Kent, Donald M. et al “Avoiding and Minimizing Impacts to Wetlands” Applied Wetlands Science and Technology Editor Donald M. Kent Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 5 Avoiding and Minimizing Impacts to Wetlands Donald M. Kent and Kevin McManus CONTENTS Planning Design and Construction Design Construction Erosion and Sedimentation Nitrogen Loading Planning Guidelines Estimating Nitrogen Loads Stormwater Runoff Planning and Nonstructural Practices Structural BMPs Pretreatment Detention Basins/Retention Ponds Vegetated Treatment Infiltration Filtration References Recent estimates of the extent of global wetlands range from 5 to 8.6 million ha (Mitsch, 1995). Increasing evidence suggests that the historic extent of global wet- lands was substantially greater. For example, in Japan, 45 percent of tidal flats have been destroyed since 1945 (Hollis and Bedding, 1994). Northern Greece has lost ©2001 CRC Press LLC 94 percent of its marshland since 1930. In the conterminous United States, an estimated 47 million ha of wetlands have been lost over the last 200 years — an average rate of 235,000 ha per year (U.S. Office of Technology Assessment, 1984; Dahl, 1990; Hollis and Bedding, 1994). This rate of loss appears to have decreased dramatically in recent years, to about 32,000 ha per year, coincident with recognition of the importance of wetlands and a “no net loss” government policy (Heimlich and Melanson, 1995). Wetland losses are attributed to filling and draining, primarily in support of development and agricultural activities. An unknown number of wetlands, not filled or drained, have been otherwise impacted by changes in watersheds or adjacent land uses. Alterations to wetland plant communities lead to increased erosion and sedimentation. Construction of buildings, parking lots, and other impervious surfaces increases the quantity and decreases the quality of surface runoff to wetlands. Septic systems and fertilizers increase the concentration of nitrogen in groundwater flow to wetlands. Activities adjacent to wetlands can disturb wildlife. Wetland impacts, both direct and indirect, can be avoided or minimized by appropriate planning, design, and construction. In this chapter, planning is discussed as a means for avoiding or minimizing direct impacts to wetlands. Design and construction techniques are discussed as a means to avoid or minimize indirect impacts to wetlands. Discussed in some detail are three design and construction issues. They are erosion and sedimentation, nitrogen loading, and stormwater. PLANNING Planning to avoid or minimize direct impacts to wetlands is fundamentally a three-step process. The first step is to identify the wetland resource. Discussed in detail in Chapter 2, this step requires applying hydrology, soils, and vegetation criteria to undeveloped areas. For large areas, off-site resource identification is an effective and appropriate approach for preliminary planning. Greater resource res- olution, typically requiring on-site identification, is more appropriate for smaller areas and for detailed planning. Characterization and classification (e.g., palustrine forested wetland, emergent marsh; see Chapter 1) of wetland resources are also helpful at this stage. The second step in effective planning is to assign functions and values to iden- tified wetland resources. Common techniques for determining functions and values include professional opinion, the use of indicators, direct measurement, and eco- nomic analysis (see Chapter 3). As with resource identification, off-site and less detailed approaches are most appropriate for large areas during preliminary planning, whereas on-site assessments are most appropriate for small areas and detailed plan- ning. Assigning functions and values will facilitate prioritization in the event that not all resource areas can be preserved and reveal functions and values that need to be protected or replaced during construction and operation. Finally, wetlands identified and evaluated for functions and values are incorpo- rated into a site selection process. Site selection typically includes identification of ©2001 CRC Press LLC several alternative sites and development of site selection criteria. Alternative sites satisfy minimal, implicit criteria such as availability and location. At a minimum, the site selection process should consider the criteria listed in Table 1 (McManus, 1994). Direct and indirect impacts to wetland and other envi- ronmental resources should be identified. Other environmental resources include fish and wildlife, navigation channels, and recreation areas. Projects not dependent upon access to water should be sited elsewhere. The minimum size required to satisfy the project purpose should