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Floods Case Study

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FLOODING: is a high flow of water which overtops the bank of a river MAIN CAUSES: Climatic forces whereas the flood-intensifying conditions tend to be drainage basin specific o3 types: -Deep depression (low pressure system) long lasting & cover a wide area (UK) -Short periods of heavy rainfall (summer rain 3 months 70% of rain INDIA) -melting snow responsible for widespread flooding (ARTIC Regions) These flood intensifying conditions involve a range of human-related factors that alter the drainage basin response to a given storm (HIGH flood runoff, HIGH drainage density, LOW carrying capacity)

More rapid discharge in urban areas due to the impermeable surface and increased number of drainage channels
Urbanisation and urban growth- increases impermeable surfaces
Floodplain development- increasing risk of damage
Bridges, dams and obstructions- leading to reducing carrying capacity oChanges in vegetation cover (deforestation)
Human-induced climate change
River engineering works (levees)

RECURRENCE: recurrence interval refers to the regularity of a flood of a given size. FORECASTING AND WARNING: Improving flood warnings (UN’s Environment Programme) Improved rainfall and snowpack estimates, and better and longer forecasts of rainfall Better gauging of rivers, collection of meteorological information and mapping of channels better and current information about human populations, infrastructure, elevation and stream channels to improve flood risk assessment models better sharing of information between forecasters, national agencies, relief organisations and the general public more complete and timely sharing of meteorological and hydrological information among countries within international drainage basins sharing of technology among all agencies involved in flood forecasting and risk assessment, both within basins and throughout the world PREVENTION AND AMELIORATION (to improve) OF FLOODS

Loss sharing adjustments include disaster aid and insurance. Disaster aid refers to any aid, such as money, equipment, staff and technical assistance that is given to a community following a disaster. However not all flood-prone households have insurance and many of those who are insured may be under insured Event modification adjustments include environmental control and hazard-resistant design. Physical control of floods depends on 2 measures

FLOOD ABATEMENT: involves decreasing the amount of runoff, thereby reducing the flood peak in a drainage basin. There are a number of ways of reducing flood peaks, including: reforestation reseeding sparsely vegetated areas to increase evaporative losses treatment of slopes, such as through contour ploughing or terracing, to reduce the runoff coefficient comprehensive protection of vegetation from wild fires, overgrazing and clear cutting clearance of sediment and other debris from headwater streams construction of small water water and sediment holding areas preservation of natural water storage zones, such as lakes

Flood diversion measures, by contrast, include the construction of levees, reservoirs, and the modification of river channels. Levees are the most common form of river engineering. Reservoirs store excess rainwater in the upper drainage basin. However, this may only be appropriate in small drainage networks HAZARD-RESISTANT DESIGN

Flood proofing includes any adjustments to buildings and their contents, which help reduce losses. Some are temporary, such as: blocking up entrances
sealing doors and windows
removal of damageable goods to higher levels
use of sandbags
Long-term measures include moving the living spaces above the likely level of the floodplain. building above the flood level, but could also include building homes on stilts LAND USE PLANNING: The process by which lands are evaluated and assessed to become a basis for decisions involving land disposition and utilization. This involves studies on the environmental effects of land use and its impact on the community. man-made levees

replacing buildings on floodplains with parks/ car parks
Drought is an extended period of dry weather leading to conditions of extreme dryness. Absolute drought is a period of at least 15 consecutive days with less than 0.22mm of rainfall. Partial drought is a period of at least 29 consecutive days during which the average daily rainfall does not exceed 0.2mm Semi-arid areas: rainfall of less of less than 500 mm per annum arid areas: rainfall of less than 250 mm

extreme arid areas: less than 125 mm per annum

Global atmospheric circulation. Dry. descending air associated with the subtropical high pressure belt is the main cause of aridity around at 20-30 N distance from sea, or continentally, limits the amount of water carries across by winds cold offshore currents limit the amount if condensation in the overlying air intense rainshadow effects, as air passes over mountains

