Construction And Analysis Of Hydrographs

Hours from start of rain storm Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb Basin lag time mm Peak flow Flood Hydrograph

Hydrograph Record of River Discharge over a period of time River Discharge = cross sectional area rivers mean (average) velocity X (at a particular point in its course) Storm Hydrographs Show the change in discharge caused by a period of rainfall

Why Construct & Analyse Hydrographs ? To find out discharge patterns of a particular drainage basin Help predict flooding events, therefore influence implementation of flood prevention measures

Construction Of Storm (flood) Hydrographs

Hours from start of rain storm Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb Basin lag time mm Peak flow Flood Hydrograph

Hours from start of rain storm Discharge (m 3 /s)

Hours from start of rain storm Discharge (m 3 /s) mm Rainfall shown in mm, as a bar graph

Hours from start of rain storm Discharge (m 3 /s) mm Discharge in m 3 /s, as a line graph

Hours from start of rain storm Discharge (m 3 /s) Rising limb mm Rising limb The rising flood water in the river

Hours from start of rain storm Discharge (m 3 /s) Rising limb mm Peak flow Maximum discharge in the river

Hours from start of rain storm Discharge (m 3 /s) Rising limb Recession limb mm Peak flow Recession limb Falling flood water in the river

Hours from start of rain storm Discharge (m 3 /s) Rising limb Recession limb Basin lag time mm Peak flow Basin lag time Time difference between the peak of the rain storm and the peak flow of the river

Hours from start of rain storm Discharge (m 3 /s) Base flow Rising limb Recession limb Basin lag time mm Peak flow Base flow Normal discharge of the river

Hours from start of rain storm Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb Basin lag time mm Peak flow Overland flow Through flow + = Storm Flow

Volume of water reaching the river from surface run off Overland flowThrough flow Volume of water reaching the river through the soil and underlying rock layers

Hours from start of rain storm Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb Basin lag time mm Peak flow

Analysis

Factors influencing Storm Hydrographs Area Shape Slope Rock Type Soil Land Use Drainage Density Precipitation / Temp Tidal Conditions

Interpretation of Storm Hydrographs Rainfall Intensity Rising Limb Recession Limb Lag time Peak flow compared to Base flow Recovery rate, back to Base flow You need to refer to: Basin lag time Hours from start of rain storm Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb mm Peak flow

Following, are some theoretical interpretations of influencing factors BUT………… When interpreting hydrographs all factors must be considered together !

Area Large basins receive more precipitation than small therefore have larger runoff Larger size means longer lag time as water has a longer distance to travel to reach the trunk river AreaRock TypeDrainage Density ShapeSoilPrecipitation / Temp SlopeLand UseTidal Conditions

Shape Elongated basin will produce a lower peak flow and longer lag time than a circular one of the same size AreaRock TypeDrainage Density ShapeSoilPrecipitation / Temp SlopeLand UseTidal Conditions

Slope Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time AreaRock TypeDrainage Density ShapeSoilPrecipitation / Temp SlopeLand UseTidal Conditions

Rock Type Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb AreaRock TypeDrainage Density ShapeSoilPrecipitation / Temp SlopeLand UseTidal Conditions

Soil Infiltration is generally greater on thick soil, although less porous soils eg. clay act as impermeable layers The more infiltration occurs the longer the lag time and shallower the rising limb AreaRock TypeDrainage Density ShapeSoilPrecipitation / Temp SlopeLand UseTidal Conditions

Land Use Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag. Steeper recession limb. WHY? RISING LIMB / LAG TIME: Shallower in rural areas – increased interception Vegetation intercepts and stores water slowing it’s passage into the river system In Urban areas concrete, tarmac etc. channels water quickly into drains and straight into river / sewage system

Land Use Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag. Steeper recession limb. WHY? STORAGE : Falling limb will be steeper on an urban hydrograph due to lack of infiltration, storage In rural areas water will continue to flow into the river for many hours from underground flow and through flow The return to base flow is much slower in rural areas

Land Use Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag. Steeper recession limb. WHY? Afforestation - intercepts the precipitation, creating a shallow rising limb and lengthening the time lag. Shallower recession limb. WHY? AreaRock TypeDrainage Density ShapeSoilPrecipitation / Temp SlopeLand UseTidal Conditions

Drainage Density A higher density will allow rapid overland flow AreaRock TypeDrainage Density ShapeSoilPrecipitation / Temp SlopeLand UseTidal Conditions

Precipitation & Temperature Short intense rainstorms can produce rapid overland flow and steep rising limb If there have been extreme temperatures, the ground can be hard (either baked or frozen) causing rapid surface run off Snow on the ground can act as a store producing a long lag time and shallow rising limb. Once a thaw sets in the rising limb will become steep AreaRock TypeDrainage Density ShapeSoilPrecipitation / Temp SlopeLand UseTidal Conditions

Tidal Conditions High spring tides can block the normal exit for the water, therefore extending the length of time the river basin takes to return to base flow AreaRock TypeDrainage Density ShapeSoilPrecipitation / Temp SlopeLand UseTidal Conditions

Remember these influencing factors will: Influence each other Change throughout the rivers course

ANALYSING HYDROGRAPHS 1.Discharge does not increase immediately (only a small amount of precipitation falls directly into the river) 2.Water reaches the river from overland flow / surface run-off 3.This is then supplemented with water from throughflow 4.Rising limb shows overland flow 5.Recession limb indicates there is still water in the system – e.g. throughflow 6.Rivers with a short lag time and high discharge are most likely to flood 7.Two basins which receive the same amount of rain can have very different flood hydrographs

ANALYSING HYDROGRAPHS 1.____ ____ will be higher in larger basins 2.If slopes are steep, ______ will be lower, therefore peak flow will be greater 3.If basin is flatter, there will be more infiltration resulting in peaks 4.The _______ the slope – the shorter the lag time A combined atmosphere / hydrosphere question can appear: ITCZ Hydrographs Peak Flow infiltration lower steeper

FLOOD HYDROGRAPHS TASK: Using pages 43  55 Answer questions 1  5 on page under the heading ‘Flows and Flood Hydrographs’

PAST PAPER QUESTIONS 2006 – Hydrosphere Remember to double marks when looking pre-2008