Lesson 8 Dam decommissioning and return of river
Rivers are connected systems , and barriers such as dams, culverts and floodgates disconnect one area from another. They prevent species from migrating – isolating previously connected populations. Human impact on river ecosystems are: pollution, flow modifications, exotic species, harvesting. Pollution is the result of human infrastructure around a river. Pollution enters the river, sometimes in small amounts, at many different locations along the length of the river. The clearing of forests to produce farmland has led to on-going erosion, with large quantities of sediment deposited into rivers. Agricultural intensification (substantial increases in fertiliser application and increased stock numbers) has resulted in nutrient and chemical loss to nearby streams and rivers. Elevated nutrient concentrations (nitrogen and phosphorus – key components of fertilisers) can result in the eutrophication of slow-moving waterways. Pollution can lower the pH of the water , affecting all organisms from algae to vertebrates. Biodiversity decreases with decreasing pH.
Flow modifications • Dams affect river systems by disconnecting different areas. • Water taken from rivers for irrigation can lower river flows • Dams alter the flow, temperature and sediment in river systems. Reduced flow alters aquatic habitats – reducing or removing populations of fish, invertebrates and plants that depend on the flow to bring food. • Reduced flow also decreases tributary stream flow, changing habitats and altering the water table in the stream aquifer. Consequently, riverside vegetation may be affected and decline in numbers. This may affect animal biodiversity, for example, bird species may leave the area if their habitat is lost or altered. • Changes in water temperature due to flow modification can affect insect development by not allowing them to complete their life cycle. • Exotic species have been introduced to river systems sometimes intentionally (for example, for fishing purposes or as food for other species) and sometimes unintentionally (for example, species come in on the bottom of boats or on fishing gear or they escape from pond areas during flooding, such as koi carp).
Urban areas add to this pollution when contaminants (PAHs and heavy metals) are washed off hard surfaces such as roads and drain into water systems. Sulfur dioxide and nitrous oxide emitted from factories and power stations enter river systems through acid rain. Sewage and effluent are discharged into rivers in some areas. Excessive fishing in river ecosystems can drastically reduce numbers of species. Ecological impacts of dam and diversion channels construction, channelization and changes in the river cross-section geometry, may be synthesized as follows: • Significant reduction of habitats diversity, since hydraulic works interfere with the natural continuity of the river physical borders, and make more homogeneous both the river bed and the flooded areas. • Reduction in the native species diversity, caused by the disappearance of the species that de- pend on seasonal flow differences, and spatial habitat variations to survive. Simultaneously, the exotic species gain a competitive advantage due to the before mentioned alterations.
River training ’ refers to the structural measures which are taken to improve a river and its banks. It is an important component in the prevention and mitigation of flash floods and flood control, as well as ensures safe passage of a flood. It reduces sediment transportation and minimize bed and bank erosion. River training structures are implemented with bioengineering techniques to lessen the negative effects on environment and landscape Check dams are used along river courses to control erosion. Gabions, concrete, logs, bamboo, and many other materials are used. They decrease the morphological gradient of the torrent bed and reduce the water velocity during a flood event by increasing the time of concentration and reducing the flood peak and solid transportation capacity of the river. Spur, spur dyke, or groyne is a structure made to project flow from a river bank into a stream or river with the aim of deflecting the flow away from the side. They are placed in series along straight or convex bank lines where the flow lines are roughly parallel to the bank
Screen dams and beam dams are sediment retention structures, designed to trap medium to large size debris and boulders carried downstream in flood events. This type of dam is often installed in alluvial fans, along stretches with a steep slope, in wooded areas, in areas with frequent mass movements, and along narrow channel beds at the end of a valley just before the stream or river enters an alluvial fan or plains area. Porcupines like structure is used as a pro-siltation protection device for a natural river bank. They are flexible, which ensures stability against extreme water forces and even earthquakes. Porcupines reduce the flow velocity, intercept and break eddies formed by floodwater, and fill up scour holes with silt. Levees or earth fill embankments are dam-like earthen structures constructed along a river in order to protect the surrounding countryside from flooding and/or to confine the course of a river to provide higher and faster water flow. They are usually constructed for long stretches along a river in low lying areas with an extended floodplain
Guide banks are constructed to: • Confine the flow to a single channel • Improve the distribution of discharge across the width of a river thus controlling the angle of attack by a flash flood, protect weirs, barrages, or other hydraulic structures constructed in the river such as intakes from flash floods • Control the meander pattern of a river • Control overtopping of natural embankments in a flash flood and protect adjacent land from flooding, reduce erosion of banks by the water current, prevent sliding of soil as a result of the draw down effect of the flood water level, facilitate smooth transportation of water, and prevent piping of water through the banks. Channel lining is a protective layer used to protect the banks and bed of a watercourse against erosion. Channel lining can help increase the velocity of flow to ensure easy transport of sediment and reduce deposition in the channel bed. It is recommended in catchments highly prone to erosion, particularly in urban and alluvial fan reaches. Bamboo can be used in the form of piles to strengthen a foundation or stabilize a flood embankment or river bed. The rows of bamboo piles should be firmly fixed with a rope or iron wire. Piling in wet soil is very easy but may otherwise require more strength. It may be necessary to excavate small holes in boulder covered parts of the river bed. Two parallel rows of piles can be prepared and the space between them filled with boulders and pebbles as a toe protection measure for flood embankments (
Reservoirs are meant to absorb a part of flood water and the excess is discharged through a spillway. It is also essential to study the relation between flood discharge, reservoirs capacity and spillway size in order to propose an economic solution to the whole project. Two main categories: (a) Impounding reservoirs into which a river flows naturally, and (b) Service or balancing reservoirs receiving supplies that are pumped or channelled into them artificially. Multi Purpose reservoirs • Human consumption and/or industrial use: • Irrigation: usually to supplement insufficient rainfall. • Hydropower: to generate power and energy whenever water is available or to provide reliable supplies of power and energy at all times when needed to meet demand. • Pumped storage hydropower schemes: water flows from an upper to a lower reservoir for generating power at times of high demand through turbines. Water is pumped back to the upper reservoir when surplus energy is available. The cycle is usually daily or twice a day to meet peak demands. • Flood control: storage capacity is required to be maintained to absorb foreseeable flood inflows to the reservoirs, so far as they would cause excess of acceptable discharge spillway opening. Storage allows future use of the flood water retained. • Amenity use: this may include provision for boating, water sports, fishing, sight seeing.
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