human activities, desertification of prior agriculture land

HARD ENGINEERING approaches are generally larger in scale and involve artificial constructions like a dam, they tend to be more expensive and have considerable impact on the environment DAMS; help regulate river flow by holding water back in time of flood and then releasing water during period of lower rainfall. (+) opportunities for leisure, water supply and hydroelectricity, income from tourism (+) when water is released gradually from the dam, make river more navigable and lessens soil erosion due to lower velocity (-) blocks fish migrations, which in some cases and with some species completely separate spawning habitats from rearing habitats. (-) traps sediments, which are critical for maintaining physical processes and habitats downstream of the dam REALIGNMENT/ DIVERSION; meandering rivers tend to be slower, and this can encourage flooding, deepen and straightening the river, helps to increase velocities and scouring of the channel.

Sedimentation is discourages that can otherwise decrease a channel’s width and depth CHANNELISATION; to increase a river’s width and depth to reduce flooding, helps reduce natural erosion, topsoil is eroded but doesn’t get re-deposited, helps prevent the water from changing directions randomly, net erosion is reduced REVETMENTS; artificial river banks, often made with stone boulders, prevent bank erosion and therefore to maintain a deep and straight channel. LEVEES; are low ridges/ embankments of soil, sand or clay and contain impermeable concrete wall surrounded by earth material with grass, reduces erosion and help maintain a deep channel. SOFT ENGINEERING approaches are more environmentally sensitive, often involve working with natural processes AFFORESTATION; trees intercept water, planting along the floodplain reduces surfaces run-off Mississippi River Case Study

Physical Characteristics
The Mississippi river is the fourth largest river in the world. It is 3,705 kilometres in length.
Its drainage basin covers 41% of the area of the United States (3.2 million square kilometres). It rises from Lake Itasca, NW Minnesota, a small glacial lake. At source it is 450m above sea level.

Nearly half the drop in elevation occurs in Minnesota State. The average discharge into the Gulf of Mexico is 16,200m3/s. The mouth of the river enters the Gulf of Mexico. At the headwater of the Mississippi it is approximately 1m deep. At the mouth of the Mississippi it is approximately 67m deep. Each day an average of 436,000 tons of sediment is transported. Over the course of a year 159 million tons of sediment is transported. The river is divided into 2 sections:

1. The Upper Mississippi, from the source south to the confluence of the Ohio river with the Mississippi. 2. The Lower Mississippi, from the Ohio to the mouth 160km south of New Orleans. The river has under gone a series of natural shifts in its course, two major earthquakes in 1811 and 1812 (estimated 8 on the Richter Scale) are thought to have temporarily reversed the course.

Ecosystem Characteristics
Home to at least 260 different species of fish, 25% of all North American fish species. 40% of migratory birds in the United States use the river corridor during their Autumn and Spring migrations. 60% of all North American migratory birds use the river basin as part of their flyway. The Upper Mississippi is home to 50 species of mammals.

At least 145 species of amphibians and reptiles are found in the Upper Mississippi. 38 species of mussel are found in the upper reaches of the river. In the Lower Mississippi there are thought to be 60 separate species of mussel. Seasonal flooding drives a highly productive and diverse ecosystem. Fire was once a dominant force maintaining floodplain grassland-savanna landscapes. Ice flows, tree falls, and log jams are all natural occurrences that help define local habitats and maintain high habitat diversity. Biological disturbances (e.g., beavers) are important in the development of floodplain landscapes.

