Misplaced Pages

#798201

95-502: Weir can also refer to the skimmer found in most in-ground swimming pools, which controls the flow of water pulled into the filtering system. The word likely originated from Middle English were , Old English wer , a derivative of the root of the verb werian, meaning "to defend, dam". The German cognate is Wehr , which means the same as English weir. Commonly, weirs are used to prevent flooding , measure water discharge, and help render rivers more navigable by boat. In some locations,

190-449: A dam , landslide, or glacier . In one instance, a flash flood killed eight people enjoying the water on a Sunday afternoon at a popular waterfall in a narrow canyon. Without any observed rainfall, the flow rate increased from about 50 to 1,500 cubic feet per second (1.4 to 42 m /s) in just one minute. Two larger floods occurred at the same site within a week, but no one was at the waterfall on those days. The deadly flood resulted from

285-646: A flash flood . Flash floods usually result from intense rainfall over a relatively small area, or if the area was already saturated from previous precipitation. The amount, location, and timing of water reaching a drainage channel from natural precipitation and controlled or uncontrolled reservoir releases determines the flow at downstream locations. Some precipitation evaporates, some slowly percolates through soil, some may be temporarily sequestered as snow or ice, and some may produce rapid runoff from surfaces including rock, pavement, roofs, and saturated or frozen ground. The fraction of incident precipitation promptly reaching

380-515: A landslide , earthquake or volcanic eruption . Examples include outburst floods and lahars . Tsunamis can cause catastrophic coastal flooding , most commonly resulting from undersea earthquakes. The primary effects of flooding include loss of life and damage to buildings and other structures, including bridges, sewerage systems, roadways, and canals. The economic impacts caused by flooding can be severe. Every year flooding causes countries billions of dollars worth of damage that threatens

475-447: A pond or lake and thus reduce thermal stratification . Once circulated water reaches the surface, the air-water interface facilitates the transfer of oxygen to the lake water. Natural resource and environmental managers have long been challenged by problems caused by thermal stratification of lakes. Fish die-offs have been directly associated with thermal gradients, stagnation, and ice cover. Excessive growth of plankton may limit

570-484: A tropical cyclone or an extratropical cyclone , falls within this category. A storm surge is "an additional rise of water generated by a storm, over and above the predicted astronomical tides". Due to the effects of climate change (e.g. sea level rise and an increase in extreme weather events) and an increase in the population living in coastal areas, the damage caused by coastal flood events has intensified and more people are being affected. Flooding in estuaries

665-410: A 74-ha oxic zone. It was upgraded in 2019 to fine-bubble injectors to provide more oxygen directly. Many water treatment processes use a variety of forms of aeration to support biological oxidative processes. A typical example is activated sludge which can use fine or coarse bubble aeration or mechanical aeration cones which draw up mixed liquor from the base of a treatment tank and eject it through

760-468: A country can be lost in extreme flood circumstances. Some tree species may not survive prolonged flooding of their root systems. Flooding in areas where people live also has significant economic implications for affected neighborhoods. In the United States , industry experts estimate that wet basements can lower property values by 10–25 percent and are cited among the top reasons for not purchasing

855-538: A drainage channel has been observed from nil for light rain on dry, level ground to as high as 170 percent for warm rain on accumulated snow. Most precipitation records are based on a measured depth of water received within a fixed time interval. Frequency of a precipitation threshold of interest may be determined from the number of measurements exceeding that threshold value within the total time period for which observations are available. Individual data points are converted to intensity by dividing each measured depth by

950-627: A few minutes for roof and parking lot drainage structures, while cumulative rainfall over several days would be critical for river basins. Water flowing downhill ultimately encounters downstream conditions slowing movement. The final limitation in coastal flooding lands is often the ocean or some coastal flooding bars which form natural lakes . In flooding low lands, elevation changes such as tidal fluctuations are significant determinants of coastal and estuarine flooding. Less predictable events like tsunamis and storm surges may also cause elevation changes in large bodies of water. Elevation of flowing water

1045-605: A few. Gastrointestinal disease and diarrheal diseases are very common due to a lack of clean water during a flood. Most of clean water supplies are contaminated when flooding occurs. Hepatitis A and E are common because of the lack of sanitation in the water and in living quarters depending on where the flood is and how prepared the community is for a flood. When floods hit, people lose nearly all their crops, livestock, and food reserves and face starvation. Floods also frequently damage power transmission and sometimes power generation , which then has knock-on effects caused by

