2.1.3 Flooding

Primary cause of flooding

Primary cause of flooding

Natural: nature of precipitation input

Years where major floods caused by monsoons in Bangladesh occured

1987, 1988, 1998

La Niña year in Bangladesh that caused flooding

1988. 15-20 million more people affected than 1987 which was not a La Niña year.

Secondary flood-intensifying conditions of flooding

Natural: snowmelt, basin characteristics

Human: floodplain encroachment, catchment modification (urbanisation, deforestation, conversion to farmland)

Environmental impacts of flooding

1. Environmental contamination

2. Threat to wildlife

3. Erosion

Human impacts of flooding

1. Renewal of soil fertility

2. Replenishing of acquirers/recharging of groundwater store

3. Flood hazards (loss of lives, assets, livelihoods, displacement)

4. Disruption to economic activities

Example of topography causing high rainfall

Kerala, India: receives heavy rainfall due to its location on the windward slopes of the Western Ghats. It received 2200mm of rainfall in 2023 alone.

What basin characteristics makes Bangladesh so prone to flooding

1. Brahmaputra-Ganges-Meghna catchment involves three major rivers.

2. Intense seasonality of rainfall causes 75% of annual rainfall to fall during June to September during the monsoon season.

3. Snowmelt is carried by the Brahmaputra and Ganges from the Himalayas, which starts in March and reaches the Bay of Bengal in June to July, contributing to high peak discharge

4. Immensely high peak discharge in the Brahmaputra of over 100,000m per second

Example of exposure to significant rainfall events that contribute to flooding

2013 Typhoon Haiyan in the Philippines was a category 5 storm that caused a storm surge. A wave 7.5m tall hit the coast an a 5m surge hit Tacloban city. 90% of all buildings were destroyed, including 5 million homes, and killed 5800 people.

Example for economic structure and resultant economic impacts of flooding

2011, Chao Phraya basin flood, Thailand.

• Agriculture and manufacturing industries saw sharp drop in output, with GDP growth for Q4 dropping to -9%, and total economic losses amounting to 6.5 billion USD.

• Manufacturing sector contributed 45% of Thailand’s GDP at the time. Car manufacturer Honda’s factory in Ayutthaya closed, as did 10 Toshiba factories producing a range of electronics.

• Agriculture contributed 12% of Thailand’s GDP at the time, and Thailand was known as the “rice basket of SEA’. Crop damages from floods resulted in rice exports halving from a monthly average of 1 million tons, a loss of 10% of the world’s total rice crop.

Example of systemically disadvantaged groups being put at higher risk of exposure to flooding hazards

2022, monsoon flood in Pakistan, Basti Ahman Din, Punjab Province: only men were permitted by community elders to travel to relied camps, as women mixing with men outside of their families at the shared emergency shelters would ”compromise the family’s honour”.

What makes floodplain zoning important in LICs?

High population densities on the floodplains, making vulnerability high.

Examples to demonstrate the limited spatial scale of effectiveness of floodplain zoning as a management strategy.

1. India, Central Water Commission prepared a bill for floodplain zoning, but after 39 years, only three states (Rajasthan, Uttarakhand, Manipur) enacted legislation. This was largely due to the need to relocate developments, which could incur the cost of compensation payments and redevelopment of entire floodplains.

2. 2011 report on Nepal’s capacity to carry out disaster risk management, lack of financial means to update decades-old flood hazard maps for proper zonation and land use planning. The only areas with updated flood risk assessments, like those around the Bishnumati River were able to manage floods experienced. However, due to the limited spatial scale at which zoning was implemented, Nepal continues to suffer enormous economic losses exceeding $140m per annum due to flooding. The variance in effectiveness across space is likely due to the prioritisation of significant urban areas, like the capital Kathmandu, which the Bishnumati River runs through.

What makes hard engineering flood protection strategies important in LICs and HICs?

For populations and developments that cannot be relocated away from the flood way, but are of high value and require protection from flooding.

Examples to demonstrate the limited effectiveness of hard engineering strategies across different flood magnitudes

Under Bangladesh’s Flood Action Plan, 3555km of embankments were enacted along major rivers. 24% of total land area and 39% of net cultivated area have been protected against small-magnitude floods. However, the embankments were not built to withstand catastrophic flood events. A 1-in-100-year flood could easily overtop the embankments, which are made of river-bed sands and clays, as opposed to sturdier embankments possibly rock-filled and concrete lined. This could also minimise the reliability of the strategy as the embankments are eroded easily every year after floods, requiring an annual cost of $500 million to repair, making them more prone to breaches.

