Analysis of Hydrographs Construction And Analysis of Hydrographs ©Microsoft Word clipart ©Microsoft Word clipart

Hydrograph Record of River Discharge (the level of water flowing down a river channel) over a period of time, they show how certain rivers respond to a rainstorm. River Discharge (the level of water flowing down a river) (is calculated) = 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

Construct & Analyse Hydrographs ? 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 ©Microsoft Word clipart

Storm (flood) Hydrographs Construction ©Microsoft Word clipart Of Storm (flood) Hydrographs

Flood Hydrograph 3 2 Discharge (m3/s) 1 0 12 24 36 48 30 72 Basin lag time Peak flow Flood Hydrograph 3 Rising limb Overland flow Recession limb 2 mm Discharge (m3/s) 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm

Hours from start of rain storm The discharge of the river is measured in cumecs - this stands for cubic metres per second 3 2 1 Discharge (m3/s) 0 12 24 36 48 30 72 Hours from start of rain storm

Rainfall shown in mm, as a bar graph 3 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm

Discharge in m3/s, as a line graph 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm

Rising limb The normal (base) flow of the river starts to rise when run-off, ground and soil water reaches the river. 3 Rising limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm

Peak flow Peak flow Maximum discharge in the river, the time when the river reaches its highest flow 3 Rising limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm

shows that water is still reaching the river but in decreasing amounts Recession limb Peak flow shows that water is still reaching the river but in decreasing amounts 3 Rising limb Recession limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm

The time it takes for the water to find its way to the river Basin lag time Basin lag time Peak flow The time it takes for the water to find its way to the river 3 Rising limb Recession limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm

Normal discharge of the river Base flow Basin lag time Peak flow Normal discharge of the river 3 Rising limb Recession limb 2 mm Discharge (m3/s) 4 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm

+ = Overland flow Through flow Storm Flow 3 2 Discharge (m3/s) 1 Basin lag time + Peak flow 3 Through flow = Rising limb Overland flow Recession limb 2 Storm Flow mm Discharge (m3/s) 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm

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

Analysis ©Microsoft Word clipart

Interpretation of Storm Hydrographs Basin lag time You need to refer to: Peak flow 3 Rising Limb Rising limb Base flow Through flow Overland flow 2 Recession limb mm Recession Limb Discharge (m3/s) 4 1 3 Lag time 2 Rainfall Intensity 0 12 24 36 48 30 72 Hours from start of rain storm Peak flow compared to Base flow Recovery rate, back to Base flow

Some Factors influencing Storm Hydrographs Area Slope Rock Type Land Use Soil Precipitation / Temp ©Microsoft Word clipart

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 Area Rock Type Soil Slope Land Use Precipitation / Temp

Slope Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time Area Rock Type Soil Slope Land Use Precipitation / Temp

Rock Type Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb Area Rock Type Soil Slope Land Use Precipitation / Temp

Soil Infiltration is generally greater on thick soil The more infiltration occurs the longer the lag time and shallower the rising limb Area Rock Type Soil Slope Land Use Precipitation / Temp

Land Use Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag In wooded areas, trees intercept/absorb the precipitation, creating a shallow rising limb and lengthening the time lag Area Rock Type Soil Slope Land Use Precipitation / Temp

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 Area Rock Type Precipitation / Temp Soil Slope Land Use

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