SECTION 10

#1732779736799

1140-601: A fishing lake. Aeration can be achieved through the infusion of air into the bottom of the lake , lagoon or pond or by surface agitation from a fountain or spray-like device to allow for oxygen exchange at the surface and the release of gasses such as carbon dioxide , methane or hydrogen sulfide . Decreased levels of dissolved oxygen (DO) is a major contributor to poor water quality. Not only do fish and most other aquatic animals need oxygen, aerobic bacteria help decompose organic matter. When oxygen concentrations become low, anoxic conditions may develop which can decrease

1235-432: A greater oxygen transfer rate. One of the drawbacks to fine bubble aeration is that the membranes of ceramic diffusers can sometimes clog and must be cleaned in order to keep them working at their optimum efficiency. Also, they do not possess the ability to mix the water column as well as other aeration techniques, such as coarse bubble aeration. ( See also Lake de-stratification ) Circulators are commonly used to mix

1330-728: A home. According to the U.S. Federal Emergency Management Agency (FEMA), almost 40 percent of small businesses never reopen their doors following a flooding disaster. In the United States, insurance is available against flood damage to both homes and businesses. Economic hardship due to a temporary decline in tourism, rebuilding costs, or food shortages leading to price increases is a common after-effect of severe flooding. The impact on those affected may cause psychological damage to those affected, in particular where deaths, serious injuries and loss of property occur. Fatalities connected directly to floods are usually caused by drowning ;

1425-408: A large surface area through which oxygen can be transferred. Upon return, these droplets mix with the rest of the water and thus transfer their oxygen back to the ecosystem. Fountains are a popular method of providing surface aeration because of the aesthetic appearance that they offer. However, most fountains are unable to produce a large area of oxygenated water. Also, running electricity through

1520-432: A mixing of the water. In comparison to other aeration techniques, coarse bubble aeration is very inefficient in the way of transferring oxygen. This is due to the large diameter and relatively small collective surface area of its bubbles. Fine bubble aeration is an efficient way to transfer oxygen to a water body. A compressor on shore pumps air through a hose, which is connected to an underwater aeration unit. Attached to

1615-486: A more distant point controls at higher water levels. Effective flood channel geometry may be changed by growth of vegetation, accumulation of ice or debris, or construction of bridges, buildings, or levees within the flood channel. Periodic floods occur on many rivers, forming a surrounding region known as the flood plain . Even when rainfall is relatively light, the shorelines of lakes and bays can be flooded by severe winds—such as during hurricanes —that blow water into

1710-488: A negative effect on fish species that migrate as part of their breeding cycle (e.g., salmonids ), but it also can be useful as a method of preventing invasive species moving upstream. For example, weirs in the Great Lakes region have helped to prevent invasive sea lamprey from colonising farther upstream. Mill ponds are created by a weir that impounds water that then flows over the structure. The energy created by

1805-414: A range of biota , including poor swimmers. Even though the water around weirs can often appear relatively calm, they can be extremely dangerous places to boat, swim, or wade, as the circulation patterns on the downstream side—typically called a hydraulic jump —can submerge a person indefinitely. This phenomenon is so well known to canoeists, kayakers, and others who spend time on rivers that they even have

1900-414: A rueful name for weirs: "drowning machines". The Ohio DNR recommends that a victim should "tuck the chin down, draw the knees up to the chest with arms wrapped around them. Hopefully, conditions will be such that the current will push the victim along the bed of the river until swept beyond the boil line and released by the hydraulic." The Pennsylvania State Police also recommends to victims, "curl up, dive to

1995-520: A solution to help improve water quality. Aeration of hypoxic water-bodies seems an appealing solution and it has been tried successfully many times on freshwater ponds and small lakes. However no one has undertaken an aeration project as large as an estuary . A 353-hectare portion of the bay connected to the Rock Creek has been aerated using pipes since 2016. The system started as a large-bubble system intended mainly for de-stratification, creating

SECTION 20

#1732779736799

2090-558: A strong flood . In large water bodies in temperate climates, autumn turn-over can introduce oxygen rich water into the oxygen-poor hypolimnion . The low speed surface aerator is a device for biology aeration with high efficiency. These devices are often in steel protected by epoxy coating and generate high torque. The mixing of water volume is excellent. The common power is going from 1 up to 250kw per unit with an efficiency (SOE) around 2 kgO2/kw. Low speed aerator are used mostly for biology plant aeration for water purification. The higher