be determined and project configuration and layout evaluated to further reduce project size. Constructability refers to project topographic, slope, soil, and backfill requirements. Extensive grading, blasting, or filling are typically associated with environmental impacts and should be avoided. Proximity to support- ing infrastructure, such as utilities and roadways, affects project size, configuration and layout, and cost. Cost prohibitive sites should be eliminated; thereafter, the costs of development should be weighed against the costs of environmental impacts. The opportunity for successfully satisfying the requirements of various international, national, regional, and local entities such as regulatory agencies and lending insti- tutions should also be evaluated. Larger and more complex projects will require a more detailed site selection process. In the United States, the National Environmental Policy Act (U.S. Con- gress/NEPA, 1978) provides guidance as to appropriate criteria for evaluating project impacts to wetlands and other environmental resources. In addition to environmental impacts, this approach considers impacts to human uses and the technical, economic, and institutional feasibility and merits of the site. Table 2 represents a hypothetical site-screening matrix consistent with the NEPA (McManus, 1994). In the example, Site 1 is technically and economically feasible, but will likely impact the environment and human use of the site, and is not publicly acceptable. Site 2 has no significant environmental, human use, or institutional constraints but has technical and eco- nomical issues. Site 3 is the preferred site, having no significant environmental or human use impacts, being technically and economically feasible and acceptable to the public. Table 1 Representative Site Selection Criteria (Adapted from McManus, 1994) Wetland impacts Other environmental impacts Water dependency Site size Constructability Supporting infrastructure Costs Regulatory/institutional issues ©2001 CRC Press LLC Table 2 A Hypothetical Site Selection Matrix (Adapted from McManus, 1994) Screening Criteria Site 1 Site 2 Site 3 Environmental Aquatic Ecosystem Substrate 0 0 0 Water quality 0 0 0 Water circulation 0 0 0 Normal water fluctuations – 0 0 Threatened and endangered species – 0 0 Other aquatic organisms and wildlife – 0 0 Special Aquatic Sites Sanctuaries/refuges – 0 0 Wetlands – 0 0 Mudflats 0 0 0 Vegetated shallows 0 0 0 Riffle and pool complexes – 0 0 Human Uses Water supplies 0 0 0 Recreational and commercial fisheries – 0 0 Water–related recreation – 0 0 Aesthetics – 0 0 Parks, preserves, wilderness areas – 0 0 Archaeological or historical sites 0 0 0 Compatibility with adjacent land uses – 0 0 Potential noise impacts 0 0 0 Potential odor impacts 0 0 0 Public health 0 0 0 Traffic increase 0 0 0 Technical Suitable foundation/soils conditions + – + Adequate land area + – + Access to existing roads and utilities – + Economic Land acquisition + – + Operation and maintenance + – + Capital cost—construction 0 – 0 Institutional Public acceptance – 0 + Compliance with existing regulations 0 0 0 Note: + indicates an expected positive impact; – is an expected negative impact; 0 is an insignificant or no impact. ©2001 CRC Press LLC DESIGN AND CONSTRUCTION Design Once the site selection process has been completed, the focus can shift to design details, site layouts, construction methods, and other specific engineering require- ments to minimize unavoidable wetland impacts. A reasoned assessment of the minimum economically and functionally viable size for a proposed structure(s) should be made, particularly if the project is not water dependent. Even for water dependent projects, such as marinas or dredging projects, project scope should be evaluated with an eye toward minimizing wetland impacts. The project should have an accurate wetland delineation line depicted on site plans to facilitate evaluation of layout options. For projects that may involve clearing of trees and other existing vegetation, care should be taken to minimize the limits of clearing to the minimum acreage needed for the project. Maintenance of existing vegetative buffers, particularly within wet- land areas, is not only a valuable means of providing a visual and auditory buffer for the facility, but it also may reduce overall facility wetland impacts. This is particularly true along active coastal shorelines, such as eroding bluffs, beaches, and dune environments. The orientation and layout of a project are generally a function of its intended purpose and use. Many projects, such as railways, roads, and retaining walls, being linear features, have limited flexibility with regard to basic configuration. However, their