TREATMENT WETLANDS - CHAPTER 18 pot

The number of flow paths in the design of surface flow wetlands is based on considerations of redundancy, maintenance, and topography3. All constructed wetland treatment systems should h

help with understanding and overcoming difficulties that may

arise during construction and start-up of wetland treatment

systems

Wetland treatment system establishment can be

subdi-vided into three topics: (1) physical design, (2) construction,

and (3) vegetation establishment Physical design

encom-passes all the choices of how the wetland basin is built,

and how the water is conveyed to and through the system

With the exception of urban stormwater wetlands (Schueler,

1992), there is no single comprehensive literature source that

explains options for layout and configuration of the wetland

cells that typically comprise a treatment wetland

Site civil construction is a broad discipline of engineering

and has been refined through centuries of experience This

chapter does not attempt to discuss all aspects of this

engi-neering practice but rather describes the highlights of site

civil construction that are most relevant to wetland treatment

system construction A brief general discussion of wetland

site civil construction is provided by Tomljanovich and Perez

(1989) All aspects of wetland construction are discussed in

detail in the Wetlands Engineering Handbook (U.S Army

Corps of Engineers, 2000) This handbook discusses

engi-neering procedures for establishing necessary hydrologic

conditions, geotechnical design and soils handling for site

modification, selecting appropriate vegetation and planting

schemes, and establishing substrate conditions conducive to

the desired functions It also discusses baseline assessments

of existing site conditions, monitoring strategies to determine

long-term success, and contracting considerations However,

the objective of this handbook is the construction of mitigation/

habitat wetlands, not treatment wetlands

Establishment of wetland vegetation seems deceptively

simple, but is not, and is frequently outside of the

experi-ence of many site civil contractors Therefore, this important

aspect of treatment wetland establishment should generally

be subcontracted to specialists, nursery owner/operators, or

wetland builders

18.1 PHYSICAL DESIGN

The size of the wetland has been determined, at least

ini-tially, by the procedures of the preceding Chapter 17 The

general characteristics of the site have been investigated in

Chapter 16 There remain a number of design decisions on

arrangement, vertical and horizontal placement, and

meth-ods of moving and controlling water

sidered during the establishment of the basis of design In general, there will remain a number of possibilities for the location of the system within the overall site confines Condi-tions that should be evaluated during physical design of a wet-land treatment system include geography, soils and geology, runoff water volume and chemistry, ecological and socioeco-nomic factors, and ancillary regulations The importance of each of these conditions may vary, but all should be inves-tigated to some extent Detailed studies may be needed to determine the importance of those site conditions that affect technical feasibility

Floodplains and Floodways

A hypothetical siting situation is shown in Figure 18.1, which indicates that the regional waterways, a river and its tributary, are geographic features to be reckoned with Constructed wetlands located in floodplains with extreme flood condi-tions, such as along major rivers, must have dikes that are designed to allow the passage of floods and/or that are sized

to exclude flood waters Constructed treatment wetlands have typically been allowed in floodplains under constrained con-ditions Designs should avoid damage due to high-frequency recurrence events, and therefore would usually be built to withstand the 25-year flood event without impairment of func-tion Similarly, wetland facilities would usually be designed

to withstand the 100-year flood event without severe damage However, floodplains may be jurisdictional wetlands, thus cre-ating regulatory questions Nevertheless, many FWS wetland systems have been constructed in floodplains such as those at Columbia, Missouri; Brawley and Imperial, California; and Tres Rios, Arizona

If a wetland is built in a floodplain, it is necessary to demonstrate that its presence will not back up floodwaters upstream of the project In other words, the project should not block the floodway of the river For most major rivers of the United States, there exist published maps showing the boundaries of the 100-year floodplain and the floodway

O THER R EGULATORY C ONCERNS

Wastewater treatment and disposal are regulated by an ever increasing number of federal, state, and local laws, rules, ordinances, and standards In some cases, the most chal-lenging part of implementing a wetland treatment project is complying with regulations through the permitting process

In fact, regulations may hinder innovative project design

for wetland treatment systems, even when the new design

improves upon existing technology Permitting an innovative

project may delay implementation and increase cost with no

incremental benefit for environmental protection As a result

of regulatory constraints, designers may opt for conventional

technology with well-known deficiencies instead of

technol-ogy that is perceived to have a greater risk—but with

poten-tially high environmental benefits

A detailed knowledge of pertinent regulations is essential

to evaluate the feasibility of a wetland treatment project This

section provides an overview of the regulations that affect

the use of wetlands for wastewater treatment in the United

States as well as examples of some specific state situations

An updated and more detailed survey of federal, state, and

local ordinances should be conducted to determine those that

might be relevant to specific projects

Endangered Species

Under the Endangered Species Act, the U.S Fish and

Wild-life Service is mandated to identify and list plant and

ani-mal species that are threatened or endangered by extinction

or that are considered likely to be threatened in the future

Threatened and endangered species cannot be harmed,

killed, or otherwise negatively affected by human activities

The U.S Fish and Wildlife Service reviews potential impacts

to federally listed species and can veto Clean Water Act

per-mits (such as a Section 404 wetland fill permit or an NPDES

permit) by issuing a “jeopardy” opinion

The potential for occurrence of threatened or endangered

species inhabiting a project site should be considered during

project planning Typically, a list of threatened and

endan-gered species that could occur at a project location is

com-piled from federal and state natural resource agency records

Road Road

City

Rural Development

Site 1

FIGURE 18.1 Siting and flood concerns for a hypothetical treatment wetlands project Communities are adjacent to a river and its tributary

The floodway occupies the central valley corridors and is bounded by the floodplain Three potential constructed wetland sites are indicated, one in the floodplain.

