5.5 Surpluses in the hydrological system Flashcards
What is flooding?
if the discharge is of sufficient quantity to cause a body of water to overflow its channel and submerge the surrounding land, flooding is deemed to have occurred.
What are the four metrological causes of flooding?
-Intense storms leading to flash flooding
-Unusually heavy or prolonged rainfall
-Extreme monsoonal rainfall
-Snowmelt
What are some examples of intense storms leading to flash flooding
e.g. November 2013 in Sardinia thunderstorms caused flash flooding resulting in 18 deaths and US$14.14 billion in damage.
What is intense rain that leads to flash flooding
Flash floods happen very quickly, often without warning. Occur in the UK in summer. Intensity of the rain exceeds capacity of the river. Short lag time – minutes or hours.
What is snowmelt?
Occurs in higher latitudes or mountainous areas due to a sudden rise in air temperature in spring. Flooding occurs snow melts and the water cannot infiltrate into the soil.
What are examples of snowmelt?
-e.g. 2009, Red River in North Dakota - due to increased air temperature, impermeable ground and additional precipitation.
-In Iceland glacial outburst floods occur due to volcanic activity known as jokulhlaup.
-In winter in 2013 Norfolk Police reported flooding caused by snowmelt after mild wet weather caused rapid thawing.
What is extreme monsoon rainfall and where does it become a problem specifically?
A seasonal change in the direction of the prevailing winds of a world region. Occurs across South and Southeast Asia between May and September when around 70% of the annual rainfall occurs in about 100 days, due to warm air from the south-west Indian Ocean blowing towards India.
What are some examples of extreme monsoon rainfalls?
-July 2015 usually heavy monsoon rains in Myanmar caused 103 deaths and critically affected over 1 million people.
-In July 2010 in Pakistan 9000mm of rain in one week caused landslides and flooding.
What causes unusually heavy or prolonged rainfall? How does this then lead to flooding
In the UK this can be associated with the passage of low-pressure systems (depressions), and usually occurs in autumn and early winter. Depressions have 2 bands of rain – showers and rain with the warm front then heavier rain with the cold front. Initially the ground will absorb the rain, but this will soon become saturated, leading to increased overland flow.
What is an example of unusually heavy or prolonged rainfall?
In the UK there was a succession of very intense storms between October and December 2015. It resulted from a sinuous jet stream in a fairly constant track which mean that high pressure was blocked.
What factors can affect flooding levels?
soil depth
vegetation
slope
rock type
drainage density
What is land use now like and how does this affect flooding levels
we have barer, drier soils; straightening and dredging channels; faster runoff; reduced lag times, higher peak discharge. Rainwater now reaches the floodplain quicker – and is where we find many impermeable surfaces. Due to climate change, we also have higher levels of rainfall.
What did land use used to be like and how did this affect flooding levels?
trees absorbed and slowly released water; meandering channels slowed the flow, and bogs held back water.
How does farming cause flooding?
-Ploughing compacts soil - overcultivation
-Grazing animals compact soil - over grazing
-Sprinkling of groundwater on to arable crops - saturated groundwater storage
How does damming and bridges cause flooding
-Dams built to supply towns with water, but can suddenly burst
-Impeding channel flow by building alongside the river
How does changing the course of a river cause flooding?
-Natural streams meander and have marshy areas; channelization does not
-Sewers feed water into channels
-Streams channelled into culverts to aid rapid drainage of farmland
Why is channel straightening seen as mismanagement?
This may speed up water flow but it doesn’t take a greater capacity. It would be more efficient to dredge the river, which involves widening and deepening the channel to increase efficiency by increasing capacity and moving water at a faster rate (although at an environmental cost)
What are the four main human causes of flooding?
-deforestation
-increased urbanisation
-climate change
-increased population
How has urbanisation caused flooding?
-60% of the population have decided to turn their gardens into driveways to accommodate more than one car
-urban drainage systems look to be as efficient as possible
-more bridges have been built for better access
-channelization in urban areas increases the velocity of a river
How does a growing population cause flooding?
-more space is needed for living
-wetland areas near towns have been drained for living space
-need to provide more land for food so land is required for agriculture
-change was quick and therefore lacked planning and thought
How does climate change cause increased flooding?
-higher levels of rainfall
How has deforestation caused flooding?
-tree roots helped to bind soil together; this no longer happens
-less interception and evapotranspiration
-animals trample land and compact soil
-bare soil is more susceptible to erosion
When did Storm Desmond happen and what happened?
It hit Cumbria in December 2015, particularly affecting the towns of Keswick and Cockermouth
341mm of rain fell in 24 hours
What caused storm Desmond?
deep Atlantic low pressure system. Low pressure of 946 millibars meant its fronts brought prolonged heavy rainfall
-brought through a mechanism known as the conveyor belt
-the jet stream remained over the north west for longer than usual- 48 hours; an atmospheric river was detected bringing warm air across from the Caribbean in a narrow band of enhanced water vapor transfer - warmer air holds moisture for longer