2185-410: A surface aerator is limited to a small area as they are unable to add circulation or oxygen to much more than a 3-metre radius. This circulation and oxygenating is then limited to the uppermost portion of the water column, often leaving the bottom portions unaffected. Low speed surface aerators can also be installed on floats. Paddlewheel aerators also utilize air-to-water contact to transfer oxygen from

2280-413: A thunderstorm over part of the drainage basin, where steep, bare rock slopes are common and the thin soil was already saturated. Flash floods are the most common flood type in normally-dry channels in arid zones, known as arroyos in the southwest United States and many other names elsewhere. In that setting, the first flood water to arrive is depleted as it wets the sandy stream bed. The leading edge of

2375-520: A type of hybrid river/areal flooding can occur, known locally as "overland flooding". This is different from "overland flow" defined as "surface runoff". The Red River Valley is a former glacial lakebed, created by Lake Agassiz , and over a length of 550 mi (890 km), the river course drops only 236 ft (72 m), for an average slope of about 5 inches per mile (or 8.2 cm per kilometer). In this very large area, spring snowmelt happens at different rates in different places, and if winter snowfall

2470-415: A very high oxygen transfer efficiency (OTE), sometimes as high as 15 pounds of oxygen / (horsepower * hour) (9.1 kilograms of oxygen / (kilowatt * hour)). On average, diffused air aeration diffuses approximately 2–4 cfm (cubic feet of air per minute) (56.6-113.3 liters of air per minute), but some operate at levels as low as 1 cfm (28.3 L/min) or as high as 10 cfm (283 L/min). Fine bubble diffused aeration

2565-449: A weir is broad-crested for much of its length, but has a section where the weir stops or is 'open' so that small boats and fish can traverse the structure. A notch weir is any weir where the physical barrier is significantly higher than the water level except for a specific notch (often V-shaped) cut into the panel. At times of normal flow all the water must pass through the notch, simplifying flow volume calculations, and at times of flood

2660-405: Is a flat-crested structure, where the water passes over a crest that covers much or all of the channel width. This is one of the most common types of weir found worldwide. A compound weir is any weir that comprises several different designs into one structure. They are commonly seen in locations where a river has multiple users who may need to bypass the structure. A common design would be one where

2755-469: Is a generic relationship and specific calculations are available for the many different types of weir. Flow measurement weirs must be well maintained if they are to remain accurate. The flow over a V-notch weir (in ft/s) is given by the Kindsvater–Shen equation: where As weirs are a physical barrier, they can impede the longitudinal movement of fish and other animals up and down a river. This can have

2850-412: Is able to maximize the surface area of the bubbles and thus transfer more oxygen to the water per bubble volume. Additionally, smaller bubbles take more time to reach the surface so not only is the surface area maximized but so are the time each bubble spends in the water, allowing it more opportunity to transfer oxygen to the water. As a general rule, smaller bubbles and a deeper release point will generate

2945-405: Is also the intentional flooding of land that would otherwise remain dry. This may take place for agricultural, military, or river-management purposes. For example, agricultural flooding may occur in preparing paddy fields for the growing of semi-aquatic rice in many countries. Flooding may occur as an overflow of water from water bodies, such as a river , lake , sea or ocean. In these cases,

Weir - Misplaced Pages Continue

3040-419: Is common when heavy flows move uprooted woody vegetation and flood-damaged structures and vehicles, including boats and railway equipment. Recent field measurements during the 2010–11 Queensland floods showed that any criterion solely based upon the flow velocity, water depth or specific momentum cannot account for the hazards caused by velocity and water depth fluctuations. These considerations ignore further

3135-493: Is commonly caused by a combination of storm surges caused by winds and low barometric pressure and large waves meeting high upstream river flows. The intentional flooding of land that would otherwise remain dry may take place for agricultural, military or river-management purposes. This is a form of hydraulic engineering . Agricultural flooding may occur in preparing paddy fields for the growing of semi-aquatic rice in many countries. Flooding for river management may occur in

3230-400: Is controlled by the geometry of the flow channel and, especially, by depth of channel, speed of flow and amount of sediments in it Flow channel restrictions like bridges and canyons tend to control water elevation above the restriction. The actual control point for any given reach of the drainage may change with changing water elevation, so a closer point may control for lower water levels until

3325-461: Is more significant to flooding within small drainage basins. The most important upslope factor in determining flood magnitude is the land area of the watershed upstream of the area of interest. Rainfall intensity is the second most important factor for watersheds of less than approximately 30 square miles or 80 square kilometres. The main channel slope is the second most important factor for larger watersheds. Channel slope and rainfall intensity become

