Static Gates & Wetlands Analysis
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Table of Contents
1. Static Levees – Page 3
2. Tetrapods – Page 8
3. Wetlands/Marshes – Page 10
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Static Levees
Introduction
A levee (or dike) is an elongated naturally occurring ridge or artificially
constructed fill or wall, which regulates water levels. Levees are prominent systems for river
flood and coastal flood prevention to protect low-lying lands (e.g. levee systems along the
Mississippi River to protect the city of New Orleans). It is usually earthen and often parallel to
the course of a river in its floodplain or along low-lying coastlines.
Levee alongside the Mississippi River
Purpose
The main purposes of an artificial levee are to prevent flooding of the
adjoining countryside and to slow natural course changes in a waterway to provide reliable
shipping lanes for maritime commerce over time; they also confine the flow of the river,
resulting in higher and faster water flow. Levees can be mainly found along the sea, where dunes
are not strong enough, along rivers for protection against high-floods, along lakes or
along polders. Furthermore, levees have been built for the purpose of empoldering, or as a
boundary for an inundation area. The latter can be a controlled inundation by the military or a
measure to prevent inundation of a larger area surrounded by levees. Levees have also been built
as field boundaries and as military defenses.
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Construction & Materials
Levees are usually built by piling earth on a cleared, level surface. They are broad at the
base, but taper to a level top, where temporary embankments or sandbags can be placed. Because
flood discharge intensity increases in levees on both river banks, and because silt deposits raise
the level of riverbeds, planning and auxiliary measures are vital. Sections are often set back from
the river to form a wider channel, and flood valley basins are divided by multiple levees to
prevent a single breach from flooding a large area. A levee made from stones laid in horizontal
rows with a bed of thin turf between each of them is known as a spetchel.
Artificial levees require substantial engineering. Their surface must be protected from
erosion, so they are planted with vegetation such as Bermuda grass in order to bind the earth
together. On the land side of high levees, a low terrace of earth known as a banquette is usually
added as another anti-erosion measure. On the river side, erosion from strong waves or currents
presents an even greater threat to the integrity of the levee. The effects of erosion are countered
by planting suitable vegetation or installing stones, boulders, weighted matting or
concrete revetments. Separate ditches or drainage tiles are constructed to ensure that the
foundation does not become waterlogged.
Since the 1940s, slurry cutoff walls have been used as seepage barriers to limit the
horizontal flow of water and provide added stability for dams and the foundations on which
dams rest. The two most common types of non-structural slurry walls are referred to as soilbentonite and cement-bentonite. For several decades, slurry cutoff walls have been used to
control seepage both through and under dams and levees. It has also been used as an effective
barrier to reduce leakage from ponds. In early 1980s, U.S. Environmental Protection Agency
began approving slurry cutoff walls in waste management applications to control flow of
groundwater at contaminated sites. With the recent heightened awareness of many inadequate
and high-hazard levees, the technology is often considered for levee rehabilitation.
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Slurry wall used to control seepage through and under the levee
Failure
Both natural and man-made levees can fail in a number of ways. The most frequent (and
dangerous) is a levee breach. A levee breach is when part of the levee actually breaks away,
leaving a large opening for water to flood the land protected by the levee. A breach can be a
sudden or gradual failure that is caused either by surface erosion or by a subsurface failure of the
levee. Levee breaches are often accompanied by levee boils, or sand boils. The under-seepage
resurfaces on the landside, in the form of a volcano-like cone of sand. Boils signal a condition of
incipient instability which may lead to erosion of the levee toe or foundation or result in sinking
of the levee into the liquefied foundation below. Some engineers think that boils lead to a form
of internal erosion called piping which undermines the levee. Others consider boils as a symptom
of generalized instability of the foundation.
Surface erosion of the surface of a levee is usually caused by the action of wind and
water (waves but also normal flow). Erosion can be worsened by pre-existing or new damage to
a levee. Areas with no surface protection are more prone to erosion.
Trees in levees are a special risk. A tree can become unstable after the soil of the levee
has become saturated with water. When the tree falls the root system will likely take a chunk of
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the saturated soil out of the levee. This shallow hole can quickly erode and result in a breach. If
the tree falls in the water and floats away it can damage the levee further downstream. Floating
trees near levees should be quickly removed by the agency responsible for the maintenance of
the levee.