Potential project impacts on any of these species are assessed

If negative impacts on a species are anticipated during this review, it may be prudent to conduct a field survey to ascer-tain whether that species actually occurs at the proposed site This confirmation step is important in eliminating the chance

of finding a “fatal flaw” after considerable planning, design,

or construction monies have been spent

Experience indicates that the presence of federally tected species does not always necessitate substantial changes

pro-to wetland treatment project siting or design In a number

of cases, the discharge of treated wastewater effluents to constructed wetlands has enhanced the population of listed wetland-dependent species by creating additional habitat and food resources For example, populations of bald eagles, wood storks, and snail kites, all federally protected bird spe-cies, have been increased in and around the 485-ha Orlando Easterly constructed wetland treatment system in Florida Thus, the potential impacts of a project on protected species must be evaluated on a species-by-species basis As a further illustration, the operation of the 2,400-ha STA1E treatment wetland in south Florida was prevented for a period of weeks

because of the nesting activities of burrowing owls (Athene cunicularin floridana) and stilts (Himantopus mexicanus)

(Figure 18.2)

C ULTURAL R ESOURCES

The U.S National Historic Preservation Act requires that sites

of significance for the early peoples of America be preserved That normally involves avoidance of construction activities that might disrupt ancient village sites and burial grounds For example, a historic archaeological site was excluded from the Coyote Hills, California, urban stormwater wetland (Figure 18.3) The Musselwhite, Ontario, mine-water treatment

wetland was sited with due recognition of the line-of-sight

from the regional First Nations burial ground

L AYOUT AND C ONFIGURATION

The area determined to achieve the project treatment goals

may typically be arranged in a number of ways The

princi-pal decisions are

1 How many independent flow paths?

2 How many cells in each?

3 What aspect ratio for the cells or flow path?

There is a need to set the compartmentalization of the

sys-tem The number of flow paths in the design of surface

flow wetlands is based on considerations of redundancy,

maintenance, and topography All constructed wetland

treatment systems should have at least two cells that can

operate in parallel to allow for operational flexibility (cell

FIGURE 18.2 Nesting black-necked stilts (Himantopus mexicanus) caused brief stoppage of the STA1E project in Florida.

FIGURE 18.3 This archaeological site was excluded from the Coyote Hills urban stormwater wetland.

resting, rotation of flows, or maintenance) The ability to take cells out of service is usually viewed as very desir-able Maintenance activities do not normally require fre-quent shutdowns, but it is usually a good idea to allow at least two flow paths so that complete system bypass is not needed Having at least two parallel cells is especially important because of unexpected events such as vegetation die-off, pretreatment failures and subsequent wetland con-tamination, and berm and other structural failures Mul-tiple flow paths allow the loading rate to be manipulated

to meet varying inflow water quality Also, parallel flow paths allow cells to be drained for replanting, rodent con-trol, harvesting, burning, leak patching, or other possible operational controls In the extreme long term, replacement

of structures and piping become necessary Some of the older FWS treatment wetlands are now reaching this point

in their service life Large systems may profitably porate more than two flow paths for purposes of internal flow control and to accommodate the site boundaries, site

incor-topography, and desired aspect ratios However,

multiplic-ity of inlet and outlet control structures can add significant

cost to the overall project

Compartmentalization

Compartmentalization interacts with the design-area

calcu-lation because more cells and higher aspect ratios lead to

higher tanks-in-series (NTIS) numbers, and hence to higher

PTIS numbers As seen in the previous chapters, higher

PTIS does not help much for low desired-removal

percent-ages but is a necessity if very high percentpercent-ages are required

Aspect ratio is also a determinant of NTIS, with fewer for

low length-to-width ratios A single FWS wetland cell of

modest aspect ratio (1 < L/W < 5) is likely to behave as three

or four NTIS in tracer testing When compounded with the

weathering factor that influences most wastewater parameter

reductions, it is likely that a P-value for the P-k-C* model

will be two or three That may be perfectly adequate if the

treatment does not attempt to achieve outlet concentrations

close to C*, but as the effluent target approaches C*, the

value of P becomes more and more important If the

compu-tation is sensitive to P, then it becomes desirable to use cells

in series Use of three cells in series will lead to P-values

between six and nine, beyond which is a region of

diminish-ing returns

Site constraints of steeply sloped ground may

man-date terraced, multicell design Because it is not possible to

maintain uniform level and shallow depths on steep slopes,

the change in elevation is accomplished via drop structures

between cells in series Inman et al (2001; 2003) reported

that site slopes of up to 25% called for 22 cells arranged on

three separate flow paths at the Clayton County, Georgia,

wastewater-polishing wetland facility That project required

Deep Zone

Emergent Vegetation Control Structure

Berm A

Cell Aspect: 2:1

N TIS: 4

Vegetated Area: 0.90 Wetted Area: 1.00 Control Structures: 2

B Cell Aspect: 1:2

N TIS: 6

Vegetated Area: 0.70 Wetted Area: 0.90 Control Structures: 4 C Cell Aspect: 1:1

N TIS: 9

Vegetated Area: 0.65 Wetted Area: 0.85 Control Structures: 8

FIGURE 18.4 Options for cells in series and parallel NTIS are rough estimates Areas are examples only; actual numbers are size-dependent.

the movement of 420,000 m3 of earth over the 22 ha of lands or close to 2 m of cut and fill over the site area

wet-Costs increase as the number of cells increases The number of cells required in each flow path must be deter-mined by balancing the cost of more cells with the need for high-hydraulic efficiency The ratio of berm area to wetland surface area increases with more cells Figure 18.4 shows the design variables associated with increasing cells in series for a hypothetical fixed rectangular project footprint Com-

partmentalization increases the NTIS for the footprint The

number of transfer structures increases with the number of cells, but the vegetated area becomes smaller due to more berm area and more water distribution zone areas If the total area were not fixed, the total footprint would increase with compartmentalization For an extreme example of dual-flow paths and multiple cells, see Figure B.24 in Appendix B for the Orlando Easterly project, which contains 34 structures for 17 cells on two flow paths

Internal Cell Arrangement

The arrangements of internal features of a single wetland cell have very large implications for the efficiency of the cell

Both the volumetric efficiency, eV, and the detention time

dis-tribution efficiency, eDTD, are very sensitive to the vegetation patterns within the cell It is probably these factors that give rise to the majority of the intersystem scatter in performance data

Each individual wetland cell may be contoured in a ber of different ways The goals of internal physical design are to provide efficiency of treatment, adequate conveyance

num-of water, and perhaps habitat values Wetlands have a dency to channelize from points of inlet to points of outlet

ten-If permitted, this operational feature reduces the gross areal efficiency of the wetland Control of the bottom elevation and

There are no hard and fast rules for design for high-areal

efficiency for an individual wetland cell Some general

rec-ommendations are

1 Avoid very small length-to-width ratios

2 Avoid blind spots in corners

3 Avoid unvegetated short-circuit paths

4 Reestablish flow distribution at intermediate points

in a flow path

5 Maintain good bottom uniformity during

con-struction and start-up: minimize the formation of

topographic channels parallel to flow

Figure 18.5 illustrates some of these ideas

Aspect Ratio

The length-to-width (aspect) ratio is important in basin design

because of its effect on flow distribution and hydraulic short

circuiting Theoretically, a constructed wetland with a

high-aspect ratio is not better for treatment than one with a lower

aspect ratio, as long as flow is distributed effectively It has

been speculated that long, narrow flow paths are closer to

plug flow than short, wide flow paths But the interior

micro-channels of a uniform depth, uniformly vegetated FWS

wet-land have small dimensions; recirculation eddies are limited

by depth (on the order of 0.3 m for a FWS wetland) or by

the lateral spacing of plant stems or clumps of stems (also a

fraction of a meter) Thus, the effective length-to-width ratio

is predetermined and widening the wetland only adds more

parallel channels

(a) Bad Open water channel

from inlet to outlet.