3420-415: Is pumped from an on-shore air compressor . through a hose to a unit placed at the bottom of the water body. The unit expels coarse bubbles (more than 2mm in diameter), which release oxygen when they come into contact with the water, which also contributes to a mixing of the lake's stratified layers. With the release of large bubbles from the system, a turbulent displacement of water occurs which results in

3515-540: Is pumped, from five sites around the Bay, through a series of steel reinforced rubber pipelines, laid on the beds of the Bay and Rivers Taff and Ely. These are connected to approximately 800 diffusers. At times this is insufficient and the Harbour Authority uses a mobile oxygenation barge built by McTay Marine with liquid oxygen stored in a tank. Liquid oxygen is passed through an electrically heated vapouriser and

3610-562: The Second World War ). Floods are caused by many factors or a combination of any of these generally prolonged heavy rainfall (locally concentrated or throughout a catchment area), highly accelerated snowmelt , severe winds over water, unusual high tides, tsunamis , or failure of dams, levees , retention ponds , or other structures that retained the water. Flooding can be exacerbated by increased amounts of impervious surface or by other natural hazards such as wildfires, which reduce

3705-409: The oxygen saturation of water in both natural and artificial environments. Aeration techniques are commonly used in pond, lake, and reservoir management to address low oxygen levels or algal blooms. Water aeration is often required in water bodies that suffer from hypoxic or anoxic conditions, often caused by upstream human activities such as sewage discharges, agricultural run-off, or over-baiting

3800-562: The water table is shallow, such as a floodplain , or from intense rain from one or a series of storms . Infiltration also is slow to negligible through frozen ground, rock, concrete , paving, or roofs. Areal flooding begins in flat areas like floodplains and in local depressions not connected to a stream channel, because the velocity of overland flow depends on the surface slope. Endorheic basins may experience areal flooding during periods when precipitation exceeds evaporation. Floods occur in all types of river and stream channels, from

3895-470: The ability of the water body to support life. Any procedure by which oxygen is added to water can be considered a type of water aeration. There are many ways to aerate water, but these all fall into two broad areas – surface aeration and subsurface aeration . A variety of techniques and technologies are available for both approaches. Natural aeration is a type of both sub-surface and surface aeration. It can occur through sub-surface aquatic plants. Through

Weir - Misplaced Pages Continue

3990-717: The adverse ecological impact of the inundation. That impact may also be adverse in a hydrogeological sense if the inundation lasts a long time. Examples for uncontrolled inundations are the second Siege of Leiden during the first part of the Eighty Years' War , the flooding of the Yser plain during the First World War , and the Inundation of Walcheren , and the Inundation of the Wieringermeer during

4085-404: The air in the atmosphere to the water body. They are most often used in the aquaculture (rearing aquatic animals or cultivating aquatic plants for food) field. Constructed of a hub with attached paddles, these aerators are usually powered by a tractor power take-off ( PTO ), a gas engine , or an electric motor . They tend to be mounted on floats . Electricity forces the paddles to turn, churning

4180-423: The amount of water damage and mold that grows after an incident. Research suggests that there will be an increase of 30–50% in adverse respiratory health outcomes caused by dampness and mold exposure for those living in coastal and wetland areas. Fungal contamination in homes is associated with increased allergic rhinitis and asthma. Vector borne diseases increase as well due to the increase in still water after

4275-501: The bottom, and swim or crawl downstream". As the hydraulic jump entrains air, the buoyancy of the water between the dam and boil line will be reduced by upward of 30%, and if a victim is unable to float, escape at the base of the dam may be the only option for survival. There are many different types of weirs and they can vary from a simple stone structure that is barely noticeable, to elaborate and very large structures that require extensive management and maintenance. A broad-crested weir

4370-501: The change in height of the water can then be used to power waterwheels and power sawmills, grinding wheels, and other equipment. Weirs are commonly used to control the flow rates of rivers during periods of high discharge. Sluice gates (or in some cases the height of the weir crest) can be altered to increase or decrease the volume of water flowing downstream. Weirs for this purpose are commonly found upstream of towns and villages and can either be automated or manually operated. By slowing

4465-505: The crest of an overflow spillway on a large dam may therefore be referred to as a weir. Weirs can vary in size both horizontally and vertically, with the smallest being only a few centimetres in height whilst the largest may be many metres tall and hundreds of metres long. Some common weir purposes are outlined below. Weirs allow hydrologists and engineers a simple method of measuring the volumetric flow rate in small to medium-sized streams/rivers or in industrial discharge locations. Since

