1. A RIVER SYSTEM ACTS LIKE A SYSTEM OF DOWNPIPES AND GUTTERING ON A HOUSE - IT ALLOWS THE MOVEMENT OF RAINWATER INTO THE SEA

2. Watershed (higher ridge of land, the boundary between one drainage basin and another) Source (the place where a river begins -a river system will have numerous sources, such as springs) Confluence (a place where two rivers meet) Tributary stream (a smaller river that flows into a larger river) A TYPICAL RIVER DRAINAGE BASIN. THIS IS THE CATCHMENT AREA FOR ALL THE RAIN THAT FALLS WITHIN THE WATERSHED Mouth

4. All rain falling within a drainage basin ends up in the river system. How does it get there?

5. OVERGROUND Water runs downhill over the ground towards a river or stream - SURFACE RUNOFF This is the fastest way for water to get to a river (other than landing directly in it). The faster rain can reach a river the higher the discharge will be. Higher discharge = More likely to flood

6. UNDERGROUND If the rocks are permeable, rain will go further down into the rock (percolation) and then move very very slowly to the river (Groundwater Flow) Raindrops INFILTRATE soil and rock. The water then moves underground towards a stream or river. Movement through the soil is slow and is known as Throughflow

8. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Flood Hydrograph

9. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s)

10. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) mm 4 3 2 Rainfall shown in mm, as a bar graph

11. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) mm 4 3 2 Discharge in m 3 /s, as a line graph

12. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb mm 4 3 2 Rising limb The rising flood water in the river

13. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb mm 4 3 2 Peak flow Maximum discharge in the river

14. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb Recession limb mm 4 3 2 Peak flow Recession limb Falling flood water in the river

15. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Basin lag time Time difference between the peak of the rain storm and the peak flow of the river

16. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Base flow Normal discharge of the river

17. Storm Hydrographs Storm hydrographs show how a river’s discharge changes after a storm. Depending on a number of factors, the hydrograph can change considerably in shape. Video

18. Flashy or Flat?

19. Peak rainfall Peak discharge 1.After how many hours does rainfall peak? 2.What is peak river discharge (in Cumecs) 3.What is the lag time in Scenario 1? 4.What kind of river basin characteristics would produce this kind of fast response? 5.What is the main way that rainfall likely travelled to the river? A FLOOD HYDROGRAPH

20. Peak rainfall Scenario 2 Peak discharge 1.After how many hours does rainfall peak? 2.What is peak river discharge (in Cumecs) 3.What is the lag time in Scenario 2? 4.What are the likely characteristics of this basin? 5.What are the main ways that rainfall likely travelled to the river? A FLOOD HYDROGRAPH

21. So: Surface runoff Shorter lag time Higher river peak discharge More likelihood of FLOODING

22. What conditions will make surface runoff more likely? The rainfall itself - very heavy, in big droplets (less likely to infiltrate) Water table already very high (little scope for infiltration if ground is already saturated) Steep slopes in drainage basin (water flows downhill rapidly -less time for infiltration to occur Non-permeable rock within drainage basin (water can’t infiltrate easily) Few trees (tree roots break the soil, providing little gaps for water to infiltrate). Trees then take up and store water from underground Urbanisation within the drainage basin - large areas concreted or tarmaced over, making surface non- permeable. (Storm drains are built in, of course, but these carry water very quickly to the river, making lag time even shorter)

23. Storm drains are connected to rivers and streams by an underground network of pipes, called the storm sewer system, which helps to prevent flooding from rain.

24. It is important that we have some understanding of drainage basin conditions and storm hydrographs (both natural and man made) if we are to understand what can be done to reduce future flood risks

25. Which rivers are most likely to flood and why? River A (Mostly rural) vs River B (Mostly Urban) River C (Steep valley) vs River D (Gentle sloping landscape) River E (Porous, permeable rock) vs River F (Impermeable rock) River G (Thin Soils) vs River H (Deep soils) River I (A month of rain) vs River J (Mostly dry in previous month) River K (Woodland area) vs River L (Deforested area) River K (In Winter) vs River K (In Summer)