Flood Hydrographs How do we know if a river will flood? How can we represent that graphically?

Essential question How can we analyze flood risk?

Flooding clips http://www.youtube.com/watch?v=XLtTrGjyrBY http://www.youtube.com/watch?v=-s1QPs5NFeQ http://www.youtube.com/watch?v=m835Off4XuM

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

Activities: 1. Create your own flood hydrograph 2. label it 3 Activities: 1. Create your own flood hydrograph 2. label it 3. Analyse the two hydrographs and decide which is more likely to flood and why... Write your answer on the back of the poster

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

Factors influencing Flood Hydrographs 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 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

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

Quick Quiz: Answer in Pairs What is a tributary? Name three ways in which the worlds water is stored From what source do we get most of the water we use? What does confluence mean? What is the term used to describe the downward flow of water through pores in the soil? In the hydrological cycle – where does the energy that causes evaporation come from? Explain how different types of rock and soil influence drainage What types of drainage basin are more likely to flood? Explain. Name one input in the drainage basin system Name one store in the drainage basin system Name an output in the drainage basin system Explain what the basin lag time is How does interception by vegetation influence the flood hydrograph? How do concrete roads influence the flood hydrograph? List three human factors that effect basin lag time List three natural factors that effect basin lag time

Quick Quiz: Answer in Pairs What is a tributary? Name three ways in which the worlds water is stored From what source do we get most of the water we use? What does confluence mean? What is the term used to describe the downward flow of water through pores in the soil? In the hydrological cycle – where does the energy that causes evaporation come from? Explain how different types of rock and soil influence drainage What types of drainage basin are more likely to flood? Explain. Name one input in the drainage basin system Name one store in the drainage basin system Name an output in the drainage basin system Explain what the basin lag time is How does interception by vegetation influence the flood hydrograph? How do concrete roads influence the flood hydrograph? List three human factors that effect basin lag time List three natural factors that effect basin lag time

Factors that affect flood risk… Natural & Human

The hydrograph – a reminder! Key Fact: Shorter Lag Time + Steeper Rising Limb = Increased Flood Risk

Natural & Human Factors causing flooding: With partner list all of the things that may increase flood risk in a drainage basin / river system… Make one list of natural factors that may increase flood risk or cause flooding and another for human factors that may increase flood risk or cause flooding Now try to decide what are the most important three factors in each list! – explain why each factor increases flood risk (think about hydrographs and lag time) Share your answers with the class.

Natural & Human Factors causing flooding: With partner list all of the things that may increase flood risk in a drainage basin / river system… Make one list of natural factors that may increase flood risk or cause flooding and another for human factors that may increase flood risk or cause flooding Now try to decide what are the most important three factors in each list! – explain why each factor increases flood risk (think about hydrographs and lag time) Share your answers with the class.

Tasks: With partner list all of the things that may influence flood risk in a drainage basin / river system… Make one list of natural factors that may influence flooding human factors that may influence flood risk or cause flooding Now try to decide what are the most important three factors in each list! – explain why each factor increases flood risk (think about hydrographs and lag time) Share your answers with the class.

Natural Factors 1 Weather & Climate Precipitation type (Rain Vs Snow) Length of storm in time (leading to saturation) Intensity of rainfall (precipitation exceeds infiltration rate) Seasonality Temperature

Natural Factors 2 Surface Type & Geology Permeability of surface Soil type Rock Type

Natural Factors 3 Vegetation Type (trees, bushes, grasses) Seasonal effects on vegetation (summer vs. winter) Coverage (bare ground Vs Vegetation cover) Interception & storage

Natural Factors 4 Relief (shape of the land) Shape of drainage basin Size of drainage basin Drainage density

Human Factors 1 Urbanization Surface permeability Drainage systems Vegetation removal

Human Factors 2 Agriculture Changes to vegetation Animals Erosion and runoff

Human Factors 3 Changes to the drainage basin River management (dams, flood relief channels) Flood prevention schemes (levees, dredging) Straightening Deforestation

Tasks 2: Now use your text book (New Wider World) Read pages 280 – 281 to reinforce your understanding of these flood risk ideas Now read the case study of flooding in Bangladesh p.288-289… make a mind map to represent this flood event – include all of the factors that led to this flooding… use one color for the natural factors and another color for the human factors.

reflection Reflect on what you have learnt about flooding in the past two lessons Things to mention: Flood hydrographs and what they tell us Natural and Human factors that influence flooding If you have a question on this topic also write this