Other forms of damage can be caused by ships or other (large) floating objects or from
objects in the levee, like traffic signs or fences that are damaged or completely removed by wind
or water. Barbed wire fences can collect large amounts of floating plant material, resulting in a
large amount of drag from the water. Whole fences can be dragged away by the water.
Sometimes levees are said to fail when water overtops the crest of the levee. Levee
overtopping can be caused when flood waters simply exceed the lowest crest of the levee system
or if high winds begin to generate significant swells (a storm surge) in the ocean or river water to
bring waves crashing over the levee. Overtopping can lead to significant landside erosion of the
levee or even be the mechanism for complete breach. Often levees are armored or reinforced
with rocks or concrete to prevent erosion and failure.
The many ways that a levee can fail (specifically during Hurricanes Katrina and Gustav)
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In certain cases, levees are breached intentionally. This can be done to protect other areas
or to give back land to nature. In most cases, an intentional breach is not without discussion since
valuable land is given up. During the Great Mississippi Flood of 1927, a levee was blown up
with dynamite to prevent the flooding of New Orleans.
Taking land from the cycle of flooding by putting a dike around it prevents it from being
raised by silt left behind after a flooding. At the same time, the drained soil consolidates and peat
decomposes leading to land subsidence. In this way, the difference between the water level on
one side and land level on the other side of the dike grew. In some areas reclaimed land is given
back to nature by breaching and removing dikes to allow flooding to occur (again). This restores
the natural environment in the area.
References
http://www.mcclatchydc.com/2008/09/01/51424/the-levees-held-thanks-to-reconstruction.html
http://www.cement.org/water/dams_sc_faqs_slurry.asp
http://science.howstuffworks.com/engineering/structural/levee.htm
http://en.wikipedia.org/wiki/Levee
http://en.wikipedia.org/wiki/Levee_breach
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Tetrapods
Introduction
The word “Tetrapod" (taken from Greek) means "four-legged" — hence in English it means
"four-legged animal”. In coastal engineering, a Tetrapod is a four-legged concrete structure
intended to prevent coastal erosion. However, the term is now used to refer to any of the concrete
blocks that come in a variety of configurations, with three to eight legs. Use of Tetrapods to
prevents coastal erosion falls under the category of "Hard Stabilization" Technique.
Materials and Cost
Depending on the design of the tetrapods and the number of interlocked layers the values
may be 5 m to 15 m USD/km. Tetrapods are usually produced in high quality mass concrete
which is adequate for their purpose. Only in very rare cases there is a reinforcement required
only to cater for special handling procedures. At a later stage reinforcement might mean
corrosion problems and a reduced life expectancy. As a consequence the shearing strength of
unreinforced tetrapods is limited.
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How Tetrapods Work
The Tetrapod's shape is designed to dissipate the force of incoming waves by allowing water
to flow around rather than against it and to reduce displacement by allowing a random
distribution of Tetrapods to mutually interlock. Tetrapods were designed to remain stable under
even the most extreme weather and marine conditions, and when arranged together in lines or
heaps, they create an interlocking, porous barrier that dissipates the power of waves and currents.
Earlier barrier material used in breakwaters, such as boulders and conventional concrete
blocks, tended to become dislodged over time by the force of the ocean constantly crashing
against them. Tetrapods and similar structures are often numbered so any displacement that
occurs can be monitored through satellite photos.
Disadvantages
Tetrapods have been shown to offer little advantage compared to other concrete armour
units. Layering and the amount of tetrapods have shown to have no appreciable effect on the
stability of the tetrapod structures although stability increases with the size of the tetrapod pad.
Tetrapods have also been criticized for causing more damage than they prevent because they
alter ocean currents and disrupt the natural cycles of erosion and deposition that form and
reshape coasts. Concrete coastal installations can also be lethally dangerous to swimmers and
surfers, as well to shipping and recreational boaters.
New shore protection innovations have shown that there are better methods that are less
obtrusive and more environmentally friendly than concrete armor structures. Alternate
placements of sand and mix sediments and modifications of incident wave conditions through
the use of reefs are examples of those.