actual alignment, relative to wetland areas, can often be optimized to reduce impacts to insignificant levels. Similarly, layouts of buildings and ancillary struc- tures such as garages, walkways, and decks can be adjusted to minimize direct wetland impacts. Specific design details for a project can also be important factors in reducing wetland impacts. For example, use of the maximum safe slopes for site preparation will minimize incursions into wetland areas. Maximum safe slopes can be achieved using vertical retaining walls, cellular confinement, sheet piling, or gabion rock walls. Backfill and other construction materials should ensure good drainage and scour protection (Nelson, 1995). Another method for minimizing impacts is to use boardwalks supported by posts or post-like anchors. Waterway crossings offer another opportunity to minimize wetland impacts. Typically, culverts are used when crossing small waterways. Culverts should be designed to pass expected flows (e.g., 100-year flood event), and to avoid changes to flow velocity and increased erosion and scour. Bridges can minimize impacts to larger waterways, especially if construction is accomplished in midair using a crawler crane. Construction For many projects, such as subsurface water, sewage, and other utility pipelines, the primary impacts to wetlands occur during construction. The use of temporary ©2001 CRC Press LLC access materials, specialized construction equipment, and the placement of staging areas can all affect the level of wetland impacts. Temporary pile-supported construction trestles can be used to significantly reduce direct wetland impacts through ecologically sensitive wetland areas such as estuarine and fresh-water marshes, beach or dune environments, and peat bogs (Figure 1). These trestles can be located either directly above, or directly adjacent to, the work area. Equipment can be brought to the work area using rail-mounted transport platforms, and the trestle can be constructed in stages to accommodate the construction schedule. Trestles provide a stable temporary work platform that directly impacts little wetland acreage. Figure 1 Temporary pile-supported construction trestles can be used to significantly reduce wetland impacts. The trestles may be located either directly above or immediately adjacent to the work area. PLAN VIEW pipeline wood planking sheet piling construction trench sheet piling wood planking pipeline wetland PROFILE upland ©2001 CRC Press LLC Another effective construction technique uses steel sheet piling to isolate the active work area, and temporary wood decking placed directly on top of the sheet piling. This allows construction equipment to access the work areas without com- pacting wetland soils. Compacted wetland soils lose their original productivity and hydrologic functions. For smaller projects that may not warrant the use of sheet piling, geotextile fabric, clean granular material, and wood decking can be placed within the project alignment. Sheet piling can also be used in intertidal or shallow freshwater areas. Combined with siltation curtains, piling can prevent the release of sediment-laden water to surrounding wetlands and waterways. The use of barge-mounted equipment can also be used in intertidal and shallow freshwater areas to access sensitive sites. Work barges can be floated into place on rising tides, and grounded out to provide suitable access with minimal or no long-term impacts. For construction of trenches in wetland areas, utility workers have developed specialized, tracked, trenching vehicles that can operate on soft, unstable soils. The vehicles work directly within the project alignment. Wide, low-pressure tires on vehicles that distribute loads across wetland soils and vegetation also reduce vehicle impacts. For dredging within wetlands, waterways, and waterbodies, clamshell dredge equipment fitted with covers and watertight buckets minimizes sediment washout and turbidity. Large construction projects typically require staging areas. Staging areas should be located outside wetlands and their designated buffer zones and should be paved to minimize erosion and groundwater impacts. Also, staging areas should include stormwater management systems designed to trap suspended sediments and to con- tain accidental releases of fuel oil, lubricants, and other potentially hazardous releases from equipment. Scheduling can minimize temporary, construction-related impacts. As a general rule, wetland work in temperate climates should be scheduled during winter and early spring when plants are dormant and the soils are frozen or well consolidated. Soil compaction is minimized, and site cleanup and rehabilitation during the coming peak growing season are facilitated. Other