Applied Wetlands Science - Chapter 5 ppsx

Table 1 Representative Site Selection Criteria Adapted from McManus, 1994 Wetland impacts Other environmental impacts Water dependency Site size Constructability Supporting infrastructu

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

WetlandsDonald M Kent and Kevin McManus

CONTENTS

PlanningDesign and ConstructionDesign

ConstructionErosion and SedimentationNitrogen Loading

Planning GuidelinesEstimating Nitrogen LoadsStormwater Runoff

Planning and Nonstructural PracticesStructural BMPs

PretreatmentDetention Basins/Retention PondsVegetated Treatment

InfiltrationFiltrationReferences

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 havebeen destroyed since 1945 (Hollis and Bedding, 1994) Northern Greece has lost

94 percent of its marshland since 1930 In the conterminous United States, anestimated 47 million ha of wetlands have been lost over the last 200 years — anaverage 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 decreaseddramatically 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 andMelanson, 1995) Wetland losses are attributed to filling and draining, primarily insupport of development and agricultural activities

An unknown number of wetlands, not filled or drained, have been otherwiseimpacted by changes in watersheds or adjacent land uses Alterations to wetlandplant communities lead to increased erosion and sedimentation Construction ofbuildings, parking lots, and other impervious surfaces increases the quantity anddecreases the quality of surface runoff to wetlands Septic systems and fertilizersincrease the concentration of nitrogen in groundwater flow to wetlands Activitiesadjacent to wetlands can disturb wildlife

Wetland impacts, both direct and indirect, can be avoided or minimized byappropriate planning, design, and construction In this chapter, planning is discussed

as a means for avoiding or minimizing direct impacts to wetlands Design andconstruction techniques are discussed as a means to avoid or minimize indirectimpacts to wetlands Discussed in some detail are three design and constructionissues They are erosion and sedimentation, nitrogen loading, and stormwater

PLANNING

Planning to avoid or minimize direct impacts to wetlands is fundamentally athree-step process The first step is to identify the wetland resource Discussed indetail in Chapter 2, this step requires applying hydrology, soils, and vegetationcriteria to undeveloped areas For large areas, off-site resource identification is aneffective and appropriate approach for preliminary planning Greater resource res-olution, typically requiring on-site identification, is more appropriate for smallerareas and for detailed planning Characterization and classification (e.g., palustrineforested wetland, emergent marsh; see Chapter 1) of wetland resources are alsohelpful at this stage

The second step in effective planning is to assign functions and values to tified wetland resources Common techniques for determining functions and valuesinclude professional opinion, the use of indicators, direct measurement, and eco-nomic analysis (see Chapter 3) As with resource identification, off-site and lessdetailed 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 thatnot all resource areas can be preserved and reveal functions and values that need to

iden-be protected or replaced during construction and operation

Finally, wetlands identified and evaluated for functions and values are rated into a site selection process Site selection typically includes identification of

incorpo-several alternative sites and development of site selection criteria Alternative sitessatisfy 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 ronmental resources should be identified Other environmental resources include fishand wildlife, navigation channels, and recreation areas Projects not dependent uponaccess to water should be sited elsewhere The minimum size required to satisfy theproject purpose should be determined and project configuration and layout evaluated

envi-to further reduce project size Constructability refers envi-to project envi-topographic, slope,soil, and backfill requirements Extensive grading, blasting, or filling are typicallyassociated with environmental impacts and should be avoided Proximity to support-ing infrastructure, such as utilities and roadways, affects project size, configurationand layout, and cost Cost prohibitive sites should be eliminated; thereafter, the costs

of development should be weighed against the costs of environmental impacts Theopportunity 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 selectionprocess In the United States, the National Environmental Policy Act (U.S Con-gress/NEPA, 1978) provides guidance as to appropriate criteria for evaluating projectimpacts to wetlands and other environmental resources In addition to environmentalimpacts, this approach considers impacts to human uses and the technical, economic,and institutional feasibility and merits of the site Table 2 represents a hypotheticalsite-screening matrix consistent with the NEPA (McManus, 1994) In the example,Site 1 is technically and economically feasible, but will likely impact the environmentand human use of the site, and is not publicly acceptable Site 2 has no significantenvironmental, human use, or institutional constraints but has technical and eco-nomical issues Site 3 is the preferred site, having no significant environmental orhuman use impacts, being technically and economically feasible and acceptable tothe 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

Table 2 A Hypothetical Site Selection Matrix (Adapted from

McManus, 1994) Screening Criteria Site 1 Site 2 Site 3

Threatened and endangered species – 00 Other aquatic organisms and wildlife – 00 Special Aquatic Sites

Technical

Suitable foundation/soils conditions + – +

Access to existing roads and utilities – +

Compliance with existing regulations 000

Note: + indicates an expected positive impact; – is an expected negative impact; 0 is an insignificant or no impact.

