WETLAND, AN ALTERNATIVE FOR FLOOD AND

STORMWATER CONTROL

Baiah Widia Utaminingtyas

Water Resource Engineering University of Brawijaya Malang

email: baiahwidia@yahoo.com

Background

Urbanization dramatically alters the natural hydrologic cycle. As urban

structures such as roads and buildings are built, the amount of impervious area within a

watershed increases. Increases in impervious area increase the volume and rate of

runoff, while decreasing groundwater recharge. Urbanization also increases the and

amount of surface runoff. The increased amount of surface runn off caused flood in

some regions.

Floods and storms are some of the most destructive hydro-meteorological

phenomena in terms of their impacts on human well-being and socioeconomic

activities. Within the last year, the papers and airwaves around the world once again

have been inundated with reports of flooding and consequent human loss and suffering.

The risks of floodplain development are well known; the failures of structural

protection are well demonstrated; the social costs of flooding are enormous. How are

we going to reform our use of floodplains?

Older approaches to stormwater management have focused on efficiently

collecting and conveying stormwater offsite. This approach can increase downstream

property damage and impacts on receiving waters. Newer approaches to stormwater

management seek to retain natural features of drainage systems and provide onsite

management to address water quality and water quantity goals. This approach views

stormwater as a resource to be used to recharge groundwater and to supply fresh water

to surface waters, including wetlands. Properly managing stormwater can avoid

problems with erosion, flooding, and adverse impacts on natural drainage features.

Sustainable flood and storm control schemes could include structural and

nonstructural measures. Design modifications of physical structures that allow the

maintenance of natural environment to a large extent could be sustainable. This,

together with the nonstructural measures (for example,water retention areas, restoration

of wetlands, land use, zoning, and risk assessment, and early warning systems), can

deliver benefits to humans and ecosystems over a long period of time. Wetland is one

of the alternatif to overcome the problem related to flood and stormwater control which

is sustainable and environmentally friendly.

What is Wetlands?

There are numerous definitions of wetlands that can be found in the literature.

Some of these definitions include: “an area of wet soil that is inundated or saturated

under normal circumstances and would support a prevalence of hydrophytic plants”

(Ward and Elliot, 1995), and “the occurrence of water in bodies that do not constitute

permanent watercourses, such as lakes or rivers” (Percy, 1993). A reference definition

given by the National Research Council (1995) includes: “an ecosystem that depends

on constant or recurrent, shallow inundation or saturation at or near the surface of the

substrate. The minimum essential characteristics of a wetland are recurrent, sustained

inundation or saturation at or near the surface and the presence of physical, chemical,

and biological features reflective or recurrent, sustained inundation or saturation.

Common diagnostic features of wetlands are hydric soils and hydrophytic vegetation.

These features will be present except where specific physicochemical, biotic or

anthropogenic factors have removed them or prevented their development”.

For this paper, however, a wetland will be defined as an area of land that is

saturated with water long enough to promote wetland or aquatic processes as indicated

by poorly drained soils, hydrophytic vegetation, and various kind of biological

activities which are adapted to a wet environment (National Wetlands Working Group,

1987).

Wetland Characteristics

Wetlands vary widely due to local and regional differences in topography,

hydrology, vegetation, and other factors, including human involvement. Because

wetland is a collective term encompassing a wide range of wet environments, all

wetlands must share three basic characteristics:

There must be water present at or near the surface of the ground for a portion of the

year.

There must be plants adapted to wet conditions (hydropylic plants).

There must be soil types that develop from wet conditions (hydric soils).

Wetlands are typically classified as distinct entities since they are neither

completely aquatic nor completely terrestrial; wetlands often represent a physical

interface between the aquatic and terrestrial ecosystems resulting in a functional

overlap (Figure 1a) (National Research Council 1995). Wetland areas can also be found

isolated from aquatic systems (Figure 1b). They can share the vascular flora of

terrestrial ecosystems, however, the flora are usually of a different species.

Figure 1a. Wetlands as physical interface between aquatic and terrestrial system

Figure 1b. Wetlands isolated from other water bodies

Hydrology

Hydrology is described by Mitsch and Gosselink (1986) as probably the most important

factor in the establishment and maintenance of specific types of wetlands and wetland

processes. Precipitation, surface water inflow and outflow, groundwater exchange, and

evapotranspiration are the major factors influencing the hydrology of most wetlands.