Diagram 1: Pre-settlement vegetation of Upper Mississippi River valley Table 1. Ecological Disturbances
Man Made
Channel migration
Sediment resuspension
Ice shear
Tree wind-throw
Log jam
Water level regulation
Dredging and dredged
material disposal
Channel training structures
Boat generated waves
Levee construction
Nutrient enrichment
Urban development

Human Influence
Population of the Mississippi corridor is 12 million people. It flows through or borders 10 states; Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Arkansas, Tennessee and Mississippi. 27 locks were built on the Upper Mississippi in the 1930s designed primarily to maintain a channel for commercial barge traffic, this created a series of lakes used mainly for boating and fishing. Although the dams make the river deeper and wider no flood control is intended. During periods of high flow the dams can be submersed and opened, ceasing to function. Below St. Louis, the Mississippi is relatively free flowing, although it is constrained by a series of levees and directed by multiple wing dams. Davenport, Iowa, is the only town over 20,000 people in the Upper Mississippi to not be protected by a permanent floodwall or levee. Impoundment and river regulation for navigation have significantly modified the hydrologic regime and the pattern of sedimentation. Impoundment, water level regulation, channelization, levee construction, logging, and urban and agricultural development are the dominant human activities affecting river habitats on the Upper Mississippi River System (UMRS).

Portions of the floodplain were permanently flooded by the dams and backwater area increased significantly in some river reaches (Fig. 1). Rock wing dams, closing dams, and bank revetments are used to maintain the navigation channel and to reduce dredging requirements, these structures reduce bank erosion and force flow into the main channel. In the Open River reach, channel training structures have greatly reduced the number and quality of secondary channels (Fig. 2), there has also been loss of channel area as sediment filled the area between wing dams. Over 1.1 million acres, mostly agricultural land, are protected from moderate floods by levees in the UMRS. Logging has caused significant habitat degradation throughout the river floodplains and northern parts of the basin. Deforestation and agricultural conversion throughout the basin increased sediment delivery to the main rivers in the watershed. Urban development displaced native habitats, but also caused indirect impacts; sewage and industrial pollution caused significant water quality problems that eradicated sensitive species downstream of large cities.

The problem has subsided since the 1970s. The separation of the river from its original floodplain and the loss of millions of acres of wetlands, many drained for agricultural purposes, have limited the river’s ability to absorb and protect against spring floodwaters. Although billions have been spent to tame the river and reduce flood damages, recent floods have cost taxpayers billions and resulted in significant loss of life. Nutrients discharged from the river system into the Gulf of Mexico have contributed to a “dead zone.” Strategies to improve and increase wetland area (Fig. 3) in the lower reaches of the Mississippi would also help reduce the problem of nutrient discharge due to uptake by plants in these areas leading to a potential reduction in size of the “dead zone” in the Gulf of Mexico.

Figure 3: Coastal wetlands project in Louisiana
HARD Engineering in the Mississppi
The US needed to prevent the yearly floods and tame the river to make it navigable in order to develop Before management schemes were implemented the river constantly shifted its channel and eroded its banks They used stone dykes to trap sediment and provoke the river to erode vertically so that the channel was deep enough for paddle steam boats to use More wing dykes were constructed along with reserviors, levees and channel straightening, channelisation (concrete matressing) and dregding were also used —–> this all made the river faster as they increased the gradient along the rivers long profile All of this management, like all river management in the America, was completed by the US Army Corps of Engineers and costs $180 million a year in maintainence as the force of the water sweeps away thousands of dollars worth of management each year. It is hard to manage rivers as they constantly change (in a state of dynamic equilbrium) and so management techiniques are based on guess work and trialed in labs.

Some people, though, think that management of the Mississippi has made the floods worse…… 1993 = 3 months of torrential rain —–> defences were not designed for the such large size of flood that occured and the local people chose not to pay for the levees to be heightened. The levees failed. However, many think that if the levees didn’t fail then the flooding would have been worse as they are believed toconstrict water movement, block up the channel and increase pressure. Floodplain development has raised the flood risk as concreteincreases surface runoff by reducing infiltration. Also the removal of vegetation reduces the interception store and, because there is nothing to trap the sediment, can raise the level of the river bank. Drains etc, which are designed to imitate the natural processes like throughflow,are a lot more efficient and so the water enters the channel quicker. Therefore scientists conclude that floodplainsshould not be built on as they are a natural flood defence that is supposed to flood.

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