River Flooding Notes 1.4 pt 2

Figure 1.44 illustrates flood risk with areas marked as significant (1-75 year flood) and moderate (76-200 year flood) risk.

Rainfall Radar: Plots the approach of rain-bearing weather systems; data is fed into the Agency's computer system.
Tipping Rain Gauges: Monitor rainfall throughout the drainage basin, feeding data into the system.
River Discharge Gauges: Automatic gauges monitor rising river levels along the river, adding data to the system.
System Database: Includes a model of the drainage basin's behavior based on past flood events (storm hydrographs).
Computer System Prediction: Compares incoming data with past events to predict river levels at different points, including flood peak height and arrival time.
Environment Agency Warnings: Issues detailed flood warnings based on predictions, allowing people time to prepare.

Death of people and animals
Damage to buildings, infrastructure, and farmland
Disruption to people's lives (social, e.g., homelessness; economic, e.g., damage to businesses)
In HICs, economic costs are typically higher, while LICs often experience higher death tolls.

Success depends on:
- Level of economic development (influences emergency services, infrastructure, recovery ability).
- Willingness to spend money on preparation and alleviation.

Water supplies to Shanghai (13 million people) are secure.
Generates 10% of China's electricity (equivalent to 15 nuclear power stations) - clean energy, reduces air pollution and impact to climate change.
Boosts economic growth in central and eastern China (Wuhan, Shanghai).
The Chang Jiang is navigable by ships up to 10000 tonnes up to Chongqing.
Expected 500% increase in river traffic will boost economic growth.

Reservoir flooded 150 towns/cities and 1300 villages.
1.2 million people resettled (compensation insufficient).
Reservoir heavily polluted by toxins from flooded mines/factories, damaging the ecosystem and endangering species (White Flag River Dolphin, Siberian Crane).
Landslides are expected due to water seeping into valley sides (earthquake risk).
Silt deposition in the reservoir reduces capacity; dredging or flushing will be needed.
Loss of downstream farmland fertilization by silt; increased need for chemical fertilizers.

More buildings on the floodplain reduce space for floodwater to flood, increasing flood levels.
Deforestation in the headwaters (1960s) reduced interception, evapotranspiration, and infiltration, increasing surface runoff and flood peaks.
Mismanagement of deforested land (overgrazing) led to soil erosion and siltation of the river channel.
Canalization straightened the river, speeding up flow, reducing lag time, and increasing flood peak.
Flood protection embankments constrain the river, leading to rapid and deadly flooding when breaches occur.
Loss of Dongting and Poyang Lakes as safety valves due to farmland reclamation (polders), increasing flood peak levels.
Potential increased rainfall due to global warming.

240 million people affected in seven provinces.
4000 people drowned.
Thousands of farm animals died.
Huge areas of crops (rice, cotton) destroyed.
Deliberate breaching of flood protection embankments to protect Wuhan, flooding countryside areas.
14 million people homeless for months.
People out of work due to factory repairs.
Thick layers of clay deposited on the fertile fields.
Enormous cost to China's economy.

Located at Sandouping in a deep, narrow section of the valley (Three Gorges).
Reservoir is 660 km long and 1 km wide, extending upstream almost to Chongqing.
Largest hard engineering project ever undertaken on a river.
Construction started in 1994 and completed in 2006; reservoir filled in 2009.
Dam is 2.3 km long and almost 200 meters high.
Gigantic ship locks built at the northeast end, and an 18000 megawatt H.E.P. station built inside the southwest end.
Cost in excess of US$ 38 billion.

Flood risk assessment looks at probability and impacts of flooding.
Flood management strategies are grouped into three categories:
- Forecasts and warnings to allow behavioral strategies (adjusting lifestyles and taking personal responsibility).
- Hard engineering solutions (building something).
- Soft engineering strategies (working with nature).

Ensure understanding of flood forecasting and warning services to allow time to move animals, furniture, and evacuate people.
Have an emergency plan at the household and town level.
Organize at-risk houses with tiled floors, moveable mats, drains, cupboards/plug sockets above flood level, wide stairs, and storage space upstairs.
Take out insurance; premiums depend on flood risk.

Dams and reservoirs can store floodwaters (if not already full).
Platforms can be built on the floodplain to raise buildings above flood level.
Embankments along the river are effective but expensive.
Dredging the river lowers the bed but is not a permanent solution.
Straightening the river speeds up flow but causes bigger floods downstream.
Retention basins divert floodwaters at times of crisis with restrictions on land use.
Flood relief channels take away excess water around a town, but cause worse flooding downstream.

Floodplain zoning restricts land use to things less affected by floods (e.g., sports pitches); avoids building houses in high-risk areas.
Rely on washlands. Land upstream is allowed to flood to protect towns.
Plant trees in the upper drainage basin to encourage interception, evapotranspiration, and infiltration.
Wetland and riverbank conservation schemes protect natural floodplain areas.
River restoration schemes return rivers and floodplains to their natural state.