Figure 2. shows a simplified diagram of a wetland hydrologic cycle.

Figure 2. Wetland Hydrological Cycle

Some wetlands remain permanently inundated or saturated, some are wet for

only a short period during the year; and others may remain dry over periods of several

years. The periods of saturation or dryout in wetlands have strong implications for the

characteristic structures that develop in wetlands (Kadlec and Knight, 1996). Each

wetland type exhibits a unique hydroperiod that is fundamental in the stability of a

wetland system. Hydroperiod is defined as the periodic or regular occurrence of

flooding and/or saturated soil conditions (Marble, 1992). Mitsch and Gosselink (1986)

suggested characterizing hydroperiod as the ratio of flood duration divided by flood

frequency over a given period of time. Cowardin and others (1979) provided general

descriptions of hydroperiod for both tidal and nontidal wetland systems. The

hydroperiod for a particular wetland is a function of the water budget (i.e., inflow and

outflow water balance) and storage capacity, which is affected by the surface contours

of the landscape and subsurface soil, geology, and groundwater conditions (Mitsch and

Gosselink, 1986).

Hydric Soil (Saturated Soil)

Wetland’s soil already saturated long enough to create an anaerobic state in the

soil horizon. Carbon is the major nutrient cycled within wetlands. Most nutrients, such

as sulfur, phosphorus, carbon, and nitrogen are found within the soil of wetlands.

Anaerobic and aerobic respiration in the soil influences the nutrient cycling of carbon,

hydrogen, oxygen, and nitrogen, and the solubility of phosphorus thus contributing to

the chemical variations in its water. Wetlands with low pH and saline conductivity may

reflect the presence of acid sulfates and wetlands with average salinity levels can be

heavily influenced by calcium or magnesium. Biogeochemical processes in wetlands

are determined by soils with low redox potential.

Biota

The biota of a wetland system includes its vegetation zones and structure as well

as animal populations. The most important factor affecting the biota is the duration of

flooding. Other important factors include fertility and salinity. In fens, species are

highly dependent on water chemistry. The chemistry of water flowing into wetlands

depends on the source of water and the geological material in which it flows through as

well as the nutrients discharged from organic matter in the soils and plants at higher

elevations in slope wetlands. Biota may vary within a wetland due to season or recent

flood regimes.

Wetland Types

The effects of stormwater on a particular wetland depend, in part, on the type of

wetland in question. Wetland type can be defined as the combination of attributes (e.g.,

physical, chemical, and biological) that make a particular wetland different from other

wetlands. A wide range of wetland types, which are the result of the cumulative effect

of many environmental variables, exist in the United States. In an effort to bring

precision and standardization to the classification of wetland types, Cowardin and

others (1979) developed Classification of Wetlands and Deepwater Habitats of the

United States for use in the National Wetlands Inventory. Their classification system

breaks wetlands into systems, subsystems, and classes analogous to plant or animal

taxonomic classifications.

Table 1. Wetland Classification

The major type of wetland which almost every countries in the world have are

categorized as:

Marshes

A marsh is a type of wetland that is dominated by herbaceous rather than

woody plant species. Marshes can often be found at the edges of lakes and streams,

where they form a transition between the aquatic and terrestrial ecosystems. They

are often dominated by grasses, rushes or reeds. If woody plants are present they

tend to be low-growing shrubs. This form of vegetation is what differentiates

marshes from other types of wetland such as swamps, which are dominated by

trees, and mires, which are wetlands that have accumulated deposits of acidic peat.

Marshes differ depending mainly on their location and salinity. Both of

these factors greatly influence the range and scope of animal and plant life that can

survive and reproduce in these environments. The three main types of marsh are

salt marshes, freshwater tidal marshes, and freshwater marshes. These three can be

found worldwide and each contains a different set of organisms.

Figure 3. A Marshes

Bogs

A bog is a mire that accumulates peat, a deposit of dead plant material—

often mosses, and in a majority of cases, sphagnum moss. It is one of the four main

types of wetlands. Other names for bogs include mire, quagmire and muskeg;

alkaline mires are called fens. They are frequently covered in ericaceous shrubs

rooted in the sphagnum moss and peat. The gradual accumulation of decayed plant

material in a bog functions as a carbon sink.