References
http://en.wikipedia.org/wiki/Tetrapod_(structure)
http://www.concretebasics.org/clctetrapods.html
http://www.imia.com/downloads/interesting_claims/claim_car_2008_28.pdf
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Wetlands
Definition
A wetland is a land area that is saturated with water, either permanently or seasonally, such that
it takes on the characteristics of a distinct ecosystem. Primarily, the factor that distinguishes
wetlands from other land forms or water bodies is the characteristic vegetation that is adapted to
its unique soil conditions: Wetlands consist primarily of hydric soil, which supports aquatic
plants.
Peat bogs are freshwater wetlands that develop in areas with standing water and low fertility.
For regulatory purposes under the Clean Water Act, the term wetlands means "those areas that
are inundated or saturated by surface or groundwater at a frequency and duration sufficient to
support, and that under normal circumstances do support, a prevalence of vegetation typically
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adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs
and similar areas."
Generally, wetlands are lands where saturation with water is the dominant factor determining the
nature of soil development and the types of plant and animal communities living in the soil and
on its surface Wetlands vary widely because of regional and local differences in soils,
topography, climate, hydrology, water chemistry, vegetation, and other factors, including human
disturbance. Indeed, wetlands are found from the tundra to the tropics and on every continent
except Antarctica.
The water found in wetlands can be saltwater, freshwater, or brackish. Main wetland types
include swamps, marshes, bogs and fens. Sub-types include mangrove, carr, pocossin, and
varzea.
Wetlands play a number of roles in the environment, principally water purification, flood control,
and shoreline stability. Wetlands are also considered the most biologically diverse of all
ecosystems, serving as home to a wide range of plant and animal life.
Wetlands occur naturally on every continent except Antarctica. They can also be constructed
artificially as a water management tool, which may play a role in the developing field of watersensitive urban design.
The largest wetlands in the world include the Amazon River basin and the West Siberian Plain.
Another large wetland is the Pantanal, which straddles Brazil, Bolivia, and Paraguay in South
America.
Ecosystem services
The function of natural wetlands can be classified by their ecosystem benefits. The United
Nations Millennium Ecosystem Assessment and Ramsar Convention found wetlands to be of
biosphere significance and societal importance in the following areas:
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Flood control, Groundwater replenishment, Shoreline stabilization and storm protection, Water
purification, Reservoirs of biodiversity, Wetland products, Cultural values, Recreation and
tourism, Climate change mitigation and adaptation.
The economic worth of the ecosystem services provided to society by intact, naturally
functioning wetlands is frequently much greater than the perceived benefits of converting them
to ‘more valuable’ intensive land use – particularly as the profits from unsustainable use often go
to relatively few individuals or corporations, rather than being shared by society as a whole.Ramsar convention
Flood control
Major wetland type: floodplain
Storage Reservoirs and Flood Protection. The wetland system of floodplains is formed from
major rivers downstream from their headwaters. Notable river systems that produce large spans
of floodplain include the Nile River (Africa), Mississippi River (USA), Amazon River (South
America), Yangtze River (China), Danube River (Central Europe) and Murray-Darling River
(Australia). "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.
Human-Impact. Converting wetlands through drainage and development have contributed to
the issue of irregular flood control through forced adaption of water channels to narrower
corridors due to loss of wetland area. These new channels must manage the same amount of
precipitation causing flood peaks to be [higher or deeper] and floodwaters to travel faster.
Water management engineering developments in the past century have degraded these wetlands
through the construction on artificial embankments. These constructions may be classified as
dykes, bunds, levees, weirs, barrages and dams but serve the single purpose of concentrating
water into a select source or area. Wetland water sources that were once spread slowly over a
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large, shallow area are pooled into deep, concentrated locations. Loss of wetland floodplains
results in more severe and damaging flooding. Catastrophic human impact in the Mississippi
River floodplains was seen in death of several hundred individuals during a levee breach in New
Orleans caused by Hurricane Katrina. Ecological catastrophic events from human-made
embankments have been noticed along the Yangtze River floodplains after the where the middle
of the river has become prone to more frequent and damaging flooding including the loss of
riparian vegetation, a 30% loss of the vegetation cover throughout the river’s basin, a doubling of
the percentage of the land affected by soil erosion, and a reduction in reservoir capacity through
siltation build-up in floodplain lakes.