4560-434: The deadliest floods worldwide, showing events with death tolls at or above 100,000 individuals. Floods (in particular more frequent or smaller floods) can also bring many benefits, such as recharging ground water , making soil more fertile and increasing nutrients in some soils. Flood waters provide much needed water resources in arid and semi-arid regions where precipitation can be very unevenly distributed throughout

4655-414: The design of a weir that ensure that fish can bypass the barriers and access upstream habitats. Unlike dams, weirs do not usually prevent downstream fish migration (as water flows over the top and allows fish to bypass the structure in that water), although they can create flow conditions that injure juvenile fish. Recent studies suggest that navigation locks have also potential to provide increased access for

4750-509: The diameter, the higher the SOE and mixing. A fountain consists of a means of squirting the water upwards into the air. Typically this may be done using a motor that powers a rotating impeller . The impeller pumps water from the first few feet of the water and expels it into the air. This process utilizes air-water contact to transfer oxygen. As the water is propelled into the air, it breaks into small droplets. Collectively, these small droplets have

4845-576: The field that is intended to impede the movement of the enemy. This may be done both for offensive and defensive purposes. Furthermore, in so far as the methods used are a form of hydraulic engineering, it may be useful to differentiate between controlled inundations and uncontrolled ones. Examples for controlled inundations include those in the Netherlands under the Dutch Republic and its successor states in that area and exemplified in

SECTION 50

#1732779736799

4940-470: The flood process; before, during and after. During floods accidents occur with falling debris or any of the many fast moving objects in the water. After the flood rescue attempts are where large numbers injuries can occur. Communicable diseases are increased due to many pathogens and bacteria that are being transported by the water .There are many waterborne diseases such as cholera , hepatitis A , hepatitis E and diarrheal diseases , to mention

5035-492: The flood thus advances more slowly than later and higher flows. As a result, the rising limb of the hydrograph becomes ever quicker as the flood moves downstream, until the flow rate is so great that the depletion by wetting soil becomes insignificant. Coastal areas may be flooded by storm surges combining with high tides and large wave events at sea, resulting in waves over-topping flood defenses or in severe cases by tsunami or tropical cyclones. A storm surge , from either

5130-403: The floods have settled. The diseases that are vector borne are malaria , dengue , West Nile , and yellow fever . Floods have a huge impact on victims' psychosocial integrity . People suffer from a wide variety of losses and stress . One of the most treated illness in long-term health problems are depression caused by the flood and all the tragedy that flows with one. Below is a list of

5225-413: The flow at outlets of lakes, ponds, and reservoirs. Flooding A flood is an overflow of water ( or rarely other fluids ) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide . Floods are of significant concern in agriculture , civil engineering and public health . Human changes to the environment often increase

5320-428: The flow rate exceeds the capacity of the river channel , particularly at bends or meanders in the waterway . Floods often cause damage to homes and businesses if these buildings are in the natural flood plains of rivers. People could avoid riverine flood damage by moving away from rivers. However, people in many countries have traditionally lived and worked by rivers because the land is usually flat and fertile . Also,

5415-656: The form of diverting flood waters in a river at flood stage upstream from areas that are considered more valuable than the areas that are sacrificed in this way. This may be done ad hoc , or permanently, as in the so-called overlaten (literally "let-overs"), an intentionally lowered segment in Dutch riparian levees, like the Beerse Overlaat in the left levee of the Meuse between the villages of Gassel and Linden, North Brabant . Military inundation creates an obstacle in

5510-470: The gas is injected into a stream of water which is pumped from, and returned to, the bay. The barge is capable of dissolving up to 5 tonnes of oxygen in 24 hours. Similar options have been proposed to help rehabilitate the Chesapeake Bay where the principal problem is lack of filter-feeding organisms such as oysters responsible for keeping the water clean. Historically the Bay's oyster population

5605-602: The geometry of the top of the weir is known and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow. However, this can only be achieved in locations where all water flows over the top of the weir crest (as opposed to around the sides or through conduits or sluices) and at locations where the water that flows over the crest is carried away from the structure. If these conditions are not met, it can make flow measurement complicated, inaccurate, or even impossible. The discharge calculation can be summarised as where However, this calculation

