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1、ConstructionAndAnalysis of HydrographsMicrosoft Word clipartMicrosoft Word clipartHigher Geography - HYDROSPHEREHydrographRecord of River Discharge over a period of timeRiver DischargeThe flow of the river recorded at a particular time at a particular point.Storm HydrographsShow the change in discha
2、rge caused by a period of rainfallWhy Construct & Analyse Hydrographs ?To find out discharge patterns of a particular drainage basinHelp predict flooding events, therefore influence implementation of flood prevention measuresMicrosoft Word clipartConstructionOf Storm (flood) HydrographsMicrosoft Wor
3、d clipart0 12 24 36 48 30 72Hours from start of rain storm321Discharge (m3/s)Base flowThrough flowOverland flowRising limbRecession limbBasin lag timemm432Peak flowFlood Hydrograph0 12 24 36 48 30 72Hours from start of rain storm321Discharge (m3/s)0 12 24 36 48 30 72Hours from start of rain storm321
4、Discharge (m3/s)mm432Rainfall shown in mm, as a bar graph 0 12 24 36 48 30 72Hours from start of rain storm321Discharge (m3/s)mm432Discharge in m3/s, as a line graph0 12 24 36 48 30 72Hours from start of rain storm321Discharge (m3/s)Rising limbmm432Rising limbThe rising flood water in the river0 12
5、24 36 48 30 72Hours from start of rain storm321Discharge (m3/s)Rising limbmm432Peak flowPeak flowMaximum discharge in the river0 12 24 36 48 30 72Hours from start of rain storm321Discharge (m3/s)Rising limbRecession limbmm432Peak flowRecession limbFalling flood water in the river0 12 24 36 48 30 72H
6、ours from start of rain storm321Discharge (m3/s)Rising limbRecession limbBasin lag timemm432Peak flowBasin lag timeTime difference between the peak of the rain storm and the peak flow of the river0 12 24 36 48 30 72Hours from start of rain storm321Discharge (m3/s)Base flowRising limbRecession limbBa
7、sin lag timemm432Peak flowBase flowNormal discharge of the river0 12 24 36 48 30 72Hours from start of rain storm321Discharge (m3/s)Base flowThrough flowOverland flowRising limbRecession limbBasin lag timemm432Peak flowOverland flowThrough flow+= Storm FlowVolume of water reaching the river from sur
8、face run offOverland flowThrough flowVolume of water reaching the river through the soil and underlying rock layersAnalysisMicrosoft Word clipartFactors influencing Storm Hydrographs Area Shape Slope Rock Type Soil Land Use Drainage Density Precipitation / Temp Microsoft Word clipartInterpretation o
9、f Storm HydrographsRainfall IntensityRising Limb Recession Limb Lag timePeak flow compared to Base flow Recovery rate, back to Base flowYou need to refer to:Basin lag time0 12 24 36 48 30 72Hours from start of rain storm321Discharge (m3/s)Base flowThrough flowOverland flowRising limbRecession limbmm
10、4 3 2Peak flowWhen interpreting hydrographs all factors must be considered together !Here are some theoretical interpretations of influencing factorsBUTMicrosoft Word clipartAreaLarge basins receive more precipitation than small therefore have larger runoffLarger size means longer lag time as water
11、has a longer distance to travel to reach the trunk riverAreaRock TypeDrainage DensityShapeSoilPrecipitation / TempSlopeLand UseTidal ConditionsShapeElongated basin will produce a lower peak flow and longer lag time than a circular one of the same sizeAreaRock TypeDrainage DensityShapeSoilPrecipitati
12、on / TempSlopeLand UseTidal ConditionsSlopeChannel flow can be faster down a steep slope therefore steeper rising limb and shorter lag timeAreaRock TypeDrainage DensityShapeSoilPrecipitation / TempSlopeLand UseTidal ConditionsRock TypePermeable rocks mean rapid infiltration and little overland flow
13、therefore shallow rising limb AreaRock TypeDrainage DensityShapeSoilPrecipitation / TempSlopeLand UseTidal ConditionsSoilInfiltration is generally greater on thick soil, although less porous soils eg. clay act as impermeable layersThe more infiltration occurs the longer the lag time and shallower th
14、e rising limbAreaRock TypeDrainage DensityShapeSoilPrecipitation / TempSlopeLand UseTidal ConditionsLand UseUrbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lagAfforestation - intercepts the precipitation, creating a shallow rising li
15、mb and lengthening the time lag AreaRock TypeDrainage DensityShapeSoilPrecipitation / TempSlopeLand UseTidal ConditionsDrainage DensityA higher density will allow rapid overland flowAreaRock TypeDrainage DensityShapeSoilPrecipitation / TempSlopeLand UseTidal ConditionsPrecipitation & TemperatureShort intense rainstorms can produce rapid overland flow and steep rising limbIf there have been extreme temperatures, the ground can be hard (either baked