seasonal restrictions are often applied for work within coastal environments based upon the expected presence of commercially and recreationally important fish and wildlife species. Species susceptible to ill- timed construction include spawning and migrating anadromous fish and shrimp, overwintering groundfish, and migratory waterfowl. Another method for minimizing the impacts of construction within wetlands is proper work sequencing. For example, minimizing the extent of clearing in front of the active trenching operation will reduce the potential for soil erosion into adjacent wetlands and reduce impacts to wildlife using the existing vegetative cover. Wherever possible, work that is required within wetland areas should be completed as quickly as possible, without excessive delays between the initial disturbance and rehabilita- tion. Trenching should be conducted as a single, continuous operation, involving clearing, installation, backfilling, and soil restoration. An open trench can act as a channel to dewater adjacent wetland areas and increase erosion and runoff impacts. ©2001 CRC Press LLC EROSION AND SEDIMENTATION Sedimentation of wetlands can be avoided or minimized by preventing soil erosion and controlling already eroded sediments. There are numerous methods for erosion and sedimentation control, all of which seek to isolate and contain, to the maximum extent possible, sediment-laden runoff generated during project construc- tion activities. The performance of these various methods in the field varies consid- erably depending upon the type of soils, water flows, exposure, and other site specific factors. Figure 2 summarizes some of the more popular sedimentation control meth- ods. Critical elements of effective erosion and sediment control plans are listed in Table 3 (Brown and Caraco, 1997). Erosion and sedimentation control methods can be used singly or in combination. By limiting the amount of incremental and total land clearing, and maintaining existing ground cover to the maximum extent possible, potential runoff, gully cre- ation, rutting, and airborne dust formation can be reduced to acceptable levels. Cleared land produces as much as 2000 times more sediment than uncleared land (Paterson et al., 1993). Where feasible, a project site layout should take advantage of existing vegetation between the clearing limits and adjacent wetlands. Buffers of at least 25 m in width are the most effective in filtering sediment from construction site runoff (Woodward, 1989). Vegetative buffers should also be preserved for projects with shoreline frontage to protect structures from wave and flooding impacts. Installation of hay bales within shallow cut-off trenches upgradient of wetland areas can be an effective and inexpensive perimeter control method. Bales should be staked to the ground, without gaps between bales. Bales should be routinely monitored, and bales damaged, moved, or destroyed during construction should be repaired. Construction specifications should provide for regular checks of the con- dition and effectiveness of the hay bale protection systems. Geotextile siltation fences can be wrapped around hay bales and staked into the ground to provide an extra measure of protection against the release of fine-grained materials. Siltation fence efficiency ranges from 35 to 86 percent depending upon site conditions (Horner et al., 1990; W&H Pacific and CH2M-Hill, 1993). Siltation curtains can also be used effectively in both wetlands and open water environments. Curtains can be used to surround subaqueous dredging operations, particularly those occurring within sheet piling, to isolate trench water from the surrounding environment. Curtains with flotation can also be installed around shore- line construction projects and anchored in place to isolate the work area. However, the effectiveness of these structures decreases significantly in areas of strong river currents, tidal flows, and large tidal ranges, particularly if the curtain is installed perpendicular to the current flow. In such cases, the siltation curtain experiences rollover or submergence and is susceptible to damage from debris. Therefore, silt- ation curtains are most effective in ponds, lakes, and other sheltered water bodies with little or no variation in water height. In any construction project, regardless of the proximity to wetlands or other adjacent sensitive habitats, construction specifications should require prompt stabi- lization of newly exposed soils, including stockpiled soil. Seeding and sodding are relatively inexpensive, and up to 99 percent effective in reducing erosion (Brown Figure 2 Erosion and sedimentation control methods (McManus, 1994). Black indicates the method is suitable for use in the environment; gray indicates the method is suitable with limitations. ©2001 CRC Press LLC [...]