W8: Floods

Water Balance

Why flooding?

Types of flooding:

1. river flooding

2. groundwater flooding

3. surface water flooding

4. coastal flooding

5. sewer flooding

6. reservoir flooding

7. canal flooding

river flooding

also known as fluvial flooding, occurs when the capacity of a rivers channel is exceeded as a result of intense or sustained rainfall across the catchment

groundwater flooding

groundwater flooding occurs when the water table rises up to the surface during a prolonged wet period.

low-lying areas, areas near aquifers and properties with cellars or basements are more likely to experience groundwater flooding

surface water flooding

surface water flooding occurs when the volume of rainfall is unable to drain away through the drainage systems or soak into the land, and instead flows over land. Blocked drains and sewers can increase the risk of surface water flooding as the water has nowhere to go.

coastal flooding

a coastal flood is when the coast is flooded by the sea

sewer flooding

most sewer flooding incidents are the result of overloaded sewers following heavy rainfall, or due to blockages caused by misuse of the sewer system such as flushing unsuitable items down the toilet

reservoir flooding

can be similar to river and surface water flooding if the water escapes slowly. However, in the unlikely event of the dam wall falling, a large amount of water could escape. It could happen with little or no warning and you may need to evacuate immediately.

canal flooding

occurs when the level of water in the canal is too high and overtops - however, this is rare.

Climate change and rainfall - Clausius-Clapeyron relation

Flooding management

1. nature-based solutions

2. gravel management

3. monitoring

4. community resilience

nature-based flooding solution

Green roof:

• Plant level → growing medium → filter sheet → drainage layer → protection layer → waterproofing → roof construction

what is return period?

Exceedance probability

what is a 100-year flood?

A 100-year flood is a highly severe flood event that has a 1% chance of happening or being exceeded in any given year. Also known as a 1% flood, its name is based on statistical probability rather than a strict timeline, meaning such a flood could theoretically occur multiple times in a decade.

controversy over 100-year flood

Easy confusion in communication

• Something people think they understand but often don’t

• Every year you will read similar explainers on newspaper
  

• The shaky – no-so-sound scientific – foundation

  • Limited observations – large uncertainty

  • Changing climate – non-stationarity