Applied Wetlands Science - Chapter 6 pot

SuccessionPlanting DesignPlant SelectionStock SelectionPlanting DensityWeed ControlCultural Issues MosquitoesWater QualityImplementation Construction Sequencing Protective FlaggingWeed R

Zentner, John “Wetland Enhancement, Restoration, and Creation”

Applied Wetlands Science and Technology

Editor Donald M Kent

Boca Raton: CRC Press LLC,2001

CHAPTER 6

Wetland Enhancement, Restoration,

and CreationJohn Zentner

Cultural ConstraintsAdjacent Site ConditionsThe Use of Template AssociationsSmall-Scale Experimental ConstructionGoal Setting

Elements of a Goal StatementGoal-Setting Process

PracticabilityConstruction DesignGeography

Size and ShapeLocationSlope

Adjacent UsesHydrology

Hydroperiod and DepthWater Supply

Soil

SuccessionPlanting DesignPlant SelectionStock SelectionPlanting DensityWeed ControlCultural Issues

MosquitoesWater QualityImplementation

Construction Sequencing

Protective FlaggingWeed RemovalSalvagingGradingPlantingWater SupplyFencing

As BuiltsMaintenance

Weed ControlErosion ControlHerbivoryPlant CareIrrigation System MaintenanceLitter Removal

General Maintenance FrequencyMinimizing Maintenance EffortsResearch Needs

References

Freshwater wetlands develop at elevations above open water aquatic habitats andbelow uplands They are found in a wide range of hydrologic conditions, frompermanently flooded (to a depth of 1 m) to seasonally saturated Freshwater wetlandsoccur on a wide variety of soil types including both organic and mineral soils, aswell as in nonsoil conditions Most freshwater wetlands are either freshwater marshes

or riparian woodlands

Freshwater marshes are dominated by herbaceous emergents and can be dividedinto three general categories reflective of hydrology (Figure 1) Wet meadows aretemporarily or intermittently flooded and dominated by graminoids and Juncaceae

In the United States, seasonal marshes are seasonally flooded or saturated anddominated by Cyperaceae and Juncaceae Perennial marshes are permanently or

Figure 1 Freshwater marshes are dominated by herbaceous emergents and can be divided into three hydrological categories: perennial marsh, seasonal

marsh, and wet meadow.

Hydrology

Dominant Plants

Temporarily flooded or intermittently flooded

Seasonally flooded or saturated

Permanently flooded or semi-permanently flooded

Cattail, bulrush, tules Sedges, rushes

Grasses, rushes

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semipermanently flooded and dominated by tall emergents such as cattails (Typha latifolia) or bulrush (Scirpus acutus).

Riparian woodlands are dominated by shrubs and trees and are characterized byimpermanent and varying periods of inundation or root zone saturation during thegrowing season Compared to freshwater marshes, riparian woodlands occur onrelatively permeable and well-oxygenated substrates As with freshwater marshes,riparian woodlands can be categorized by hydrological regime (Figure 2) Highterrace woodlands are temporarily flooded and dominated by a variety of species,especially oaks (Quercus spp.), that are typified by heavy seeds with relatively longerviability Mid-terrace woodlands are seasonally flooded and generally dominated bygreen ash (Fraxinus pennsylvanica), sycamore (Platanus occidentalis), and otherspecies with medium weight seeds And low terrace woodlands are semipermanentlyflooded and generally dominated by willows (Salix spp.),silver maple (Acer sac- charinum), and similar species with relatively light seeds of limited viability Thesecategories correspond to Categories V (higher hardwood wetlands), IV (mediumhardwood wetlands), and III (lower hardwood wetlands), as described by Cowardin

et al (1979), Larson et al (1981), and Clark and Benforado (1981)

Coastal wetlands share many of the characteristics of freshwater wetlands andare generally defined as those wetlands that lie within the realm and effects, howeverminor, of tidal salt water As such, coastal wetlands include saltmarsh, fresh andbrackish tidal marsh, and, in tropical waters, mangrove

Saltmarsh is the most ubiquitous type of coastal wetland, occurring on all coastswhere appropriate substrate and tidal regimes are present (Figure 3) Saltmarshesare distributed over a relatively broad salinity range, from the high intertidal zone

to the oligohaline habitats upstream on tidal tributaries where salinity may neverexceed 10 parts per thousand In the United States, various species of cordgrass(Spartina spp.) are dominant, with rushes (Juncus spp.) also common On the PacificCoast, Spartina foliosa and pickleweed (Salicornia virginica) are common On theGulf Coast and in the southeast, other species of cordgrass (Spartina alterniflora,

