flooding

bernd

types of flood

flood- temporary inundation of normally dry land by water

fluvial (river floods)

pluvial (rainfall-induced flash floods)

costal (storm surge)

physical causes of floods

RIVER FLOODS:

• atmospheric hazards (rainfall, snowmelt, ice jam)

• tectonic hazards (landslides)

• technological hazards (dam failures)

COASTAL FLOODS:

• atmospheric hazards (storm surges)

• tectonic hazards (tsunamis)

coastal floods: 1953 North Sea storm surge

• northerly gales- force wind driven water southwards

• 1-2m surge

• 2,000 people lost their lives

drainage basin

conditions (hydraulic geometry, size of basin) determine speed of flood onset, flow velocity, peak flow and event duration

floodplain

flat landform adjacent to river channel composed of alluvium and subject to flooding

floodway

part of the floodplain where there is rapidly flowing water in floods

flood prone environments

• low lying parts of floodplains (Bangladesh)

• low-lying coasts and deltas (Vietnam)

• Basins subject to flash floods (Kuala Lumpur, Malaysia)

• Areas below safe or inadequate dams (Derna, Libya)

• Low-lying inland shorelines (Great Lakes, USA)

• Alluvial fans (arid American West)

Measuring floods

1. discharge- volume of water through a cross-section

2. stage- level of water over a datum

3. flood stage- stage at which water body rises to a level that causes inundation of land and danger to human life

flood frequency: before observational records

Palaeoflood data

Tree rings

Geomorphic processes can cause a disturbance and bring about changes in the morphologyFl

Flood intensification

CLIMATIC CHANGE

• increased hourly to daily rainstorm magnitude-frequency

• increased monthly/seasonal rainfall

• more extreme depressions and tropical cyclones

• sea-level rise

HUMAN INFLUENCES

• land use change

• higher runoff + situation= reduced channel capacity

• increased occupancy of floodplains; urbanisation

• poverty and migration

• inadequate flood protection measures

flood hazards- primary

• damage and loss of property and infrastructure

• sediment deposited

• loss of crops and livestock

• drowning

flood hazards- secondary

• drinking water quality

• disease

• mental health

• services disrupted

flood hazards- long term

• insurance rates increase

• river channels change

• agricultural land destroyed

flood events- europe

• low fatality rates but high economic damages

• since 1970: most frequent disaster

flood events- asia (Bangladesh and China)

• 90% if all flood-related deaths and 50% of economic damages

• Since 1900: 5 most disastrous river floods worldwide

populations exposed to flood risk

france- 2.5%

uk- 4.8%

us- 12%

netherlands- 50%

vietnam- 70%

bangladesh- 80%

exposure is related to high rural population densities and the location of urban areas along rivers and coasts

2021 European floods

affected- UK, germany, belgium, austria, netherlands, luxembourg, italy

causes- intense rainfall events (12-15 july), 271.5mm over 48h in belgium

effects- 243 deaths (196 in germany), estimated £8 billion total damages

forecasting and warning

• modelling of storm rainfall and storm flows

• real time data handling

• coupling of weather, precipitation data and hydrological models

• flash flood forecast- problematic

• warning systems

• communication

protection

• embankments and seawalls (compacted earth mounds, deep foundations)

• storage dams (temporary, upstream)

• flood abatement (soft engineering e.g. afforestation, local scale)

embankments

• design criteria and location are crucial

• ongoing maintenance (hurricane katrina, embankments failed due to construction defects and poor maintenance)

• impacts on natural riparian environments if accompanied by channel enlargements

vulnerability modification- preparedness

• especially important in LDCs

• overseas aid/ red cross

• traditional practices

• hazard awareness & education

• emergency and evacuation response

vulnerability modification

• land use planning

• balance between maximising benefits of floodplain occupation & minimising flood losses

• flood risk mapping

• purchase of flood-prone land & relocation

• living with floods approach

what makes people vulnerable?

• demographic characteristics (age, race, gender, family)

• socioeconomic status (wealth, education, job)

• health

• risk perception (awareness, prior experience)

• coping capacity

flood response & recovery

disaster aid

• agricultural crop losses (LDCs)

• pakistan 2010 (food security= main requirement)

-1.3 million ha of standing crops destroyed

-1.2 million animals drowned

-land recovery difficult

insurance

• loss-reducing decision making

• uk insurers not automatic