2 Flood HydrographsLO: To describe what a flood hydrograph shows and how it can be affected by the drainage basinbbc footageAll will be able to name the features shown on a flood hydrographMost will be able to read data from a flood hydrographSome will be able to describe how the stages in a flood hydrograph can affect people2 hour hydrograph
3 HydrographA hydrograph is a line that shows river discharge over a period of time.Hydrographs of rivers in flood help show the link between rainfall and river level. This helps people to predict possible floods.
7 LagtimeLag time is the amount of time it takes rain to get into a river.It can be affected by lots of things including; rock type, steepness of hills, vegetation slopes and whether the area is built up.1. What is the peak rainfall on the Boscastle hydrograph? What time did it occur? 2. What is the discharge of the river Valency before the rain (the baseflow)? 3. What time was the peak discharge at Boscastle? How much water was in the river then? 4. What is the lag time on the Boscastle hydrograph i.e. how long did it take the rain water to reach the river?
9 Boscastle Floods 16th August 2004 In the middle of their peak tourist season in 2004 Boscastle wasBoscastle
10 Factor Short lag time Long lag time Rock type Vegetation Hill slope Land usePut the statements into their correct spot in the tableBare earth with few plantsPermeable (water soaks into it)Steep valley sidesRural farmlandLots of trees and plantsImpermeable (water doesn’t soaks in)Built up area – lots of tarmacGentle valley sides
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