... 0. 75 mg NO3/L = 5, 539.7 L/day 4, 154 .8 mg/day Paved 4,000 m2 × 1 m/yr × 1,011 L/m3 × 1 yr/3 65 day L runoff/day × 1 .5 mg NO3/L = 11079 .5 L/day 16,619.3 mg/day Natural 20,000 m2 × 0. 45 m/yr × 1,011 L/m3 × 1 yr/3 65 24,928.8 L/day Lawn 10,000 m2 × 1000 mg/100 m2/yr × 1 yr/3 65 days 68 .5 mg/day Wastewater 3 .5 bedrooms × 400 L/bedroom × 20 bedrooms L wastewater per day × 35 mg NO3/L= 28,000 L wastewater per... MADEP, 1997 McManus, K., Wetlands avoidance and impact minimization, in Applied Wetlands Science and Technology, Kent, D M., Ed., Lewis Publishers, Boca Raton, FL, 1994, 1 05 Mitsch, W., The world’s wetlands and SWS—a call for an international view, Wetlands Bull., September, 1, 4, 19 95 Mitsch, W H and Knight, R L., Treatment Wetlands, Lewis Publishers, Boca Raton, FL, 1997 Nassau-Suffolk Regional Planning... per day 980,000 mg/day Cumulative nitrate nitrogen load 4, 154 .8 + 16, 619.3 + 68 .5 + 980, 000 mg - = 14.39 mg/L 5, 53 9.7 + 11079 .5 + 24, 928 + 28, 000 liters Figure 5 Table 5 Nitrate nitrogen loading calculations for a hypothetical 20 house residential development with an average of 3 .5 bedrooms per house Percent Nitrogen Input to the Watershed,... Wastewater NO3 35 mg/L Roof Area 2,000 m2 Roof runoff NO3 0. 75 mg/L Paved Area 4,000 Paved runoff NO3 1 .5 mg/L Natural Area 20,000 m2 Fertilizer 1000 g/100 m2 Lawn Area 10,000 m2 Fertilizer leach rate 0. 25 Wastewater 400 L/bedroom Impervious surface recharge rate 1 meter/year Natural area recharge rate 0. 45 meter/year Roof 2,000 m2 × 1 m/yr × 1,011 L/m3 × 1 yr/3 65 day L runoff/day × 0. 75 mg NO3/L = 5, 539.7 L/day... 0.2 to 2 ha (0.4 to 4 acres) for most development situations A design filtration rate of 5 cm (2 in.) per hour is typical, and the filter should drain within 24 h REFERENCES Bingham, D., Wetlands for stormwater treatment, in Applied Wetlands Science and Technology, Kent, D M., Ed., Lewis Publishers, Boca Raton, FL, 19 95 Brach, J., Protecting Water Quality in Urban Areas: Best Management Practices for Minnesota,... the active work area in both wetlands and open water areas is an effective method to limit the horizontal extent of disturbance, particularly in areas where significant dredging is required In such cases, dredging open trenches beyond 1 m in depth requires side slopes which can range from 3:1 to 5: 1 or greater, meaning that a 3-m-deep trench would disturb a minimum 2 0- to 33-m width of sediments Clearly,... elements of natural wetlands, constructed wetlands reduce peak discharge and reduce the occurrence of downstream flooding, settle particulate pollutants, and facilitate the uptake of pollutants by vegetation Constructed wetlands require relatively large contributing drainage areas to maintain dry weather base flows, and construction costs are relatively high Chapter 10 discusses constructed wetlands for the... States range from 0.14 to 1. 15 ppm nitrate nitrogen (Loehr, 1974) Dry deposition of nitrogen may double this concentration (Valiela et al., 1997) Nitrogen loading off of impervious surfaces is, however, significant, ranging from 0.41 to 1. 75 ppm nitrate nitrogen and 1.13 to 10 ppm total nitrogen (IEP, 1988) Recharge rate off of impervious surfaces is poorly understood The TR -5 5 stormwater modeling program... The Cape Cod Golf Course Monitoring Project, Cape Cod Commission, Water Resources Office, Barnstable, MA, 1990 Heimlich, R and Melanson, J., Wetlands lost, wetlands gained, Natl Wetlands Newsl., May–June, 17(3), 1, 23, 19 95 Hollis, T and Bedding, J., Can we stop wetlands from drying up?, New Sci., July, 31, 1994 Horner, R R., Skupien, J J., Livingston, E H., and Shaver, H E., Fundamentals of Urban Runoff... Thus, it is recommended that on-site resident inspectors monitor the success of the installed erosion control devices ©2001 CRC Press LLC Ground Slope 15' - 20 (Typ.) or as Directed Suitable Device to Dissipate Velocity To Natural Water Course Sediment Laden Water Sediment Free Water Pump Discharge Flat Stone Ground Slope Approved Filter fabric Mat Baled Hay or Straw 10 '-1 5' (Typ.) or as Direcled Pump . Watershed Input to Estuaries Atmospheric 56 89 30 Fertilizer 14 79 15 Wastewater 27 65 48 4 154 .8 16 619.3 68 .5 980 000 mg,++,+, 5 539.7 11079 .5 24 928 28 000 liters,+,++, 14.39 mg/L= ©2001. Impacts to Wetlands Applied Wetlands Science and Technology Editor Donald M. Kent Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 5 Avoiding and Minimizing Impacts to Wetlands . meter/year Natural area recharge rate 0. 45 meter/year Roof 2,000 m 2 × 1 m/yr × 1,011 L/m 3 × 1 yr/3 65 day 5, 539.7 L/day L runoff/day × 0. 75 mg NO3/L = 4, 154 .8 mg/day Paved 4,000