DESIGN AND CONSTRUCTION Design

Once the site selection process has been completed, the focus can shift to designdetails, site layouts, construction methods, and other specific engineering require-ments to minimize unavoidable wetland impacts A reasoned assessment of theminimum economically and functionally viable size for a proposed structure(s)should be made, particularly if the project is not water dependent Even for waterdependent projects, such as marinas or dredging projects, project scope should beevaluated 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, careshould be taken to minimize the limits of clearing to the minimum acreage neededfor the project Maintenance of existing vegetative buffers, particularly within wet-land areas, is not only a valuable means of providing a visual and auditory bufferfor the facility, but it also may reduce overall facility wetland impacts This isparticularly true along active coastal shorelines, such as eroding bluffs, beaches, anddune environments

The orientation and layout of a project are generally a function of its intendedpurpose and use Many projects, such as railways, roads, and retaining walls, beinglinear features, have limited flexibility with regard to basic configuration However,their actual alignment, relative to wetland areas, can often be optimized to reduceimpacts to insignificant levels Similarly, layouts of buildings and ancillary struc-tures such as garages, walkways, and decks can be adjusted to minimize directwetland impacts

Specific design details for a project can also be important factors in reducingwetland impacts For example, use of the maximum safe slopes for site preparationwill minimize incursions into wetland areas Maximum safe slopes can be achievedusing vertical retaining walls, cellular confinement, sheet piling, or gabion rockwalls Backfill and other construction materials should ensure good drainage andscour protection (Nelson, 1995) Another method for minimizing impacts is to useboardwalks 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 bedesigned 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 tolarger waterways, especially if construction is accomplished in midair using acrawler 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

access materials, specialized construction equipment, and the placement of stagingareas can all affect the level of wetland impacts.

Temporary pile-supported construction trestles can be used to significantlyreduce direct wetland impacts through ecologically sensitive wetland areas such asestuarine 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-mountedtransport platforms, and the trestle can be constructed in stages to accommodate theconstruction schedule Trestles provide a stable temporary work platform thatdirectly 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

Another effective construction technique uses steel sheet piling to isolate theactive work area, and temporary wood decking placed directly on top of the sheetpiling This allows construction equipment to access the work areas without com-pacting wetland soils Compacted wetland soils lose their original productivity andhydrologic functions For smaller projects that may not warrant the use of sheetpiling, geotextile fabric, clean granular material, and wood decking can be placedwithin the project alignment.

Sheet piling can also be used in intertidal or shallow freshwater areas Combinedwith siltation curtains, piling can prevent the release of sediment-laden water tosurrounding wetlands and waterways The use of barge-mounted equipment can also

be used in intertidal and shallow freshwater areas to access sensitive sites Workbarges can be floated into place on rising tides, and grounded out to provide suitableaccess with minimal or no long-term impacts

For construction of trenches in wetland areas, utility workers have developedspecialized, tracked, trenching vehicles that can operate on soft, unstable soils Thevehicles work directly within the project alignment Wide, low-pressure tires onvehicles that distribute loads across wetland soils and vegetation also reduce vehicleimpacts For dredging within wetlands, waterways, and waterbodies, clamshelldredge equipment fitted with covers and watertight buckets minimizes sedimentwashout 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 includestormwater management systems designed to trap suspended sediments and to con-tain accidental releases of fuel oil, lubricants, and other potentially hazardousreleases from equipment