5700-774: The intensity and frequency of flooding. Examples for human changes are land use changes such as deforestation and removal of wetlands , changes in waterway course or flood controls such as with levees . Global environmental issues also influence causes of floods, namely climate change which causes an intensification of the water cycle and sea level rise . For example, climate change makes extreme weather events more frequent and stronger. This leads to more intense floods and increased flood risk. Natural types of floods include river flooding, groundwater flooding coastal flooding and urban flooding sometimes known as flash flooding. Tidal flooding may include elements of both river and coastal flooding processes in estuary areas. There

5795-403: The land in quantities that cannot be carried within stream channels or retained in natural ponds, lakes, and human-made reservoirs . About 30 percent of all precipitation becomes runoff and that amount might be increased by water from melting snow. River flooding is often caused by heavy rain, sometimes increased by melting snow. A flood that rises rapidly, with little or no warning, is called

SECTION 60

#1732779736799

5890-614: The livelihood of individuals. As a result, there is also significant socio-economic threats to vulnerable populations around the world from flooding. For example, in Bangladesh in 2007, a flood was responsible for the destruction of more than one million houses. And yearly in the United States, floods cause over $ 7 billion in damage. Flood waters typically inundate farm land, making the land unworkable and preventing crops from being planted or harvested, which can lead to shortages of food both for humans and farm animals. Entire harvests for

5985-417: The location of the flood. Damage to roads and transport infrastructure may make it difficult to mobilize aid to those affected or to provide emergency health treatment. Flooding can cause chronically wet houses, leading to the growth of indoor mold and resulting in adverse health effects, particularly respiratory symptoms. Respiratory diseases are a common after the disaster has occurred. This depends on

6080-436: The loss of power. This includes loss of drinking water treatment and water supply, which may result in loss of drinking water or severe water contamination. It may also cause the loss of sewage disposal facilities. Lack of clean water combined with human sewage in the flood waters raises the risk of waterborne diseases , which can include typhoid , giardia , cryptosporidium , cholera and many other diseases depending upon

6175-423: The maximum height a species can jump or creates flow conditions that cannot be bypassed (e.g., due to excessive water velocity) effectively limits the maximum point upstream that fish can migrate. In some cases this can mean that huge lengths of breeding habitat are lost, and over time this can have a significant impact on fish populations. In many countries, it is now a legal requirement to build fish ladders into

6270-399: The natural process of photosynthesis, water plants release oxygen into the water providing it with the oxygen necessary for fish to live and aerobic bacteria to break down excess nutrients. Oxygen can be driven into the water when the wind disturbs the surface of the water body and natural aeration can also occur through a movement of water caused by an incoming stream , waterfall , or even

6365-457: The period of time between observations. This intensity will be less than the actual peak intensity if the duration of the rainfall event was less than the fixed time interval for which measurements are reported. Convective precipitation events (thunderstorms) tend to produce shorter duration storm events than orographic precipitation. Duration, intensity, and frequency of rainfall events are important to flood prediction. Short duration precipitation

6460-438: The plant populations , resulting in a reduction of dissolved oxygen levels, rendering areas of the bay unsuitable for aerobic aquatic life. In a symbiotic relation the plants provide the oxygen needed for underwater organisms to proliferate, and in exchange the filter feeders keep the water clean and thus clear enough for plants to have sufficient access to sunlight. Researchers have proposed oxygenation through artificial means as

6555-413: The rate at which water moves downstream even slightly, a disproportionate effect can be had on the likelihood of flooding. On larger rivers, a weir can also alter the flow characteristics of the waterway to the point that vessels are able to navigate areas previously inaccessible due to extreme currents or eddies . Many larger weirs will have construction features that allow boats and river users to "shoot

6650-431: The recreational use of lakes and the commercial use of lake water. With severe thermal stratification in a lake, the quality of drinking water also can be adversely affected. For fisheries managers, the spatial distribution of fish within a lake is often adversely affected by thermal stratification and in some cases may indirectly cause large die-offs of recreationally important fish. One commonly-used tool to reduce

6745-586: The result of sustained rainfall, rapid snow melt, monsoons , or tropical cyclones . However, large rivers may have rapid flooding events in areas with dry climates, since they may have large basins but small river channels, and rainfall can be very intense in smaller areas of those basins. In extremely flat areas, such as the Red River Valley of the North in Minnesota , North Dakota , and Manitoba ,

6840-480: The risks associated with large debris entrained by the flow motion. Floods can be a huge destructive power. When water flows, it has the ability to demolish all kinds of buildings and objects, such as bridges, structures, houses, trees, and cars. Economical, social and natural environmental damages are common factors that are impacted by flooding events and the impacts that flooding has on these areas can be catastrophic. There have been numerous flood incidents around