S patens), saltgrass (Distichlis spicata), and black needlerush (Juncus roemerianus)are typical Sawgrass (Cladium jamaicense) marshes, such as the Everglades, occur

in more brackish water On the Atlantic Coast and to the northeast, Spartina suroides, S alterniflora, J roemerianus, and some other species are common Waxmyrtle (Myrica cerifera) and groundsel tree (Baccharis halimifolia) are typicalshrubs associated with salt marshes from the Gulf Coast to New England

cyno-Oligohaline (salinity of 0.5 to 5.0 ppt) and tidal freshwater marshes (salinity

is less than 0.5 ppt) are herbaceous wetlands located in tidally influenced rivers

or streams The plant community exhibits a diverse mixture of true marine speciesand typical freshwater taxa that tolerate low salinities (Cowardin et al., 1979;Lewis, 1990)

Mangrove forests are limited in distribution to subtropical and tropical zones(Figure 4) In the United States, they occur predominantly in southern Florida,sparsely along the Gulf Coast to the Laguna Madre of Texas, and extensively inPuerto Rico (Kuenzler, 1974) Many species of trees of different families are calledmangroves, with black mangrove (Avicennia germinans), red mangrove (Rhizo- phora mangle), and white mangrove (Laguncularia racemosa) common to the

Figure 2 Riparian woodlands are dominated by shrubs and trees As with freshwater marshes, riparian woodlands can be categorized by hydrological

regime: semipermanently flooded, seasonally flooded, and temporarily flooded.

UPLAND

HIGH TERRACEWOODLAND

Temporarily flooded Seasonally flooded

Semi-Permanently flooded

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United States There are an additional 31 species worldwide (Tomlinson, 1986).All mangroves have features in common that are adaptations for survival in thesaline conditions of the intertidal zone (Tomlinson, 1986) Morphological adapta-tions include the aerial roots (pneumatophores) of black mangrove that provide forgas exchange and the viviparous, floating propagules of red mangroves In fact,nearly all species of mangroves are viviparous, with the white mangrove being anexception Physiological mechanisms for dealing with salt excretion or exclusionare also characteristic of mangroves.

Lewis (1990) recently defined wetland enhancement, restoration, and creation.Enhancement is an increase in values afforded to a specific vegetation association

by construction actions Restoration is the recreation of a specific vegetation ciation on a site where that association was once known to occur Creation is theconstruction of a wetland from an upland or aquatic site The term construction will

asso-be used to encompass enhancement, restoration, and creation in this chapter.The earliest wetland construction projects may have occurred many thousands

of years ago with the manipulation and creation of freshwater and coastal wetlands

to enhance rice or fish harvests Waterfowl conservation and hunting organizationshave been responsible for numerous wetland construction projects in this century.New York, especially, had numerous small freshwater marshes created in the mid-1950s (Dane, 1959) Specific problems associated with wetlands, such as nuisancemosquitoes, have also resulted in a number of useful research and enhancementpractices More recently, interest in wetland construction is a response to regulatoryrequirements Nevertheless, there is also a greater public interest and understanding

Figure 3 Saltmarshes are the most widely distributed of coastal wetlands They occur over

a relatively broad salinity range from the high intertidal zone to oligohaline habitats

on tidal tributaries.

of the environmental values of wetlands and a desire for the recreation of lost ordiminished landscapes and values.

This chapter discusses the construction of freshwater marshes, riparian lands, and coastal wetlands The construction process is described in five stepsinherent to designing, building, and maintaining wetland landscapes These stepsare site analysis, goal setting, construction design, implementation, and maintenance.Implicit in these steps are two tenets First, specification of a target vegetationassociation (TVA) is the primary goal A TVA provides the habitat for any plant orwildlife populations that may be desired on the project site and, because of itsstructural nature, facilitates maintenance and monitoring efforts Second, in con-struction planning there is no substitution for directed observation of the project siteand the TVA

wood-SITE SELECTION AND ANALYSIS

The ideal construction site will most closely meet the requirements of thecommunity to be constructed Therefore, the goals of the project will have substantialbearing on final site selection Construction projects inherently require understandingthe initial cause of habitat differentiation or degradation and determining the prob-ability that the habitat can be constructed and maintained The question of habitatdisplacement inevitably needs to be addressed in the site selection process as well.Site analysis is the first tangible step in a wetland construction project Theanalysis may be completed during a wetland delineation effort, as the first step in a

Figure 4 Mangrove forests are found throughout the subtropics and tropics The tree species

which comprise mangrove forests have morphological and physiological tions for surviving in intertidal saline conditions.

adapta-neighborhood restoration program, or from many other perspectives This step vides the basic framework for goal setting, identification of the TVAs, and develop-ment of performance standards.

pro-The site analysis typically includes a topographic assessment, wetland vegetationassociation mapping and analyses, and a review of historic conditions The analyseswill also include an assessment of hydrologic, soil, and cultural conditions, andexamination of any nearby templates, or examples, of TVAs Small-scale experi-mental wetland construction efforts should be initiated at this time if possible.Generally, the initial site analysis is completed within a period of three weeks tothree months, with a relatively intensive effort in defining site topography, vegetationassociations, soils, and historic and cultural conditions in the first few weeks Lessextensive effort is expended in monitoring hydrology and constructing and observingexperimental wetlands over the remaining period

