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Geography Lecture 10 part 1 (3. Factors affecting hydrographs (Factors…
Geography Lecture 10 part 1
1. Introduction
Hydrographs
Line graphs that plot changes in a river's discharge over time
Shows discharge in response to variation in precipitation
2. Types of hydrographs
Types
Few days
Shows effects of antecedent moisture in the soil
Storm (individual event)
Shows how quickly the river discharges in response to the precipitation event
Annual
Shows a river's regime
Components
Rainfall Histogram
Shows intensity and amount
Shows rainfall/discharge relationships
Rising limb
Increase in discharge as initial runoff reaches the river
Steeper = higher chance of flood
Peak discharge
Highest level (multiple peaks may mean arrival of water from multiple sources)
Falling limb
River begins to subside
Base flow
Throughflow and Baseflow slowly increase
Bankfull discharge
Height at which a flood will occur
Discharge line
Amount of discharge in a river
Approach segment
Discharge levels before rainfall
Peak rainfall
Highest rainfall
Lag time
Amount of time between peak rainfall and peak discharge
Shows time taken for runoff to reach river
(short = flood prone)
Storm runoff
Runoff from rainfall that reaches river channel
3. Factors affecting hydrographs
Most important parts
Lag time
Short lag time = high flood risk
Factors
Length of precipitation event
Shorter lag time
Too much rain, causes overland flow
Intensity of precipitation
Rainfall faster than infiltration, excess overland flow
Type of precipitation
Snow stays on the surface and water is released slowly over long periods of time
Drainage basin size
Small basin = shorter lag time
Takes less time for water to reach river in a smaller basin
Shape of basin
Round = more flood prone
Water reaches river at the same time
Relief
Steeper = more runoff
Vegetation
Vegetation intercepts rainfall and slows down water from reaching rivers
Soil type
Sandy soils have more spaces for water to flow, thus resulting in lower lag times
Human activity
Urbanisation
Higher surface runoff, and sewers may take water quickly to rivers
Deforestation
Less interception and evapotranspiration, more surface runoff
Dams
Increasing lag time by holding back water
Key processes
Surface runoff
Throughflow
Groundwater
Direct precipitation
Additional Information
Shorter lag times = "Flashy" catchment
4. Purpose of hydroraphs
Allows researches with info about flood management and prevention
Previous floods will also show how a river will react to high input of precipitation
Allows early warning and reduce flood risks
5. Case Study
Carlislie, 2005
Jan 10, 2005, heavy storms
River Eden flooded
The natural floodplain was developed
70k left without power, 3 died
Bocastle, 2004
16th and 17th August, 2004
Wet month, soils were mostly saturated
Sudden heavy rainfall
River Valency, steep valleys
2.75m of water at peak
Tourism destroyed
River Ribble, Lancashire
Short lag time, rising and falling limbs are steep, but little floods
Rock is largely impermeable
Soils are a major store of groundwater (peat)
Very long drainage basin
Consistent rainfall annually
Urbanisation and agriculture has reduced infiltration rates