Floods Flashcards
What are the main causes of flooding?
High intensity rainfall
Periodic climate cycles like ENSO
Heavy snowmelt
Prolonged rainfall
Human factors which increase the risk of flooding
Channelisation to aid rapid drainage into farmland
Dams- Accumulated water in the reservoir may lead to flooding upstream.
Ploughing - this compacts soils reducing pore space. This reduces infiltration and increases surface runoff
Deforestation
Sewers feed water into channels
Urbanisation
Pasture land - grazing animals trample soil reducing soil pore space. This reduces infiltration - increasing surface runoff.
Why is Bangladesh so flood-prone?
It is a land of flood plains built by large rivers such as the Ganges, Padma and Meghna. These rivers are swollen twice per year. once due to snowmelt from the Himalayas and again due to the Summer monsoon.
Bangladesh also experiences coastal flooding due to storm surges or when high spring tides meet high river discharge.
What are storm surges?
Storm surges are formed by a combination of low pressure, strong winds and high tides. The low pressure forms a Bulge. Strong winds then push this region of higher water towards the coast. This often breaches coastal defences and floods large areas.
Causes of flooding along the River Severn in 2020.
A very wet October and November in 2019. Soil moisture deficits were at 0% from October 2019 to February 2020. This meant that soils were already saturated. Intense rain also meant that by November 2019 theatre level of the Severn reached 5.56 m in some areas compared to the usual range of 0.3 to 3.3 metres.
In the first week of February, the UK was hit by a series of depressions causing heavy rainfall. In February, The UK was also hit by 3 named storms. Meaning rainfall in February 2020 was 3x the monthly average.
Discharge in Ironbridge reached 400 cumecs in February 2020.
Deforestation along the Severn
A change in the position of the jet stream. In 2010, it moved further south pushing more depressions over the UK
Impacts of the 2020 River Severn Floods
Areas in Bewdley, which had been flood-free for over 100 years, were inundated with water.
40 evacuated from Ironbridge
Tewkesbury water pumping stations damage - affecting water supply
Power stations flooded affecting electricity supply.
Many shops were forced to close and couldn’t reopen due to high insurance costs.
Whilst flooding destroyed crops and temporarily prevent agriculture. the deposition of fertile sediment from the river sediemnt caused and increase in agricultural productivity in the following months.
What strategies had been used to reduce risk of flooding along the Severn.
Clywedog dam and reservoir built between 1963 and 197 can regulate water flow. Storing water in times of surplus may increase flood risk upstream.
Flood walls at Bewdley, Ironbridge Shrewsbury and Worcester. These contain water within the Severn but can be breached by powerful floods.
Artifical levees constructed. This increases maximum capacity of the river channel decreasing the likelihood of flooding. However in the case of a flood, water will be unable to return to the channel.
Widening and deepening the river. This increases capacity and discharge but may cause flooding downstream where the channel cannot handle the increase discharge.
Environmental impacts of flooding
Flooding can have both positive environmental impacts - such as recharging groundwater stores, filling wetlands, increasing connectivity between aquatic habitats and moving sediment containing nutrients into marine environments. Flood events may also trigger breeding migration and dispersal
However, excessive overland flow may lead to eutrophication, leading to the destruction of aquatic ecosystems.
In many developing countries subsistence farmers rely on annual inundation to bring sediment and nutrients to the fields. The construction of the Aswan Dam to control flooding in the Ule had a negative impact on subsistence farmers and fisherman as the sardines migrated away from the Nile Delta