Topography

Elevation and slope are two of the more critical factors in determining the success

of wetland construction projects It is difficult to recommend an absolute plantingelevation for a given species because of local and geographic variability Synergisticeffects may also alter growth at given elevations For example, reduced salinity maypermit growth of smooth cordgrass at higher and lower elevations than those typicallyrecommended for the species

The optimum elevation can be determined empirically by observing and suring the lower and upper elevation limits of a nearby natural wetland Lewis (1990)recommends that the lowest and highest points should be disregarded, and only themiddle range used for planting If reference information is unavailable, an adequatetest program should be conducted prior to initiating construction

mea-Some degree of slope is essential for proper drainage A gradual slope willincrease the area available for planting and will dissipate hydrologic energy over agreater area, thereby reducing the possibility of erosion (Broome, 1990) Slopeshould be toward water sources to minimize ponding as substrates settle followingsite preparation Gentle slopes do not drain as extensively as steeper ones, which isgenerally beneficial to most wetland vegetation However, gentle slopes are moresusceptible than steeper slopes to ponding, which can be especially problematic forriparian woodland species unless the soils are also relatively permeable

Vegetation Association Mapping

Wetland vegetation associations for the construction site and surrounding areacan be identified, and their extent determined, by mapping from an aerial photograph.Aerial photographs are usually available as stock or library film from an aerial surveycenter These centers fly important regions every two to three years, providing abacklog or library of film that can be reproduced at a variety of scales Requesting

a half-tone mylar as well as a print of the project site is important The mylar can

be used to make blueprints for field use, and blackline prints can be reproduced andincluded in reports

The borders of the vegetation associations are more precisely defined throughon-site analysis after initial mapping from an aerial photograph For marshes, ran-domly selected 1-m plots representative of each vegetation association can be sam-pled for species and cover Randomly selected 0.25 hectare (ha) polygons (defined

by tree cover and separation among woodland vegetation associations) are effective

in woodlands

Table 1 provides an example of vegetation association mapping from a thetical freshwater wetland project site in California characterized by gently slopinghills surrounding a central creek The result is a table that describes the vegetationassociations and the absolute and relative cover of marsh and riparian woodlandvegetation A scan of the table and knowledge of the region reveal that threevegetation associations are present and that the site is dominated by nonnative speciesand species representative of disturbed areas It is also appropriate at this time toidentify and survey any upland areas that may be considered for wetland construction

hypo-to ensure that valuable upland habitats are not inadvertently lost

Site History and Current Status

Shisler (1990) states that if wetlands are not present there has to be a reason,especially for tidal wetlands Determining this reason is fundamental to reviewing

a site for potential wetland habitat construction If possible, past functioning of the

Table 1 Example of Vegetation Association Mapping from a

Hypothetical Project Site in California 1

Vegetation Association Plant Species

Area (ha)

Cover (%)

**Species indicative of disturbance.

1 The table provides absolute and relative information for use in site

analysis and target vegetation association selection.

wetland should be evaluated, and all past uses of the site should be reviewed prior

to habitat construction Current and future land use, zoning regulations, and projectedsea-level rise may also have a bearing on site selection

Historic aerial and other site photographs, and any available topographic or othermaps, are reviewed to define the past conditions and boundaries of the site vegetationassociations Important issues to review with these sources include the location ofhistoric wetland vegetation associations and their relationship to current vegetationassociations, water sources, and cultural elements Historic aerials are often availablefrom the U.S Geologic Survey (USGS) and local aerial photography sources His-toric maps are also found at USGS as well as local historical societies Costs forhistoric aerials are generally less than current aerials, however, reproduction costsfor older maps may be significant

Hydrological Analysis

The quantity, quality, and timing of water entering the wetland are of criticalimportance to its survival If the correct hydrology is not present, or is insufficient,the project will fail (Shisler, 1990)

The initial site analysis provides an opportunity to define the hydrologic tions that create the site or template vegetation associations Generally, hydroperiod,the duration of inundation or saturation, and water depth best define the vegetationassociation for freshwater wetlands Accordingly, hydrology stakes and shallowobservation wells are placed in wetland and upland areas to define surface andgroundwater conditions The wells typically consist of 10 cm diameter PVC pipeinserted 1.2 to 2.4 m into the ground, with a surrounding backfill of gravel or otherpermeable material Wells and stakes can be observed weekly during the analysis

condi-to define the depth of ponding and the surface approach of groundwater However,short-term monitoring may not reflect typical groundwater levels Long-term ground-water level data may be available from local water districts or farmers

For coastal wetlands, hydrologic factors that need to be evaluated during the siteselection process include drainage features, such as channels and ponds, the tidalregime and range, wave intensity, and salinity Obviously, some of these can bemanipulated through design while others cannot It is sometimes necessary to basesite selection upon the most practical trade-off In general, coastal wetland construc-tion projects are more likely to be successful in sheltered locations where waveenergy and current velocity are minimal Salinity variation at the site should bemonitored so that appropriate plant species can be selected Adequate drainage isanother requirement, because standing water is typically detrimental to the estab-lishment of wetland vegetation Wide, shallow channels that retain water at low tidemaximize flushing and plant health Water quality (e.g., dissolved oxygen, turbidity)

is an especially important factor determining site selection for seagrass restorationprojects Other factors affecting hydrology are precipitation, surface flows, groundwater, and evapotranspiration Each of these factors affects habitat productivity andspecies diversity