Scheduling can minimize temporary, construction-related impacts As a generalrule, wetland work in temperate climates should be scheduled during winter andearly spring when plants are dormant and the soils are frozen or well consolidated.Soil compaction is minimized, and site cleanup and rehabilitation during the comingpeak growing season are facilitated Other seasonal restrictions are often applied forwork within coastal environments based upon the expected presence of commerciallyand 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 isproper work sequencing For example, minimizing the extent of clearing in front ofthe active trenching operation will reduce the potential for soil erosion into adjacentwetlands and reduce impacts to wildlife using the existing vegetative cover Whereverpossible, work that is required within wetland areas should be completed as quickly

as possible, without excessive delays between the initial disturbance and tion Trenching should be conducted as a single, continuous operation, involvingclearing, installation, backfilling, and soil restoration An open trench can act as achannel to dewater adjacent wetland areas and increase erosion and runoff impacts

rehabilita-EROSION AND SEDIMENTATION

Sedimentation of wetlands can be avoided or minimized by preventing soilerosion and controlling already eroded sediments There are numerous methods forerosion and sedimentation control, all of which seek to isolate and contain, to themaximum 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 specificfactors 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 maintainingexisting 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 constructionsite runoff (Woodward, 1989) Vegetative buffers should also be preserved forprojects with shoreline frontage to protect structures from wave and flooding impacts.Installation of hay bales within shallow cut-off trenches upgradient of wetlandareas can be an effective and inexpensive perimeter control method Bales should

be staked to the ground, without gaps between bales Bales should be routinelymonitored, and bales damaged, moved, or destroyed during construction should berepaired Construction specifications should provide for regular checks of the con-dition and effectiveness of the hay bale protection systems Geotextile siltation fencescan be wrapped around hay bales and staked into the ground to provide an extrameasure of protection against the release of fine-grained materials Siltation fenceefficiency 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 waterenvironments Curtains can be used to surround subaqueous dredging operations,particularly those occurring within sheet piling, to isolate trench water from thesurrounding 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 rivercurrents, tidal flows, and large tidal ranges, particularly if the curtain is installedperpendicular to the current flow In such cases, the siltation curtain experiencesrollover or submergence and is susceptible to damage from debris Therefore, silt-ation curtains are most effective in ponds, lakes, and other sheltered water bodieswith little or no variation in water height

In any construction project, regardless of the proximity to wetlands or otheradjacent sensitive habitats, construction specifications should require prompt stabi-lization of newly exposed soils, including stockpiled soil Seeding and sodding arerelatively 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

and Caraco, 1997) Seeding is the least expensive option and is appropriate whentemporary stabilization is required Seeds can be broadcast by hand or hydroseeded.The latter is a mixture of seeds, water, fertilizer, lime, and mulch sprayed onto thesoil Sodding is more appropriate for permanently vegetated areas and providesimmediate cover and greater resistance to higher flow velocities.

The construction schedule should allow time for vegetation to become lished prior to the end of the growing season In cases where this is not possible,more expensive but generally less effective measures, such as mulching or coveringexposed areas with erosion control blankets, jute mats, or geotextile mats, should

reestab-be employed Mulches, blankets, and mats protect seeds from erosion, dehydration,and animals until the next growing season (Brown and Caraco, 1997) Mulches,consisting of straw, hay, fiber, or wood chips, are effective on flat or gently slopingareas Erosion control blankets consist of a mulch material held together by a plasticnetting, and jute mats are sheets of woven jute fiber Effective on relatively levelground, both the blankets and the mats are stapled to the ground after seeding anddegrade over time Geotextile mats are more appropriate for steeper slopes andchannels The mats are typically laid on the soil surface and covered with topsoiland seed

As previously discussed, isolation of the active work area in both wetlands andopen water areas is an effective method to limit the horizontal extent of disturbance,particularly in areas where significant dredging is required In such cases, dredgingopen 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 20- to33-m width of sediments Clearly, this size dredging operation would require thehandling and disposal of large amounts of excess dredged material Conducting thiswork within sheet pilings allows a vertical sidewall, thereby reducing the volume

of material to be handled and isolating the silt-laden trench water from surroundingmarsh and other wetland areas

Open dredging within or adjacent to wetland areas can produce significantamounts of turbidity If typical dredging equipment is used, for example, a barge-mounted crane with a clamshell dredge bucket, methods are available which canreduce turbidity These include establishing requirements that all lifts of a clamshelldredge bucket through the water column are vertical, that dredge buckets be used

Table 3 Critical Elements of an Erosion and

Sediment Control Plan (Adapted from Brown and Caraco, 1997)