(d) Better yet Inlet spreader ditch

and outlet collector ditch.

(e) Still Better Spreader, collector

and redistribution ditches.

(b) Poor Large corner zones

not in flow path.

(c) Better Multiple inlets

and outlets.

FIGURE 18.5 Concepts of cell internals: inlet distribution, outlet collection, and internal redistribution (From Kadlec and Knight (1996)

Treatment Wetlands First Edition, CRC Press, Boca Raton, Florida.)

an increase with L:W Since that time, computational bilities have permitted theoretical exploration of the values of

capa-eV and eDTD for basins of different shapes and inlet tions (see Chapter 2 for a discussion of these efficiencies) For

configura-instance, Persson et al (1999) investigated the effect of L:W

using the two-dimensional, depth-integrated code of

MIKE-21 (Warren and Bach, 1992) Jenkins and Greenway (2005) also investigated empty ponds using the two-dimensional, depth-integrated code of TDFLOW (Jenkins and Keller, 1990) Both numerical studies utilized point inlets and point

outlets, and found a theoretical increase in eDTD (increase in

NTIS) with increasing L:W, up to 10–30 TIS for L:W = 15

However, both studies neglect wind mixing, which may be the dominant driving force in open water systems

Just as importantly, increasing aspect ratio will decrease the inefficiency of utilizing a point inlet or outlet, but there are other steps that may be taken to minimize poor inlet

or outlet distribution of flows A large aspect ratio tends

to improve volumetric efficiency, quantified by the ratio of actual to theoretical detention time In other words, it is not possible to get closer to the plug-flow model, but it is possible

to utilize the entire wetland area For empty ponds, a L:W

of over 10 may be needed to minimize the effect of jet flow short-circuiting and corner dead zones (Figure 18.6)

FWS treatment wetland data show that “longer and rower” does not necessarily mean less effect of mixing, and hence a closer approximation to plug flow For example, the

nar-tracer studies at Richmond, Australia, showed NTIS = 4.6 for an open water unit of L:W = 25 and NTIS = 5.0 for a Myriophyllum FWS wetland of the same aspect (Bavor

et al., 1988) A number of tracer tests of the wetland cells

7B and 9B, both with L:W = 12.5, at the Sacramento,

Califor-nia, project found NTIS = 4.7 ± 0.8 and 5.0 ± 1.4, respectively

(Nolte and Associates, 1998b) On the other hand, the data

from the Champion, Florida, project show NTIS = 11 for a

FWS wetland with L:W = 10 (Knight et al., 1994) Likewise,

the South Florida Water Management District (SFWMD)

periphyton treatment wetland project found NTIS = 9 for

L:W = 5 and NTIS = 25 for L:W = 45 (CH2M Hill, 2003a)

The effect of aspect ratio in these studies is confused by

issues of wind, vegetation patterns, and deep zones Thus, it

is dangerous to assume that aspect ratio alone can provide the

desired improvements in efficiency

The literature contains a few “recommendations” for

L:W ratios to be used in design For instance, Crites et al.

(2006) recommend 2:1 < L:W < 4:1 U.S EPA (2000a)

comments that, in general, FWS wetlands are built with

L:W > 1:1 These appear to be reasonable suggestions that

go with the recognition that lower system aspect ratios may

be used provided cells in parallel are used However, there

is not much quantifiable treatment performance motivation

to prefer one aspect ratio over another, as long as the design

stays in a reasonable range, such as 2 < L:W < 10 However,

there are sometimes limitations on aspect ratio due to the

hydraulic profile (see Chapter 2 and subsequent sections of

this chapter)

If one looks beyond the volumetric improvements, data

from FWS wetland studies indicate somewhat better

perfor-mance for higher length-to-width ratios (Herskowitz, 1986;

Knight et al., 1994; CH2M Hill, 2003a), but the margin is

not large when the pollutant reductions are low-to-moderate

(0–50%)

Higher aspect ratios increase the area of external berms

that must be constructed to enclose a given wetland area

Therefore, economics may argue for low L:W ratios (Knight,

1987) Other methods for maintaining effective flow

distribu-tion, such as deep zones, should be considered as alternatives

to reduce the need for higher length-to-width ratios

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Aspect Ratio, L:W

eV

FIGURE 18.6 The effect of aspect ratio on the volumetric efficiency of unvegetated ponds with point inlets and point outlets (Simulation

data from Jenkins and Greenway (2005) Ecological Engineering 25(1): 61–72.)

Sinuousity

One of those alternatives is to use smaller internal divider berms to create a sinuous flow path of high-aspect ratio (Figure 18.7) An extreme example of this design is the Tucush, Peru, wetland with 14 cross-passes in each of two flow paths (see Figure 16.1) The interior berms create longer flow paths at the expense of cost and area Not only do the berms subtract from the wetted area but the frequent flow reversals invite dead zones in the corners of the switchbacks These corner zones may be minimized by orienting the flow

in the long dimension of the cell

FIGURE 18.7 Options for interior berms creating sinuosity NTIS

are rough estimates Areas are examples only; actual numbers are size-dependent.