Soil Analysis

Soil analysis can be one of the most important steps in defining wetland tion opportunities and constraints, especially for the drier freshwater marshes andriparian woodlands The soil analysis can identify the edaphic requirements of thevegetation, thereby defining the appropriate TVA Soil analysis will also identify anyrestrictions to construction activities because of soil permeability or soil chemistry.Generally, the best source of information on freshwater wetland soil characteristicsfor an area is direct site observations Direct observations can be supplemented bylocal, state, or federal information, such as Natural Resource Conservation Service(NRCS, formerly U.S Soil Conservation Service) soil series maps The latter areavailable at no cost from NRCS offices The NRCS and associated services such ascounty agricultural offices often have aerial photographs for review as well Wheresoil series maps are unavailable, NRCS field personnel may provide on-site assistance.Site observations typically consist of a series of soil pits, 30 to 50 cm in depth,dug by hand throughout the site for the purpose of defining surface conditions Use

construc-of a backhoe to define conditions at depth is generally helpful and is essential ifsignificant excavation is required Test pits will help identify any significant variation

in permeability among soil layers It is also useful to identify similar soils onproximal, undisturbed sites using the NRCS soil survey so that natural vegetationassociations can be observed and used as a template Soil chemical properties can

be analyzed in a laboratory If the site requires the addition of soil, or if the site hasbeen filled in the past, the possibility of contamination should be investigated.Generally, sandy soils are more amenable to grading and planting than are siltand clay However, some degree of organic content is recommended as a nutrientsource (Broome, 1990) The soil must be of sufficient depth to support planting.Garbisch (1986) recommends a minimum depth of 0.3 m for marshes Trees forwoodland construction will typically require a greater soil depth, depending on thespecies

Based on a literature review and field inspections of created wetlands in NewJersey, substrate preferences for coastal marshes were ranked as follows:

1 Natural marsh peat

2 Clay and silty clay

3 Estuarine sediments (dredge and fill)

4 Sand (Shisler, 1990)

For riparian woodlands, however, the list is almost completely reversed Some anoxicsoils can become highly acidic upon exposure to air, such as during earthwork, andmay warrant the addition of a calcareous material such as crushed shell as a bufferingagent At sites subject to tidal or riverine inundation, sedimentation processes should

be carefully studied Some degree of sedimentation may be desirable to stabilizeinitial plantings; however, burial of seedlings should be avoided

Cultural Constraints

Few sites are unconstrained by human artifacts Artifacts may range from historic remains to buried sewer lines Visible artifacts such as roads that affect theproject site (e.g., drainage impediment) should be identified Identification of sub-surface artifacts is more difficult Nevertheless, cultural resources can be prelimi-narily identified in the United States through record searches with the State HistoricPreservation Office (SHPO) or local universities Buried infrastructure may be iden-tified through interviews and research at local Public Works Departments

pre-Adjacent Site Conditions

Conditions near the proposed construction site should also be considered Sitesadjacent to existing functioning wetlands offer the greatest chance for success.Conversely, an increasing wetland area in the watershed may alter hydrologicregimes

The presence of aggressive exotic species close to the proposed site may also

be a problem In many cases, a buffer zone should be included in the project design

A buffer zone may increase the diversity of flora and fauna present (Josselyn et al.,1990) and will protect the site from human and vehicular traffic The buffer zonemust be kept clear of exotics and other nuisance species that may invade the projectsite Buffer zones with tall vegetation provide animals, especially birds and mam-mals, with refuge from cold winds, high temperatures, and high waters

The proximity or attractiveness of the potential construction site to predationshould also be considered Geese (e.g., Branta canadensis), muskrat (Ondatra zibe- thica), and nutria (Myocastor coypus) have all been identified as voracious consum-ers of newly planted marsh vegetation (Shisler, 1990; Broome, 1990) Conversely,placement of a well-designed created wetland near a disturbed wetland may offerrefuge to dwindling populations of desirable species

Open water near potential sites should be assessed for boat traffic and offshoredepth because both can affect wave energy Shallow offshore water reduces theseverity of waves reaching the shore (Broome, 1990) The degree of wave energycan be modified to a certain extent (see below), but sites facing more than5.5 nautical miles of open water are generally not recommended for planting (Crewzand Lewis, 1991)

