CIVE 633 - ENVIRONMENTAL HYDROLOGY
WETLANDS: LANDFORM AND OCCURRENCE

Wetlands lie on a continuum between dry lands (uplands) and deeply flooded lands (aquatic systems).

There is no absolute hydrologic demarcation between these ecosystems, and all definitions (of wetland) are somewhat arbitrary.

The most consistent attribute of wetlands in the presence of water during some or all of an average annual period.

Wetlands are areas where the soil is saturated with water or where shallow standing water results in the absence of plant species which depend on aerobic soil conditions.

Wetlands are dominated by plant species that are adapted to growing in seasonally or continuously flooded soils with resulting anaerobic or low-oxygen conditions.
Wetlands remain flooded or saturated for more than 7 to 30 days per year.
Wetlands grade to aquatic ecosystems which are flooded to a depth or at a duration where emergent, rooted plants cannot survive.
The average depth of water which typically separates wetlands from adjacent aquatic ecosystems is in the range of 1 to 2 m.

WETLAND CLASSIFICATION

Wetlands can be classified in terms of salinity:

Freshwater wetlands, with salinities less than 1000 mg/L.

Saltwater wetlands, with salinities greater than 1000 mg/L.

Natural saltwater wetlands can be classified in terms of the type of vegetation:

Salt marsh, dominated by emergent herbaceous plant species.

Mangrove, dominated by woody plant species.

Natural freshwater wetlands can be classified in terms of the type of vegetation:

Freshwater marsh, dominated by emergent herbaceous plant species adapted to various flooding patterns.

Freshwater swamp, dominated by woody plant species adapted to flooding.

ANATOMY OF A WETLAND

Figure 4.3 shows typical components of a wetland system. These are:

Underlying strata, unaltered organic, mineral or lithic strata, typically saturated or impervious, below the rooted zone.
Hydric soils, containing roots, rhizomes, tubers, tunnels, burrows, and other connections to the surface system.
Detritus, the accumulation of live and dead organic material.
Seasonally flooded zone, providing habitat for aquatic organisms, algae and microbes.
Emergent vegetation, vascular, rooted plant species which contain structural components that emerge above the water, including both herbaceous and woody plants species.

Technically, wetlands occur when one or more of these components occur.

CONSTRUCTED WETLANDS

The purposeful construction of wetland ecosystems is a relatively new technology.
Constructed wetlands are systems that are built expressly for water quality treatment.
Wetlands are engineered and constructed for the following reasons:

To compensate for and help offset the loss of natural wetlands to development (habitat wetland).
To improve water quality (treatment wetland).
To provide flood control (flood control wetland)
To produce food and fiber (aquaculture wetland).

CONSTRUCTED TREATMENT WETLANDS

Constructed treatment wetlands have a water quality objective.
Wastewaters to be treated can be municipal, industrial, and agricultural, and also stormwater.
Treatment wetlands are built at, above, or below the existing land surface.
Earthwork is primarily fills to create berms or levees.
A low cost design will attempt to balance cut and fill.
Liners or relatively impervious site soils are critical to the success of treatment wetlands in areas where groundwater levels are well below the ground surface.
An unlined constructed treatment wetland can become dessicated and unable to support wetland vegetation.
Soils of low permeability or groundwater at or close to the ground surface will avoid the need for liners.
To protect groundwater quality, liners will be necessary.
Liners can be made of clay, clay-bentonite mixtures, poly-vinyl-chloride (PVC), or high-density polyethylene (HDPE).
Wetland plants require an adequate rooting medium.
Rooting substrate must allow the development of ample roots for stability and nutrition of the mature plants.
Most wetland plants will grow slowly or die in dense clay soils or angular rocks.
A loamy or sandy topsoil layer is ideal for propagation of most wetland plant species.
The Army Corps of Engineers, which implements Section 404 of the Clean Water Act, has excluded treatment ponds and lagoons from its definition of regulated waters of the U.S.
This policy has generally been followed for wetlands.
However, in cases where treatment ponds and constructed wetlands have been abandoned and have developed significant environmental and wetland functions, the Corps has ruled that these areas have become waters of the U.S.

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