Weekly lesson objectives Review the plotting and analysis of flood hydrographs Develop your knowledge and understanding of the effect of global warming on flooding in the Brahmaputra river Prepare and submit your binders for the quarterly binder check (minor – Thursday) Complete the Hydrology Decision Making exercise post test (Medium)

Quick Quiz 1) On the hydrograph – show the peak flow/ discharge 2) On the hydrograph – show the rising limb 3) On the hydrograph – show the falling limb 4) On the hydrograph - What the maximum rainfall? 5) Explain what the basin lag time is 6) How does interception by vegetation influence the flood hydrograph? 7) How do tides influence the flood hydrograph? 8) How do concrete roads influence the flood hydrograph? 9) List three factors that effect basin lag time

Activity: create your own flood hydrograph with the information given, and answer the questions

Flood Hydrographs

Key words Discharge – Depends on the rivers velocity and volume Velocity – is the speed of the river. It is measured in meters per second Discharge is the velocity of a river times its volume. It’s the amount of water in a river passing a given point at a given time, measured in cumecs (cubic meters a second)

Hydrograph River Discharge Flood Hydrographs = cross sectional area X 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) Flood Hydrographs Show the change in discharge caused by a period of rainfall

Measuring river velocity with a current meter

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

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, (cumecs) 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 3 2 Discharge (m3/s) 1 0 12 24 36 48 30 72 The rising flood water in the river (when the storm begins, the river does not respond immediately – the first water to reach the river is through surface runoff and throughflow) 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

Maximum discharge in the river (you can see this is after the rain) Peak flow/discharge Peak flow Maximum discharge in the river (you can see this is after the rain) 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

Falling flood water in the river Recession limb Peak flow Falling flood water in 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

Basin lag time Basin lag time Peak flow Time difference between the peak of the rain storm and the peak flow of 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

Note sharing activity In groups – go around and get info of other students to complete your hydrograph analysis mind map

Factors influencing Flood Hydrographs Paragraph: Describe the hydrograph and give reasons for why it looks the way it does Shape Rock Type Slope Precipitation / Temp Land Use Factors influencing Flood Hydrographs Area Tidal Conditions Soil Drainage Density ©Microsoft Word clipart

Factors influencing Flood Hydrographs Shape Rock Type Porous Slope Precipitation / Temp Land Use Factors influencing Flood Hydrographs Area Tidal Conditions Soil Drainage Density ©Microsoft Word clipart

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

Here are some theoretical interpretations of influencing factors BUT…… When interpreting hydrographs all factors must be considered together ! ©Microsoft Word clipart

Area Large drainage basins receive more precipitation than small therefore have larger runoff (area the water has to travel to reach the trunk river) Larger size means longer lag time between peak rainfall and peak discharge as water has a longer distance to travel to reach the trunk river Area Rock Type Drainage Density Shape Soil Precipitation / Temp Slope Land Use Tidal Conditions

Shape Elongated drainage basin will produce a lower peak flow and longer lag time than a circular one of the same size – because the water takes longer to reach the river Area Rock Type Drainage Density Shape Soil Precipitation / Temp Slope Land Use Tidal Conditions

Slope Channel flow can be faster down a steep slope therefore steeper rising limb (because the water is moving faster – higher velocity) and shorter lag time Area Rock Type Drainage Density Shape Soil Precipitation / Temp Slope Land Use Tidal Conditions

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

Land Use Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag Forests - intercepts the precipitation, creating a shallow rising limb and lengthening the time lag Area Rock Type Drainage Density Shape Soil Precipitation / Temp Slope Land Use Tidal Conditions

Drainage Density A higher density will allow rapid overland flow (because there are many streams) Area Rock Type Drainage Density Shape Soil Precipitation / Temp Slope Land Use Tidal 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 Area Rock Type Drainage Density Shape Soil Precipitation / Temp Slope Land Use Tidal 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 Area Rock Type Drainage Density Shape Soil Precipitation / Temp Slope Land Use Tidal Conditions

These influencing factors will: Change throughout the rivers course Remember! These influencing factors will: Influence each other Change throughout the rivers course ©Microsoft Word clipart

A flood hydrograph

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 Activity: Label your flood hydrograph

Activity: Interpret the graph and complete the following questions on your worksheet a) Using the words below, copy and complete the following paragraph: peak rainfall falling limb rising limb lag time peak discharge The graph shows a steep ____________ as the flow of the river increases. When the ______________ is reached a gentler _______________ records the gradual decrease in discharge as the rainwater drains away. The gap between _____________ and peak discharge is called the _________________. b) i) After how many hours from the time of peak rainfall did the river reach its peak discharge? _________________ ii) For how many hours did the river flow above its normal level? ___________________ iii) Give two reasons why most rivers do not respond immediately to rainstorms. c) Which of the following rivers is most likely to flood:River ___ River A A low discharge and lengthy lag time. River B A high discharge and a short lag time.

Answers a) Using the words below, copy and complete the following paragraph: peak rainfall falling limb rising limb lag time peak discharge The graph shows a steep rising limb as the flow of the river increases. When the peak discharge is reached a gentler falling limb records the gradual decrease in discharge as the rainwater drains away. The gap between peak rainfall and peak discharge is called the lag time. b) i) After how many hours from the time of peak rainfall did the river reach its peak discharge? 8 hours ii) For how many hours did the river flow above its normal level? 21 hours iii) Give two reasons why most rivers do not respond immediately to rainstorms. Soil: Infiltration is generally greater on thick soil, although less porous soils eg. clay act as impermeable layers 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 c) Which of the following rivers is most likely to flood: River River B River A A low discharge and lengthy lag time. River B A high discharge and a short lag time.

Quick Quiz 1) On the hydrograph – show the peak flow/ discharge 2) On the hydrograph – show the rising limb 3) On the hydrograph – show the falling limb 4) On the hydrograph - What the maximum rainfall? 5) Explain what the basin lag time is 6) How does interception by vegetation influence the flood hydrograph? 7) How do tides influence the flood hydrograph? 8) How do concrete roads influence the flood hydrograph? 9) List three factors that effect basin lag time