The Use of Template Associations

Recreation of an historic vegetation association will in many instances increasethe probability of success In the event that the TVA is missing or poorly represented

on the project site, a proximally located vegetation association can be used as atemplate The template association should have hydrology and soils similar to existing

or intended conditions for the construction site In addition to determining speciescomposition, the template vegetation association should be studied to determinespecies evenness and distribution, and identify potential additions to the planting listand likely successional patterns The specific TVA to be constructed may be difficult

to predict at this stage of the project However, initial review of several differentvegetation associations may have a significant effect on goal setting For example,participants in a riparian woodland construction project for a degraded waterway inSolano County, CA, wished to plant the California sycamore (Platanus racemosa)extensively due to its stature and appearance However, the sycamore is adapted toalluvial flats with relatively permeable soils and would not do well on the projectsite's clay soils Taking the participants to a regional park that included a well-preserved riparian woodland native to this region and similar to the project site resulted

in the selection of several alternative trees, all better adapted to the project site

Small-Scale Experimental Construction

When time is available, small-scale experimental projects or pilot studies should

be undertaken to identify soil and hydrology constraints and to refine planningalternatives Appropriate studies include determining the water-holding capacity ofunmodified soils, determining germination and salvage survival rates for targetvegetation, refining transplanting techniques, and evaluating construction manage-ment abilities The importance of small-scale pilot projects for building confidenceand consensus among the planning group should not be underestimated

GOAL SETTING

The setting of specific goals is crucial to wetland construction projects Goalshelp define monitoring elements and protocols, and establish standards for judgingsuccess The latter allows others to objectively evaluate the project and serves as abasis for practitioner and general public educational efforts

Elements of a Goal Statement

Goal statements are typically comprised of a substantive objective, for example,construct 1.3 ha of coastal salt marsh Specification of a TVA may be the mostappropriate substantive goal as noted in the introduction to this chapter A TVA can

be related to wetland functions (e.g., provision of wildlife or fisheries habitat),facilitates clear performance standards and monitoring, and can be readily measured.This is most easily accomplished through comparison with an existing or historicallywell-described vegetation association from the project site or a nearby site One ofthe more difficult tasks is to keep the TVA general enough to be realizable, yetspecific enough that the goal is meaningful Typically, selection of a species typeassociation, for example a rush-dominated wet meadow, is an effective approach.Other commonly used options for the substantive element of the goal statementinclude a specific wildlife or plant population In this case, the measurement param-eter would identify a particular population level as the target Wildlife goals aregenerally less useful than TVAs because wildlife populations vary so greatly year

to year as a result of regional conditions Regional climate changes can have a muchgreater effect on wildlife populations, for example, than the maturation of the

construction site Moreover, the construction project will build a specific type ofhabitat or assemblage of habitats to produce the desired change in wildlife popula-tions Therefore, designation of the TVA(s) makes as much sense as designation ofthe wildlife goal and is more readily measured.

A wetland function, such as water quality improvement, is also often used asthe substantive element of the goal and the measurement parameter defined to include

an indicator such as percent reduction in target pollutants Again, functional goalsare still dependent on a specific TVA

Equally important, and often overlooked or unstated, is the procedural element.The procedural element of a goal statement refers to the method by which the goalstatement is completed Too often, goals are developed without discussion with, orconsensus among, all interested parties Conversely, a large committee may developgoals that are nonspecific, thereby negating effective evaluation at any point

Goal-Setting Process

Several points should be considered in the development of goals Constructiongoals should address the concerns of all those meaningfully affected by the project.Affected parties will include landowners on or adjacent to the project site, regulatingagencies, and operation and maintenance entities such as mosquito abatement andflood control districts Table 2 lists parties potentially affected by wetland construc-tion activities, and goals common to these parties All parties should be contactedand their comments solicited on initial goal statements For smaller-scale projectswhere the affected party’s interests are relatively similar (e.g., neighborhood groupsreconstructing a local creek), comments should be used to develop a consensus onproject goals For most projects this is not practicable, and the consultative processwill involve reviewing each comment and incorporating a response into the con-struction plan

Experience suggests that a flexible, two-step goal development process is mosteffective The initial goals are developed following the initial site analysis anddiscussions with the affected parties Final goals are selected at the conclusion ofthe construction design process Even then, the goals may require modificationduring the construction, or even post-construction monitoring phase, as new oppor-tunities and constraints arise

Practicability

A substantial body of literature exists on the design and goals of wetlandconstruction projects (Kusler and Kentula, 1989) However, a significant gap existsbetween those who develop the goals and write the plans and those who constructwetlands This gap mirrors a similar chasm found between landscape architectsand landscape contractors and is likely to exist because of education dissimilarities,professional practices, and even insurance requirements This gap poses a signifi-cant danger to the development of ecological construction because it may ensurethat important knowledge gained during construction is not incorporated into sub-sequent designs

Building a wetland shares many basic elements with any landscape constructionproject, whether it is gardening or reforestation The most important issue in anysuch project is to determine what conditions create the TVA Subsequent issues mayarise that threaten vegetation association growth, whether one is planting corn, taro,

or rushes Wetlands are among the wettest landscapes and, accordingly, the quantityand quality of the water, as well as the use of soil types which ensure water retention,are extremely important Also important are those conditions that result from theapplication of significant amounts of water to the soil such as erosion and saltbuildup, which reduce the viability of the TVA

