IGCSE Geography: Flooding: Why it happens, and how to prevent it Flooding Causes of Flooding 1. Precipitation: Rain, Snowmelt High levels of Precipitation- River bursts its banks and floods 2. Steep Slopes: Gradient of Land If land is particularly steep, normal storm events may escalate into small floods. This is because of a short lag time. When there is rain, steepness of land means not long after, there will be discharge/ flood. 3. Permeability of surface: Soil, Vegetation, Rock type ability to absorb water Good permeable surface: Water is absorbed during rainfall, less chance of flood Impermeable: Water can not be absorbed, and instead flows off the surface leading to flood 4. Human activities: Developments, Settlements If a area is heavily built up, eg. Tarmac roads (impermeable)- Water has no way of being absorbed naturally (no vegetation, rocks and soil) , thus the lack of absorbent material in urban landscapes means rainfall could lead to flooding here. How Flooding is controlled Classifications NIMBY- (acronym: Not in my backyard) – This regards to saving floods from locals ONLY Catch and Hold- Prevent the Flood, Slow it down Run away- Accept floods occur, be in a safe zone, eg. Stilted buildings when floods occur Settlements can be impacted by flooding on both the higher and lower courses of rivers, and stopping a flood at one location means that the flood may still be visible downstream. Preventions Here are a few of the main ones you should remember!
1. Floodwalls : A nimby technique, this is a reinforced, retractable wall which blocks off water level rises, to protect a certain location 2. Channelization : This means the river stream is straightened using certain equipment, the river flood is diverted away downstream at a high velocity. 3. Control Dams : These are reservoirs, large areas of space which take in excess water rises for control. Similar to wells and catchments. The water trapped can be purified and used by the population Floodplain – is the area of land likely to flood in the event of rainfall, storms. 4. Floodways : Prohibiting development in certain areas, the floodplain, no human settlements etc. Floodways allow for protection against 100 year floods 100 year floods : Big scale floods which occur rarely, “once in a hundred years” 5. Afforestation : Planting trees improves the surface area for infiltration, and water being absorbed reduces risk of flooding 6. Dredging – A costly method to remove silt in streams. Taking out silt gives higher water capacity . Disadvantage: Damages ecosystem, heavy machinery, change in water habitat. 7. Levees – These can be natural or artificially made. Natural – When river speed decreases, from friction with the floodplain, the load it is carrying is deposited. The coarsier, heavier material is deposited first onto the floodplain. Over time, the material layers together, and actually forms a barrier on the river edge. This barrier prevents small floods when a river bursts its banks. Artificial – Humans often use local resources, wood, bark etc. to create small levees on the river edge and floodplain. This acts as local protection, protecting their houses, animals, crops. It is also important to understand there are Hard flood management techniques and Soft Flood management techniques Hard : Long Lasting, Slightly expensive (eg. Dredging) Soft : Local scale, done by locals and not expensive, done with local material, very small scale (eg. Levees)