Plenary Why did the Mississippi floods of 1993 occur? How do waterfall’s form? What do we mean by competence and capacity?
Learning Objectives For students to understand the principle of flood frequency and how to calculate this Students to appreciate and demonstrate that there are both human and physical causes of flooding That there are various strategies that can help alleviate the effects of flooding
The Story so far... Students have already provided an in-depth analysis of the Mississippi floods of 1993. Have some knowledge of why Bangladesh floods We understand the principles of flooding So why does it happen and what can we do?
Why do Rivers Flood? Rivers flood naturally People live near rivers from both a historical as well as a cultural perspective Floods are a natural hazard Many rivers flood consistently Humans alter natural landscapes flood risk can increase.
Flood Frequency How to calculate flood frequency: Size of largest flood for each year is ranked with 1 being the largest for all available records for any given year T = n+1 m T = Recurrence intervals n = The number of years of observation m = The rank Order
What does this tell us? It indicates the number of years within which a flood of this size can be expected What problems can you see in this equation? It’s based on historical data It doesn’t mean floods can’t occur less or more frequently The EA recommend that that flood defences should be built to withstand a 1 in 50 year flood The EA however build flood defences for a 1 in a 100yr flood
Activity Complete the flood frequency activity handed out
Physical Causes Flooding occurs when a river’s discharge exceeds the capacity of it’s channel to carry the discharge Excessive levels of precipitation over a prolonged period of time. Leads to soil saturation when water table reaches ground surface there is an increase in overland flow. Intensive Precipitation over a short period of time especially on hard sun baked ground.
Physical Causes Snowmeltespecially when subsoil is still frozen as there is limited infiltration capacity. Climatic Hazards – Cyclones in Bangladesh, Hurricanes in Gulf of Mexico. Deep low pressure systems in mid latitude climates Relief– The slope of the river Basin Vegetation – Can help increase infiltration and thus reduce surface run off
Physical Causes Geology – Rock type both pervious and porous Soil Type – Sand to clay type soils and changes in infiltration Drainage Density – Amount of surface streams within the basin
Example of Floods Physical causes of floods have caused floods such as: Boscastle August 2004 Bangladesh 1998
What factors Increased the risks of floods in Boscastle?Watch the video and you decide.
Human Causes Urbanisation – Reduces infiltration and increases runoff Deforestation – Reduces interception River Management – Dams can cause issues along with other techniques Climate Change – Heavier storms, more frequent, melting of ice caps
Activity Complete the table activity on your sheets