Wetland construction strives to achieve ecological function and presumes thatthe project, once constructed, will evolve in a natural fashion Ideally, no maintenancewill occur following an initial establishment period making an understanding of thesuccessional aspects of the TVA very important On the other hand, many wetlandconstruction projects, especially those specifically for waterfowl habitat, will usesources of water that require continued operation and maintenance (Payne, 1992).Unless the project is built in an extremely remote area, human actions willinfluence the project Conversely, the project may influence humans For thesereasons, wetland construction must consider the role of people in the design and, insome cases, identify methods for ensuring that potential interactions between theconstructed habitat and people are mutually beneficial

These elements are considered in more detail in the Construction Design section

of this chapter However, they must also be considered in the goal setting stage.Ignoring these issues and proceeding without consideration of the practicability ofthe project may shorten the planning phase of the project but will likely decreasethe probability of success

Table 2Parties Potentially Affected by Wetland Construction Projects and

Goals Common to These Parties

Landowners

Adjacent landowners Increase aesthetics

Adjacent residents Eliminate off-site issues (e.g., mosquitos,

odors, floods) Increase access Reduce vandalism Permitting and commenting agencies

Corps of Engineers Provide appropriate data

Wildlife agencies Respond to all concerns

Public land agencies Restore single species habitat

Provide appropriate public access Ensure success

Operation and maintenance entities

Flood control districts Minimize maintenance costs

Park districts Provide adequate flood conveyance

Mosquito abatement Ensure compatibility with park needs

Minimize mosquito production

CONSTRUCTION DESIGN Geography

Size and Shape

Market or regulatory forces well in advance of any design analysis oftendetermine the size and configuration of the construction site Even when strictlimitations exist, the designer must consider the appropriate extent and shape forthe different vegetation associations to be constructed Recent work stemming fromMacArthur and Wilson’s (1967) research on island biogeography suggests thatareas of at least 16 ha contain significantly greater numbers of species of birds,mammals, and invertebrates than do smaller areas (Tilghman, 1987; Faeth andKane, 1978) Species richness is also positively correlated with edge length andhabitat diversity (O’Meara, 1984)

Location

Exposure to wave energy is particularly important for coastal wetland tion projects The relationship between wave energy and effects on cordgrass plantingcan be related to fetch (the distance traveled by waves) At less than one nauticalmile of fetch, plantings will be unaffected Between 1 and 3.5 nautical miles, somereplanting is necessary, and over 3.5 nautical miles, some kind of wave-barrierstructure is necessary to protect plantings (Crewz and Lewis, 1991)

construc-Orientation of the site with respect to colonizing (or invading) plants and animals

is another factor influencing design Exposure to desirable flora and fauna should

be provided Conversely, exposure to exotic and nuisance species should berestricted Prevailing winds and currents may enhance or deter establishment ofvolunteers The presence and condition of adjacent wetlands may also affect designspecifics For example, preserving isolated patches of desirable vegetation mayinhibit water circulation by creating berms Sound ecological judgment must be used

in assessing whether existing vegetation impairs or contributes to developing desiredfunctional attributes at the site (Crewz and Lewis, 1991)

Slope

Perhaps the most critical aspect in successful wetland construction design isgrading the soil surface to the elevation that provides the optimal hydrologic regimefor the TVA (Broome, 1990) It would not be an overstatement to consider accuratetopographic surveys and grading the keystone to meeting wetland construction goals.Slopes should be as gradual as possible, while still allowing good surface drain-age at low water conditions Gentle slopes at the perimeters of the project site arerecommended to reduce erosion and filter runoff reaching the site If steep slopesare necessary, they should be stabilized to serve as a deterrent to exotic or nuisancespecies invasion Gradual slopes at the perimeter of construction sites also serve to

prevent compression of vegetative zones owing to sea-level rise, the rate of which

is predicted to increase rapidly after the year 2025 (Crewz and Lewis, 1991)

Adjacent Uses

Significant opportunities exist for a larger native landscape or a corridor whenadjacent uses include natural or restored lands Whereas a great deal of informationhas been developed on the value of larger sites, the case for creating corridors is moreintuitive MacClintock et al (1977), Fahrig and Merriam (1985), and Wegner andMerriam (1979) all provide study results that suggest that birds and small mammalsactively use natural corridors, and that patches of native landscape connected by suchcorridors to larger habitats receive greater wildlife use than isolated patches.Construction projects should consider potential use of the wetland by people andthe effects of the human landscape on the natural system Adams and Dove (1989)document widespread interest in viewing and interacting with wildlife within subur-ban and urban settings Providing a trail that offers views and interaction with thewetland is often the best means to ensure visitors do not impact sensitive areas.Leaving the edge of the wetland open so that views of the wetland are created may

be the best means to ensure that the wetland is not used for trash and debris dumping.Glare is best treated with tall, rapidly growing trees as a buffer Off-road vehicleaccess should be eliminated through the use of post and cable barriers between anypublic roadways and the wetland, unless steep vertical barriers or moats are provided.Domestic and feral cats and dogs are significant predators on ground-nesting wetlandbirds Both can be restrained with fences, and open water is an effective barrier to cats