6935-426: The river bottom) that reduces the water oxygen content and smothers invertebrate habitat and fish spawning sites. The oxygen content typically returns to normal once water has passed over the weir crest (although it can be hyper-oxygenated), although increased river velocity can scour the river bed causing erosion and habitat loss. Weirs can have a significant effect on fish migration . Any weir that exceeds either

7030-454: The rivers provide easy travel and access to commerce and industry. Flooding can damage property and also lead to secondary impacts. These include in the short term an increased spread of waterborne diseases and vector-bourne disesases , for example those diseases transmitted by mosquitos. Flooding can also lead to long-term displacement of residents. Floods are an area of study of hydrology and hydraulic engineering . A large amount of

7125-498: The severity of these lake management problems is to eliminate or lessen thermal stratification through aeration. Many types of aeration equipment have been used to reduce or eliminate thermal stratification. Aeration has met with some success, although it has rarely proved to be a panacea. During heavy rain, London's sewage storm pipes overflow into the River Thames , sending dissolved oxygen levels plummeting and threatening

7220-497: The shore areas. Extreme flood events often result from coincidence such as unusually intense, warm rainfall melting heavy snow pack, producing channel obstructions from floating ice, and releasing small impoundments like beaver dams. Coincident events may cause extensive flooding to be more frequent than anticipated from simplistic statistical prediction models considering only precipitation runoff flowing within unobstructed drainage channels. Debris modification of channel geometry

7315-538: The smallest ephemeral streams in humid zones to normally-dry channels in arid climates to the world's largest rivers. When overland flow occurs on tilled fields, it can result in a muddy flood where sediments are picked up by run off and carried as suspended matter or bed load . Localized flooding may be caused or exacerbated by drainage obstructions such as landslides , ice , debris , or beaver dams. Slow-rising floods most commonly occur in large rivers with large catchment areas . The increase in flow may be

7410-473: The species it supports. Two dedicated McTay Marine vessels, oxygenation barges Thames Bubbler and Thames Vitality are used to replenish oxygen levels, as part of an ongoing battle to clean up the river, which now supports 115 species of fish and hundreds more invertebrates, plants and birds. The dissolved oxygen concentration within Cardiff Bay are maintained at or above 5 mg/L. Compressed air

7505-527: The supply of vegetation that can absorb rainfall. During times of rain, some of the water is retained in ponds or soil, some is absorbed by grass and vegetation, some evaporates, and the rest travels over the land as surface runoff . Floods occur when ponds, lakes, riverbeds, soil, and vegetation cannot absorb all the water. This has been exacerbated by human activities such as draining wetlands that naturally store large amounts of water and building paved surfaces that do not absorb any water. Water then runs off

7600-410: The terms dam and weir are synonymous, but normally there is a clear distinction made between the structures. Usually, a dam is designed specifically to impound water behind a wall, whilst a weir is designed to alter the river flow characteristics. A common distinction between dams and weirs is that water flows over the top (crest) of a weir or underneath it for at least some of its length. Accordingly,

7695-440: The third most important factors for small and large watersheds, respectively. Time of Concentration is the time required for runoff from the most distant point of the upstream drainage area to reach the point of the drainage channel controlling flooding of the area of interest. The time of concentration defines the critical duration of peak rainfall for the area of interest. The critical duration of intense rainfall might be only

7790-699: The two Hollandic Water Lines , the Stelling van Amsterdam , the Frisian Water Line , the IJssel Line , the Peel-Raam Line , and the Grebbe line in that country. To count as controlled , a military inundation has to take the interests of the civilian population into account, by allowing them a timely evacuation , by making the inundation reversible , and by making an attempt to minimize

7885-470: The unit are a number of diffusers. These diffusers come in the shape of discs, plates, tubes or hoses constructed from glass-bonded silica, porous ceramic plastic, PVC or perforated membranes made from EPDM (ethylene propylene diene Monomer) rubber . Air pumped through the diffuser membranes is released into the water. These bubbles are known as fine bubbles . The EPA defines a fine bubble as anything smaller than 2mm in diameter. This type of aeration has

7980-490: The water and allowing oxygen transfer through air-water contact. As each new section of water is churned, it absorbs oxygen from the air and then, upon its return to the water, restores it to the water. In this regard paddlewheel aeration works very similarly to floating surface aerators. Subsurface aeration seeks to release bubbles at the bottom of the water body and allow them to rise by the force of buoyancy. Diffused aeration systems utilize bubbles to aerate as well as mix