Groundwater replenishment
The surface water which is the water visibly seen in wetland systems only represents a portion of
the overall water cycle which also includes atmospheric water and groundwater. 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. Karst (cave) systems are a unique
example of this system and are a connection of underground rivers influenced by rain and other
forms of precipitation. These wetland systems are capable of regulating changes in the water
table on upwards of 130 metres (430 ft).
Human-Impact. Groundwater is an important source of water for drinking and irrigation of
crops. Over 1 billion people in Asia and 65% of the public water sources in Europe source 100%
of their water from groundwater. Irrigation is a massive use of groundwater with 80% of the
world’s groundwater used for agricultural production.
Unsustainable abstraction of groundwater has become a major concern. In the Commonwealth of
Australia, water licensing is being implemented to control use of the water in major agricultural
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regions. On a global scale, groundwater deficits and water scarcity is one of the most pressing
concerns facing the 21st century.
Shoreline stabilization and storm protection
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.
Human-Impact. The sheer number of people who live and work near the coast is expected to
grow immensely over the next 50 years. From an estimated 200 million people that currently live
in low-lying coastal regions, the development of urban coastal centers is projected to increase the
population by 5 fold within 50 years. The United Kingdom has begun the concept of managed
coastal realignment. This management technique provides shoreline protection through
restoration of natural wetlands rather than through applied engineering.
Water purification
Wetland Type: Floodplain, Mudflat, Saltmarsh, Mangroves
Nutrient Retention. Wetlands cycle both sediments and nutrients balancing terrestrial and
aquatic ecosystems. A natural function of wetland vegetation is the up-take and storage of
nutrients found in the surrounding soil and water. These nutrients are retained in the system until
the plant dies or is harvested by animals or humans. Wetland vegetation productivity is linked to
the climate, wetland type, and nutrient availability. The grasses of fertile floodplains such as the
Nile produce the highest yield including plants such as giant reed, papyrus, and reed.
Sediment Traps. Rainfall run-off is responsible for moving sediment through waterways. These
sediments move towards larger and more sizable waterways through a natural process that moves
water towards oceans. All types of sediments which may be composed of clay, sand, silt, and
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rock can be carried into wetland systems through this process. Reed beds or forests located in
wetlands act as physical barriers to slow water flow and trap sediment.
Water purification. Many wetland systems possess bio filters, hydrophytes, and organisms that
in addition to nutrient up-take abilities have the capacity to remove toxic substances that have
come from pesticides, industrial discharges, and mining activities. The up-take occurs through
most parts of the plant including the stems, roots, and leaves . Floating plants can absorb and
filter heavy metals water hyacinth, duckweed and water fern store iron and copper commonly
found in wastewater. Many fast-growing plants rooted in the soils of wetlands such as cattail and
reed also aid in the role of heavy metal up-take. Animals such as the oyster can filter more than
200 liters (53 gallons) of water per day while grazing for food, removing nutrients, suspended
sediments, and chemical contaminants in the process.
Capacity. The ability of wetland systems to store nutrients and trap sediment is highly efficient
and effective but each system has a threshold. An overabundance of nutrient input from fertilizer
run-off, sewage effluent, or non-point pollution will cause eutrophication. Upstream erosion
from deforestation can overwhelm wetlands making them shrink in size and see dramatic
biodiversity loss through excessive sedimentation load. The capacity of wetland vegetation to
store heavy metals is affected by waterflow, number of hectares (acres), climate, and type of
plant.
Human-Impact. Introduced hydrophytes in different wetland systems can have devastating
results. The introduction of water hyacinth, a native plant of South America into Lake Victoria in
East Africa as well as duckweed into non-native areas of Queensland, Australia, have overtaken
entire wetland systems suffocating the ecosystem due to their phenomenal growth rate and
ability to float and grow on the surface of the water.
References:
http://en.wikipedia.org/wiki/Wetland
http://water.epa.gov/lawsregs/guidance/wetlands/definitions.cfm
http://wetlands.fws.gov/
http://www.wetlands.org/
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http://water.epa.gov/type/wetlands/what.cfm
http://el.erdc.usace.army.mil/wetlands/
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