Hydrology

Wetland hydroperiod, including the duration and frequency of inundation orsaturation, and depth of inundation, are generally the major determinants of wetlandvegetation associations However, the manner in which the water is supplied to theconstruction site may be equally crucial Artificial supply systems, such as drip orspray irrigation and pumps, require a significant amount of maintenance and reducethe naturalness of the wetland However, these systems eliminate much of theuncertainty involved in water supply planning Natural water supply systems such

as open channels are less manageable but can result in a self-maintaining systemthat mimics natural systems Use of natural water supply systems requires a goodunderstanding of ecology and engineering and will require more effort in the inves-tigation and planning stages

Hydroperiod and Depth

Two models are useful in estimating wetland hydroperiod and depth of tion in freshwater wetlands Slope models are used to define the hydroperiod anddepth of inundation for wetlands abutting a river or lake Basin models are used tomodel wetlands isolated from riverine or lacustrine systems For tidal wetlands, avariety of models have been used to describe tidal flows In any case, tidal ranges

inunda-and important elevations (mean high water, etc.) must be identified; these are erally derived from local tidal gauges and other records.

gen-Slope wetlands are characterized by seasonally varying water levels with noimplied storage The relationship between wetland vegetation associations and watersurface elevations is well described (Dickson et al., 1965; Harris et al., 1975) How-ever, water surface elevation analyses of streamflows for storms of various intensity

or duration are complex Channelized streamflow equations have been developedfor flood control analyses These equations consider the amount of flow, the cross-sectional area and slope of the channel, and a friction element that defines channelroughness The latter reflects the capability of channel vegetation to reduce flowvelocity The HEC-1 model developed by the Corps of Engineers (1981) is anexample of this type of model Used in conjunction with an accurate topographicmap of a site, these models can predict the depth of water and duration of flooding

at a specific elevation for a specific storm event However, these models requireaccurate data that are typically very expensive to generate

Slope wetland hydrology can often be more simply defined by directed vations of the vegetation associations and of the channel during specific storm events.The locations of wetland vegetation associations in a slope environment relate towater level, which is related to topographic position Identifying the elevation of aTVA relative to the channel and then predicting the creation of a new and similarassociation based on a similar channel position is crude but relatively effective(Figure 5)

obser-Observations during storm conditions are crucial for determining channel ior during high flows Widening it to pre-alteration conditions and thus reestablishingthe TVA might easily restore a channelized stream However, other factors may also

behav-be critical For example, soil or underlying behav-bedrock may behav-be radically different onadjacent reaches Greater or lesser permeability in the construction reach might result

in less or more water than predicted Other factors such as channel roughness in theform of dense vegetation or in-channel morphological features may slow flows andsignificantly reduce peak flows A comparison of stormflow peak and duration inthe template and construction reaches can eliminate most sources of error

Basin wetland hydrology is best defined by a water budget That is, the wateravailable to support the wetland is a function of water inflow minus water outflow.This is illustrated by

S = P + SI + GI – ET – SO – GO

where

S equals the storage in the wetland basin.

P equals the precipitation.

SI equals the surface inflow.

GI equals the groundwater inflow.

ET equals the evapotranspiration.

SO equals the surface outflow.

GO equals the groundwater outflow or infiltration.

Water sources for wetlands include precipitation falling directly on the wetland,

surface inflow, and groundwater inflow Precipitation amounts should be identified

on at least a monthly basis Data are usually available from local weather stations or

airports In the United States, National Oceanic and Atmospheric Administration

(NOAA) handbooks are also a good source of information Surface inflow is the

product of precipitation within the watershed less the amount lost to

evapotranspira-tion and infiltraevapotranspira-tion or inflow from adjacent lacustrine or riverine sources For inflow

from adjacent sources, a stage-discharge model may be used to determine surface

inflow or the elevation of the flood flow can be estimated based upon corresponding

vegetation association boundaries Groundwater inflow is often extremely difficult to

determine and is best accomplished through the use of piezometers

Figure 5 Determining the appropriate elevation of a constructed slope wetland can

some-times be accomplished through observation of existing vegetation associations

and the elevations at which the associations occur.

Water is lost from the wetland (and its watershed) through evapotranspiration,

surface outflow, and groundwater outflow Evapotranspiration is often described on

a monthly basis in NRCS soil surveys, NOAA handbooks, and other sources The

low point in the basin will control the basin water surface elevations and surface

outflow The average storage capacity is then the remaining volume of water in the

basin Some water flowing into the basin will be lost to outflow through the soil,

with the amount of outflow determined by the height of groundwater and the type

of soil Where groundwater levels are high or the soil underneath the basin is at field

capacity, no or little loss will occur Where groundwater levels are low or the soils

beneath the basin are not at field capacity, water loss to the soil will be more

significant The amount of loss during these conditions is dependent upon soil type,

