The Storm Hydrograph Specification Focus: Factors affecting discharge: the storm hydrograph. Bar and line graphs Learning Outcomes: recognise the regimes of different rivers and explain the differences. be able to draw and label a storm hydrograph. explain the physical and human factors that alter the storm hydrograph.

Stop! Starter Draw a simple water budget graph. You have 5 minutes to (from memory): Draw a simple water budget graph. Label the points where there is water surplus, soil moisture utilisation and soil moisture recharge and explain these terms. 10 River Regime: the variability in the discharge of a river during the year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics Stop! 2

River Regimes Rivers can have distinct patterns of A river regimes shows the annual pattern of discharge (water level) in a river. Copy the definition into your notes: River Regime: the variability in the discharge of a river during the year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics Rivers can have distinct patterns of discharge depending on a range of factors. These include climate, drainage basin characteristics and human activities 15 3

Vegetation and Land use Rainfall Climate Human activities Size and shape of river basin Shape of the land Soil type and depth Bedrock 10 How will these factors influence water flows within a drainage basin? Write down some ideas then discuss them with the person next to you. 4

River Regimes An Alpine river Discharge (Q) 15 J F M A M J J A S O N D An Alpine river The drainage basin contains glaciers and snowfields, which feed the river. Snow falls in the winter and melts in the summer. This is reflected in the discharge of the river. 6

River Regimes A river in the Sahara Desert Discharge (Q) 15 J F M A M J J A S O N D A river in the Sahara Desert The climate is very dry as most of the drainage basin is desert, so river levels are very low or non existent. It only rains occasionally, but when it does rain it rains a lot in a short period of time. This can cause a flash flood, and temporarily raise the river levels very rapidly. 7

River Regimes A medium river in England… Discharge (Q) 15 J F M A M J J A S O N D A medium river in England… This drainage basin has well drained soils and is covered by vegetation. The rock underneath is permeable chalk so the river levels remain relatively constant.... 8

River Regimes A large river in the Mediterranean… Discharge (Q) 15 J F M A M J J A S O N D A large river in the Mediterranean… The Mediterranean climate has hot, arid summers but there can be intense downpours of rain during the winter. The rock underneath this drainage basin is impermeable chalk. 9

Hydrographs River Regime Storm Hydrograph The water levels in a river can be represented in a special type of graph, the hydrograph. You already know one type of hydrograph… Long period of time (a year) Shows response to climate River Regime Short period of time (hours – days) Shows response to a single weather event Storm Hydrograph (or Flood Hydrograph) 10

Hydrographs The amount of water flowing in a channel is called the discharge and is calculated by multiplying velocity by cross sectional area of the river: Q (discharge) = V (velocity)  A (cross sectional area) Q = VA 20 Use your textbook to: 1) write a definition for discharge 2) give the name of the unit it is measured in and 3) explain how the discharge of a drainage basin can be calculated 11

The Storm Hydrograph

The Storm Hydrograph The time from peak rainfall to peak discharge is the LAG TIME. The soil becomes saturated and overland flow and through flow reach the river and discharge increases. Overland flow arrives first. The discharge starts to fall slowly as water is added from through flow and groundwater flows which are much slower. The base flow supplies the river with water between storms and keeps it flowing in summer. Start of the storm there is a slow rise in discharge, as only a small amount of water falls into the channel Rainfall is intercepted or infiltrated into the soil moisture store

Key words you need to know! The Storm Hydrograph Key words you need to know! Estimate of the water that flows into a river from groundwater stores. LAG TIME BASEFLOW The time at which river flow is at its maximum. PEAK RAINFALL Rate at which river Q increases after a weather event. RISING LIMB Period of time between peak rainfall and peak discharge RECEDING LIMB 15 The time at which precipitation is at its maximum. PEAK DISCHARGE The rate at which river discharge falls. ANTECENDENT DISCHARGE The discharge before the storm event. 14