A Cell Aspect: 2:1

NTIS: 4

Flow Area: 0.90 Wetted Area: 1.00 Control Structures: 2 B Cell Aspect: 20:1

NTIS: 12

Flow Area: 0.80 Wetted Area: 0.90 Control Structures: 2 C Cell Aspect: 12:1

NTIS: 15

Flow Area: 0.60 Wetted Area: 0.90 Control Structures: 2

to-width was effective in promoting biochemical oxygen

demand (BOD) reduction, but more internal baffles were

yet more effective Ratios 1:1 < L:W < 100:1 were

investi-gated for point inlets and outlets The swing in BOD

reduc-tions was from 10 to 95%

Field testing of pathogen reduction was done in a

side-by-side test at Lidsey, United Kingdom, utilizing baffled

and unbaffled ponds, 1 m deep, with point inlets and outlets

(Bracho et al., 2006) The unbaffled pond had L:W = 9:1; the

adjacent pond of the same shape and size had three

longi-tudinal baffles, creating L:W = 79:1 Tracer testing indicated

NTIS = 2.1 and eV = 0.33 for the unbaffled pond, and NTIS =

7.3 and eV = 0.38 for the baffled pond The low efficiencies

were attributed to “jet flow short-circuiting.” The reduction of

fecal coliforms was 82.1% in the unbaffled pond and 99.4%

in the baffled pond

Interestingly, the use of divider berms to create

back-and-forth sinuous flow paths has been found to provide

low-volumetric efficiency while showing moderate-to-high NTIS

values in some systems Tracer tests at the sinuous Hillsdale,

Michigan, site showed NTIS = 4.3 and 7.4, and eV = 0.33 and

0.51, and for the east cell of L:W ≈ 16 (four cross-passes in

a 1:1 footprint); and NTIS = 37, and eV = 0.34, and for the

west cell of L:W ≈ 18 (five cross-passes in a 1.6:1 footprint)

The reason was found to be the fact that the flow path became

channelized (deeper) in the center portion of the sinuous path,

leaving excluded sides and corners

A side-by-side comparison of sinuous and nonsinuous

cells for phosphorus removal was conducted by the SFWMD

(Figure 18.8) (CH2M Hill, 2003a) The hydraulics of the

sin-which may create bad short-circuits This was the case at the SFWMD project Also, the divider berms occupy some mea-surable fraction of the total footprint, which detracts from overall areal effectiveness Results to date indicate that inter-nal sinuosity may have little net effect

Flow Distribution

Both field results and computational studies have shown that point inlets and point outlets are not a good idea Pond studies are not a good model in this regard, because inlets to ponds are very frequently point inlets The corner zones of the pond are often not easily accessed by the incoming water However, the presence of vegetation in FWS wetlands can lead to even more difficulties for water to reach corner zones Computational fluid mechanics suggests that if the water is distributed uniformly from side-to-side of the wetland inlet, rather than at a point, there will be a significant improve-

ment of the hydraulics (Persson et al., 1999) The changed distribution is forecast to increase NTIS from 1.5 to 6.7, and

eV from 79 to 89% for a basin of L:W  2:1 Wörman and Kronnäs (2005) computed that as the width of the water inlet

increases relative to the wetland width, both eV and NTIS

should increase

Stairs (1993) experimentally determined the effect of inlet cross-spreading by performing tracer tests both before and after the removal of a perforated pipe spreader With the

spreader, NTIS = 4.47 and eV = 83.6% Without the spreader,

for a point inlet, NTIS = 3.27 and eV = 66.5% Thus, the retical results of simulations have been verified in the field

theo-Cell 2

Cell 1

FIGURE 18.8 Cells 1 and 2 of the South Florida Water Management District periphyton treatment wetland project Cell 1 had an aspect

ratio of 5:1, whereas Cell 2 had L:W = 45:1, because of a three-pass sinuous design Both were two hectares in size and are shown empty in

this photo Cell 1 tracer tested at NTIS = 9, whereas Cell 2 tested at NTIS = 25 (Photo courtesy CH2M Hill.)

The collection of the water at the wetland outlet poses the

same problems of corner dead zones Therefore, collection

across the wetland width is advantageous

Bathymetry: Deep Zones and Speed Bumps

Wetlands have a tendency to channelize from points of

inlet to points of outlet If permitted, this operational

fea-ture reduces the gross areal efficiency of the wetland (see

Figure 4.21, for instance) Control of the bottom elevation and

vegetation density can, in principle, prevent poor flow

distri-bution But in practice, the bottom of the wetland can neither

be constructed nor maintained at tolerances that promote full

areal contacting Bottoms do not start completely level, and

a number of processes contribute to the formation of local

highs and lows such as soil heaving and sediment deposition

Nevertheless, care must be taken to degrade any

preexist-ing ditches, roads, or berms on the site because these will

exert possibly undesirable flow control in the FWS wetland

Dierberg et al (2005) demonstrated the poorer treatment that

occurs along such channelized short circuits

Compartmentalization involves emergent berms with

transfer structures, but such cross-berms may also be

con-structed so as to be always or occasionally under water For

example, the treatment wetland at Bulwer Island, Australia,

was constructed with a repeated pattern of 0.4 l 0.2 l 1.0 m

depths along the flow path (Simi and Mitchell, 1999) The

intent of this design was to break up longitudinal channeling

and to promote different microenvironments Underwater

cross berms have also been placed in some of the stormwater

treatment areas in South Florida It is not known if these have

positive effects on treatment The use of cross-benches was

forecasted to promote good hydraulics, NTIS = 23 and eV =

0.80, by Persson et al (1999).

Deep zones in surface flow constructed wetlands are

sup-posed to serve several purposes (Knight and Iverson, 1990)

These deeper areas extend below the bottom of the vegetated basin areas by at least 1 m to exclude the development of rooted macrophytes (Figures 18.9 and 18.10) Such unveg-etated cross ditches provide a low-resistance path for water

to move laterally and provide a nearly constant head across the wetland They also provide for extra detention time, but

in a deep water zone Such ditches often become covered

with duckweed (Lemna spp.) and can be used by wetland

birds and fish as reliable habitat These redistribution ditches change the potential for short-circuiting within the wetland, because high-speed rivulets are intercepted and mixed with quiescent water in the deep zone However, the redistribution ditch adds a potential for wind mixing that compensates the reduced short circuiting But water is more effectively distrib-uted over the wetland, improving the gross areal efficiency to

some extent Lightbody et al (2007) provided evidence from

laboratory models and computations that indicated ment due to deep zones at high degrees of removal However, that improvement was contingent on the assumption that seri-ous short-circuiting occurred in the vegetated sections of the system

improve-Knight et al (1994) operated a set of six treatment

wet-lands receiving treated paper-pulp mill effluent for two years Two pairs of these received the same influent water, one of each pair containing two internal deep zones comprising 25% and 45% of the area (Table 18.1) There was variability in flow (different HLRs) and aspect ratios (2.5 and 10) between the pairs Tracer data from that project were here reanalyzed and characterized by a tanks-in-series (TIS) detention time distribution The effect of deep zones was to decrease the number of TIS (Table 18.1) And, for the larger cell pair (A and B), the volumetric efficiency was improved by deep-zone addition The load reductions for all contaminants were increased by the inclusion of deep zones but by only a very small margin for the higher aspect ratio pair (E and F) One

Deep Zone

Emergent Vegetation Control Structure

Berm A

Cell Aspect: 2:1

N TIS: 4

Vegetated Area: 0.90 Wetted Area: 1.00 Control Structures: 2

B Cell Aspect: 2:1

N TIS: 4

Vegetated Area: 0.70 Wetted Area: 1.00 Control Structures: 2 C Cell Aspect: 4:1

N TIS: 6

Vegetated Area: 0.65 Wetted Area: 0.90 Control Structures: 4

FIGURE 18.9 Options for deep zones in series and parallel NTIS are rough estimates Areas are examples only; actual numbers are

size-dependent.

