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
What types of rain was storm Desmond?
convectional and orographic rainfall over the lake district
What were some of the factors that meant towns like Keswick were not well prepared for Storm Desmond?
Too much rainfall fell for an already saturated ground to absorb any water. Overland flow carried water to rivers , which flooded and steep slopes accelerated this process
Impermeable surfaces and blocked sewers meant water rapidly flowed into already filled channels
What were the social impacts of Storm Desmond?
5,200 homes flooded
a landslide closed a section of the west coast mainline between Preston and Carlisle
61,000 homes lost power
One found dead after falling in a river outside Kendall
-40 schools closed
What were the economic impacts of Storm Desmond?
Dublin and Leeds Bradford airport closed
Storm Desmond and Eva cost 1.3 billion together
Insurance claims caused by flooding across the UK in 2015 exceeded £6billion
House prices fell in areas at risk
What were the environmental impacts of Storm Desmond?
-Many river banks were eroded, adding to future flood risk
-rivers were choked with debris and contaminated sewage
-saturated ground led to landslides and decomposition of dead plants and animals- giving off poisonous gases
What were the responses to storm Desmond?
8 response and rescue centres set up
the government emergency committee - Cobra- released £50 million to local councils
Why was the flood defenses in Keswick not appropriate?
There were hard engineering defences in Keswick. However, these weren’t always appropriate given changes to land use and climate change. After the 2009 floods a defense wall was built to 5m, but the flood in 2015 reached 5.9 metres and so the water went over the top
What soft engineering techniques do people in Cumbria think would be the way forward?
-reafforestation
-restoration of river channels in their natural meandering state
-restoration of flood plains
-refusal of planning permission to build or expand developments near the river
Why was the government criticised in Storm Desmond
there was an anger over the lack of flood defences, but there are questions over whether it would have actually done anything as 2005 flood was half a metre less
Additionally they were criticised for failing to apply for EU funding for clean up