Example of successful dam building in terms of scale of strategy

China

• 1990-1999 annual death toll due to flooding was more than 1000

• Due to rapid dam building over past few decades, now with a total to 98000 dams, the annual death toll has only exceeded 1000 twice since 2011 despite the increase in intense rain events.

• Large-scale dams like the Three Gorges Dam minimised flood risk in the middle and lower reaches of the Yangtze RIver. The Three Gorges Dam is Capable of holding back 6.6 million cumec of water and has a lifespan of over 100 years, showing adequate scale of the strategy and sustainability of effectiveness over time. However, it cost US$37 billion to construct.

Evaluate the effectiveness river restoration as a flood management strategy using an example.

Singapore Bishan-Ang Mo Kio Park previously had a concrete channel, but the river was channelised - made shallower, but elongated from 2.7km to 3.2km.

• Biodiversity increased by 30% in the park after the naturalisation of the canal, with regular sightings of otters and egrets.

• However, most river restoration projects have limited effectiveness on a spatial scale as they only aim to restore short stretches of river. The regular flooding that occurs in a naturalised canal limits development on the floodplain and the adjacent environment. Many channels in Singapore remain concretised , putting many areas at risk of flash floods. Bukit Timah Canal overflowed in August last year, causing flooding in the area.

Example of afforestation as a flood management strategy

Recharge Pakistan Project

• Government collaborated with international organisation World Wildlife Fund to establish ecosystem-based flood management interventions in the Indus Basin.

• This includes the restoration of degraded forests in the Dera Ismail Khan district, estimated to reduce flood extent by 50800 hectares and capture 20 million cubic metres of water, reducing flood magnitude.

• The project received investment of $77.8 million from public and private funders, including a $66 million grant from the Green ClimateFund. $12 million of investment an technical support came from the Coca-Cola Foundation, the US Agency for International Development (USAID) and the WWF.

Loess Pleateau Project in western China

• The plateau is dry and hence highly erodable, contributing to severe flooding of the Yellow River

• Upstream restoration (regeneration of grasslands, trees and ship cover) on previously cultivated slope-lands allowed perennial vegetation cover to increase from 17% to 34%.

• Sediment flow has been reduced by more than 100 million tons per year, significantly reducing the risk of flooding.

Examples of sponge city infrastructure types

1. Detention lates: temporarily store storm water, then release it gradually into the channel to allow it time to drain

2. Permeable pavements which increase infiltration adn decrease infiltration excess flow

Example of sponge city infrastructure as a flood management strategy

China Sponge City Programme was initiated in 2014.

• substantial economic costs estimated at $15-22 million per square kilometre

• However, due to high costs of implementation, sponge city infrastructure is generally small-scale, and can only work effectively in certain areas, withstanding only low-magnitude floods.

• Since initiation in 2014, only 64 out of 654 cities introduced legislation to implement sponge city guidelines.

• Despite Zhengzhou city allocating 60 billion yuan to the programme, it was unable to deal with rainfall of 200mm per day.

Evaluate the effectiveness of flood warning and evacuation systems as a flood management strategy.

While lacks of communication technology and satellite technology limit the scale at which warnings can be disseminated, as well as the reliability of systems to predict floods efficiently, they can be improved over time with international aid and partnerships.

1. The National Centre for Atmospheric Research in the United States provides forecasts to Bangladesh. The country’s forecast system is also funded buy the United States to improve flood warnings. While this allowed Bangladesh to have a lead time of 2 to 3 days, further improvements were made through partnerships with the international organisation Asia Disaster Preparedness Centre, which helped establish a network of govt. and non-govt. agencies to ensure warnings reached people even rural areas, and with a lead time of up to 10 days, enabling large-scale evacuations and better emergency preparedness during floods.

2. China’s flood preparedness has improved over the years. In 2020’s rainy season, 4.7 million people were evacuated from floods, 50% more than the average of the previous five years. This can be attributed to weather forecasts increasing in accuracy and communication systems improving with technology, such as the setting up of flood forecasts through messaging app WeChat.