A. Freshwater issues and conflicts - Drainage basins and flooding

Drainage basins and flooding

Drainage basins

Source: http://www.bbc.co.uk/schools/gcsebitesize/geography/water_rivers/background_rivers_rev2.shtml

 

Source: http://greenfieldgeography.wikispaces.com/Drainage+basins

Source: https://prezi.com/9cvqkugz43am/elements-of-the-drainage-basin-systems/

Feedback Loops: A system is always trying to maintain a balance (equilibrium). Feedback loops explain when a system (drainage basin) loses and attempts to regain its balance. Positive feedback causes an instability by changing inputs (increase in precipitation). Negative feedback attempts to regain the equilibrium by reducing inputs or possibly increasing outputs. Humans can alter feedback loops by changing the amount of water released from dams, changing the amount of water used or even increasing run-off through deforestation.

Source: http://greenfieldgeography.wikispaces.com/Drainage+basins

Negative and positive feedback systems keep a system in dynamic equilibrium. A negative feedback decreases the amount of change by reducing some of the inputs, returning the system to stability. Positive feedback is less common. It increases the amount of change. This leads to an imbalance.

Source: http://www.geographyalltheway.com/in/ib-freshwater/drainage-basins-tasks.htm

Discharge

In hydrology, discharge is the volume rate of water flow (velocity), which is transported through a given cross-sectional area. 

Discharge is normally measured in cumecs (cubic metres a second). 

Source: http://www.thegeographeronline.net/freshwater---issues-and-conflicts.html

Bed: The bottom of the river channel

Bank: The sides of the river channel.

Channel: The confines of the river, encompassing the bed and two banks.

Wetted Perimeter: The total length of the bed and the banks in contact with the river.

Cross-sectional area: The width of the river multiplied by the depth of the river. Because the depth of the river will vary across its width, an average depth reading is normally taken. The cross sectional area is normally given in m2.

Velocity: This is the speed that the water in a river is travelling at. The unit of measurement is normally metres a second (m/s). River velocity can be measured using a flowmeter (pictured right), or more commonly by timing a floating object over a set distance (pictured left). Velocity is then calculated by dividing the time (seconds) by the distance (metres).

Discharge: This is the amount of water in a river at a given point. Discharge is normally measured in cumecs (cubic metres a second). It is calculated by multiplying the cross-sectional area by the velocity.

As you move from the source to the mouth, both the discharge and velocity of a river increases.

Source: http://greenfieldgeography.wikispaces.com/Discharge

Source: http://www.thegeographeronline.net/freshwater---issues-and-conflicts.html

Source: http://greenfieldgeography.wikispaces.com/Discharge

Hydrographs

A hydrograph or storm hydrograph is a type of graph that shows how a rivers' discharge responds to a period of precipitation. Discharge is measured in one point (using the units cumecs), but precipitation is measured over the whole drainage basin. On the hydrograph, precipitation is shown using a bar graph and discharge is shown using a line graph.

Peak rainfall: The highest rainfall (usually measured in mm) during a storm.

Peak discharge: The highest discharge as a result of a storm event.

Lag time: The period of time between peak rainfall and peak discharge

Response time: The time between the first rain falling and the first change in discharge.

Rising limb: The period of time that the river is experiencing an increase in discharge.

Falling or recessional limb: The period of time that a rivers' discharge is falling after a storm event and returning to its normal flow.

Bankfull discharge: The maximum amount of discharge that a river can hold before it burst its banks and floods.

As the period from the initial storm increases, throughflow and baseflow will become increasingly more important in contributing to a rivers' discharge. Initially surface run-off will be the most important factor in contributing to a rivers' discharge because this is the quickest way for rainfall to enter a river.

Source: http://greenfieldgeography.wikispaces.com/Hydrographs

Source: http://www.thegeographeronline.net/freshwater---issues-and-conflicts.html

Source: http://www.thegeographeronline.net/freshwater---issues-and-conflicts.html

The hydrograph below illustrates two storm hydrographs for the same rainfall event in two areas of contrasting land use.

Source: http://www.geographyalltheway.com/in/ib-freshwater/hydrographs.htm

Floods

Rio de janeiro 2011

Source: http://www.bbc.com/news/world-latin-america-12180079

Brazil flood and mudslide deaths rise as rescue goes on

Brazil floods: More than 500 dead

HUMAN CAUSES

PHYSICAL CAUSES

Deforestation of hillsides. This reduces the strength of hills by removing route systems. It also reduces the amount of interception and transpiration. The reduced interception and transpiration means that the soil becomes saturated more quickly increasing surface run-off and the stress on slopes. This increased stress can cause landslides to happen. Building on marginal land. Increasing rates of rural-urban migration have meant that more and more people are building on marginal land. Marginal land includes floodplains and steep slopes which are unsuitable for settlement building. No building regulations. Most settlements built on marginal land are informal. This means that residents have built the houses themselves, using any available material. The houses will be weak with no real foundations so are very vulnerable during times of flood. In Brazil and Rio many of these informal settlements are called favelas. Most of these settlements will not have any drainage. This will increase the saturation of soil and increase the likelihood of floods. Population density. Most informal settlements have extremely high population densities. These high population densities mean that any floods are going to affect a larger number of people. Poor transport and communication. Poor areas were worse affected, which meant many people received no warning of the floods because they did not have access to media sources. In addition they would have had little opportunity to live the area, even if they did know. The poor transport and communication also meant that rescue efforts were made much more difficult.

Steep drainage basins and valley sides. Much of Rio and the surrounding area is very mountainous. The steep valleys means that any rainfall reaches streams and rivers very quickly causing flash floods. High levels of precipitation. It is estimated that areas around Rio received the equivalent of a months rainfall in just two days, 11th January and 12th January. Brazil experiences a tropical climate and the south east of Brazil (around Rio de Janeiro) can experience over 4 metres of rain a year. The majority of the rains falls during the summer months (including January). This means that during the summer the ground remains largely saturated, reducing infiltration rates and increasing surface run-off. La Nina - During 2010 and early 2011 Brazil has suffered increased rainfall caused by La Lina. La Lina is an oceanic-atmospheric system that can cause increased rainfall by changing temperatures. Mudslides. Although many of the causes of mudslides were human (deforestation, building on marginal land) the mudslides themselves were a secondary hazard of the flood water. The flood water saturated the ground, increasing the stress on the slopes and causing mudslides to happen.

MORE HUMAN FACTORS

MORE PHYSICAL FACTORS

Urbanisation can increase flooding, because the construction of roads and buildings increase the amount of impermeable surfaces. Global warming can increase the frequency of tropical storms, cause greater snow melt and increase the magnitude and frequency of storm events. Terracing and contour ploughing can actually reduce the speed of surface run-off, but other farming methods like building irrigation canals, deforestation and ploughing against the contours of the land can reduce interception, increase surface run-off and therefore the risk of flooding. Protecting one area with embankments or river straightening can cause increased risk of flooding further downstream.

Impermeable rock tends to increase the risk of flooding because less precipitation can infiltrate. Some countries suffer snow melt during late spring and summer, this can greatly increase the discharge of rivers and therefore increase the risk of flooding. Tropical storms can increase precipitation and also create storm surges which can increase the height of the water table and increase the risk of flooding. Sedimentation of rivers either naturally or because of human actions can cause rivers' cross-sections to become smaller and increase the risk of flooding.

Source: http://greenfieldgeography.wikispaces.com/Floods