Flood Management Strategies

What are some ways to control floods? [LDF]

• Levees

• Dams

• Flood abatement

What are levees?

Increase height of channel bank & increase storage capacity of the channel.

Limitations of levees

• Silt deposition can raise the river bed and increase flood heights (can lead to breaching).

• When breached/overtopped, levees result in longer periods of inundation as floodwaters cannot drain into the river as easily.

Example of levee failure due to breaches

New Orleans, Hurricane Karina (2005); Led to breaches in levees of over 50 locations, flooding 80% of the city and resulting in deaths of over 1500 people and over $100 billion in damages. Most of the flooding was as deep as 10 feet, which took weeks to recede. Inadequate design and construction, negligent maintenance and overestimation of soil strength led to failure.

Example of levee failure due to silt deposition

Yangtze Basin; About 3600km of main river levees and 30,000km of tributary levees that protect farmland, oilfields and cities. However, slit deposition led to the riverbed rising higher and higher over the years. The level of the Yangtze in a high flood is now 10m above the land behind the 16m tall levee, which is being held in only by the levees. Many levees are old and subject to breaching.

Functions of Dams

Offer temporary storage of water such that flood peaks downstream can be reduced. Often, they are multi-purpose (flood control, hydropower, tourism etc.)

Limitations of dams

• Sedimentation reducing effective capacity of reservoir, shortening its lifespan

• Dam failures; human error or triggered by earthquakes, causing severe floods downstream.

• Conflict between different groups of people (social and environmental factors)

Example of limitation of dams: Dam failure

2018 floods in Kerala India; Dams were quickly filled due to the high rainfall (310mm in 24hrs), which is more than 250% of its average rainfall. 35 of 54dams were opened simultaneously to avoid overtopping, but led to flooding of low-lying areas downstream, leading to the deaths of around 500 people and economic losses of about $2.8 billion, with entire harvests of coffee, tea and rubber being lost.

Example of limitation of dams: Conflicts between different groups of people

China’s dam construction on the upper Mekong; suffered declining fisheries and changing water levels along the Thai-Laos border.

Flood Abatement

Reducing overland flow contributing to floods across an entire river basin through land use management.

Limitations of flood abatement

• Need to be implemented in more than half the drainage basin (better for small to medium sized basins)

  • Time lag in larger basins due to extensiveness of strategy needed

  • Drainage basins spanning multiple countries: downstream nations unable to influence adoption of such measures in upstream areas.

How Do We Adapt To Floods?

• Land use zoning

• Flood proofing

• Forecasting & warning systems

What is land-use zoning?

Dividing the floodplain into areas which experience different degrees of flood risk and restricting development of high risk areas to minimize damage.

Limitations of Land-Use Zoning

• Unrealistic for urban areas; face resistance from property owners that are forced to relocate

• Shortage of housing — need for development to meet housing demands

• Harder to enforce planning regulations in developing countries.

What is Flood Proofing?

Modifying vulnerable buildings and their contents to withstand flood damage better.

• Elevation, wet flood proofing, dry flood proofing and flood walls are some strategies.

• Temporary measures: sealing doors and windows with shields, using sandbags to protect structures from water.

What is Forecasting & Warning?

Technology (satellite, radar and real-time gauge data) have improved accuracy of flood forecasting through better modeling of rainfall and river flow.

Limitations of Forecasting & Warning

• Human factors:

  • Vulnerable populations do not respond in time,

  • LDCs face further constraints due to limited tech, poor communication and low literacy levels.

• Tech limitations: Respond times are short; warnings are often inaccurate or delayed in smaller basins.

What is Disaster Aid?

Meet immediate needs of survivors: Search and rescue + provisioning of basic amenities and medical care.

LDCs: international aid (reconstruction)

• Mozambique, Feb-March 2000; Following exceptionally heavy rainfall, severe flooding in Mozambique inundated about 12% of cultivated land, causing total losses of around US$980 million. The scale of damage highlights how recovery from such disasters often exceeds the capacity of developing countries, necessitating international financial assistance.