Applied Wetlands Science - Chapter 5

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 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. 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 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 1is 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. ©2001 CRC Press LLC Table 2 A Hypothetical Site Selection Matrix (Adapted from McManus, 1994) ScreeningCriteria Site 1 Site 2 Site 3 Environmental Aquatic Ecosystem Substrate Water Water Normal quality circulation water 000 000 000 fluctuations – 00 Threatened and endangered species – 00 Other aquatic organisms and wildlife – 00 Special Aquatic Sites Sanctuaries/refuges – 00 Wetlands – 00 Mudflats 000 Vegetated shallows 000 Riffle and pool complexes – 00 Human Uses Water Recreational supplies and commercial 000 fisheries – 00 Water–related recreation – 00 Aesthetics – 00 Parks, preserves, wilderness areas – 00 Archaeological or historical sites 000 Compatibility Potential Potential Public Traffic with noise odor health increase adjacent impacts impacts land uses – 00 000 000 000 000 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 Compliance acceptance with existing – 0+ regulations 000 Note: + indicates an expected positive impact;– is an expected negative impact; 0 is an insignificant or no impact. ©2001 CRC Press LLC ... - tailieumienphi.vn