FIGURE 18.10 The Saginaw, Michigan, treatment wetland system contains two flow paths, each bifurcated by one cross-deep zone.

interpretation of these results is that the net effect of better

efficiency and worse NTIS results from interception of short

circuits in the wide cell but that these are less important for the high-aspect cells Thus, wide cells profit from deep zones but narrow cells do not Another important feature of these results was the fact that the added volume provided by deep zones resulted in smaller volumetric rate constants, an obser-vation in agreement with similar results detailed in Part I.The artifact of L:W was not present in a study of 12 research wetlands operated under varying conditions at a site west of the city of Phoenix, Arizona (Kadlec, 2006a) These were constructed as a triplicated design with zero, one, two, and three internal deep zones, all containing an inlet distribu-tion and an outlet collection deep zone, together comprising 12.5–35% of the wetland areas There were no differences in the internal hydraulics with deep-zone numbers Deep-zone numbers in the wetlands did not affect nitrogen treatment performance No differences with deep-zone numbers were found for temperature, dissolved oxygen, pH, or nitrogen removals or rate constants In this study, there was no large treatment benefit or detriment of incorporating internal deep zones in free water surface wetlands

Moore and Niswander (1997) operated a set of six treatment wetlands receiving diluted dairy wastewater for two years Two wetlands had a central internal deep zone comprising 45% of the area, whereas the other four had no internal deep zone The authors concluded that deep-center sections did not show any significant impact on treatment efficiency Data for the second year, past the startup period, are shown in Table 18.2, as reported in NADB v.2 (NADB data-base, 1998) None of the outlet concentrations are different at the 5% level The wetland areas were all identical and, hence, the hydraulic loading to all six wetlands was uniform at 3.95 cm/d For a presumed 4 TIS hydraulic pattern, the areal

k-values do not differ at the 5% level For instance, for total Kjeldahl nitrogen (TKN), k = 10.1 ± 1.1 m/yr compared to

Source: Hydraulic and concentration data are from Knight et al.

(1994) TAPPI Journal 77(5): 240–245; the DTD analyses are

from this work.

k = 11.0 ± 1.1 m/yr for the two cells with deep zones

How-ever, the inclusion of the deep zone added to the water

vol-ume, and increased the average depth from 30 to 75 cm, and

consequently increasing the nominal detention time from 7.6

to 19 days As a consequence, the kV values for TKN were

different at the 5% level, 0.093 ± 0.01 d−1 for no deep zones

versus 0.040 ± 0.004 d−1 for deep zones As for the Champion

study, these Oregon State University results suggest that the

extra depth added by deep zones carries a penalty in

volu-metric performance

Importantly, all three studies demonstrate a penalty for

deep zones in terms of lower volumetric rate constants for

systems with deep zones The presumption of constant kV,

which is reiterated in much of the treatment wetland literature

(Crites and Tchobanoglous, 1998; U.S EPA, 1999; U.S EPA,

2000a; Water Environment Federation, 2001), is, therefore,

inappropriate for design of FWS wetlands with deep zones

This further substantiates the growing body of knowledge

that wetland areal models are to be preferred over volumetric

models for FWS wetlands, in general For deep zones, the

added water volume and its attendant-added detention time

do not contribute proportionately to pollutant removal

Taken together, these studies do not demonstrate a clear

advantage or a clear disadvantage for deep zones Other

wet-land attributes, such as aspect ratio, vegetation community

type, and compartmentalization, may be equally or more

important for water quality improvement Nevertheless, deep zones may be important for habitat value, may provide for increased reaeration, and may provide opportunity for patho-gen reduction via UV radiation The amount of open water that may be included in a cell remains a controversial issue Based upon anecdotal evidence, U.S EPA (2000a) contends that the central part of a flow path must be open water/SAV

to provide oxygenation for ammonia reduction However, Kadlec (2005e) found no particular support for that view in

an examination of data from many systems, including ponds

as the open water extreme Open water sections in a cell vide better habitat value than does a uniform dense stand of emergent plants (Weller, 1978) However, habitat values must

pro-be balanced by the potential for wildlife to interfere with treatment Excessive waterfowl use can add pathogens to the water Muskrats, nutria, and beavers want the deep water for refuge and can seriously impair the vegetation, hydraulics,

and possibly treatment (Kadlec et al., 2007).

An important function of an inlet deep zone is the tling of particulate matter that might otherwise create dif-ficulties in the inlets of vegetated sections This has been discussed in Chapter 7 but deserves consideration here as part of the process of developing a layout for the treatment wetland It is useful to remove large sediment loads before they reach the vegetation, so that the eventual cleanout

set-is from a pond rather than from vegetation The Imperial, California, system employs a 3.9-ha, two-cell sedimenta-tion basin before wetlands (Figure 18.11) Over a four-year operating period, the basins trapped 3,820 tons of sediments, amounting to approximately 10 cm of accretion Those solids would have been detrimental to the inlet zone of the 4.7-ha follow-on wetlands Table 7.8 indicates that it is easily pos-sible to settle river-borne solids in just a few days detention

in a presettling basin with removals of over 90% The ing of this inlet element of the treatment train is dependent upon the settling characteristics of the specific solids in ques-tion, as well as the accumulation rate of the solids An inlet deep zone built for flow distribution may fill up quickly and require emptying at frequent intervals (see Figure 7.13) A simple mass balance suffices for the sizing of sedimentation basin storage for the desired loading and emptying frequency, whereas a settling calculation determines the area needed for

siz-a given bsiz-asin-opersiz-ating depth

Fringing, Banded, and Clumped Vegetation

Many natural wetlands are riverine, meaning that the land contains a central stream channel, which may meander through a zone of emergent vegetation Indeed, this wetland form is so common that it is mistakenly presumed to be a good prototype for a treatment wetland Nothing could be further from the truth A central channel will convey most

wet-of the water, and the fringing vegetation will effectively receive flow only on rising flood conditions and discharge

it on the ebbing stage Therefore, configurations similar to Figure 18.12A are usually to be avoided Simply stated, if the water needs to go through the plants, do not plant them off

to the side of the flow path Jenkins and Greenway (2005)

TABLE 18.2

Oregon State University Pilot Wetland

Performance, Means, and Standard Deviations

Note: IDZ  inlet deep zone; data are averages of biweekly

data, January 1994–February 1995.

