Urban wetlands :Impact and Overview

wetlands-indiaWetlands are defined as transitional between terrestrial and aquatic eco-systems where the water table is usually at or near the surface or the land is covered by shallow water .

Wetlands have been categorized both as biomes and ecosystems. A patch of land that develops pools of water after a rain storm would not be considered a “wetland” though the land is wet. Wetlands have unique characteristics: they are generally distinguished from other water bodies or landforms based on their water level and on the types of plants that thrive within them. Specifically, wetlands are characterized as having a water table that stands at or near the land surface for a long enough season each year to support aquatic plants. Put simply, wetlands are lands made up of hydric soil.

Wetlands have also been described as ecotones, providing a transition between dry landbio at wetlands and water bodies. Mitsch and Gosselink write that wetlands exist “…at the interface between truly terrestrial ecosystems and aquatic systems, making them inherently different from each other, yet highly dependent on both.”

 An ecotone is a transition area between two adjacent but different patches of landscape, such as forest and grassland. It may be narrow or wide, and it may be local (the zone between a field and forest) or regional (the transition between forest and grassland ecosystems).An ecotone may appear on the ground as a gradual blending of the two communities across a broad area, or it may manifest itself as a sharp boundary line.

The word ecotone was coined from a combination of eco(logy) plus -tone, from the Greek tonos or tension – in other words, a place where ecologies are in tension.

Urban wetlands are places within city limits where water and soils mingle. They range from remnants of water bodies that once existed on sites where buildings now stand, to drainage ditches resulting from inadequate development.

The filling of wetlands to create solid ground for development was one established urban trend in past and to some extent till date. Unintentional creation of “accidental” wetlands is an interesting trend of urban phenomenon. This is often the result of increased stormwater runoff and snow melt collecting in depressions on the surface.

Urbanization of areas that surrounds a wetland has serious consequences for the local ecosystem. Wetland areas are desirable for development for many reasons. Even if the wetland area is not immediately impacted by urbanization, any development in upstream areas, and in some instances downstream areas, can negatively impact the wetland’s functions. Wetlands provide many ecological benefits such as pollutant removal, flood attenuation, ground water discharge, shoreline protection, recreational areas, and support of natural resources. The cost of replacing a natural wetland is greater than the benefits gained by developing it. To replace the functions of natural wetlands, cities need to build and maintain complex storm water drainage and treatment plants, green spaces, and flood control structures. The impact or loss of habitats of flora and fauna is often not addressed at all, and some original functionality of the wetland such as effective ground water recharge is lost forever without rehabilitating the original system. Some communities have begun to take a proactive environmental approach to issues such as storm water runoff management by creating artificial wetlands in their communities.

Impacts of Urbanization

Wetland impacts can be direct or indirect. Direct impacts include the development of the wetlands for other purposes which often involve dredging, filling, and draining the area. Indirect impacts originate outside of the wetlands through the alteration of the hydrologic system. Hydrologic changes through land development compound as vegetation is removed that intercepts rainfall, soil is compacted, impervious surfaces are created, and drainage systems are installed. This creates hydrologic stressors of increased ponding, increased water level fluxuation, flow constrictions, decreased groundwater discharge, and hydrologic drought. Ponding alters the ecosystem and paves the way for invasive species that can more rapidly adapt to the new conditions. Water level fluxuation, mostly a result of impeded buffering of storm runoff, creates acute flood hazards and also greatly impacts the stability of water ways and emergent habitat. Flow constrictions at crossing points (viaducts and bridges) can fill with sediment, reducing the flow volumes across the barrier, but also limiting communication between upstream and downstream areas for wildlife, many of which are bound to the boundaries of the flow path. Impervious structures and storm sewer systems combine to reduce ground water discharge. Hydrologic drought is brought by channel deepening which occurs during episodic high discharge events. As the channel cuts down it lowers the local water table and the extent of the lowering can keep tap roots from accessing the groundwater.

Economic Value

Development of wetlands is often determined by an economic benefit.

Value considerations for wetlands are complex and contrary to contemporary economic thinking. If value considerations are in the short term then the wetland functions rarely exceed the value of economic development.

Scale: value of the wetlands versus development shifts over longer time scales. Typically development for industry or agriculture will exhaust the land to the point that maintaining wetland functions are more expensive to leaving the land undeveloped. Residential development brings a short term benefit from construction and if significant adversity is experienced by the environment due to the loss of wetland function then flooding reduces property values or flood control exceeds property values.

Marginal Value Paradox: Typically economic evaluations place a greater value on scarce resources. True to this thought, as wetlands are reduced their social value increases. However if the wetlands is reduced to a non-sustainable portion its function is negated completely. Common examples would be thin remnants of wetlands overloaded with pollution or flood waters and incapable to hold the water in storage for filtering and delayed release.

Hydrogeologic Principle: the value of the system is based on relationship and location. Wetlands along a stream likely have a greater benefit for downstream environments than a recreated wetland that is isolated from the stream as a result of development. Aside from position, the landscape of the between the natural wetland and a potential created wetland matter, engineered wetlands designed to replace a developed wetland would need intense study to determine the effectiveness if located substantially up or down stream or on steep versus flat terrain. Steep wetlands are typically groundwater discharge areas and flat wetlands are ground water recharge areas.

While wetlands may have greater value per acre compared to grassland, forest, or other land cover, it is not always practical to replace those systems with engineered wetlands. Different than the hydrogeologic principle that assumes a successful long term conversion, the ecosystem may reject an artificial wetland system by not supporting it with necessary surface and groundwater inputs or erosion and wildlife may not be able to develop in the new location.

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About Rashid Faridi

I am Rashid Aziz Faridi ,Writer, Teacher and a Voracious Reader.
This entry was posted in Cities, Urban Studies, water. Bookmark the permalink.

7 Responses to Urban wetlands :Impact and Overview

  1. lenrosen4 says:

    In most coastal cities planning climate change actions wetland strategies are important. When I wrote my blog posting yesterday I described what New York City has planned. See: http://www.21stcentech.com/york-city-preparing-consequences-climate-change/

    Happy New Year, Rashid.



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