Extremes: (flash) floods
12 important questions on Extremes: (flash) floods
Give 2 definitions for flash floods?
What is the Brown Willy effect?
Occurs on peninsulas, which are more elevated and rough, causing friction. These areas often have severy precipitation.
Friction and uplift creates a convergence zone near the central spine of the peninsula, leading to condensation of the moisture borne by tehm.
It is thought that the Boscastle flood of 2004 was caused by a particularly extreme example.
Which meteo phenomena cause a Hupsel vs a catchment scale peak discharge, possibly causing floods?
Meso-Convective Scale: catchment scale
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What is the relation between watershed area and peak discharge (in relation to flash floods)?
Knick point whereafter lag time increases more rapidly with increasing watershed area.
Accurate flash flood estimates are difficult to obtain, because (name 2):
- Instrumentation might overflood, thus afterward individual field estimation of floodmarks, slope of peak, etc must be obtained
- Hupsel was lucky case, since peak discharge occurred right there
- They are rare
Runoff coefficient: how much rainfall ends up as runoff
- Storage capacity of the soil needs to fill up first
- High volume of P necessary to get a reasonable runoff coefficient
How does the rating curve influence flash flood modelling?
Post-event surveys and interviews can help to estimate and constrain flow conditions during event.
What is the impact of rainfall distribution, which was also discussed by Lobligeois?
- Aggregated data (min 33)
- Forced model with different P resolution
- Lumped run, different resolutions for distributed runs
- Lumped approach does not really work
- Some distribution really improves the model results
- Especially for summer estimations of flash floods this is imperative!
- In winter it is not as relevant'
- Also for highly variable catchments (spatial?) it is relevant
What are the main influence factors on modelling flash floods?
- Watershed area(?)
- Runoff coefficient
- Initial conditions
- Rating curve
- Rainfall distribution
- Storm movement
Min 38/slide 27: What is effective rainfall?
Influence of storm movement
Rainfall moved from upstream to outlet at more or less same speed as catchment response(=physical flood wave propagation). Good motivation of rainfall data, only source of info on how individual storms are moving.
Track movement, direction of movement wrt catchment area (in addition to P dist) is very important for flash flood forecasting and modelling.
Slide 30: wrapping up (min 41)
Storm characteristics (distribution and movement)
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