non-growing season also has shown good results.

Top: Wetland mitigation area immediately following container plant installation. Bottom: Same area showing 100% cover from container plants just 5 weeks following installation.

Species Selection, Materials and Quantities. Species mixes for wetland mitigation sites should be customized based on anticipated soils and hydrology conditions because of the strong influence these factors have on plants. These are the most important factors to consider when preparing a species mix.

To reduce upfront costs, seeding is typically proposed to establish wetland vegetation. Under ideal conditions, seeding a wetland can be effective. However, seeding opportunities are very limited in wetlands that rely chiefly on surface runoff because periodic flooding prevents seed incorporation. If the site has an adjustable weir, seeding conditions can be improved by lowering water levels, but only to a limited extent. Even if the seed is properly incorporated, a subsequent flooding event can drown newly emerged seedlings as well as flush the seed downstream, outside of the intended area. Furthermore, seeding results are extremely unpredictable because most emergent species reproduce vegetatively (e.g., rhizomes, runners). Often, only a few of the species in the seed mix establish, resulting in wetlands with minimal diversity and low vegetative coverage. To achieve greater vegetative establishment success, a combination of seeding and installation of grown-out plants should be conducted.

The main benefit of installing grown-out plants is that they are more tolerant of flooding events, due to their established roots and shoots. Flooding following planting is one of the main reasons of planting failure. Plants also provide immediate coverage, which reduces the opportunity for establishment of weedy species resulting in less long-term weed management costs. In periodically flooded wetlands, vegetative establishment should depend more upon plant installation, with only particular species installed as seed. The seed mix proposed should only include those species that characteristically reproduce from seed under flooded conditions. The dependence on a seed mix to provide vegetative cover should decrease as water levels and duration of flooding increase.

The quantity of plants is typically specified on a per acre basis. More is better. Standard wetland mitigation designs include 2,000 to 3,000 plants per acre. Based on spacing of 1 to 2 foot on center, 3,000 plants will cover approximately 4,500 ft2, which is only 10% of 1 acre. Increasing the amount of plants to 5,000 per acre will provide additional cover of desirable species, which can dramatically improve mitigation results. Installation of more plants will increase upfront costs. However, a decrease in long-term costs will be realized as a result of less weed control needs and supplemental planting.

Additionally, the type of plant material installed can improve establishment success. Due to the harsh conditions of typical surface water wetland mitigation areas, container-grown plants provide the best option. Container plants have well established roots and tall shoots, which result in improved survivorship under harsh conditions such as a large flooding event or drought. Container plants are resistant to transport shock and desiccation. Not all container plants, however, provide the same level of value. Container plants should have established roots that hold the soil-plug intact and have shoot heights of at least 16 inches at the time of installation. To ensure robust root growth and well-established shoots, plants should be provided in square or circular, deep open bottom pots.

Installation, most important step. Installation of the seed and plant mix is probably the most important site activity that will be performed. Inappropriate installation will result in supplemental planting and seeding, extend site monitoring, and delay Corps of Engineers sign-off. All of those factors can increase long-term costs significantly. Additionally, results achieved from a replanting usually are not as good as the results that could have been achieved if done correctly the first time. Retaining a reputable contractor that specializes in native wetland plantings will improve results and provide significant long-term savings.

There are numerous firms that advertise themselves as native planting contractors. To identify reputable ones, verify that the contractor has an experienced wetland plant ecologist on staff who supervises the labor crew throughout the installation. This is essential to the success of the installations because it is crucial to appropriately place the plant species in a favorable environment. Because slight changes in conditions can greatly affect the survivorship of many wetland species, appropriate placement is important. Furthermore, the difficulty of plant installation increases with the microtopography of most constructed wetlands. As a result, knowledgeable staff will be needed throughout the installation to examine the wetland topography to determine appropriate species placement.

Follow-up Irrigation increases success. Periodic watering of newly installed wetland plants during the first summer can significantly increase survivorship. Most wetland plant installations in the Chicago region occur in late spring or early summer, due to high water levels or incomplete construction that prohibit early spring installation. This typically results in immature plants being subjected to hot and dry conditions during July and August. Watering newly installed plants should be performed during these dry, hot months to encourage establishment and prevent desiccation. Watering will provide an environment conducive for maximum plant growth, which will increase plant health and hardiness. Healthy plants have higher over-wintering potential, will be more tolerant of the subsequent spring flooding, and have an increased resilience to depredation.

U.S. Army Corps of Engineers wetland mitigation requirements include a minimum five-year management and monitoring period. That time period is used to foster the desired development of the mitigation areas through standard applied management techniques and to evaluate the mitigation area’s progress towards achieving predetermined goals. Standard ecological management activities include: chemical, mechanical, and biological activities to control weedy species; prescribed burn management; manipulation of hydrology; supplemental planting and seeding; and, regrading. Predetermined goals set by the Corps for mitigation areas are stated as performance standards. Meeting particular performance standards imply that mitigation efforts have been successful and meet regulatory requirements.

If performance standards are not met at the end of five years, additional compensation will be required to receive a Corps of Engineer project sign-off. Additional compensation may include: the purchase of mitigation credits from a mitigation bank; the extension of the management and monitoring period with the goal of performance standard achievement; and/or, a monetary donation to a Corps approved in-lieu fee program or fund. At any rate, failure to meet the performance standards will result in extra mitigation costs above and beyond what was allocated in the five-year management and monitoring budget.

To further promote success of a mitigation site, inspections may need to be stepped up above and beyond the biannual visits required by the Corps of Engineers – especially during the first and second year of the management period. Weed control during this time is essential for establishment of desirable vegetation during the early stages of the project. Gaining control of weedy species early increases mitigation success and reduces long-term weed control efforts and associated costs. To accomplish this, sufficient site inspections to quickly identify weed problems are needed to ensure that control activities occur within specified “control windows.” Every species has an optimal time when it is most vulnerable to control activities. Conducting activities within this period will maximize results and increase control effectiveness. Taking a stewardship approach during the management and monitoring period instead of meeting the minimum regulatory monitoring requirements will improve mitigation success.

Despite advances in the science of mitigation, the unpredictability of nature such as a large flood can cause significant plant mortality and replanting costs. However, an increased chance of success can be realized by creating the best possible environment for wetland plant establishment. This can be accomplished by minimizing soil compaction during excavation and grading to ensure that adequate soil structure is retained, and by reducing the depth and duration of flooding to provide a more natural wetland hydroperiod. Additionally, proper execution of certain implementation and management strategies can improve mitigation results. These implementation and management strategies include: water level manipulation to create optimal seeding and planting conditions as well as weed management; selection of species suitable to anticipated conditions including adequate plant quantities and appropriate plant materials; the use of high-quality native wetland planting contractors to provide premium installation services; and a stewardship approach during the management and monitoring period. Creating the best possible environment and employing these strategies will increase the probability of obtaining a Corps of Engineers sign-off at the end of the prescribed management period, which will lower the chance of additional costs associated with potential supplementary mitigation activities. L&W

For more information contact George Milner, V3 Companies, 7325 Janes Avenue, Suite 100, Woodridge, IL 60517, (630)724-9100, fax (630)724-9111.

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Tuesday, February 11, 2003 - http://www.landandwater.com/features/vol47no1/vol47no1_2.html