Source: Data from NADB database (1998) North American

Treatment Wetland Database (NADB), Version 2.0 Compiled

by CH2M Hill Gainesville, Florida.

provide numerical calculation results for several aspect ratios

and cover percentages, which show that hydraulic

efficien-cies are low for fringing vegetation

The Jenkins and Greenway (2005) computational

research also demonstrated that cross-banded vegetation is a

better option for hydraulic efficiency than fringing vegetation

Therefore, any open water that may be included in the cell

should be oriented across the flow direction, whether or not

it is deeper than the average However, as will be discussed

in the following text, adequate conveyance may be difficult to maintain if large aspect ratios are combined with significant banded or uniform vegetation

These concepts were utilized in the Brawley, nia, treatment wetland (Figure 18.13) The first cell was built with a sinuous path and fringing vegetation; the second cell was built with banded vegetation Only overall system data were taken, thus no information on relative efficiencies is available

Califor-A third concept for the vegetation pattern is the use of small islands or hummocks, either emergent or submerged

(Thullen et al., 2005) At the present time, there is no

infor-mation on the hydraulic efficiency of this alternative or any pollutant removal comparison

F ITTING THE W ETLANDS TO THE S ITE

Given the total required wetland area and the concepts

of system configuration, there still remains the placement

of the wetland on the site The principal considerations are adaptation to the boundaries and contours of the site, minimization of intercell conveyance, and minimization of earthmoving

Site boundaries often determine the external shape of the overall system, because there is often no extra land or the ability to choose the shape of the available land In that event, the various pieces of the overall system must conform

to the space available The topology of the conceptual out is retained, but shapes and perhaps areas are sacrificed

lay-A good example is the initial layout of the Columbia,

Mis-souri, treatment wetlands (Brunner et al., 1992; Brunner and

Kadlec, 1993) The conceptual configuration was selected

to be three banks of cells in parallel The available lands were bounded by streams, roads, towns, railroads, and the floodway of the Missouri River As a consequence, the actual

FIGURE 18.11 Aerial view of the Imperial, California, treatment wetland Two sedimentation basins, in parallel, precede the four wetland

cells The first cell is built sinuous with fringing vegetation The second is built with cross-banded vegetation The final two cells are out of the view to the upper right.

(a) Bad Bench and channel design provides fringing vegetation,

dead zones and a short circuit due to topography.

(b) Better Inlet distribution, outlet collection, and hummocks.

(c) Still Better Inlet distribution, outlet collection, and banded

vegetation.

FIGURE 18.12 Concepts of cell internal vegetation patterns

Fringing, patterned, and banded vegetation may be used.

layout was not completely rectangular nor were the cells all

in close proximity (Figure 18.14) Despite the long-transfer

distances, this complex of cells is gravity driven A transfer

pump is provided at the system outlet, to supply the treated

water to the Eagle Bluffs wildlife wetlands, where it is used

to foster habitat A fourth bank of wetland cells was added to

the system in 2003

18.2 HYDRAULICS

Linear velocity can become a consideration in design for very large wetlands For instance, total suspended solids (TSS) removal depends on sedimentation and trapping within the wetland Excessive linear velocities lead to large values of shear stress on deposited solids, and therefore can lead to

FIGURE 18.13 Aerial view of the Brawley, California, treatment wetland The first cell of two in series is built in a hairpin, sinuous with

fringing vegetation The second is also a hairpin but with cross-banded vegetation.

From WWTP

1A 1B 1C

Wetland Unit Number 1

Wetland Unit Number 2

Wetland Unit Number 3

To conservation wetlands 1.2 Mile transfer

0.2 Mile transfer

1D

2D

3D 3B

FIGURE 18.14 Arrangement of cells within the initial configuration of the Columbia, Missouri, wetland treatment units All cells receive

water via submerged, gated distribution pipes along the inlet edge; each cell has three outlet structures (From Kadlec and Knight (1996)

Treatment Wetlands First Edition, CRC Press, Boca Raton, Florida.)

suggested to be about 20 cm/s (U.S Army Corps of

Engi-neers, 2000) (Figure 18.15) However, it is well to bear in

mind that existing FWS wetlands operate at velocities lower

than this, mostly below 100 m/d, and, hence, there is no field

test of the criterion The large Florida phosphorus control

wetlands have all been subjected to a maximum velocity

cri-terion of 3.0 cm/s (2,600 m/d)

Theory indicates that higher water linear velocities may

favor improved pollutant removal because of improved mass

transfer (Kadlec and Walker, 2004) In stagnant water,

pollut-ant transport to removal sites is by molecular diffusion, whereas

in turbulent flow, eddies carry materials to solid surfaces

As the length-to-width ratio is increased, both the linear

velocity and the head loss increase At some point, the head

loss will produce an inlet water depth that is unacceptably

large, as detailed in Chapter 2 A hydraulic analysis would

usually be used to check whether an aspect ratio, set from

site considerations and constraints, produces an acceptable

hydraulic profile

H YDRAULIC P ROFILES

The primary purpose of calculating the expected hydraulic

profile is to determine if the proposed wetland can pass the

design flow without an excessive “pile-up” of water at the

inlet The water depths for any particular cell configuration

and flow may be calculated using known procedures of

vary-ing complexity Complicated, two-dimensional calculations

may be required for complex wetland geometries Examples

of readily obtainable computer codes include the suite of

Hydraulic Engineering Center (HEC) models (Feldman,

a b c B

water depth, m

h H

 wwater depth, mwetland length, mvolume

L Q

vegetative resistance to flow The coefficients a, b, and c are

selected to be representative of the type and density of the vegetation (see Chapter 2) The usual specification for the outlet depth is determined as a weir setting plus the height over that weir, but other devices may also result in an outlet depth specification Because the depth is known (set) at the outlet, the integration of Equation 2.24 proceeds from the outlet upstream to the inlet, thus resulting in the designation

as a backwater calculation

The wetland-sizing procedure will have determined a

hydraulic loading for the design, q = Q/LW, where Q is the

design flow The wetland width is set from the selected aspect

FIGURE 18.15 Relationship between grain size, velocity, and sediment movement At low velocities, deposition is dominant, whereas at

higher velocities, resuspension and transport may occur (From Hayes et al (2000) Wetlands Engineering Handbook ERDC/EL

TP-WRP-RE21, U.S Army Corps of Engineers: Washington, D.C.)

