Static Gates & Wetlands Analysis 1 Table of Contents 1. Static Levees – Page 3 2. Tetrapods – Page 8 3. Wetlands/Marshes – Page 10 2 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. 3 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. 4 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 5 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) 6 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 7 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. 8 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 9 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 10 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: 11 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 12 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 13 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 14 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/ 15 http://water.epa.gov/type/wetlands/what.cfm http://el.erdc.usace.army.mil/wetlands/ 16