flood management - exeter Flashcards
physical causes of flooding (precipitation)
prolonged periods of rainfall saturate soil, subsequent rainfall flows quickly over surface causing river levels to rise rapidly, overflows
heavy rainfall can lead to flashflooding, occur with little warning, devastating
physical causes of flooding (geology)
impermeable rocks promote rapid overland flow of water towards river channels, speeds up water transfer, increases risk of flooding
physical causes of flooding (relief)
mountain environments, steep slopes encourage rapid transfer of water to river channels, increases risk of flooding
human causes of flooding (urbanisation)
building on floodplains creates impermeable surfaces (tarmac, concrete, slate) water in transferred quickly to drains, sewers and into river channels, rapid surge of water increases risk of flooding downstream, flash flooding often affects urban areas as drains can’t cope with volume of water
human causes of flooding (deforestation)
much of the water that falls on trees is intercepted by branches and leaves before being evaporated to air, or dripping slowly down, which slows the flow of water to river channels, also take in water through roots, without trees, water falls straight to floow and transfers rapidly to channels
human causes of flooding (agriculture)
in arable farming, soil is left unused and exposed to elements for several months, particularly in winter, leads to more surface runoff, increases risk of flooding, overgrazing and over-cultivation can damage and compact soil, increasing flood risk
what is a hydrograph
plots river discharge (cumecs - cubic metres per second) after a storm against time, how it rises after storm, reaces its peak and then returns to normal rate of flow
important aspect is lag time, the time in hours between the highest rainfall and highest discharge, shows how quickly water is transferred into river channel - key factor i flood risk, shorter lag time = greater risk of flooding
affects on shape of ‘flashy’ hydrograph with short lag time and high peak
basin size - small basins lead to rapid water transfer
drainage density - high density speeds up water transfer
rock type - impermeable rocks encourage rapid overland flow
land use - urbanisation encourages rapid water transfer
relief - steep slopes = rapid water transfer
soil moisture - saturated soil results n rapid overland flow
rainfall intensity - heavy rain may exceed infiltration capacity of vegetation, leads to rapid overland flow
affects on shape of low flat hydrograph with a low peak
basin size - large basins result in relatively slow water transfer
drainage density - low density leads to slower transfer
rock type - permeable rocks encourage slow transfer by groundwater flow
land use - forests slow down water transfer, because of interception
relief - gentle slopes slow down water transfer
soil moisture - dry soil soaks up water and slows down its transfer
rainfall intensity - light rain will transfer slowly and most will soak into the soil
hard engineering (dams and reservoirs
regulates river flow, reduces risk of flooding, irrigation, water supply, hydro-electric power generation, recreation
water can be stored in reservoir during periods of high rainfall, released when rainfall is low, construction can be controversial, v. costly, reservoir often floods, many people moved from homes
clywedog reservoir, prevents flooding on river severn, still a little bit of flooding downstream, prevented major floods
hard engineering (channel straightening)
cut through meanders to create straight channel, speeds flow of water, reduces risk of flooding in one location, could increase further downstream, shifts problem
sometimes concrete lines channel, prevents collapse and banks and speeds up flow, reduces river capacity, reduces range of aquatic habitats, reduces biodiversity, looks unattractive
hard engineering (embankments)
raised river bank, allows river channel to hold more water, relatively cheap as made from mud dredged from river, look like natural levees, in towns or cities, concrete embankments can be made but can block views, costly, reduces biodiversity
hard engineering (flood relief channels)
artificial channel constructed to bypass urban area, in times of high flow, gates can open to let water flow down relief channel, reduces risk of flooding on main channel
supports biodiverse aquatic ecosystems, provide leisure and recreation, can affect community access if footpaths are changed, farmers may lose land
jubilee river has been constructed on river thames near maidenhead, reduces risk of flooding, creates new wetlands - good for environment, popular for recreational activities
soft engineering (flood warnings and preparation)
river (basins) monitored remotely using satellites and technology, instruments measure rainfall, check river levels, models used to predict discharge and identify areas at risk of flooding
flood warnings sent out if flooding is likely
plan what to do if theres a flood warning, use flood gates to prevent property damage, use sand bags to keep water away
local authorities use flood maps to plan responses - evacuating people, closing roads, securing buildings
perceived by public as being accurate and appropriate, some areas blighted by being marked as “at risk” increase insurance premiums and reduce property value
soft engineering (flood plain zoning)
restricts different land uses to certain locations on floodplain, areas closer to the river kept clear of high-value land uses such as housing and industry
highly cost effective, reduces vulnerability and exposure to flooding
hard to apply to towns and cities, may blight certain areas and property prices may fall