Use the data below to construct your own Storm hydrograph Hydrographs Day Rainfall (mm) Discharge (cumecs) 1 2 5 3 30 4 45 8 6 12 7 14 9 10 11 15

The football referee is deciding if the match should be postponed. 12 108642 Label the 6 key words to your graph: RISING LIMB BASEFLOW PEAK DISCHARGE PEAK RAINFALL LAG TIME RECEDING LIMB Then add the following labels to make a ‘living hydrograph’ Luke’s new carpet is ruined as water starts to come in under the door. The football referee is deciding if the match should be postponed. The Old Bridge at Singleton is re-opened as river levels start to subside Lucy hears a flood warning and places sandbags by the door. A canoe race is restarted as the river returns to safe levels Extension activity: What is the peak discharge? What is the lag time? Add your own labels to the graph. 16

It should look something like this! Lag time Rising Limb Falling Limb Peak rainfall 45 Baseflow Peak discharge 17

Factors affecting The Storm Hydrograph Imagine that these two areas have had the same amount of rain over a 24-hour period. How would the hydrographs be different for these two areas? 18

FACTORS WHICH INFLUENCE STORM HYDROGRAPHS VEGETATION COVER This varies seasonally. The type and amount will affect interception and stemflow/throughfall. Overland flow is reduced. Lag time will be increased. ROCK TYPE Impermeable rocks prevent groundwater flow and encourage through flow and overland flow. These rocks will decrease lag time. Permeable rock will have the opposite effect. CLIMATE The distribution of rainfall over the year and the temperatures will affect the lag times. LAKES & RESERVOIRS These will store floodwater and thus reduce lag time and control river response to heavy rainfall. FACTORS WHICH INFLUENCE STORM HYDROGRAPHS SLOPES Steep slopes will encourage overland flow and gentle slope will slow run off down. DRAINAGE BASIN Higher drainage density = greater risk of flood. Smaller the basin = faster the water reaches the channel. RAINFALL INTENSITY & DURATION Intense rain will increase overland flow and reduce lag times. Gentle rain over a longer time will allow more infiltration. LAND USE Impermeable surfaces created by urbanisation will reduce infiltration and encourage overland flow. Different types of crops affect interception rates e.g. cereals 7-15%. SOIL TYPE & DEPTH Deep soils store more water, pipes in the soil encourage through flow. Soils with small pore spaces will reduce infiltration and increase overland flow. 19

Flashy or Delayed? High Discharge Gentle rising limb Gentle falling limb Steep falling limb Steep rising limb Long lag time Short lag time Low discharge

Flashy or Delayed? You need to decide whether these factors would cause a flashy or a delayed hydrograph First sort them into 2 piles. Then you can mix them up again and test each other on individual factors. For each one you have to explain WHY the factor would cause a flashy or delayed hydrograph 15 High drainage density Low drainage density 21

Answering Extended Questions What is the question asking? Think: command words, limits, examples Structure – beginning (introduction), middle (argument), end (conclusion). PEE paragraphs – Point, evidence/example and explanation. Evaluate – show you’ve considered more than one view and show how the evidence affects your argument. Topic sentence – “There are several theories why flood damage is increasing in the UK”. Evidence – examples, facts, figures Diagrams – only if useful! Good to example key processes or concepts that are hard to concisely explain. Timing – writing for about 15 mins Conclusion – draw together your argument and answer the question. Make a rough plan of key points 22

The Storm Hydrograph What have we learnt today? Specification Focus: Factors affecting discharge: the storm hydrograph. Bar and line graphs What have we learnt today? - With the person next to you decide on:  One thing you learnt  One thing you already knew  One thing you found difficult Learning Outcomes: recognise the regimes of different rivers and explain the differences. be able to draw and label a storm hydrograph. explain the physical and human factors that alter the storm hydrograph.

Homework Read pages 6-7 about the Carlisle floods Complete the questions on page 8 – pay close attention to question 5 which should be written as an extended answer (essay). Complete homework sheet 1 24