Bogs occur where the water at the ground surface is acidic and low in

nutrients. In some cases, the water is derived entirely from precipitation, in which

case they are termed ombrotrophic (rain-fed). Water flowing out of bogs has a

characteristic brown colour, which comes from dissolved peat tannins. In general,

the low fertility and cool climate results in relatively slow plant growth, but decay

is even slower owing to the saturated soil. Hence peat accumulates. Large areas of

landscape can be covered many metres deep in peat.

Bogs have distinctive assemblages of plant and animal species and are of

high importance for biodiversity, particularly in landscapes that are otherwise

settled and farmed.

Figure 4. A Bog

Swamps

A swamp is a wetland that is forested. Many swamps occur along large

rivers where they are critically dependent upon natural water level fluctuations.

Other swamps occur on the shores of large lakes. Some swamps have hammocks,

or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates

periodic inundation. The two main types of swamp are "true" or swamp forests and

"transitional" or shrub swamps.

Figure 5. A Swamps

The Values of Wetlands

Wetlands provide a variety of ecological values and functions that directly and

indirectly benefit people. Most residents of rural communities have taken advantage of

recreational opportunities afforded by wetlands, such as boating, hunting, and fishing.

However, many important functions are far less obvious. For example, a wetland may

enhance downstream water quality by filtering chemicals, excess nutrients, and

sediments. Wetlands can also act as natural flood control areas through retaining

floodwaters and delaying their release downstream. Because of their numerous

ecological and sociological functions, every effort should be made to protect existing

wetlands and to restore those degraded by human activities.

While not all wetlands provide all functions and values, most wetlands provide

several. Under appropriate circumstances, wetlands can provide:

water quality improvement

flood storage and the desynchronization of storm rainfall and surface runoff

cycling of nutrients and other materials

habitat for fish and wildlife

recreation

education and research

aesthetics and landscape enhancement

Wetland can be alternative solution for flood and stormwater control because of

its physical and hydrological values. Physical and hydrological can be described as:

Flood Control

The wetland system of floodplains is formed from major rivers downstream

from their headwaters. Notable river systems that produce large spans of

floodplain. The floodplains of major rivers act as natural storage reservoirs,

enabling excess water to spread out over a wide area, which reduces its depth and

speed. Wetlands close to the headwaters of streams and rivers can slow down

rainwater runoff and spring snowmelt so that it doesn’t run straight off the land

into water courses. This can help prevent sudden, damaging floods downstream.

Wetlands act as protective natural sponges by capturing, storing and slowly

releasing water over a long period of time, thereby reducing the impact of floods.

Ground Water Recharge

Wetland systems are directly linked to groundwater and a crucial regulator

of both the quantity and quality of water found below the ground. Wetland systems

that are made of permeable sediments like limestone or occur in areas with highly

variable and fluctuating water tables especially have a role in groundwater

replenishment or water recharge. Sediments that are porous allow water to filter

down through the soil and overlying rock into aquifers which are the source of

95% of the world’s drinking water. Wetlands can also act as recharge areas when

the surrounding water table is low and as a discharge zone when it is too high.

Sediment Trap

Wetlands improve water quality by acting as sediment sinks or basins.

They are especially effective at trapping sediments in slow moving water. Wetland

vegetation slows water velocity and particles settle out.

Coastal Protection

Coastal marshes, mangrove swamps and other estuarine wetlands act as

effective storm buffers. Studies have concluded that more than half of normal wave

energy is dissipated within the first 3 meters of encountering marsh vegetation such

as cord grass. The erosive nature of tides is also dampened by wetland plants

because their roots hold soil in place and their stalks reduce the destructive energy

of waves and wind. Tidal and inter-tidal wetland systems protect and stabilize

coastal zones. Coral reefs provide a protective barrier to coastal shoreline.

Mangroves stabilize the coastal zone from the interior and will migrate with the

shoreline to remain adjacent to the boundary of the water. The main conservation

benefit these systems have against storms and tidal waves is the ability to reduce

the speed and height of waves and floodwaters.