8075-514: The water level can rise and submerge the weir without any alterations made to the structure. A polynomial weir is a weir that has a geometry defined by a polynomial equation of any order n . In practice, most weirs are low-order polynomial weirs. The standard rectangular weir is, for example, a polynomial weir of order zero. The triangular (V-notch) and trapezoidal weirs are of order one. High-order polynomial weirs are providing wider range of Head-Discharge relationships, and hence better control of

8170-487: The water overtops or breaks levees , resulting in some of that water escaping its usual boundaries. Flooding may also occur due to an accumulation of rainwater on saturated ground. This is called an areal flood . The size of a lake or other body of water naturally varies with seasonal changes in precipitation and snow melt. Those changes in size are however not considered a flood unless they flood property or drown domestic animals . Floods can also occur in rivers when

8265-455: The water to the fountain can be a safety hazard. Floating surface aerators work in a similar manner to fountains, but they do not offer the same aesthetic appearance. They extract water from the top 1–2 feet of the water body and utilize air-water contact to transfer oxygen. Instead of propelling water into the air, they disrupt the water at the water surface. Floating surface aerators are also powered by on-shore electricity. The effectiveness of

8360-471: The water. Water displacement from the expulsion of bubbles will cause a mixing action to occur, and the contact between the water and the bubble will result in an oxygen transfer. Subsurface aeration can be accomplished by the use of jet aerators , which aspirate air, by means of the Venturi principle, and inject the air into the liquid. Coarse bubble aeration is a type of subsurface aeration wherein air

8455-515: The waters in a flood are very deep and have strong currents . Deaths do not just occur from drowning, deaths are connected with dehydration , heat stroke , heart attack and any other illness that needs medical supplies that cannot be delivered. Injuries can lead to an excessive amount of morbidity when a flood occurs. Injuries are not isolated to just those who were directly in the flood, rescue teams and even people delivering supplies can sustain an injury. Injuries can occur anytime during

8550-486: The weir" and navigate by passing up or down stream without having to exit the river. Weirs constructed for this purpose are especially common on the River Thames , and most are situated near each of the river's 45 locks . Because a weir impounds water behind it and alters the flow regime of the river, it can have an effect on the local ecology . Typically, the reduced river velocity upstream can lead to increased siltation (deposition of fine particles of silt and clay on

8645-450: The world which have caused devastating damage to infrastructure, the natural environment and human life. Floods can have devastating impacts to human societies. Flooding events worldwide are increasing in frequency and severity, leading to increasing costs to societies. Catastrophic riverine flooding can result from major infrastructure failures, often the collapse of a dam . It can also be caused by drainage channel modification from

8740-557: The world's population lives in close proximity to major coastlines , while many major cities and agricultural areas are located near floodplains . There is significant risk for increased coastal and fluvial flooding due to changing climatic conditions. Floods can happen on flat or low-lying areas when water is supplied by rainfall or snowmelt more rapidly than it can either infiltrate or run off . The excess accumulates in place, sometimes to hazardous depths. Surface soil can become saturated, which effectively stops infiltration, where

8835-434: The year and kills pests in the farming land. Freshwater floods particularly play an important role in maintaining ecosystems in river corridors and are a key factor in maintaining floodplain biodiversity . Flooding can spread nutrients to lakes and rivers, which can lead to increased biomass and improved fisheries for a few years. Water oxygenation Water aeration is the process of increasing or maintaining

8930-765: Was heavy, a fast snowmelt can push water out of the banks of a tributary river so that it moves overland, to a point further downstream in the river or completely to another streambed. Overland flooding can be devastating because it is unpredictable, it can occur very suddenly with surprising speed, and in such flat land it can run for miles. It is these qualities that set it apart from simple "overland flow". Rapid flooding events, including flash floods , more often occur on smaller rivers, rivers with steep valleys, rivers that flow for much of their length over impermeable terrain, or normally-dry channels. The cause may be localized convective precipitation (intense thunderstorms ) or sudden release from an upstream impoundment created behind

9025-457: Was in the tens of billions, and they circulated the entire Bay volume in a matter of days. Due to pollution, disease and over-harvesting their population is a fraction of historic levels. Water that was once clear for meters is now so turbid and sediment-ridden that a wader may lose sight of their feet before their knees are wet. Oxygen is normally supplied by submerged aquatic vegetation via photosynthesis but pollution and sediments have reduced

#798201