1 10 100 1,000

ratio and the design area The backwater calculation then

determines the hydraulic profile, h(x).

As an illustration, consider a wetland designed for a

hydraulic loading of 10 cm/d (10,000 m3/d on 10 ha)

Differ-ent aspect ratios may be chosen in recognition of site

con-straints and the desire for high-hydraulic efficiency provided

by high-aspect ratio For this illustration, three ratios are

selected: 1:1 (316 m r 316 m), 5:1 (710 m r 142 m), and 10:1

(1,000 m r 100 m) The exit weir is set for a depth of 30 cm

For the three cases, the inlet depths for dense vegetation are

33, 41, and 46 cm, respectively (Figure 18.16) These are all

probably acceptable hydraulic profiles

A field example of an unacceptable head loss

situa-tion occurred for the Imperial, California, wetland (see

Figure 18.11) This system is comprised of four cells with

a combined aspect ratio of 30:1 (TTI and WMS, 2006) In

2003, the hydraulic-loading rate to the Imperial wetland was

increased to determine the impact it would have on wetland

performance However, during this period, the water

over-topped the inlet levees, and thus loading had to be reduced

However, the reduced hydraulic-loading rates used after the

overtopping event were still well above the original design

flow rate at which the wetland was operated throughout 2001

and most of 2002 The levees overtopped because physical

constraints made it impossible for water to pass through the

wetland at the rate it was being applied This demonstrates

that when considering different wetland designs, it is

impor-tant to take into account the fact that the physical

characteris-tics of the wetland may constrain the hydraulic-loading rate

18.3 EARTHMOVING: DIKES,

BERMS, AND LEVEES

B ERM D ESIGN

Berms are designed based on hydraulic and geotechnical

considerations The purpose of berms is to regulate and

contain water within specific flow paths Figures 18.17 and

18.18 show typical design features of constructed wetland berms for lined and unlined systems Exterior wetland berms are kept as small as possible while still providing adequate containment and freeboard to prevent unauthor-ized flow releases Interior berms may be used to augment flow distribution but do not have to be designed to control offsite water releases

Exterior berm freeboard should be adequate to prevent overtopping during sudden storm events (based on a storm event frequency of 10, 25, or more years) and allow overflow

of less frequent storm events through controlled and tected emergency overflow points Berm freeboard should also consider that the wetland will gradually fill with vegeta-tion and with mineral and organic sediments that increase flow resistance and decrease freeboard during system life Berm height should equal the sum of the maximum desired normal water level (for example, 45 cm), the return storm rainfall amount (for example, 20 cm for a 25-year storm event), and the lifetime loss of freeboard due to sediment and plant accumulation (approximately 1 cm/yr in some wet-lands receiving municipal wastewaters) For a 20-year life, this hypothetical wetland should have an emergency spill-way height of at least 85 cm with an additional 20 cm or so

of berm above that level Any additional berm height vides additional system life and insures against unauthorized discharges In the event that an open water system is con-templated, the freeboard should also be adequate to contain wind setup and wave action The foregoing example provides

pro-1 m of freeboard, which would be sufficient to control wind set and waves for an unobstructed fetch of about 1 km (U.S Army Corps of Engineers, 2000)

Berms should be constructed on the basis of standard geotechnical considerations The materials that are available dictate how berms will be designed and constructed Surface liners or internal clay plugs may be required to minimize berm seepage if sandy or other permeable materials are used for berm construction External seepage collection channels may

be necessary if soils are unconsolidated An exterior slurry

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

0 100 200 300 400 500 600 700 800 900 1000

Distance (m)

Aspect 10:1 Aspect 5:1 Aspect 1:1 Bottom 10:1

FIGURE 18.16 Wetland hydraulic profiles for aspect ratios of L/W = 1:1, 5:1, and 10:1 The height over the weir is set at 0.3 m (left-hand

side) Bottom has been set as nearly level The design hydraulic loading is 10 cm/d Note that water surface is not a level pool nor is it parallel to the bottom.

wall tied into deeper, low-permeability sediments can be used

to limit offsite infiltration

Berm slope is dictated by geotechnical considerations and

a slope-stability analysis Minimum berm slopes typically

used in constructed wetlands are 2:1 (horizontal:vertical), and

slopes up to 10:1 or 20:1 are used when a shallow littoral shelf

is desired to create habitat diversity Berm width should be

adequate for the intended use For example, if the berm will be

used for vehicular traffic, it should be more than 3 m in width

Dikes are used for access by walking or driving A

vehi-cle access dike needs to be more than 3 m wide at the top;

interior divider berms designed for pedestrian access may be

as narrow as 1 m Dikes greater than about 5 m in width are less likely to be fully penetrated by muskrats The side slopes

of these are typically at a three-to-one slope and may be rapped with stone to prevent erosion or rodent burrowing (Figure 18.19) The interior of a containment dike usually is designed to contain a compacted core and a layer of sealant, which extends above water level Water containment dikes are subject to local dam safety regulations

rip-B ASIN B OTTOM C ONTOURING : C UT AND F ILL

The constructed wetland complex of cells and dikes requires earthmoving The cost for this activity consists of two distinct

FIGURE 18.17 Design considerations for constructed wetland berms (From Kadlec and Knight (1996) Treatment Wetlands First

Edition, CRC Press, Boca Raton, Florida.)