Figure 6. How Wetland Works

Constructed Wetland

A constructed wetland consists of a properly designed basin that contains water,

a substrate, and, most commonly, vascular plants. These components can be

manipulated in constructing a wetland. Other important components of wetlands, such

as the communities of microbes and aquatic invertebrates, develop naturally.

Constructed wetlands satisfy a range of urban design objectives such as

improving water quality, regulating flow rates and enhancing landscape and ecological

values. They also provide a range of passive recreational and aesthetic benefits to the

community. The priority of design criteria for constructed stormwater treatment

wetlands are:

1. stormwater treatment

2. enhanced aesthetic, recreational and cultural values

3. habitat provision

It should be noted that constructed water bodies can attract flocking birds,

which may be a concern under some circumstances. Further information should be

sought from the relevant planning authority as to whether any particular design

considerations are required.

The design intent/concept design should include a site analysis as well as

describe the preliminary assessment of the treatment performance of the proposed

wetland, landscaping objectives and recreational use objectives. Site analysis involves

an audit of regional land-use planning (such as green corridors and conservation areas),

climate and landscape characteristics. Design considerations which can used for

wetland drainage are:

Subsurface Flow Systems. This system are commonly used in Europe

Surface Flow Systems. This kind of system is more common in US/North America.

This system is a marsh-like system.

Vertical Flow Systems. This is a new design which is used to overcome oxygen

depletion problem and boost nitrification

Figure 7. Drainage System in Wetland Design

Wetlands are likely to form where landforms direct surface water to shallow

basins and where a relatively impermeable subsurface layer prevents the surface water

from seeping into the ground. These conditions can be created to construct a wetland. A

wetland can be built almost anywhere in the landscape by shaping the land surface to

collect surface water and by sealing the basin to retain the water.

Hydrology is the most important design factor in constructed wetlands because

it links all of the functions in a wetland and because it is often the primary factor in the

success or failure of a constructed wetland. While the hydrology of constructed

wetlands is not greatly different than that of other surface and near-surface waters, it

does differ in several important respects:

small changes in hydrology can have fairly significant effects on a wetland and its

treatment effectiveness

because of the large surface area of the water and its shallow depth, a wetland

system interacts strongly with the atmosphere through rainfall and

evapotranspiration (the combined loss of water by evaporation from the water

surface and loss through transpiration by plants)

the density of vegetation of a wetland strongly affects its hydrology, first, by

obstructing flow paths as the water finds its sinuous way through the network of

stems, leaves, roots, and rhizomes and, second, by blocking exposure to wind and

sun.

While wetlands are primarily treatment systems, they provide intangible

benefits by increasing the aesthetics of the site and enhancing the landscape. Visually,

wetlands are unusually rich environments. By introducing the element of water to the

landscape, constructed wetlands, as much as natural wetlands. Add diversity to the

landscape. The complexity of shape, color, size, and interspersion of plants, and the

variety in the sweep and curve of the edges of landforms all add to the aesthetic quality

of the wetlands. Constructed wetlands can be built with curving shapes that follow the

natural contours of the site, and some wetlands for water treatment are

indistinguishable, at first glance, from natural wetlands.

Reference

EPA (Environmental Protection Agency). 1993. Wetland Creation and Restoration:

Status of the Science. Washington, DC.

EPA (Environmental Protection Agency). 1995. A Handbook of Constructed Wetlands.

Washington, DC.

EPA (Environmental Protection Agency). 1999. Manual Constructed Wetlands

Treatment of Municipal Wastewater. Washington, DC.

Juliano, Kristine. and Slobodan P. Simonovic. 1999. The Impact of Wetlands on Flood

Control in the Red River Valley Of Manitoba. Natural Resources Institute University of

Manitoba. Winnipeg, Manitoba.

Percy, David R. 1993. Wetlands and the Law in the Prairie Provinces of Canada.

Alberta Environmental Centre Society. Edmonton, Alberta, Canada. 128p.

Ward, Andy D. and William J. Elliot. 1995. Environmental Hydrology. CRC Press

Inc. Boca Raton, Florida. 462p.

http://www.epa.gov

http://www.helsinki.fi/urbanoases/

http://news.wef.org/

https://www.wetlands-initiative.org

http://whyfiles.org/107flood/