Minimize clearing and grading Protect waterways and stabilize drainage ways Phase construction to limit soil exposure Stabilize exposed soils immediately Protect steep slopes and cuts Install site perimeter control to filter sediments Use settlement traps and basins for larger volumes Use experienced contractors to implement the plan Tailor the plan to specific site conditions

Assess plan effectiveness after storms

with covers and gasket seals to prevent washout of sediments and suitable filtering

of water released from stockpiled dredged material

Hydraulic dredging can also be used in certain unconsolidated sediments toreduce turbidity With this method, sediments are removed and pumped as a slurry

to a settling barge or disposal site While initial turbidity at the point of dredging isminimal, large amounts of water must be filtered and removed from sediments atthe disposal site, and pumping limitations require that disposal occur in close prox-imity to the point of dredging

Construction will often require temporary stockpiling of soils, and care should

be taken to continually spray these piles with water, or cover them, in order toprevent wind erosion and transport of fines Similarly, newly graded access roadsshould be frequently sprayed with water or dust suppressants to reduce dust forma-tion The construction schedule should attempt to minimize the period of time whereexposed stockpiles or unpaved road surfaces are required

Site grading and excavation activities in areas already served by drainage systemsare a potential concern for sedimentation Many parking lots, roadways, and otherfacilities use stormwater drainage systems that discharge directly into adjacentwetland areas In order to minimize the impacts from run-off of sediment-ladenwater, existing catch basins and storm drains should be completely ringed with stakedhaybales and a layer of filter fabric Other inlet protection methods include concreteblock wrapped with wire and stones and placing geotextile fabric and stones directlyover the inlet (Brown and Caraco, 1997) These sediment traps will allow stormwaterflow to pass through, but will filter out significant amounts of suspended sediments.These structures also provide protection in the event of an accidental fuel oil spill,hydraulic hose rupture, or other hazardous material release, providing some measure

of initial containment upgradient of adjacent wetland areas As with all hay balestructures, the sediment traps need to be maintained and periodically replaced toensure their effectiveness

Excavation for foundations, utility trenches, and other facilities will often extendbelow the existing water table, resulting in collection of groundwater within theexcavation In order to dewater these areas and prevent discharge of sediment-ladenwater into surrounding areas, various types of settling basins and detention structurescan be constructed Sediment removal efficiencies generally range from 60 to 90percent, with higher efficiencies associated with wet storage (Brown and Caraco,1997) These structures allow particulate matter to settle and gradually dischargefiltered runoff Figure 3 is a schematic representation of a typical settling basin whichcan be constructed upgradient of a wetland area using filter fabric and clean rip-rapmaterial to effectively filter silts and sediments at a construction site Concrete orfiberglass settling basins are also available for use as sedimentation control structuresduring dewatering operations and are often used on barges during dredging opera-tions to filter water discharged from stockpiled dredged materials Geotextile wetlandfilter bags have also been developed to serve as sedimentation and erosion controldevices on construction sites (Figure 4)

The true test of any sedimentation and erosion control plan will occur duringthe first significant rainfall event during construction Thus, it is recommended thaton-site resident inspectors monitor the success of the installed erosion control devices

during and immediately after a rainstorm or snowmelt The hay bales, siltationfences, and other structures should be observed on, at least, a weekly basis to detectdamage from wildlife, machinery, or other activities on site.

Equally important, resident inspectors should conduct frequent visual tions of the adjacent wetlands or open water bodies to detect turbidity plumesresulting from on-site runoff For certain subaqueous activities, significant short-term increases in turbidity are unavoidable Nevertheless, attention should focus

observa-on the effectiveness of the siltatiobserva-on curtains, dredging methods, and dewatering

Figure 3 Settling basins are used in conjunction with dewatering operations to prevent

discharge of sediment-laden water into wetlands The basins are constructed upgradient of wetlands using filter fabric and clean rip-rap material.

Ground Slope

Sediment Laden Water

Pump Discharge

Flat Stone Approved Filter fabric Mat

10'-15' (Typ.) or as Direcled

15' - 20 (Typ.)

or as Directed

To Natural Water Course

Sediment Free Water

Ground Slope

Baled Hay or Straw

Pump Discharge Line

Flat Stone Approved Filter Fabric Mat

Sediment

Clean Stones (If Required) Suitable Velocity Dissipator

Baled Hay or Straw

as Directed

TYPICAL SECTION SEDIMENT TRAP

Suitable Device

to Dissipate Velocity