with clay soils generally appropriating less water than sandy soils

The factors used in a water budget calculation are often dependent upon each

other and interaction between the surface and groundwater components can confound

the results of the budget analysis (Brown and Stark, 1987) Additionally, many other

factors may affect the parameters or their measurement In all cases, the models

should be verified, validated, and calibrated through direct observation of existing

or experimental wetlands

Water Supply

Providing a relatively natural water supply system, or designing a wetland within

a system such as a channel also used for flood conveyance, is relatively simple in

tidal conditions but increasingly problematic with greater freshwater inflow Much

experience, often not positive, has been gained from the design of flood channels

that also host wetland creation efforts Concrete or earth-lined trapezoidal channels

have been extremely popular in flood control design because they require little land,

are relatively inexpensive to build, keep flood flows below existing ground levels,

and are easy to maintain As these systems were expanded to provide for wetland

construction, however, the inherent problems associated with this channel design

have become apparent The substantive problem is the perennial saturation of the

flat bottom of the channel, which encourages the development of a low-terrace

riparian woodland Woody species tend to grow densely and rapidly, creating a

significant impedance to flood flows

One resolution of these maintenance conflicts would be the establishment of a

wetland vegetation association with relatively low channel impedance Riparian

woodland associations vary tremendously in their effect on channel impedance but

generally follow a pattern of greater impedance with greater affinity for water That

is, a low-terrace woodland impedes flood flows more than a mid-terrace association,

which in turn impedes flow more than a high-terrace association In a study of riparian

woodland establishment patterns at several wetland creation projects in the Central

Valley of California, Zentner and Zentner (1992) observed that the low-terrace

wood-land in this region occurs from the upper edge of the summer water level to

approx-imately the mean annual flood line The mid-terrace zone occurs from the mean

annual flood line to about the 10- to 15-year storm line, and the high-terrace woodland

occurs from the mid-terrace zone to the 100-year flood line Therefore, to circumvent

blockage of flood waters by dense low-terrace riparian growth while still providing

for wetlands, broad low terraces could be designed that can accommodate flood flows

despite the dense nature of the vegetation Alternatively, the low-terrace zone could

be restricted, and terraces would be constructed just above the mean annual flood

line where mid-terrace riparian woodland, in friable soils, or marshes in indurated

soils can be maintained (Figures 6 and 7)

Additional hydrological factors, including water velocity, sediment loading, and

erosion potential, will also be important Table 3 is a checklist for reviewing channel

design issues The checklist can be used to review both upstream and downstream

preconstruction and postconstruction conditions In the western United States, and

most likely for other regions as well, seasonal low water, mean annual flood level,

and the 100-year flood level are the most pertinent water surface elevations Seasonal

low water level defines the upper limit of perennial marsh and the lower limit of

low-terrace riparian woodland Mean annual flow approximates the upper limit of

seasonal marsh and low-terrace woodland The 100-year flood level is an important

cultural limit, and flows above that level resulting from a wetland construction project

will require special consideration The relevant tidal wetland elevations include mean

tide level, mean higher water (MHW), and mean higher high water (MHHW), all

important boundaries for vegetation associations (Lewis, 1994)

Impedance associated with the vegetation association has an important effect on

channel flow, as does channel bottom slope microtopography Each of these factors,

as well as channel soil characteristics, are important factors in erosion and

sedimen-tation Sandy soils erode more easily than clays under most conditions, and

imped-iments to channel flow such as bridges increase scouring through local increases in

flow velocity Decreases in flow velocity, especially when there is a high sediment

load, will result in silting-in of channel bottom basins

For coastal wetlands, some kind of proximal offshore structure may be needed

for many sites exposed to moderate wave energy The protection offered by berms

or breakwaters can be substantial and are a critical factor in successful establishment

of newly planted vegetation At some sites, construction of a breakwater may be all

that is necessary to stabilize a site, both subtidally and intertidally, and consequently

facilitates significant habitat improvement For example, a small spoil island in

Clearwater Harbor, FL, that provided good wildlife habitat for local populations

(especially birds) was experiencing severe shoreline erosion Offshore seagrass beds

were also showing signs of stress related to excess wave energy In 1989, a

140-m-long breakwater of large rocks was constructed approximately 25 m offshore of the

island, seaward of the grass beds Following breakwater placement, the shoreline

stabilized through formation of a small beach and the seagrass beds appear healthy

and may be expanding in area

Large rocks and rubble are the material recommended for construction of

break-waters in most circumstances Smaller material may be less stable, or can consolidate

excessively, prohibiting adequate tidal flushing Constructed properly, rock berms

used in conjunction with mangrove restoration projects aid in trapping propagules,

thereby reducing or eliminating the need for planting

Figure 6 To avoid blockage of flood waters by dense, low riparian growth, the low-terrace zone can be restricted and terraces can be constructed above

the mean annual flood line.

©2001 CRC Press LLC

Figure 7 Another method for avoiding blockage of flood waters while providing for wetlands is to construct seasonal marsh in low-terrace areas.

©2001 CRC Press LLC