30 cm

30 cm Rooting soil Rip rap

3:1 Side slope

3 m or standard vehicle width Slope as needed

components: the cost for moving soils within the site

bound-aries (cut and fill) and the costs for importing or exporting

material from the site, from external borrow sources or to

spoil disposal sites Equal cut and fill is the preferred option

because it avoids import/export costs Selection of the

bot-tom elevation of the cells, together with proper positioning

on the site with respect to its topography, generally allows

balancing of cut and fill

Rough estimates of the internal volume of a wetland cell

may be made based upon the intended geometry For the

common rectangular geometry, the excavated volume may

The volume of earth needed for the levees for one cell,

neglecting the corners, is

VL y§©2(L WT2shL)2(W WT2shL) (¶¸ W hT LshL2)

(18.2)where

= height of levee above grade, m

of earthmoving to warrant the use of one of the several able software packages that allow very accurate balancing of cut and fill over variable topography

avail-L INERS AND R OOTING M EDIA

The bottom of the wetland, as well as the core of ment dikes, may be formed of compacted clays or benton-ite Locally available clays are preferred from the standpoint

contain-of cost reduction Plastic liners may be feasible for smaller size wetlands This clay layer or other sealant should not

be penetrated by plant roots, so that it retains its integrity The topsoil from the site should be stockpiled and replaced within the wetland to form a rooting medium The roots and rhizomes of emergent macrophytes now in use—cattails,

bulrushes, and Phragmites—usually occupy the top 30–40

cm of the soil column Therefore, a layer of that thickness should be used The original topsoil from the site may be useable; and if so, it should be stockpiled separately from the other soils during construction This topsoil will contain seeds of the wetland plants of the region, which may assist in vegetating the wetland If acceptable topsoil is not available

at the site, it may need to be imported or soil amendments used to optimize plant survival and growth Final grading to tolerances of about +3 cm is necessary to maintain sheet flow conditions in shallow marsh areas

For some treatment wetland applications involving ardous substances, double liners may be required For exam-ple, landfill leachate wetlands may need to be double lined Liner costs are the key factor in design decisions and are addressed in Chapter 23

haz-FIGURE 18.19 Wetland berm protection using stone riprap at the Lake Nebagamon, Wisconsin, treatment wetland This divider berm is

not drivable Photo shows wetland just after planting.

depends on the project goals and regulatory requirements

Some wetlands are unlined, either because the in situ native

soils are deemed to have sufficient sealing properties or

because groundwater recharge is a function of the system If

the wetland must be lined, there are a variety of liner materials

available The two most common liner materials used are

30-mil (0.76 mm) polyvinyl chloride (PVC) and 40-30-mil (1.0 mm)

high-density polyethylene (HDPE) PVC liners are generally

factory-seamed, one-piece liners used on small projects less

than 0.1 ha in size HDPE liners generally come in rolls and

are field seamed for larger projects

Liner materials include: polyvinyl chloride (PVC),

polyethylene (high-density HDPE, and linear low-density

LLDPE), polypropylene (PPE), and compacted clay Some

liners can include a scrim, that is, a woven net of nylon or

polypropylene embedded in the plastic material or encloses

the clay (Interstate Technology and Regulatory Council,

2003) Scrims provide extra strength and resistance to tears

in the material, and liners with scrims will cost more

For a given thickness, PVC is generally the least

expen-sive material and the easiest to work with in the field PVC

has the best puncture resistance of all the materials (except

those with scrims) It has the most flexibility but also has the

least resistance to ultraviolet degradation, and therefore must

be covered to protect it from degradation Because the

wet-land construction process generally covers the liner with soil

or gravel, PVC is a good first choice A minimum thickness

is generally considered to be 0.76 mm (30 mil)

Polyethylene (PE) comes in two forms for liners:

lin-ear low-density polyethylene (LLDPE) and high-density

PE (HDPE) HDPE is harder to work with in the field than

LLDPE, especially in cool weather The thickness should

be 1.00 mm (40 mil) to be equivalent to 0.76 mm PVC for

puncture resistance UV resistance is good for both PE

mate-rials Field repairs are easy Seaming with tape should

pro-vide good waterproof characteristics and is easily performed

in the field, at though welds are usually required to affect a

more permanent seal at the seams It is usually 10% more

expensive than PVC On large projects, this cost

differen-tial can be offset by the ease of field seaming Polypropylene

(PPE) has good installation properties because field seaming

and repairs are easy It has the best puncture resistance of

all three plastics A major disadvantage is that it is the most

expensive of the three materials

Reinforced Plastics

Some plastics can include a scrim, that is, a woven net of

nylon or PPE embedded in the plastic material If the project

can afford them, liners with scrims such as 1.5 mm (60 mil)

Hypalon, 1.5 mm (60 mil) XR-5, or 1.1 mm (45 mil) PPE are

placing the gravel media or soil may offset the difference in cost of liner materials Hypalon is a chloro-sulfonated PE with a nylon scrim It has excellent UV properties and punc-ture resistance It is expensive, and repair is difficult after ageing Hypalon may be less palatable to nutria, muskrats, and beavers than other synthetic liners based on anecdotal evidence

XR-5 is a PVC material with a scrim, which has very high strength and excellent puncture resistance, and is easy

to repair XR-5 is the most expensive of all synthetic liners and requires specialized equipment for installation Rein-forced 1.1 mm (45 mil) PPE has all of the advantages of PPE but with higher strength and puncture resistance

Clay Liners

Natural materials, such as clays or bentonite, may also be used as liners Some are composed of a layer of clay between two scrims of finely woven PPE or PE These have a distinct advantage for irregular-shaped cells because of difficulties

of installation of plastic materials in nonrectangular etries Bentonite may be considered, but costs depend on the

geom-in situ soil and the distance from the bentonite supply, which

in North America is in the western United States (Wyoming) However, source location is also a key issue for native clays because a nearby source may not be available In most cases, clay with permeability less than 10−6–10−7 cm/s would be required The installed thickness would usually be on the order of 30 cm with compaction These factors combine to make the use of clay a costly alternative, requiring economic comparison with synthetic materials

E ROSION AND F LOOD P ROTECTION

A treatment wetland site may be unavoidably in the plain of a river The question of protection of the wetland from flooding then arises There are two aspects to potential flooding: maintaining the physical integrity of the system and the effect on treatment during flood events

flood-Physical damage to dikes and structures is not likely to

be serious if the system is not placed in the floodway of the river The main flood currents should not impinge upon the wetlands Several treatment wetlands have easily survived gentle inundation without any significant damage, for exam-ple, Des Plaines, Illinois; Jackson Bottoms, Oregon; Tarrant County, Texas; and Columbia, Missouri Care must be taken

to allow uniform flooding to prevent uplift of a sealed bottom

by hydrostatic forces In some cases, it may be necessary to install underdrains beneath a wetland liner to relieve hydro-static pressure in the event of rising regional groundwater Outlet structures should be designed to allow backflooding (not through-flooding) of wetland cells